# AC 25-7A: Flight Test Guide for Certification of Transport Category Airplanes - Type: Advisory circular (guidance, not mandatory unless a rule or AD requires it) - Status: Historical - Version in force: [AC 25-7D CHG 1](https://openairdocs.org/ac/25-7), approved September 16, 2025. This is a historical version - Approved: March 31, 1998 - Last reviewed: June 8, 2015 - Family: AC 25-7 - Cancels: [AC 25-7](https://openairdocs.org/ac/25-7) - Cancelled by: AC 25-7B Change 1 Incorporated - 14 CFR parts: 25 - Pages: 346 - PDF: https://openairdocs.org/files/sha256/62/79/62795057ac9c608484d9a2a71cdf8d4af3053ba78013275e95c4d7876af64624 - FAA DRS: https://drs.faa.gov/browse/excelExternalWindow/4144773715D01E9A86256B9F0072F02A.0001 - DRS record last changed: November 21, 2025 - Page: https://openairdocs.org/ac/25-7A ## Versions - [AC 25-7D CHG 1](https://openairdocs.org/ac/25-7) (Current), approved Sep 16, 2025, 495 pages - [AC 25-7D](https://openairdocs.org/ac/25-7D) (Historical), approved May 4, 2018, 481 pages - [AC 25-7C](https://openairdocs.org/ac/25-7C) (Historical), approved Oct 16, 2012, 425 pages - [AC 25-7B](https://openairdocs.org/ac/25-7B) (Historical), approved Dec 7, 2011, 410 pages. Change 1 Incorporated - [AC 25-7A](https://openairdocs.org/ac/25-7A) (Historical), approved Mar 31, 1998, 346 pages - [AC 25-7](https://openairdocs.org/ac/25-7?v=59084AF5ED98820B8625733E006D28B2) (Historical), approved Apr 9, 1986, 100 pages ## Text Each page's text from the PDF; open the PDF at a page with #page=N. ### Page 1 Subject: FLIGHT TEST GUIDE FOR **CERTIFICATION OF TRANSPORT** CATEGORY AIRPLANES Date: 3/31/98 Initiated By: ANM-110 AC No: 25-7A Change: 1. PURPOSE. This revised advisory circular (AC) provides updated guidance for the flight test evaluation of transport category airplanes. These guidelines provide an acceptable means of demonstrating compliance with the pertinent regulations of 14 CFR part 25. The methods and procedures described herein have evolved through many years of flight testing of transport category airplanes and, as such, represent current certification practice. Like all AC material, these guidelines are not mandatory and do not constitute regulations. They are derived from previous FAA experience in finding compliance with the airworthiness requirements and represent methods and procedures found to be acceptable by that experience. Although mandatory terms such as "shall" and "must" are used in this AC, because the AC method of compliance is not in itself mandatory, these terms apply only to applicants who seek to demonstrate compliance by use of the specific methods described in this AC. 2. CANCELLATION. AC 25-7, "Flight Test Guide for Certification of Transport Category Airplanes," dated April 9, 1986. 3. BACKGROUND. Since AC 25-7 was released on April 9, 1986, it has been the primary source of guidance for flight test methods and procedures to show compliance with the regulations contained in subpart B of part 25, which are related to airplane performance and handling characteristics. For certification flight testing to show compliance with other part 25 regulations, Order 8110.8, "Engineering Flight Test Guide for Transport Category Airplanes," provided guidance for internal FAA use in determining acceptable means of compliance. Order 8110.8, as revised on September 26, 1974, has been subject to five "change" updates to reflect significant policy changes; the last change being the removal of the subpart B-related material concurrent with the original release of AC 25-7. Order 8110.8 reflected the policy in place when Amendment 25-29 to part 25 was adopted, and the original release of AC 25-7 reflected an Amendment 25-59 time frame. Part 25 has been amended significantly since the two referenced documents were last revised and, likewise, guidance and policy have changed in many areas as experience has been gained. During this time period, many advances in technology have also taken place that require new or modified test techniques. This revision updates the original AC to incorporate the latest policy and guidance material ### Page 2 applicable to all sections of part 25. The material related to regulations outside of subpart B supersedes that contained in Order 8110.8, which has been cancelled accordingly upon issuance of this AC. 4. APPLICABILITY. These methods and procedures are promulgated, in the interest of standardization, for use during all transport category airplane flight test certification activities. This material is not to be construed as having any legal status and must be treated accordingly. The procedures set forth herein are one acceptable means of compliance with applicable sections of part 25. Any alternate means proposed by the applicant should be given due consideration. Applicants are encouraged to use their technical ingenuity and resourcefulness in order to develop more efficient and less costly methods of achieving the objectives of Part 25. Since these methods and procedures are only one acceptable means of compliance, individuals should be guided by the intent of the methods provided in this AC. As deviations from the methods and procedures described in this AC may occur, FAA certification personnel will coordinate what they consider to be major deviations with the Transport Standards Staff (ANM-110) of the Transport Airplane Directorate. If in their judgment, however, a deviation is considered to be minor, coordination with ANM-110 may not be necessary. 5. RELATED PUBLICATIONS. Certification personnel should be familiar with FAA Order 8110.4A, "Type Certification Process," and FAA Order 8100.5, "Aircraft Certification Directorate Procedures." In this AC, reference is made to other FAA advisory circulars that provide guidance on various aspects of type certification and supplemental type certification. /s/ Ronald T. Wojnar Manager, Transport Airplane Directorate Aircraft Certification Service ### Page 3 CONTENTS Paragraph Pa ge No. **CHAPTER 1. GENERAL** 1. FAR 25.1 Applicability [RESERVED] 1 2. FAR 25.2 Special Retroactive Requirements [RESERVED] 1 CHAPTER 2. FLIGHT Section 1. GENERAL 3. FAR 25.21 Proof of Compliance … 31 Figure 3-1 Weight Tolerance Limits … 33 Figure 3-2 Equivalent Weight Extrapolation … 34 Figure 3-3 Test Parameters That Normally Can Be Corrected … 35 Figure 3-4 Wind Profile Variation … 36 4. FAR 25.23 Load Distribution Limits [RESERVED] … 42 5. FAR 25.25 Weight Limits [RESERVED] … 42 FAR 25.27 Center of Gravity Li mits [RESERVED] … 42 6. FAR 25.29 Empty Weight and Corresponding Center of Gravity [RESERVED] … 42 7. FAR 25.31 Removable Ballast [RESERVED] … 42 8. FAR 25.33 Propeller Speed and Pitch Limits … 43 Section 2. PERFORMANCE 9. FAR 25.101 General … 71 FAR 25.103 Stalling Speed (see paragraph 29) 10. FAR 25.105 Takeoff … 71 FAR 25.107 Takeoff Speeds … 71 11. FAR 25.109 Accelerate-Stop Distance … 80 Figure 11-1 Accelerate-Stop Time Delays … 80-7 Figure 11-2 Accelerate-Stop Speed Vs. Distance … 80-9 Figure 11-3 Anti-Skid System Response Characteristics (On-Off System) … 80-13 Figure 11-4 Anti-Skid System Response Characteristics (Quasi-Modulating System) … 80-15 Figure 11-5 Anti-Skid System Response Characteristics (Fully Modulating System) … 80-17 Figure 11-6 Instantaneous Brake Force and Peak Brake Force … 80-21 Figure 11-7 Anti-Skid Efficiency-Wheel Slip Relationship … 80-23 Figure 11-8 Substantiation of the Optimal Slip Value … 80-24 12. FAR 25.111 Takeoff Path … 81 13. FAR 25.113 Takeoff Distance and Ta keoff Run … 88 Figure 13-1 Takeoff Distance on a Dry Runway … 89 Figure 13-2 Takeoff Distance (All Engines Operating) … 89 Figure 13-3 Takeoff Distance on a Wet Runway … 90 Figure 13-4 Takeoff Run (Critical Engine Fails at V EF) … 91 Figure 13-5 Takeoff Run (All Engines Operating) … 91 Figure 13-6 Clearway Profiles … 92 14. FAR 25.115 Takeoff Flight Path … 92 ### Page 4 Figure 14-1 Takeoff Segments & Nomenclature … 93 Figure 14-2 Net Takeoff Flight Path … 94 15. FAR 25.117 Climb: General … 94 16. FAR 25.119 Landing Climb: All-Engines Operating … 95 17. FAR 25.121 Climb: One-Engine-Inoperative … 96 18. FAR 25.123 En route Flight Paths … 97 19. FAR 25.125 Landing … 98 Figure 19-1 Landing Time Delays … 103 Section 3. CONTROLLABILITY AND MANEUVERABILITY 20. FAR 25.143 General … 121 Figure 20-1 Sample Pitch Tracking Task … 129 Figure 20-2 A-PC Rating Criteria & Comparison to MIL Standard … 130 Figure 20-3 Example of Minimum Permitted HQ Rating for A-PC Tendencies … 131 21. FAR 25.145 Longitudinal Control … 135 22. FAR 25.147 Directional and Lateral Control … 140 23. FAR 25.149 Minimum Control Speed … 143 Section 4. TRIM 24. FAR 25.161 Trim [RESERVED] … 157 Section 5. STABILITY 25. FAR 25.171 General [RESERVED] … 171 26. FAR 25.173 Static Longitudinal Stability … 171 FAR 25.175 Demonstration of Static Longitudinal Stability … 171 Figure 26-1 Longitudinal Static Stability … 173 27. FAR 25.177 Static Directional and Lateral Stability … 173 28. FAR 25.181 Dynamic Stability … 175 ### Page 5 Section 6. STALLS 29. Stall Testing … 189 FAR 25.201 Stall Demonstration … 189 FAR 25.203 Stall Characteristics … 194 FAR 25.207 Stall Warning … 197 Figure 29-1 Stall Test Time History … 199 Figure 29-2 Stall CLCG vs. Entry Rate … 200 Figure 29-3 Stall CL vs. Weight and Flap Setting … 200 Figure 29-4 Stall Speeds vs. Weight and Flap Setting … 201 Figure 29-5 Thrust Effect on Stall C L … 201 Section 7. GROUND AND WATER HANDLING CHARACTERISTICS 30. General … 211 FAR 25.231 Longitudinal Stability and Control … 211 FAR 25.233 Directional Stability and Contro l … 211 FAR 25.235 Taxiing Condition [RESERVED] … 212 FAR 25.237 Wind Velocities … 212 FAR 25.239 Spray Characteristics, Control, and Stability on Water … 213 Section 8. MISCELLANEOUS FLIGHT REQUIREMENTS 31. FAR 25.251 Vibration and Buffeting … 221 Figure 31-1 Maneuvering Characteristics … 224 (Speeds up to the Lesser of V FC or MFC or Buffet Onset) Figure 31-2 Maneuvering Characteristics … 225 (Speeds between Figure 31-1 and V DF or MDF) 32. FAR 25.253 High Speed Characteristics … 225 33. FAR 25.255 Out-of-Trim Characteristics … 229 Figure 33-1 Mistrimmed Maneuvering Characteristics … 231 CHAPTER 3. STRUCTURE [RESERVED] CHAPTER 4. DESIGN AND CONSTRUCTION Section 1. GENERAL [RESERVED] Section 2. CONTROL SURFACES [RESERVED] ### Page 6 Section 3. CONTROL SYSTEMS 34. FAR 25.671 General … 261 [35-45] [RESERVED] … 261 46. FAR 25.701 Flap and Slat Interconnections … 261 47. FAR 25.703 Takeoff Warning System [RESERVED] … 262 Section 4. LANDING GEAR [48-51] [RESERVED] … 283 52. FAR 25.729 Retracting Mechanisms … 283 53. FAR 25.731 Wheels … 284 54. FAR 25.733 Tires [RESERVED] … 286 55. FAR 25.735 Brakes … 286 56. FAR 25.737 Skis [RESERVED] … 301 Section 5. FLOATS AND HULLS [57-59] [RESERVED] 311 Section 6. PERSONNEL AND CARGO ACCOMMODATIONS [60-61] [RESERVED] … 323 62. FAR 25.773 Pilot Compartment View … 323 [63-72] [RESERVED] … 323 Section 7. EMERGENCY PROVISIONS 73. FAR 25.801 Ditching [RESERVED] … 333 74. FAR 25.803 Emergency Evacuation … 333 [75-83] [RESERVED] … 333 Section 8. VENTILATION AND HEATING 84. FAR 25.831 Ventilation … 343 85. FAR 25.832 Cabin Ozone Concentration [RESERVED] … 343 86. FAR 25.833 Combustion Heating Systems [RESERVED] … 343 Section 9. PRESSURIZATION 87. FAR 25.841 Pressurized Cabins … 353 88. FAR 25.843 Tests for Pressurized Cabins … 355 ### Page 7 Section 10. FIRE PROTECTION [RESERVED] Section 11. MISCELLANEOUS [RESERVED] CHAPTER 5. POWERPLANT Section 1. GENERAL 89. FAR 25.901 Installation [RESERVED] … 387 90. FAR 25.903 Engines … 387 FAR 25.903(b) Engine Isolation … 387 FAR 25.903(c) Control of Engine Rotation … 387 FAR 25.903(d) Turbine Engine Installations … 388 FAR 25.903(e) Engine Restart Capability … 389 91. FAR 25.904 Automatic Takeoff Thrust Control Systems (ATTCS) … 390 92. FAR 25.905 Propellers … 392 93. FAR 25.907 Propeller Vibration [RESERVED] … 392 94. FAR 25.925 Propeller Clearance [RESERVED] … 392 95. FAR 25.929 Propeller Deicing … 392 96. FAR 25.933 Reversing Systems … 393 97. FAR 25.934 Turbojet Engine Thrust Reverser Sys. Tests [RESERVED] … 396 98. FAR 25.937 Turbopropeller-Drag Limiting Systems … 396 99. FAR 25.939 Turbine Engine Operating Characteristics … 398 100. FAR 25.941 Inlet, Engine, and Exhaust Capability (RESERVED … 398 101. FAR 25.943 Negative Acceleration … 399 102. FAR 25.945 Thrust or Power Augmentation System [RESERVE D] … 399 Section 2. FUEL SYSTEM [103-108] [RESERVED] … 419 109. FAR 25.959 Unusable Fuel Supply … 419 110. FAR 25.961 Fuel System Hot Weather Operation … 421 [111-116] [RESERVED] … 423 117. FAR 25.975 Fuel Tank Vents and Carburetor Vapor Vents … 423 [118-120] [RESERVED] … 424 Section 3. FUEL SYSTEM COMPONENTS [121-126] [RESERVED] … 445 127. FAR 25.1001 Fuel Jettisoning System … 445 Section 4. OIL SYSTEM [RESERVED] ### Page 8 Section 5. COOLING [128-130] [RESERVED] … 481 Section 6. INDUCTION SYSTEM 131. FAR 25.1091 Air Induction … 497 132. FAR 25.1093 Induction System Icing Protection … 499 [133-136] [RESERVED] … 501 Section 7. EXHAUST SYSTEM 137. FAR 25.1121 General … 511 [138-140] [RESERVED] Section 8. POWERPLANT CONTROLS AND ACCESSORIES [141-154] [RESERVED] … 521 Section 9. POWERPLANT FIRE PROTECTION [155-158] [RESERVED] … 537 159. FAR 25.1187 Drainage and Ventilation of Fire Zones … 537 [160-164] [RESERVED] … 537 165. FAR 25.1197 Fire Extinguishing Agents … 537 [166-169] [RESERVED] … 538 CHAPTER 6. EQUIPMENT Section 1. GENERAL 170. FAR 25.1301 Function and Installation … 559 171. FAR 25.1303 Flight and Navigation Instruments … 592 172. FAR 25.1305 Powerplant Instruments [RESERVED] … 594 173. FAR 25.1307 Miscellaneous Equipment [RESERVED] … 594 174. FAR 25.1309 Equipment, Systems, and Installations … 594 Section 2. INSTRUMENTS: INSTALLATION 175. FAR 25.1321 Arrangement and Visibility [RESERVED] … 605 176. FAR 25.1322 Warning, Caution, and Advisory Lights [RESERVED] … 605 177. FAR 25.1323 Airspeed Indicating System … 605 178. FAR 25.1325 Static Pressure Systems … 608 179. FAR 25.1326 Pitot Heat Indicator Systems [RESERVED] … 609 180. FAR 25.1327 Magnetic Direction Indicator [RESERVED] … 609 181. FAR 25.1329 Automatic Pilot System … 609 Figure 181-1 Altitude Loss vs. Altitude Go-Around Mode … 617 [182-185] [RESERVED] … 625 Section 3. ELECTRICAL SYSTEMS AND EQUIPMENT 186. FAR 25.1351 General … 635 187. FAR 25.1353 Electrical Equipment and Installations … 637 188. FAR 25.1355 Distribution System … 639 189. FAR 25.1357 [RESERVED] … 639 190. FAR 25.1363 Electrical System Tests … 639 ### Page 9 Section 4. LIGHTS 191. FAR 25.1381 Instrument Lights … 651 192. FAR 25.1383 Landing Lights … 651 193. FAR 25.1385 Position Light System Installation … 651 [194-200] [RESERVED] … 652 201. FAR 25.1401 Anticollision Light System … 652 202. FAR 25.1403 Wing Icing Detection Lights … 652 Section 5. SAFETY EQUIPMENT [203-207] [RESERVED] … 659 Section 6. MISCELLANEOUS EQUIPMENT 208. FAR 25.1431 Electronic Equipment … 669 [209-223] [RESERVED] … 669 CHAPTER 7. OPERATING LIMITATIONS & INFORMATION Section 1. [RESERVED] Section 2. Operating Limitations [RESERVED] Section 3. Markings and Placards [RESERVED] Section 4. Airplane Flight Manual 224. FAR 25.1581 General … 679 [225-227] [RESERVED] … 679 ### Page 10 **CHAPTER 8. AIRWORTHINESS-MISCELLANEOUS ITEMS** 228. Design and Function of Artificial Stall Warning and Identification Systems … 701 229. Reduced and Derated Thrust Takeoff Operations … 704 230. Runway Gradients Greater than +2 Percent … 705 231. Performance Requirements for Flight in Icing Conditions … 707 232. Flying Qualities in Natural Icing Conditions … 708 233. Criteria for Approval of Steep Approach to Landing … 709 234. Takeoff and Landing on Unpaved Runways … 713 235. Accountability of Performance De gradation Relative Both Minor Design Changes (External Exposure) and Configuration Deviation List (CDL) Items … 716 236. Configuration Deviation List (CDL) … 717 237. Spare Engine Pod 719 238. Authorization for Ferry Flight with One Engine Inoperative … 719 239. Instrument Landing System Weather Minima … 720 240. Takeoff Performance Credit for Alternate Forward Center of Gravity Limits … 721 241. Performance Credit for Automatic Propeller Drag Reduction Devices … 724 242. Airplane Backing Using Reverse Thru st … 725 APPENDICES Appendix 1 History of Jet Transport Performance Standards (CAR 4b Special Regulations 422, 422A and 422B) Appendix 2 History of Accelerate-Stop Transition Time Development Appendix 3 Correction of Air Minimum Control Speed (V MCA) to Standard Conditions Appendix 4 Rudder Pedal Force-Limited Air Minimum Control Speed (V MCA) Appendix 5 One-G Stall Compliance Basis Appendix 6 Sample Flight Test Program [RESERVED Appendix 7 FAA Handling Qualities Rating System Appendix 8 Functional and Reliability Tests (§ 21.35(b)(2)) ### Page 11 CHAPTER 1 – GENERAL 1.-2. [RESERVED] ### Page 12 Chap 2 12 CHAPTER 2 - FLIGHT Section 1. GENERAL 3. PROOF OF COMPLIANCE - § 25.21. a. Explanation. In an effort to provide the necessary guidelines for the flight test evaluation of transport category airplanes, without producing a cumbersome document, this advisory circular assumes a conventional transport airplane configuration. In general, a “conventional” airplane configuration is one with: distinct wing and fuselage elements that are joined together; aft-mounted horizontal and vertical stabilizers that are attached to the fuselage; and propulsion provided either by turbojet/turbofan engines that do not provide any significant increase in lift due to their operation or engine-driven propellers. The effects of non-conventional airplane configurations (e.g., blown flaps) on the compliance methods should be evaluated and determined based on the intent of the guidelines presented for conventional airplane configurations. (1) Section 25.21(a) - Proof of Compliance. (i) The burden of showing compliance with the flight requirements for an airworthiness certificate or a type certificate rests with the applicant. The applicant should, at his own expense and risk, conduct such official flight tests as required by the FAA to demonstrate compliance with the applicable requirements. During the certification process, the applicant should make available the airplane, as well as all of the personnel and equipment necessary to obtain and process the required data. (ii) If the airplane flight characteristics or the required flight data are affected by weight and/or center of gravity (c.g.), the compliance data should be presented for the most critical weight and c.g. position. Unless it can be shown that the allowable c.g. travel in one or more axes (e.g., lateral fuel imbalance) has a negligible effect on the airworthiness requirements, compliance at the critical c.g. must be substantiated. (iii) The gross weight and c.g. tolerances specified in paragraphs 3a(3)(ii)(A) and (D) are test tolerances and are not intended to allow compliance to be shown at less than critical conditions. (iv) Section 25.21(a)(1) requires, in part, that the test airplane be in conformity with its type design specifications. (A) This is interpreted as meaning the test airplane must be in conformity with its type design specification as relates to the particular test being conducted. (B) Any deviation from conformity must be clearly shown to be of no consequence to the particular test being conducted. For example, if the slip resistant ### Page 13 Chap 2 13 escape surface required by §25.810(c) is not installed, it must be shown that its presence would have no effect on measured airplane performance and flight characteristics. (2) Section 25.21(c) - Proof of Compliance (Altitude Effect on Flight Characteristics). (i) Any of the flying qualities, including controllability, stability, trim, and stall characteristics, affected by altitude should be investigated for the most adverse altitude conditions approved for operations and limited, as appropriate, in accordance with § 25.1527. (ii) Consideration should be given in the test program to aerodynamic control system changes with altitude (e.g., control throws or auto slats which are sometimes inhibited by Mach number at altitude). (3) Section 25.21(d) - Proof of Compliance (Flight Test Tolerances). (i) To allow for variations from precise test values, certain tolerances during flight testing should be maintained. The purpose of these tolerances is to allow for variations in flight test values from which data are acceptable for reduction to the value desired. They are not intended for tests to be routinely scheduled at the lower weights, or to allow for compliance to be shown at less than the critical condition; nor are they to be considered as allowable inaccuracy of measurement. As an example, when demonstrating stability with a specified trim speed of 1.4VS1, the trim speed may be 1.4VS1 +3 knots or 3 percent; however, no positive tolerance is permitted when demonstrating the minimum prescribed trim speed of 1.4VS1 (Ref. § 25.161). (ii) Where variation in the parameter on which a tolerance is allowed will have an effect on the results of the test, the results should be corrected to the most critical value of that parameter within the operating envelope being approved. If such a correction is impossible or impractical (e.g., performance at forward c.g.), the average test conditions should assure that the measured characteristics represent the actual critical value. (A) Weight limits. Figure 3-1, below, presents weight tolerances that have been found acceptable for the specified flight tests. Many flight tests need to be conducted at or very near the maximum operating weight for the airplane configuration, particularly those tests used to establish Airplane Flight Manual performance information. As noted in paragraph (i), above, the purpose of the test tolerances is to allow for variations in flight test values, not to routinely schedule tests at less than critical weight conditions or to allow for compliance to be shown at less that the critical weight condition. ### Page 14 Chap 2 14 FIGURE 3-1. WEIGHT TOLERANCE LIMITS Flight Test Conditions Weight Tolerance Limit +5% +10% Stall Speeds X Stall Characteristics X Climb Performance X Takeoff Flight Paths X Landing Braking Distance X Landing Air Distance X Takeoff Distance & Speed X Accelerate-Stop Distance X Maximum Energy RTOs X Minimum Unstick Speed X (1) For follow-on airplane certification programs (e.g., minor gross weight increase), where it is desired to use an existing certificated data base from an aerodynamically similar model of the same airplane type, the test weight tolerance limits of Figure 3-1 have also been applied as extrapolation limits to minimize additional test requirements. (2) For follow-on airplane certification programs, test data may be extrapolated to higher gross weights than specified in Figure 3-1, if the available test data includes an adequate range of weights and an appropriate number of points at the highest weight tested. If the test data analysis verifies the predicted effect of weight, then other extrapolation limits may apply. (3) Equivalent weight extrapolation limits. For follow-on airplane certification programs where it is desired to increase a maximum operating weight based on existing certified performance parameters that have weight as one of their independent terms, those parameters should be examined for equivalent compliance with the weight tolerance limits of Figure 3-1. An example would be the reduction of an airplane's landing flap position, to one approved on a similar model of the same airplane type, which would incur an increase in landing speeds and brake energy, relative to the original certificated landing flap, at any given weight. The brake energy, at the maximum certificated landing weight, should be calculated for the reduced landing flap. This brake energy should account for the increased landing speeds and reduced aerodynamic drag associated with the reduced flap setting. It should then be determined what "equivalent" gross weight would have rendered that brake energy with the original landing flap (see Figure 3-2). The resulting "equivalent" gross weight should not exceed the certificated maximum landing weight by more than the five percent weight extrapolation limit specified in Figure 3-1. If the "equivalent" gross weight (at the existing landing flap setting) is not more than five percent greater than the certificated ### Page 15 Chap 2 15 maximum landing weight, the reduced flap certification may be eligible for a limited flight test program (e.g., stall speed verification, handling characteristics, and qualitative landing demonstration). Further limitations may be imposed by the criteria of Technical Standard Order (TSO) C26c ("Aircraft Wheels and Wheel-Brake Assemblies, with Addendum I," dated May 18, 1984), wheel fusible plug energy limits, quick turnaround considerations, the maximum demonstrated kinetic energy level, wheel spin-up loads, etc. FIGURE 3-2. EQUIVALENT WEIGHT EXTRAPOLATION (B) Wind Limits. For takeoff and landing tests, a wind velocity limit of 10 knots (from any direction) or 0.12VS1 (whichever is lower) at the height of the mean-aerodynamic-chord (MAC), as determined with the airplane in a static ground attitude, has been considered the maximum acceptable. When test wind velocity is to be measured at the wing MAC height, a measurement height of six feet above the ground should be considered as a limit to avoid possible measurement inaccuracies due to surface interference. Because of likely unsteady wind conditions, it is generally considered that takeoff and landing performance data obtained under runway wind conditions greater than 5 knots are likely to be inconsistent and unreliable. If performance data are obtained with winds greater than 5 knots, these data should not necessarily be ignored. (C) C.G. Limits. A test tolerance of +7 percent of the total c.g. range is intended to allow some practical relief for inflight c.g. movement. This relief is only acceptable when the test data general scatter is on both sides of the limiting c.g. or when c.g. correction from test c.g. to limit c.g. is acceptable. (D) Airspeed Limits. Normally, tests conducted within 3 percent or 3 knots (whichever is the higher) of the desired test speed are considered acceptable. ### Page 16 Chap 2 16 (E) Thrust Limits. Thrust critical tests, such as minimum control speeds, should be conducted at the highest thrust (power) allowable on the engine given the constraints of temperature and altitude. It is then permitted to calculate further corrections to allow extrapolation of data to cover the entire operating envelope. These thrust corrections are limited to 5 percent of test day thrust, unless a detailed analysis is performed (see Appendix 3 of this AC). (iii) Because the tolerance values normally are not considered in the airplane design substantiation, it is not the purpose of these tolerances to allow flights at values in excess of those authorized in the type design. If such flights are to be conducted, adequate structural substantiation for the flight conditions should be available. These flights, however, are always conducted under controlled conditions and with the flight test crew's full cognizance of the situation. Examples of such flights are: (A) Takeoff at greater than maximum takeoff weight for the purpose of reaching a test area at the maximum takeoff weight. (B) Landing at greater than maximum landing weights during the course of conducting takeoff tests. (C) Flights to obtain data for future approvals beyond that substantiated for the initial type design. (iv) The table in Figure 3-3 indicates the cases in which corrections are normally allowed. Any corrections to flight test data should be made by methods that are agreed to by the FAA. FIGURE 3-3. TEST PARAMETERS THAT NORMALLY CAN BE CORRECTED Flight Test Condition Correctable Parameters Wt. C.G. Airspeed Altitude Power/ Thrust Wind Airspeed calibration X --- --- --- --- --- Stall speeds X X --- --- X --- Climb performance X X X X X --- Landing performance X --- X X --- X Takeoff performance X X --- X X X Accelerate-stop perf. X X --- X X X Minimum control speed --- --- --- --- X --- Minimum unstick speed X X X --- X --- Buffet boundary X X --- X --- --- (v) All instrumentation used in the flight test program should be appropriately calibrated and acceptable to the FAA test team. ### Page 17 Chap 2 17 (4) Section 25.21(f) - Proof of Compliance (Wind Measurement and Corrections). The relationship between the wind measured at one height and the corresponding wind at another height may be obtained by the following equation: VW2 = VW1(H2/H1)1/7 Where: H = Height above the runway surface VW2 = Wind velocity at H2 VW1 = Wind velocity at H1 This equation is presented graphically below. Values of H less than 5 feet should not be used in this relationship. **FIGURE 3-4. WIND PROFILE VARIATION** (5) Wind Profile Variation for Test Data. The performance data of airplanes should be obtained in such a manner that the effect of wind on the test data may be determined. The test wind velocity should be corrected from the recorded height above the test surface to the height of the airplane wing mean-aerodynamic-chord. If the wind profile variation is not measured, the variation may be calculated using the equation in paragraph (4) above. The following examples are methods of handling wind profile variation data. Other methods have also been found acceptable. 0 10 20 30 40 50 60 70 0 10 20 30 40 50 60 WIND VELOCITY - KNOTS 10 meters ### Page 18 Chap 2 18 Example: Test Data Given: o Height of mean-aerodynamic-chord with airplane on surface 8 ft. o Height of wind measurement 6 ft. o Measured wind velocity 4.8 kts. Results: o Test wind velocity with airplane 50 ft. above landing surface 4.8((50 + 8)/6)1/7 = 6.6 kts. o Test wind velocity with airplane 35 ft. above takeoff surface 4.8((35 + 8)/6)1/7 = 6.4 kts. o Test wind velocity with airplane on surface 4.8(8/6)1/7 = 5.0 kts. (6) Wind Profile Variation for Airplane Flight Manual (AFM) Data. When expanding the data to the AFM conditions, the result should include the effective velocity, at the airplane's wing mean-aerodynamic-chord, which corresponds to the wind condition as measured at 10 meters (32.81 ft.) above the takeoff surface, and corrected for wind factors of § 25.105(d)(1). Example: Airplane Flight Manual Data Given: o Height of mean-aerodynamic-chord with airplane on surface 8.0 ft. o Reported head wind at 10 meters 40.0 kts. o Section 25.105(d)(1) wind factor 0.5 Results: o Factored wind velocity with airplane 50 ft. above landing surface (0.5)(40)((50 + 8)/32.81)1/7 = 21.7 kts. o Factored wind velocity with airplane 35 ft. above takeoff surface (0.5)(40)((35 + 8)/32.81)1/7 = 20.8 kts. o Factored wind velocity with airplane on surface (0.5)(40)(8/32.81)1/7 = 16.3 kts. (7) Airplane Airspeed Variation Due to Wind Profile Variation With Simultaneous Speed Changes Due to Airplane Dynamic Performance. In the reduction of test data and in the expansion of such data to AFM conditions, the increase or decrease of speed due to the dynamic effect of the forces on the airplane are manifested by only the change in ground speed. These changes in speed (zero wind speed changes) may be ### Page 19 Chap 2 19 generalized either as speed increments or speed ratios. The changes in airspeed due to wind profile are superimposed on these speed changes. Example: Takeoff Test Data Given: o True airspeed at liftoff, VLOF 144 kts. o True airspeed at 35 ft. above takeoff surface 147 kts. o Test wind at liftoff 5.0 kts. o Test wind with airplane 35 ft. above takeoff surface 6.4 kts. Results: o Speed change due to airplane dynamic performance, zero wind speed change (V35-VLOF) = (147-6.4)-(144-5.0) 1.6 kts. Example: Takeoff--Airplane Flight Manual Data Given: o Factored wind at liftoff 16.3 kts. o Factored wind with airplane 35 ft. above takeoff surface 20.8 kts. o Zero wind speed change, (V35-VLOF) 1.6 kts. o Zero wind speed change, (VLOF-VR) 0.5 kts. o True airspeed required at 35 ft. 150 kts. Results: o Ground speed required at 35 ft. 150-20.8 = 129.2 kts. o Ground speed at liftoff 129.2-1.6 = 127.6 kts. o True airspeed at liftoff 127.6+16.3 = 143.9 kts. o Ground speed at rotation 127.6-0.5 = 127.1 kts. o True airspeed at rotation (for distance calculations) 127.1+16.3 = 143.4 kts. o Airplane flight manual rotation speed, true airspeed 150-0.5-1.6 = 147.9 kts. NOTE: The indicated airspeed at rotation (VR), given in the AFM, should be predicated on the required speed at 35 ft. minus the speed increment (V35-VR) assuming no airspeed change due to wind profile (i.e., use the "zero wind speed change" increments shown above). The proper rotation speed, assuming the airplane will gain 4.5 knots due to wind profile, is 143.4 knots. However, if this airspeed increase does not materialize, the airplane will be slow by 4.5 knots at 35 feet. It is therefore more desirable to allow the airplane to attain this 4.5 knots on the ground. Any reduction in field length margin of safety by this increase in VR is adequately compensated for by the 50 percent wind factor. ### Page 20 Chap 2 20 This also eliminates a requirement to adjust VR for wind. However, in calculating the AFM field lengths, the airspeed attained due to wind profile may be included. Example: Landing--Airplane Flight Manual Data Given: o Factored wind with airplane 50 ft. above landing surface 21.7 kts. o Factored wind with airplane on landing surface 16.3 kts. o Zero wind speed change for 50 ft. to touchdown (V50-VTD) 4.0 kts. o True airspeed required at 50 ft. 130 kts. Results: o Ground speed at 50 ft. 130-21.7 = 108.3 kts. o Ground speed at touchdown 108.3-4.0 = 104.3 kts. o True airspeed at touchdown 104.3+16.3 = 120.6 kts. (8) Expansion of Takeoff Data for a Range of Airport Elevations. (i) These guidelines are applicable to expanding takeoff data above the altitude at which the basic or verifying tests were obtained. (ii) In general, takeoff data may be extrapolated above and below the altitude at which the basic test data was obtained without additional conservatism within certain constraints. The extrapolation limits and constraints for expansion of the measured takeoff performance parameters (e.g., data used to develop V/VS=f(T/W)) to operating altitudes is addressed in sub-paragraph 3a(9)(iii), below. Extrapolation of takeoff performance data may be further limited by engine data extrapolation limits, typically to 3,000 feet above the highest altitude at which engine parameters were evaluated for the pertinent power/thrust setting. (See paragraph 9 of this AC for more information on engine testing and data extrapolation.) Certification testing for operation at airports that are above 8,000 feet should also include functional tests of the cabin pressurization system in accordance with paragraph 87b(3) of the AC. (iii) When the basic takeoff test data are obtained between sea level and approximately 3,000 ft., the maximum allowable extrapolation limits are 6,000 ft. above, and 3,000 ft. below, the test field elevation. If it is desired to extrapolate beyond these limits, when proven data reduction and expansion methods are used, one of the two following procedures may be employed: (A) Extrapolation of Performance Data for a Range of Altitudes When Verifying Tests are Not Conducted. The approval of performance data for airport elevations beyond the maximum elevation permitted by basic tests may be allowed without conducting verifying tests, if the calculated data include a conservative factor. This conservatism should result in an increase of the calculated takeoff distance at the ### Page 21 Chap 2 21 desired airport elevations by an amount equal to zero percent for the highest airport elevation approved on the results of the basic tests, and an additional cumulative 2 percent incremental factor for each 1,000 ft. of elevation above the highest airport elevation approved for zero percent conservatism. The 2 percent incremental factor should have a straight line variation with altitude. When performance data are calculated for the effects of altitude under this procedure, the following provisions are applicable: (1) Previously established calculation procedures should be used, taking into account all known variables. (2) The calibrated installed engine power for the pertinent speed and altitude ranges should be used. (3) The brake kinetic energy limits established by airplane ground tests should not be exceeded. (B) Extrapolation of Performance Data When Verifying Tests Are Conducted. (1) If data approval is desired for a greater range of airport elevations, the performance may be calculated from the basic test data up to the maximum airport elevation, provided verifying tests are conducted at appropriate elevations to substantiate the validity of the calculations. The actual airplane performance data from the verifying tests should correspond closely to the calculated performance values. (2) For the verifying tests, it has been found that normally three takeoffs at maximum weights for the elevations tested will provide adequate verification. (3) If verifying tests substantiate the expanded takeoff data, the data may be further expanded up to 6,000 ft. above the altitude at which the verifying tests were conducted. At altitudes higher than 6,000 ft. above the verifying test altitude, the 2 percent per 1,000 ft. cumulative factor discussed in paragraph 3a(9)(iii)(A), above, should be applied, starting at zero percent at the verifying test altitude plus 6,000 feet. (9) Expansion of Landing Data for a Range of Airport Elevations. (i) For turbine powered airplanes without propellers, the landing distance data may be extrapolated to more than 6,000 ft. above the test altitude without the penalty described in paragraph 3a(9), provided the true airspeed effect on the distance is accounted for. (ii) For turbopropeller and reciprocating engine-powered airplanes, the extrapolated performance should include the conservative factor described in paragraph 3a(9). This may not be necessary if it can be shown that the effects of altitude on the ### Page 22 Chap 2 22 landing performance, as affected by propeller drag, are known from data available for the airplane configuration. (10) Tailwind Takeoff and Landing. (i) Wind Velocities of 10 Knots or Less - Approval may be given for performance, controllability, and engine operating characteristics for operations in reported tailwind velocities up to 10 knots, measured at a 10 meter height, without specific flight tests. (ii) Wind Velocities Greater than 10 Knots. (A) Performance. It is considered that takeoff, rejected takeoff, and landing distances, measured in tailwind conditions greater than 10 knots, are unreliable for use in determining airplane performance. Wind conditions of such magnitude are generally not sufficiently consistent over the length of the runway or over the time period required to perform the test maneuver. The 150 percent operational tailwind velocity factor, required by §§ 25.105(d)(1) and 25.125(e), affords a satisfactory method for determination of airplane takeoff and landing performance information and limitations up to a limiting tailwind velocity of 15 knots when using a flight test data base obtained under zero wind conditions. NOTE: The design requirements of § 25.479 (Level landing conditions) also require the effects of increased contact speeds to be investigated if approval for landings with tailwinds greater than 10 knots is desired. (B) Control Characteristics. Airplane control characteristics should be evaluated under the following conditions with the center of gravity at the aft limit and the test tailwind velocity equal to the proposed limit tailwind factored by 150 percent: (1) Takeoff. At light weight with maximum approved takeoff flap deflection, both all-engines operating and one-engine inoperative takeoffs should be evaluated. (2) Landing. Approach and landing at light weight with maximum approved landing flap deflection. (3) Determination of the increased ground speed effect on gear vibration, shimmy, flight director and/or autopilot Instrument Landing System (ILS) approaches, Ground Proximity Warning Systems (GPWS) sink rate modes, etc. (4) If engine idle thrust is increased to account for the increased tailwind velocity, ensure that deviations above the glideslope are recoverable. (C) Weight Limits. In accordance with the requirements of §§ 25.105(d)(1) and 25.125(e), maximum takeoff weight and maximum quick turnaround ### Page 23 Chap 2 23 weight should be determined using brake energies and tire speeds, as appropriate, calculated with the limit tailwind velocity factored by 150 percent. (D) Engine Operating Characteristics. Satisfactory engine operation should be demonstrated at the limit tailwind velocity factored by 150 percent. The demonstrations should include: (1) Zero groundspeed operation. (2) Takeoff power setting procedure used for AFM performance (typically completed by approximately 80 knots), both manually and automatically (autothrottle). (3) Reverse thrust operations. (E) Airplane Flight Manual. The AFM should contain a statement that the limitation for tailwinds greater than 10 knots reflects the capability of the airplane as evaluated in terms of airworthiness but does not constitute approval for operation in tailwinds exceeding 10 knots. b. Procedures. (1) The performance-related flight test procedures are discussed in each of the following paragraphs of this AC: 10. Takeoff and Takeoff Speeds 15. Climb: General 11. Accelerate-Stop Distance 16. Landing Climb 12. Takeoff Path 17. Climb: One Engine Inoperative 13. Takeoff Distance and Takeoff Run 18. Enroute Flight Path 14. Takeoff Flight Path 19. Landing (2) Performance Data for Multiple Flap or Additional Flap Positions. If approval of performance data is requested for flap settings at which no test data is available, the data may be obtained from interpolation of flight data obtained at no less than four flap settings which are within a constant configuration of other lift devices. If the span of flap settings is small and previously obtained data provides sufficient confidence (i.e., the shape of the curves are known and lend themselves to accurate interpolation), data from three flap settings may be acceptable. (3) Flight Characteristics for Abnormal Configurations (Ref. § 25.671(c)). (i) For purposes of this AC, an abnormal configuration is an operational configuration that results from any single failure or any combination of failures not shown to be improbable. ### Page 24 Chap 2 24 (ii) Flight characteristics for abnormal configurations may be determined by test or analysis to assure that the airplane is capable of continued safe flight and landing. Flight tests, if required, should be conducted at the critical conditions of altitude, weight, c.g., and engine thrust associated with the configuration, and at the most critical airspeed between stall warning (see paragraph 29d(2)(ix) of this AC) and the maximum operating airspeed for the configuration. 4.-7. [RESERVED] ### Page 25 Chap 2 25 8. PROPELLER SPEED AND PITCH LIMITS - § 25.33. a. Explanation. None. b. Procedures. The tachometers and the airspeed indicating system of the test airplane must have been calibrated in the last six months. With those conditions satisfied, the following should be accomplished: (1) Determine that the propeller speeds and pitch settings are safe and satisfactory during all tests that are conducted in the flight test program within the certification limits of the airplane, engine, and propeller. This includes establishing acceptable low pitch (flight idle) blade angles on turbopropeller airplanes and verifying that propeller configurations are satisfactory at VMO/MMO to prevent propeller overspeed. (2) Determine that the propeller speeds and pitch settings are safe and satisfactory during all tests that are conducted to satisfy the performance requirements. (3) With the propeller governors operative and the propeller controls in full high r.p.m. position, determine that the maximum takeoff power settings do not exceed the rated takeoff r.p.m. of each engine during takeoff and climb at the best rate-of-climb speed. (4) With the propeller governors made inoperative by mechanical means, maximum power, no-wind, static r.p.m.'s must be determined. On reciprocating engines with the propeller governors operating on the low pitch stop, the engine speeds must not exceed 103 percent of the maximum allowable takeoff r.p.m. On turbopropeller engines, the engine speeds must not exceed the maximum engine speeds allowed by engine and propeller type designs. Note which systems were disabled and how the disablement was done. If maximum takeoff power torque or sea level standard conditions cannot be obtained on the test day, the data must be corrected to these conditions by an acceptable means. A no-wind condition is considered to be a wind of 5 knots or less. The static r.p.m. should be the average obtained with a direct crosswind from the left and a direct crosswind from the right. (5) If the above determinations are satisfactory, the low-pitch stop setting and also the high-pitch stop setting must be measured. These data may have been obtained from the propeller manufacturer and may be used, provided the pitch stops have not been changed since the manufacturer delivered the propeller. If measured, the blade station should be recorded. These blade angles must be included in the Type Certificate Data Sheet. ### Page 26 Chap 2 26 Section 2. PERFORMANCE 9. GENERAL - § 25.101 a. Explanation - Engine Behavior . Section 25.101(c) requires that airplane “performance must correspond to the propulsive thrust available under the particular ambient atmospheric conditions, the particular flight conditions….” Therefore, engine parameter evaluations, engine torque, thrust lapse, propeller thrust, and throttle sensitivity, especially with the Electronic Control Unit off (if installed), should be accounted for at all altitudes up to and including the maximum approved takeoff altitude. b. Procedures . This aspect of the rule may be accomplished by actual flight test, with approved takeoff ratings, at the maximum desired takeoff altitude, or with acceptable parameter variation analysis associated with tests conducted at less than the maximum objective takeoff altitude. (1) One acceptable method of demonstrating high altitude engine operability and performance is to simulate the maximum operating altitude conditions by overboosting (i.e., using a higher than normal power setting) at a lower altitude. (2) It is the applicant’s responsibility, in conjunction with the engine manufacturer, to define the test plan for the engine parameter evaluation up to the maximum approved takeoff altitude. Supportive data from the engine manufacturer on the same “type or series” of engine may also be included to reduce the amount of testing conducted. (3) Extrapolation of engine performance data to 3,000 feet above the highest altitude tested (up to the maximum takeoff altitude to be approved) has been accepted, provided the supportive data, including the flight testing accomplished, establishes a consistent baseline. (4) Detailed guidance for the testing necessary to show compliance with § 25.101(c) may be found in Advisory Circular 25-XX, “Turbojet Operations at High Altitude Airports,” dated (to be released). 10. TAKEOFF AND TAKEOFF SPEEDS - §§ 25.105 AND 25.107 (PART 25 AS AMENDED THROUGH AMENDMENT 25-42) . a. Explanation. The primary objective of the takeoff tests required by § 25.107 is to determine the takeoff speed schedule for all takeoff configurations at all weight, altitude, and temperature conditions within the operational limits selected by the applicant. The provisions of § 25.105 are self-evident and are not repeated or amplified in this discussion. Guidance material for unpaved runway evaluation is contained in Chapter 8 (paragraph 234) of this Advisory Circular (AC). b. Procedures . Although the following speed definitions are given in terms of calibrated airspeed, the Airplane Flight Manual (AFM) presentations shall be given in terms of indicated airspeed. Attention should be paid to all potential sources of airspeed error, but special consideration should be given to airplanes with electronic instruments in the cockpit that apply electronic filtering to the airspeed data. This filtering, which causes a time delay in the airspeed indication, can be a source of significant systematic error in the presentation of airspeed to the flightcrew. During a normal takeoff acceleration, the airplane will be at a higher speed than is indicated by the cockpit instrument, which can result in longer distances than are presented in the AFM, particularly in the event of a rejected takeoff near the indicated V 1 speed. The effects of any time delays caused by electronic filtering, pneumatic system lag, or other sources should be adequately addressed in the AFM speed and distance presentations. Further explanation of airspeed lag, particularly pertaining to airplanes with electronic instruments in the cockpit, and procedures for calibrating the airspeed indicating system (§ 25.1323(b)) are presented in paragraph 177 of this AC. (1) Section 25.107(a)(1) - Engine Failure Speed ( VEF). The engine failure speed (V EF) is defined as the calibrated airspeed at which the critical engine is assumed to fail and must be selected by the ### Page 27 Chap 2 27 applicant. VEF cannot be less than the ground minimum control speed (V MCG) as described in §§ 25.149(e) and 25.107(a)(1). * (2) Section 25.107(a)(2) - V 1. V1 may not be less than V EF plus the speed gained with the critical engine inoperative during the time interval between V EF and the instant at which the pilot takes action after recognizing the engine failure (with the proviso that the time interval may not be less than one second). This action is indicated by pilot initiation of the first action to stop the airplane, such as brakes, throttles, spoilers, etc., during accelerate-stop tests, or by the first control input during V MCG testing. The applicant may choose the sequence of pilot actions. Refer to paragraph 11 of this AC, addressing § 25.109, for a more complete description of rejected takeoff transition procedures and associated time delays. If it becomes evident in expanding the takeoff data for presentation in the AFM that excessive variation in V 1 exists due to the many performance variables involved (variations of +1.5 knots or +100 ft. have been found acceptable), then measures must be taken to reduce the variability displayed in the AFM presentation. Examples of such measures are field length factors, or increments, and multiple web charts (accelerate-go/stop, V 1/VR) for a particular configuration. (3) Section 25.107(b) - Minimum Takeoff Safety Speed (V 2MIN). (i) V2MIN, in terms of calibrated airspeed, cannot be less than: (A) 1.1 times the VMCA defined in § 25.149. (B) 1.2 times VS for two-engine and three-engine turbopropeller and reciprocating engine-powered airplanes and for all turbojet airplanes that do not have provisions for obtaining significant reduction in the one-engine inoperative power-on stalling speed (i.e., boundary layer control, blown flaps, etc.). The value of V S to be used in determining V 2-MIN is the stall speed in the applicable takeoff configuration, landing gear retracted, except for those airplanes with a fixed landing gear or for gear-down dispatch. (ii) V2MIN may be reduced to 1.15 times V S for turbopropeller and reciprocating enginepowered airplanes with more than three engines, and turbojet powered airplanes with adequate provisions for obtaining significant power-on stall speed reduction through the use of such things as boundary layer control, blown flaps, etc. (iii) For propeller-driven airplanes, the diff erence between the two margins, based upon the number of engines installed on the airplane, is because the application of power ordinarily reduces the stalling speed appreciably. In the case of the two-engine propeller-driven airplane, at least half of this reduction is eliminated by the failure of an engine. The difference in the required factors therefore provides approximately the same margin over the actual stalling speed under the power-on conditions that are obtained after the loss of an engine, no matter what the number of engines (in excess of one) may be. Unlike the propeller-driven airplane, the turbojet/turbofan-powered airplane does not show any appreciable difference between the power-on and power-off stalling speed. This is due to the absence of the propeller, which ordinarily induces a slipstream with the application of power causing the wing to retain its lift to a speed lower than the power-off stalling speed. The applicant’s selection of the two speeds specified will influence the nature of the testing required in establishing the takeoff flight path. (4) Section 25.107(c) - Takeoff Safety Speed (V2 ). V2 is the calibrated airspeed that is attained at or before the airplane reaches a height of 35 ft. above the takeoff surface after an engine failure at V EF using an established rotation speed (V R). During the takeoff speeds demonstration, V 2 should be continued to an altitude sufficient to assure stable conditions beyond the 35-ft height. V 2 cannot be less than V 2MIN. In addition, V2 cannot be less than the liftoff speed, V LOF, as defined in § 25.107(f). In accordance with § 25.107(c), V2 in terms of calibrated airspeed “...may not be less than V R plus the speed increment attained before reaching a height of 35 feet above the takeoff surface.” Section 25.111(c)(2) stipulates that the airplane must reach V2 before it is 35 feet above the takeoff surface and continue at a speed not less than V 2 until it is 400 feet above the takeoff surface. These requirements were first expressed in SR-422A, paragraphs 4T.114(b)(4) and (c)(3) and 4T.116(e). The intent of these requirements is discussed in the ### Page 28 Chap 2 28 preamble to SR-422A, which states, in part, “For these reasons, this regulation permits the airplane to lift off the ground at a speed lower than the V 2 speed.” The concern that the regulation change was addressing was the overshoot of V 2 after liftoff when it was required that the airplane attain V 2 on, or near, the ground. It was therefore the intent of the regulation to allow an acceleration to V 2 after liftoff but not to allow a decrease in the field length required to attain a height of 35 feet above the takeoff surface by attaining a speed greater than V 2, under low drag ground conditions, and using the excess kinetic energy to attain the 35 foot height. (i) In the case of turbojet powered airplanes, when the bulk of the one-engineinoperative data have been determined with idle cuts, V 2, and its relationship to V R, should be substantiated by a limited number of fuel cuts at V EF. For derivative programs not involving a new or modified engine type (i.e., a modification that would affect thrust decay characteristics), fuel cuts are unnecessary if thrust decay characteristics have been adequately substantiated. (ii) For propeller-drive n airplanes, the use of fuel cuts can be more important in order to ensure that the takeoff speeds and distances are obtained with the critical engine’s propeller attaining the position it would during a sudden engine failure. The number of tests that should be conducted using fuel cuts, if any, depends on the correlation obtained with the idle cut data and substantiation that the data analysis methodology adequately models the effects of a sudden engine failure. (5) Section 25.107(d) - Minimum Unstick Speed (V MU). (i) An applicant should comply with § 25.107(d) by conducting minimum unstick speed (VMU) determination tests with all engines operating and with one engine inoperative. If a stick pusher is installed, it should normally be active and set to the minimum angle of attack side of its rigging tolerance band. If desired, artificial stall warning systems may be disabled for this demonstration. (See paragraph (vi) below for further discussion regarding the setting of stick pusher and stall warning systems during V MU testing.) During this demonstration, the takeoff should be continued until the airplane is out of ground effect. The airplane pitch attitude should not be decreased after liftoff. (ii) In lieu of conducting actual one-engine-in operative VMU tests, the applicant may conduct all-engines-operating V MU tests that simulate and account for all pertinent factors that would be associated with an actual one-engine-inoperative V MU test. To account fully for pertinent factors, it may be necessary to adjust the resulting V MU test values analytically. The factors to be accounted for must at least include the following: (A) Thrust/weight ratio for the one-engine-inoperative range. (B) Controllability (may be related to one-engine-i noperative free air tests, such as VS, VMCA, etc.). (C) Increased drag due to lateral/directional control systems. (D) Reduced lift due to devices such as wing spoilers used for lateral control. (E) Adverse effects of any other systems or de vices on control, drag, or lift. (iii) The number of VMU tests required may be minimized by testing only the critical allengines-operating and one-engine-inoperative thrust/weight ratios, provided that the V MU speeds determined at these critical conditions are used for the range of thrust/weights appropriate to the allengines-operating and one-engine-inoperative configurations. The critical thrust/weight is established by correcting, to the V MU speed, the thrust that results in the airplane achieving its limiting one-engineinoperative climb gradient at the normally scheduled speed and in the appropriate configuration. (iv) Amendment 25-42, effective March 1, 1978, revised §§ 25.107(d) and 25.107(e)(1)(iv) in order to permit the one-engine-inoperative V MU to be determined by all-engines- ### Page 29 Chap 2 29 operating tests at the thrust/weight ratio corresponding to the one-engine-inoperative condition. As revised, § 25.107(d) specifies that V MU must be selected for the range of thrust/weight ratios to be certificated, rather than for the all-engines-operating and one-engine-inoperative conditions as was previously required. In determining the all-engines-operating thrust/weight ratio that corresponds to the one-engine-inoperative condition, consideration should be given to trim and control drag differences between the two configurations in addition to the effect of the number of engines operating. The minimum thrust/weight ratio to be certificated is established by correcting, to the V MU speed, the thrust that results in the airplane achieving its limiting engine-out climb gradient in the appropriate configuration and at the normally scheduled speed. (v) To conduct the VMU tests, rotate the airplane as necessary to achieve the V MU attitude. It is acceptable to use some additional nose-up trim over the normal trim setting during V MU demonstrations. If additional nose-up trim is required, the additional considerations of paragraph (vii), below, apply. VMU is the speed at which the weight of the airplane is completely supported by aerodynamic lift and thrust forces. Some judgment may be necessary on airplanes that have tilting main landing gear bogies. Determining the liftoff point from gear loads and wheel speeds has been found acceptable in past programs. After liftoff, the airplane should be flown out of ground effect. During liftoff and the subsequent climbout, the airplane should be fully controllable. (vi) VMU testing is a maximum performance flight test maneuver, and liftoff may occur very near the angle of attack for maximum lift coefficient. Also, even though pitch attitude may be held fairly constant during the maneuver, environmental conditions and transiting through ground effect may result in consequential changes in angle of attack. It is permissible to lift off at a speed that is below the normal stall warning speed, provided no more than light buffet is encountered. An artificial stall warning system (e.g., a stick shaker) may be disabled during V MU testing, although doing so will require extreme caution and depend upon a thorough knowledge of the airplane’s stall characteristics. If the airplane is equipped with a stick pusher, for flight test safety reasons it should normally be active and set to the minimum angle of attack side of its rigging tolerance band. However, depending on the airplane’s stall characteristics and the stick pusher design, disabling the pusher or delaying activation of the system until a safe altitude is reached may be the safer course. Again, this decision should be made only with a thorough knowledge of the airplane’s stall characteristics combined with a complete understanding of the stick pusher design. (vii) VMU Testing for Airplanes Having Limited Pitch Control Authority. (A) For some airplanes with lim ited pitch control authority, it may not be possible, at forward c.g. and normal trim, to rotate the airplane to a liftoff attitude where the airplane could otherwise perform a clean flyaway at a minimum speed had the required attitude been achieved. This may occur only over a portion of the takeoff weight range in some configurations. When limited pitch control authority is clearly shown to be the case, V MU test conditions may be modified to allow testing aft of the forward c.g. limit and/or with use of more airplane nose-up trim than normal. The V MU data determined with this procedure should be corrected to those values representative of the appropriate forward limit; the variation of VMU with c.g. may be assumed to be like the variation of free air stalling speed with c.g. Although the development of scheduled takeoff speeds may proceed from these corrected V MU data, additional tests are required (see paragraph (B) below) to check that the relaxed V MU criteria have not neglected problems that might arise from operational variations in rotating airplanes with limited pitch control authority. (B) In the following assurance test, the airplane must demonstrate safe flyaway characteristics. (1) Minimum speed liftoff should be demonstrated at the c ritical forward c.g. limit with normal trim. For airplanes with a cutback forward c.g. at heavy weight, two weight/c.g. conditions should be considered. The heavy weight tests should be conducted at maximum structural or maximum sea level climb-limited weight with the associated forward c.g. The full forward c.g. tests should be conducted at the highest associated weight. These tests should be conducted at minimum thrust/weight ### Page 30 Chap 2 30 for both the simulated one-engine-inoperative test (symmetrical reduced thrust) and the all-enginesoperating case. (2) One acceptable test technique is to hold full nose-up control column as the airplane accelerates. As pitch attitude is achieved to establish the minimum liftoff speed, pitch control may be adjusted to prevent overrotation, but the liftoff attitude should be maintained as the airplane flies off the ground and out of ground effect. (3) Resulting liftoff speeds do not affect AFM speed schedules if the test proves successful and the resulting liftoff speed is at least 5 knots below the normally scheduled liftoff speed. Adjustments should be made to the scheduled V R, forward c.g. limit, etc., if necessary, to achieve this result. (4) This minimum 5-knot reduction below the scheduled liftoff speed provides some leeway for operational variations such as mis-trim, c.g. errors, etc. that could further limit the elevator authority. The reduced V MU margins of the minimum liftoff speeds demonstrated in this test, relative to those specified in § 25.107(e)(1)(iv), result from the reduced probability of a pitch control authority-limited airplane getting into a high drag condition due to overrotation. (viii) VMU Testing for Geometry Limited Airplanes. (A) For airplanes that are geometry limited, the 11 0 percent of VMU required by § 25.107(e)(1)(iv) may be reduced to an operationally acceptable value of 108 percent on the basis that equivalent airworthiness is provided for the geometry-limited airplane. For acceptance of the 108 percent of VMU liftoff speed, safeguards protecting the geometry-limited airplane against both overrotation on the ground and in the air must be provided. Also, the airplane should be geometry limited to the extent that a maximum gross weight takeoff with the tail dragging will result in a clean liftoff and fly-away in the allengines-operating condition. During such a takeoff for the all-engines-operating condition, the resulting distance to the 35 ft. height must not be greater than 105 percent of the normal takeoff distance un der similar weight, altitude, and temperature conditions before the 15 percent margin is added. Lastly, the V MU demonstrated must be sound and repeatable. Compliance with these criteria should be formally addressed in a finding of equivalent safety. (B) The criteria for demonstrating the capability for a clean liftoff and fly-away are as follows: (1) The airplane’s pitch attitude from a speed of 96 percent of the actual liftoff speed must be within 5 percent (in degrees) of the tail-dragging attitude to the point of liftoff. (2) During the above speed range (96 to 100 percent of the actual liftoff speed), the aft under-surface of the airplane must have achieved actual runway contact. It has been found acceptable in tests for contact to exist approximately 50 percent of the time that the airplane is in this speed range. (3) Beyond the point of liftoff to a height of 35 ft., the airplane’s pitch attitude must not decrease below that at the point of liftoff, or the speed must not increase more than 10 percent. (4) The airplane shall be at the critical thrust/weight condition, as defined in § 25.107(d), but with all engines operating. (ix) VMU for a Stretched Version of a Tested Airplane. (A) It has been considered that the VMU speed represents the aerodynamic potential minimum liftoff speed related to a safe liftoff and fly-away, even though the body contact attitude may ### Page 31 Chap 2 31 prevent achieving the lift coefficient (C L) for this speed. Such is the case when V MU is determined with body contact, whether or not geometry-limited “credit” is sought. It is therefore concluded that a V MU schedule obtained on one model of an airplane type may be applied to a geometry-limited stretched version of that tested airplane. The 108 percent speed factor of paragraph 10b(5)(viii)(A), above, is only applicable to the stretched version if it was applicable to the tested airplane. (B) Since the concern for tail strikes is increased with the stretched airplane, the following shall be accomplished, in addition to normal takeoff tests, when applying a shorter body airplane’s VMU schedule to the stretched derivative: (1) Scheduled rotation speeds (V R) for the stretched airplane should result in at least the required liftoff speed margins above V MU (i.e., 1.05 and 1.10 or 1.08, as applicable) of the shorter body airplane, corrected for the reduced runway pitch attitude capability and revised c.g. range of the stretched airplane. (2) At both the forward and aft c.g. limits, and over the th rust-to-weight range for each takeoff flap, the following abuse takeoff tests must be accomplished. The tests described in paragraphs ( i) and (ii), below, should be accomplished with not more than occasional, minor (i.e., nondamaging) tail strikes. (i) All-engines-operating, early rotation abuse tests specified in paragraph 10b(6)(iii)(B), including both the rapid rotations and over-rotations as separate test conditions. (ii) One-engine-inoperative, early rotation abuse tests specified in paragraph 10b(6)(ii). (iii) All-engines-operating, moderate rotation rate (i.e., more rapid than normal) takeoff abuse tests, using the scheduled V R, and normal pitch attitude after liftoff. No tail strikes may occur for this condition. (6) Section 25.107(e) - Rotation Speed (V R). (i) The rotation speed, (V R) in terms of calibrated airspeed, must be selected by the applicant. VR has a number of constraints that must be observed in order to comply with § 25.107(e): (A) VR may not be less than V 1; however, it can be equal to V 1 in some cases. (B) VR may not be less than 105 percent of the air minimum control speed (V MCA). (C) VR must be a speed that will allow the airplane to reach V 2 at or before reaching a height of 35 ft. above the takeoff surface. (D) VR must be a speed that will result in liftoff at a speed not less than 110 percent of VMU (unless geometry limited) for the all-engines-operating condition and not less than 105 percent of the VMU determined at the thrust/weight ratio corresponding to the one-engine-inoperative condition for each set of conditions such as weight, altitude, temperature, and configuration when the airplane is rotated at its maximum practicable rate. (ii) Early rotation, one-engine-inoperativ e abuse test. (A) In showing compliance with § 25.107(e)(3), some guidance relative to the airspeed attained at the 35-ft. height during the associated flight test is necessary. As this requirement dealing with a rotation speed abuse test only specifies an early rotation (V R-5 knots), it is interpreted that pilot technique is to remain the same as normally used for a one-engine-inoperative condition. With these considerations in mind, it is apparent that the airspeed achieved at the 35-ft. point can be somewhat below ### Page 32 Chap 2 32 the normal scheduled V 2 speed. However, the amount of permissible V 2 speed reduction must be limited to a reasonable amount as described below. (B) These test criteria are applicable to all unapproved, new, basic model airplanes. They are also applicable to previously approved airplanes when subsequent abuse testing is warranted. However, for those airplanes where the criteria herein are more stringent than that previously applied, consideration will be given to permitting some latitude in the test criteria. (C) In conducting the flight tests required by § 25.107(e)(3), the test pilot shall use a normal/natural rotation technique as associated with the use of scheduled takeoff speeds for the airplane being tested. Intentional tail or tailskid contact is not considered acceptable. Further, the airspeed attained at the 35-ft. height during this test must not be less than the scheduled V 2 value minus 5 knots. These speed limits should not be considered or used as target V 2 test speeds, but rather are intended to provide an acceptable range of speed departure below the scheduled V 2 value. (D) In this abuse test, the simulated engine failure should be accomplished sufficiently in advance of the V R test speed to allow for engine spin-down, unless this would be below the VMCG, in which case V MCG should govern. The normal one-engine-inoperative takeoff distance may be analytically adjusted to compensate for the effect of the early thrust reduction. Further, in those tests where the airspeed achieved at the 35-ft. height is slightly less than the V 2-5 knots limiting value, it will be permissible, in lieu of conducting the tests again, to analytically adjust the test distance to account for the excessive speed decrement. (iii) All-engines-operating abuse tests. (A) Section 25.107(e)(4) states that there must not be a “marked increase” in the scheduled takeoff distance when reasonably expected service variations such as early and excessive rotation and out-of-trim conditions are encountered. This has been interpreted as requiring takeoff tests with all engines operating with: (1) An abuse on rotation speed, and (2) Out-of-trim conditions, but with rotation at the scheduled V R speed. NOTE: The expression “marked increase” in the takeoff distance is defined as any amount in excess of 1 percent of the scheduled takeoff distance. Thus, the abuse tests should not result in field lengths more than 101 percent of the takeoff field lengths calculated in accordance with the applicable requirements of Part 25 for presentation in the AFM. (B) For the early rotation abuse condition with all engines operating, and at a weight as near as practicable to the maximum sea level standard day takeoff weight limit, it should be shown by test that when the airplane is overrotated at a speed below the scheduled V R, no “marked increase” in the scheduled AFM field length will result. For this demonstration, the airplane should be rotated at a speed 7 percent or 10 knots, whichever is less, below the scheduled V R. Tests should be conducted at a rapid rotation rate or should include an overrotation of 2 degrees above normal attitude after liftoff. Tail strikes during this demonstration are acceptable if they are minor and do not result in unsafe conditions. (C) For reasonably expected out-of-trim conditions with all engines operating and as near as practicable to the maximum weight allowed under sea level standard day conditions, it should be shown that there will not be a “marked increase” in the scheduled AFM takeoff distance when rotation is initiated in a normal manner at the scheduled V R speed. (See paragraph 21(c)(7)(ii) for additional guidance regarding the evaluation of flight characteristics for out-of-trim conditions.) The amount of mistrim should be the maximum mistrim that would not result in a takeoff configuration warning, including taking into account the takeoff configuration warning system-rigging tolerance. It is permissible to accept an analysis ### Page 33 Chap 2 33 in lieu of actual testing if the analysis shows that the out-of-trim condition would not present unsafe flight characteristics or “marked increase” in the scheduled AFM field lengths. (iv) Stall Warning During Takeoff Speed Abuse Tests. The presumption is that if a n operational pilot was to make an error in takeoff speeds that resulted in an encounter with stall warning, the likely response would be to aggressively recover to a safe flight condition rather than making a conscious effort to duplicate the AFM takeoff performance data. Therefore, the activation of any stall warning devices, or the occurrence of airframe buffeting during takeoff speed abuse testing, is unacceptable. (7) Section 25.107(f) - Liftoff Speed (V LOF). The liftoff speed (V LOF) is defined as the calibrated airspeed at which the airplane first becomes airborne (i.e., no contact with the runway). This allows comparison of liftoff speed with tire limit speed. V LOF differs from VMU in that VMU is the minimum possible VLOF speed for a given configuration, and depending upon landing gear design, V MU liftoff is shown to be the point where all of the airplane weight is being supported by airplane lift and thrust forces and not any portion by the landing gear. For example, after the V MU speed is reached, a truck tilt actuator may force a front or rear wheel set to be in contact with the runway, even though the flyoff is in progress by virtue of lift being greater than weight. *11. ACCELERATE-STOP DISTANCE - § 25.109 . a. Explanation. This section describes test demonstrations and data expansion methods necessary to determine accelerate-stop distances for publication in the FAA-approved Airplane Flight Manual (AFM), as required by § 25.1583(h) (by reference to § 25.1533). Amendment 25-92 revised some aspects of the Part 25 accelerate-stop criteria and added new requirements related to the stopping capability of the airplane as affected by brake wear and wet runways. The changes imparted to the accelerate-stop requirements by Amendment 25-92 are listed below. (For other material related to the use of acceleratestop distances, see Parts 121 and 135 of the FAR.) * ### Page 34 Chg. 1 Chap 2 80- 1 (1) Section 25.101(i) was added to require accelerate-stop distances to be determined with all the airplane wheel brake assemblies at the fully worn limit of their allowable wear range. (2) Section 25.105(c)(1) was revised to require takeoff data to be determined for wet, in addition to dry, hard surfaced runways. At the applicant’s option, takeoff data may also be determined for wet runways that have grooved or porous friction course surfaces. (3) Section 25.107(a)(2) was revised to remove the reference to “takeoff decision speed” from the definition of V1. (4) Section 25.109 was revised to add a requirement to determine acce lerate-stop distances for wet runways. Additionally, the requirement for the AFM expansion to include two seconds of continued acceleration beyond V1, with the operating engines at takeoff thrust, as introduced by Amendment 25-42, was replaced with a distance increment equivalent to two seconds at V 1. Also, the text of § 25.109(a) was modified to clarify that the accelerate-stop distances must take into account the highest speed reached during the rejected takeoff maneuver, including, as applicable, speeds higher than V 1. (5) Section 25.109(f) was added to permit credit for the use of reverse thrust in determining wet runway accelerate-stop distances (subject to the requirements of § 25.109(e)) and to explicitly deny reverse thrust credit for determining dry runway accelerate-stop distances. (6) Section 25.109(i) was added to require a maximum brake energy accelerate-stop test to be conducted with not more than 10 percent of the allowable brake wear range remaining on each individual wheelbrake assembly. (7) Section 25.735(h) was added to require the maximum rejected takeoff brake energy absorption capacity rating used during qualification testing to the applicable Technical Standard Order to be based on the fully worn limit of the brake’s allowable wear range. (8) Section 25.1533(a)(3) was revised to add runway surface condition (dry or wet) as a variable that must be accounted for in establishing minimum takeoff distances. Section 25.1533(a)(3) was also revised to allow wet runway takeoff distances on grooved and porous friction course (PFC) runways to be established as additional operating limitations, but approval to use these distances is limited to runways that have been designed, constructed, and maintained in a manner acceptable to the FAA Administrator. b. The applicable Federal Aviation Regulations (FAR) are § 25.109, and the following: § 25.101(f) Airplane configuration and procedures. § 25.101(h) Pilot retarding means time delay allowances. § 25.101(i) Worn brake stop ping performance. § 25.105 Takeoff configuration and environmental and runway conditions. §§ 25.107(a)(l) & (2) Defines V1 and VEF speeds. § 25.735 Brakes. § 25.1301 Function and installation. § 25.1309 Equipment, systems, and inst allation. § 25.1533 Additional operating limitations - maximum takeoff weights and minimum takeoff distances. ### Page 35 Chg. 1 Chap 2 80- 2 § 25.1583(h) FAA-approved Airplane Flight Manual - operating limitations. § 25.1587 FAA-approved Airplane Flight Manual - performan ce information. c. Procedures . The following paragraphs provide guidance for accomplishing accelerate-stop flight tests and expanding the resulting data for the determination of AFM performance information. (1) Accelerate-stop testing . The following guidance is applicable to turbine-powered airplanes with and without propellers. Guidance regarding flight testing applies only to dry runway accelerate-stop distances. Guidance for expanding the flight test data to determine AFM distances applies to both dry and wet runways, unless otherwise noted. Further guidance for determining wet runway accelerate-stop distances is provided in paragraph 11c(4). (i) In order to establish a distance that would be representative of the distance needed in the e vent of a rejected takeoff, where the first action to stop the airplane is taken at or below V 1, a sufficient number of test runs should be conducted for each airplane configuration specified by the applicant. (For intermediate configurations, see paragraph 3 of this advisory circular.) (ii) The guidance outlined in paragraph 11c(3) describes how to apply the appropriate time delays, as required by § 25.101(h)(3), for the flightcrew to accomplish the rejected takeoff operating procedures. (iii) Section 25.101(i) states that the accelerate-stop distances must be determined with all the airplane wheel-brake assemblies at the fully worn limit of their allowable wear range. The fully worn limit is defined as the amount of wear allowed before the brake must be removed from the airplane for overhaul. The allowable wear should be defined in terms of a lineardimension in the axial direction, which is typically determined by measuring the wear pin extension. (A) The only accelerate-stop test that mu st be conducted at a specific brake wear state is the maximum brake kinetic energy demonstration, which must use brakes that have no more than 10 percent of the allowable brake wear range remaining (see paragraph 11c(2)(iii)). The remainder of the accelerate-stop tests may be conducted with the brakes in any wear state as long as a suitable combination of airplane and dynamometer tests is used to determine the accelerate-stop distances corresponding to fully worn brakes. For example, dynamometer testing may be used to determine whether there is a reduction in brake performance from the wear state used in the airplane tests to a fully worn brake. The airplane test data could then be adjusted analytically for this difference without additional airplane testing. (B) Either airplane-worn or mechanically-worn brakes (i.e., machined or dynamometer worn) may be used. If mechanically-worn brakes are used, it must be shown that they can be expected to provide similar results to airplane-worn brakes. This comparison can be based on service experience on the test brake or an appropriate equivalent brake, or on dynamometer wear test data when service data are unavailable. (iv) Section 25.109(f)(1) denies credit for the use of reverse thrust as a decelerati ng means in determining the accelerate-stop distance for a dry runway. This provision applies to both turbine engine and propeller engine reverse thrust. Credit for the additional deceleration available from reverse thrust is permitted for wet runway accelerate-stop distances, provided the thrust reverser system is shown to be safe, reliable, capable of giving repeatable results, and does not require exceptional skill to control the airplane. (See paragraph 11c(4)(v) for guidance related to obtaining accelerate-stop performance credit for reverse thrust on wet runways.) (v) The accelerate-stop test runs should be conducted at weight/speed combinations that will provide an even distribution of test conditions over the range of weights, speeds, and brake energies for which takeoff data will be provided in the AFM. The effects of different airport elevations can be simulated at one airport elevation, provided the braking speeds employed are relevant for the range of airplane energies that must be absorbed by the brakes. The limiting brake energy value in the AFM shall not exceed the maximum demonstrated in these tests or the maximum for which the brake has been approved. (See paragraph 11c(2) for further guidance related to tests and analyses for the demonstration of the maximum brake energy absorption capability.) ### Page 36 Chg. 1 Chap 2 80- 3 (vi) The V1 speeds used in the accelerate-stop tests need not correspond precisely to the AFM values for the test conditions since it may be necessary to increase or decrease the AFM V 1 speed to fully investigate the energy range and weight envelope. (vii) A total of at least six accelerate-stop flight tests should be conducted. Unless sufficient data are available for the specific airplane type showing how braking performance varies with weight, kinetic energy, lift, drag, ground speed, torque limit, etc., at least two tests should be conducted for each configuration when the same braking coefficient of friction is being claimed for multiple aerodynamic configurations. These tests must be conducted on hard surfaced, dry runways. (viii) For approval of dispatch capability with anti-skid inoperative, nose wheel brakes or specified main wheel-brake(s) inoperative, autobraking systems, etc., a full set of tests, as in paragraph (vii) above, should normally be conducted. A lesser number of tests may be accepted for “equal or better” demonstrations, to establish small increments or if adequate conservatism is used during testing. (ix) Either ground or airborne instrumentation sh ould include means to determine the horizontal distance time-history. (x) The wind speed and direction relative to the test runway should be determined and corrected to a height corresponding to the approximate height of the mean aerodynamic chord. (See paragraph 3 of this advisory circular.) (xi) The accelerate -stop tests should be conducted in the following configurations: (A) Heavy to light weight as required. (B) Most critical c.g. position. (C) Wing flaps in the takeoff pos ition(s). (D) Tire pressure: before taxi and with cold tires, set to the highest value appropriate to the takeoff weight for which approval is being sought. (E) Engine idle thrust: set at the recommended upper limit for use on the ground or t he effect of maximum ground idle thrust may be accounted for in data analyses. For maximum brake energy and fuse plug no-melt tests, data analysis may not be used in place of maximum ground idle thrust. (xii) Engine thrust should be appropriate to ea ch segment of the rejected takeoff and should include accounting for thrust decay rates (i.e., spindown) for failed or throttled back engines. At the speed that corresponds to the energy level defined for the test demonstration, the stopping sequence is initiated by employing the first acceptable braking means. (A) Turbojet powered airplanes . For AFM calculation purposes, the critical engine failure accelerate-stop data may be based on the failed engine spinning down to a windmilling condition. (Note: If, due to the certification basis of the airplane, all-engine-accelerate-stop distances are not being considered, the one-engineinoperative AFM distances should be based on the critical engine failing to maximum ground idle thrust rather than the windmilling condition.) The thrust from the operative engine(s) should be consistent with a throttle chop to maximum ground idle thrust. For determining the all-engines-operating dry runway accelerate-stop AFM distances, the stopping portion should be based on all engines producing maximum ground idle thrust (after engine spindown), as noted in paragraph 11c(1)(xi)(E). The accelerate-stop tests may be conducted with either concurrent or sequential throttle chops to idle thrust as long as the data are adjusted to take into account pilot reaction time, and any control, system, or braking differences (e.g., electrical or hydraulic/mechanical transients associated with an engine failing to a windmilling condition resulting in reduced braking effectiveness). Test data should also be analytically corrected for any differences between maximum ground idle thrust and the idle thrust level achieved during the test. For the criteria relating to reverse thrust credit for wet runway accelerate-stop distances, see paragraph 11c(4)(v). ### Page 37 Chg. 1 Chap 2 80- 4 (B) Turbopropeller-powered airplanes . For the one-engine-inoperative accelerate-stop distances, the critical engine’s propeller should be in the position it would normally assume when an engine fails and the power levers are closed. For dry runway one-engine-inoperative accelerate-stop distances, the high drag groundidle position of the operating engines’ propellers (defined by a pitch setting that results in not less than zero total thrust, i.e., propeller plus jet thrust, at zero airspeed) may be used provided adequate directional control is available on a wet runway and the related operational procedures comply with §§ 25.109(f) and (h). Wet runway controllability may either be demonstrated by using the guidance available in paragraph 11c(4)(v)(F) at the appropriate power level, or adequate control can be assumed to be available at ground idle power if reverse thrust credit is approved for determining the wet runway accelerate-stop distances. For the all-engines-operating accelerate-stop distances on a dry runway, the high drag ground-idle propeller position may be used for all engines (subject to § 25.109(f) and (h)). For the criteria relating to reverse thrust credit for wet runway accelerate-stop distances, see paragraph 11c(4)(v). (xiii) System transient effects (e.g., engine spin-down, brake pressure ramp-up, etc.) should be determined and properly accounted for in the calculation of AFM accelerate-stop distances (see paragraph 11c(3)(ix)). (2) Maximum Brake Energy Testing . The following paragraphs describe regulatory requirements and acceptable test methods for conducting an accelerate-stop test run to demonstrate the maximum energy absorption capability of the wheel-brakes. (i) The maximum brake energy accel erate-stop demonstration should be conducted at not less than the maximum takeoff weight and should be preceded by at least a 3-mile taxi with all engines operating at maximum ground idle thrust, including three full stops using normal braking. Following the maximum brake energy stop, it will not be necessary to demonstrate the airplane’s ability to taxi. (ii) Section 25.735(h) requires the rejected takeoff brake kinetic energy capacity rating of each main wheel-brake assembly to be determined at the fully worn limit of its allowable wear range. The calculation of maximum brake energy limited takeoff weights and speeds, for presentation in the AFM performance section, must therefore be based on each airplane main wheel-brake being in the fully worn condition. (iii) Section 25.109(i) requires a flight test demonstration of the maximum brake kinetic energy accelerate-stop distance to be conducted with not more than 10 percent of the allowable brake wear range remaining on each of the airplane wheel-brakes. The 10 percent allowance on the brake wear state is intended to ease test logistics and increase test safety, not to allow the accelerate-stop distance to be determined with less than fully worn brakes. If the brakes are not in the fully worn state at the beginning of the test, the accelerate-stop distance should be corrected as necessary to represent the stopping capability of fully worn brakes. (iv) The maximum airplane brake energy allowed for dispatch should not exceed the value for which a satisfactory after-stop condition exists, or the value documented under the applicable Technical Standard Order (TSO) (or an acceptable equivalent), whichever value is less. A satisfactory after-stop condition is defined as one in which fires are confined to tires, wheels, and brakes, such that progressive engulfment of the rest of the airplane would not occur during the time of passenger and crew evacuation. The application of fire fighting means or artificial coolants should not be required for a period of 5 minutes following the stop. (v) Landings are not an acceptable means for demonstrating the maximum rejected takeoff brake energy. Though permitted in the past, service experience has shown that methods used to predict brake and tire temperature increases that would have occurred during taxi and acceleration, as specified in paragraph 11(c)(2)(i), were not able to accurately account for the associated energy increments. (3) Accelerate-Stop Time Delays . Section 25.101(h) of the FAR requires allowance for time delays in the execution of procedures. Amendment 25-42 (effective March 1, 1978) amended the airworthiness standards to clarify and standardize the method of applying these time delays to the accelerate-stop transition period. Amendment 25-42 also added the critical engine failure speed, V EF, and clarified the meaning of V 1 with relation to VEF. The preamble to Amendment 25-42 states that “V 1 is determined by adding to V EF (the speed at which the critical engine is assumed to fail) the speed gained with the critical engine inoperative during the time interval between the instant at which the critical engine is failed and the instant at which the test pilot recognizes and reacts ### Page 38 Chg. 1 Chap 2 80- 5 to the engine failure, as indicated by the pilot’s application of the first retarding means during accelerate-stop tests.” Thus it can be seen that V 1 is not only intended to be at the end of the decision process, but it also includes the time it takes for the pilot to perform the first action to stop the airplane. The purpose of the time delays is to allow sufficient time (and distance) for a pilot, in actual operations, to accomplish the procedures for stopping the airplane. The time delays are not intended to allow extra time for making a decision to stop as the airplane passes through V 1. Since the typical transport category airplane requires three pilot actions (i.e., brakes-throttles-spoilers) to achieve the final braking configuration, Amendment 25-42 defined a two-second time period, in § 25.109, to account for delays in activating the second and third deceleration devices. Amendment 25-92 (effective March 20, 1998) redefined, and reinterpreted the application of that two-second delay time as a distance increment equivalent to two seconds at V1. No credit may be taken for system transient effects (e.g., engine spin-down, brake pressure ramp-up, etc.) in determining this distance. The following paragraphs provide guidance related to the interpretation and application of delay times to show compliance with the accelerate-stop requirements of Amendment 25-92. (i) Figure 11-1 presents a pictorial representation of the accelerate-stop time delays considered acceptable for compliance with § 25.101(h) as discussed above. FIGURE 11-1. ACCELERATE-STOP TIME DELAYS Initiation of first pilot action Initiation of second pilot action Initiation of third pilot action Engine failure V1VEF tact 1 tact 1 Event Demonstrated Time Delays Flight Manual Expansion Time Delays Initiation of subsequent pilot actions tact 4 n tact+ 1 sec. tact 2 tact 2 tact 3 tact 3 Distance equivalent to 2 sec. at V1 (ii) VEF is the calibrated airspeed selected by the applicant at which the critical engine is assumed to fail. The relationship between V EF and V1 is defined in § 25.107. (iii) ∆tact 1 = the demonstrated time interval between engine failure and initiation of the first pilot action to stop the airplane. This time interval is defined as beginning at the instant the critical engine is failed and ending when the pilot recognizes and reacts to the engine failure, as indicated by the pilot’s application of the first retarding means during accelerate-stop tests. A sufficient number of demonstrations should be conducted using both applicant and FAA test pilots to assure that the time increment is representative and repeatable. The pilot’s feet should be on the rudder pedals, not the brakes, during the tests. For AFM data expansion purposes, in order to provide a recognition time increment that can be executed consistently in service, this time increment must be equal to the demonstrated time or one second, whichever is greater. If the airplane incorporates an engine failure warning light, the recognition time includes the time increment necessary for the engine to spool down to the point of warning light activation, plus the time increment from light “on” to pilot action indicating recognition of the engine failure. (iv) ∆tact 2 = the demonstrated time interval between initiation of the first and second pilot actions to stop the airplane. (v) ∆tact 3 = the demonstrated time interval between initiation of the second and third pilot actions to stop the airplane. (vi) ∆tact 4→n = the demonstrated time interval between initiation of the third and fourth (and any subsequent) pilot actions to stop the airplane. For AFM expansion, a one-second reaction time delay to account for in-service variations should be added to the demonstrated time interval between the third and fourth (and any subsequent) pilot actions. If a command is required for another crewmember to initiate an action to stop the ### Page 39 Chg. 1 Chap 2 80- 6 airplane, a two-second delay, in lieu of the one-second delay, should be applied for each action. For automatic deceleration devices that are approved for performance credit for AFM data expansion, established systems actuation times determined during certification testing may be used without the application of the additional time delays required by this paragraph. (vii) The sequence for ini tiation of pilot actions may be selected by the applicant, but it must match the sequence established for operation in service, as prescribed by § 25.101(f). If, on occasion, the specified sequence is not achieved during testing, the test need not be repeated; however, sufficient testing must be conducted to establish acceptable values of ∆tact. (viii) Sections 25.109(a)(1)(iv) and (a)(2)(iii) require the one-engine-inoperative and all-enginesoperating accelerate-stop distances, respectively, to include a distance increment equivalent to two seconds at V 1. (Although the requirement for the distance increment equivalent to two seconds at V 1 is explicitly stated in the “dry runway” criteria of § 25.109, it is also applied to the “wet runway” accelerate-stop distances by reference in § 25.109(b).) This distance increment is represented pictorially on the right side of the “Flight Manual Expansion Time Delays” presentation in Figure 11-1, and in the speed versus distance plot of Figure 11-2, below. The twosecond time period is only provided as a method to calculate the required distance increment, and is not considered to be a part of the accelerate-stop braking transition sequence. Consequently, no credit for pilot actions, or engine and systems transient responses (e.g., engine spin-down) may be taken during this two-second time period. Similarly, the two-second time period may not be reduced for airplanes incorporating automated systems that decrease the number of pilot actions required to obtain the full braking configuration (e.g., autospoiler systems). ### Page 40 Chg. 1 Chap 2 80- 7 FIGURE 11-2. ACCELERATE-STOP SPEED VS. DISTANCE SPEED DISTANCE 3rd Action 2nd Action V1 (1st Action) VEF AFM model must accurately represent demonstrated energy state of the airplane after V1. } Distance for 2 secs. at V1 (ix) Any residual acceleration that occurs after V1, while the airplane and its systems become stabilized in the braking configuration, must be accounted for in the expansion of accelerate-stop performance data for presentation in the AFM. The effects of system transients, such as engine spin-down, brake pressure ramp-up, spoiler actuation times, etc., should be accounted for in this time period. The area of interest is noted at the top of the graphical representation of the speed versus distance relationship in Figure 11-2. (x) All-Engine Accelerate-Stop Distance . For the all-engines-operating accelerate-stop distance prescribed by § 25.109(a)(2), apply the demonstrated time intervals, and associated delays, of paragraphs 11c(3)(iv) through (vi) after the airplane has accelerated to V 1. (xi) The procedures used to determine the accelerate-stop distance must be described in the performance section of the AFM. (4) Wet Runway Accelerate-Stop Distance . The following guidance is provided for showing compliance with the requirements stated in §§ 25.109(b) through (d) for determining accelerate-stop distances applicable to wet runways. In general, the wet runway accelerate-stop distance is determined in a similar manner to the dry runway accelerate-stop distance. The only differences are in reflecting the reduced stopping force available from the wheel brakes on the wet surface and in provisions for performance credit for the use of reverse thrust as an additional decelerating means. The general method for determining the reduced stopping capability of the wheel brakes on a smooth wet runway is as follows: First, determine the maximum tire-to-ground wet runway braking coefficient of friction versus ground speed from the relationships provided in § 25.109(c)(1). Then, adjust this braking coefficient ### Page 41 Chg. 1 Chap 2 80- 8 to take into account the efficiency of the anti-skid system. (See paragraph 11c(4)(ii) for a definition of anti-skid efficiency.) Next, determine the resulting braking force and adjust this force for the effect of the distribution of the normal load between braked and unbraked wheels at the most adverse center-of-gravity position approved for takeoff, as prescribed by § 25.109(b)(2)(ii). In accordance with § 25.109(b)(2)(i), apply further adjustments, if necessary, to ensure that the resulting stopping force attributed to the wheel brakes on a wet runway never exceeds (i.e., during the entire stop) the wheel brakes stopping force used to determine the dry runway accelerate-stop distance (under § 25.109(a)). Neither the dry runway brake torque limit nor the dry runway friction (i.e., anti-skid) limit should be exceeded. Alternative methods of determining the wet runway wheel brakes stopping force may be acceptable as long as that force does not exceed the force determined using the method just described. (i) Maximum Tire-to-Ground Braking Coefficient of Friction . The values specified in § 25.109(c)(1) were derived from data contained in Engineering Sciences Data Unit (ESDU) 71026, “Frictional and Retarding Forces on Aircraft Types - Part II: Estimation of Braking Force,” (August 1981). The data in ESDU 71026 is a compilation from many different sources, including the National Aeronautics and Space Administration, the British Ministry of Aviation, and others. ESDU 71026 contains curves of wet runway braking coefficients versus speed for smooth and treaded tires at varying inflation pressures. These data are presented for runways of various surface roughness, including grooved and porous friction course runways. Included in the data presentation are bands about each of the curves, which represent variations in: water depths from damp to flooded, runway surface texture within the defined texture levels, tire characteristics, and experimental methods. In defining the standard curves of wet runway braking coefficient versus speed that are prescribed by the equations in § 25.109(c)(1), the effects of the following variables were considered: tire pressure, tire tread depth, runway surface texture, and the depth of the water on the runway. (A) Tire Pressure : Lower tire pressures tend to improve the airplane’s stopping capability on a wet runway. The effect of tire pressure is taken into account by providing separate curves (equations) in § 25.109(c)(1) for several tire pressures. As stated in the rule, the tire pressure used to determine the maximum tireto-ground braking coefficient of friction must be the maximum tire pressure approved for operation. Linear interpolation may be used for tire pressures other than those listed. (B) Tire Tread Depth : The degree to which water can be channeled out from under the tires significantly affects wet runway stopping capability. The standard curves of braking coefficient versus speed prescribed in § 25.109(c)(1) are based on a tire tread depth of 2 mm. This tread depth is consistent with tire removal and retread practices reported by airplane and tire manufacturers and tire retreaders. It is also consistent with FAA guidance provided in AC 121.195(d)-1A, regarding the tread depth for tires used in flight tests to determine operational landing distances on wet runways. Although operation with zero tread depth is not prohibited, it is unlikely that all of the tires on an airplane would be worn to the same extent. (C) Runway Surface Texture : ESDU 71026 groups runways into five categories. These categories are labeled “A” through “E,” with “A” being the smoothest and “C” the most heavily textured ungrooved runways. Categories “D” and “E” represent grooved and other open textured surfaces. Category A represents a very smooth texture (an average texture depth of less than 0.004 inches), and is not very prevalent in runways used by transport category airplanes. The majority of ungrooved runways fall into the category C grouping. The curves represented in § 25.109(c)(1) represent a texture midway between categories B and C. (D) Depth of Water on the Runway : Obviously, the greater the water depth, the greater the degradation in braking capability. The curves prescribed in § 25.109(c)(1) represent a well-soaked runway, but with no significant areas of standing water. (ii) Anti-Skid System Efficiency . Section 25.109(c)(2) requires adjusting the maximum tire-toground braking coefficient determined in § 25.109(c)(1) to take into account the efficiency of the anti-skid system. The anti-skid system efficiency is defined as the relative capability of the anti-skid system to obtain the maximum friction available between the tire and the runway surface ( μmax). It is expressed as either a percentage of μmax or a factor based on that percentage (e.g., 85% or 0.85). Applicants can either use one of the anti-skid efficiency values specified in § 25.109(c)(2), or derive the efficiency from flight tests on a wet runway. Regardless of which method is used, § 25.109(c)(2) requires that an appropriate level of flight testing mu st be performed to verify that the antiskid system operates in a manner consistent with the efficiency value used, and that the system has been properly tuned for operation on wet runways. ### Page 42 Chg. 1 Chap 2 80- 9 (A) Classification of Types of Anti-Skid Systems . (1) The efficiency values specified in § 25.109(c)(2) are a function of the type of antiskid system installed on the airplane. Three broad system types are identified in the rule: on/off, quasi-modulating, and fully modulating. These classifications represent evolving levels of technology and differing performance capabilities on dry and wet runways. The classification of anti-skid system types and the assigned efficiency values are based on information contained in Society of Automotive Engineers (SAE) Aerospace Information Report (AIR) 1739, titled “Information on Anti-Skid Systems.” (2) On/off systems are the simplest of the three types of anti-skid systems. For these systems, full-metered brake pressure (as commanded by the pilot) is applied until wheel locking is sensed. Brake pressure is then released to allow the wheel to spin back up. When the system senses that the wheel is accelerating back to synchronous speed (i.e., ground speed), full-metered pressure is again applied. The cycle of full pressure application/complete pressure release is repeated throughout the stop (or until the wheel ceases to skid with pressure applied). (3) Quasi-modulating systems attempt to continuously regulate brake pressure as a function of wheel speed. Typically, brake pressure is released when the wheel deceleration rate exceeds a preselected value. Brake pressure is re-applied at a lower level after a length of time appropriate to the depth of the skid. Brake pressure is then gradually increased until another incipient skid condition is sensed. In general, the corrective actions taken by these systems to exit the skid condition are based on a pre-programmed sequence rather than the wheel speed time history. (4) Fully modulating systems are a further refinement of the quasi-modulating systems. The major difference between these two types of anti-skid systems is in the implementation of the skid control logic. During a skid, corrective action is based on the sensed wheel speed signal, rather than a pre-programmed response. Specifically, the amount of pressure reduction or reapplication is based on the rate at which the wheel is going into or recovering from a skid. (5) In addition to examining the control system for the differences noted above, a time history of the response characteristics of the anti-skid system during a wet runway stop should be used to help identify the type of anti-skid system. Comparing the response characteristics between wet and dry runway stops can also be helpful. (6) Figure 11-3 shows an example of the response characteristics of a typical on-off system on both dry and wet runways. In general, the on-off system exhibits a cyclic behavior of brake pressure application until a skid is sensed, followed by the complete release of brake pressure to allow the wheel to spin back up. Full-metered pressure (as commanded by the pilot) is then re-applied, starting the cycle over again. The wheel speed trace exhibits deep and frequent skids (the troughs in the wheel speed trace), and the average wheel speed is significantly less than the synchronous speed (which is represented by the flat-topped portions of the wheel speed trace). Note that the skids are deeper and more frequent on a wet runway than on a dry runway. For the particular example shown in Figure 11-3, the brake becomes torque-limited toward the end of the dry runway stop, and is unable to generate enough torque to cause further skidding. ### Page 43 Chg. 1 Chap 2 80- 10 FIGURE 11-3. ANTI-SKID SYSTEM RESPONSE CHARACTERISTICS On-Off System (7) The effectiveness of quasi-modulating systems can vary significantly depending on the slipperiness of the runway and the design of the particular control system. On dry runways, these systems typically perform very well; however, on wet runways their performance is highly dependent on the design and tuning of the particular system. An example of the response characteristics of one such system is shown in Figure 11-4. On both dry and wet runways, brake pressure is released to the extent necessary to control skidding. As the wheel returns to the synchronous speed, brake pressure is quickly increased to a pre-determined level and then gradually ramped up to the full-metered brake pressure. On a dry runway, this type of response reduces the depth and frequency of skidding compared to an on-off system. However, on a wet runway, skidding occurs at a pressure below that at which the gradual ramping of brake pressure occurs. As a result, on wet runways the particular system shown in Figure 11-4 operates very similarly to an on-off system. ### Page 44 Chg. 1 Chap 2 80- 11 FIGURE 11-4. ANTI-SKID SYSTEM RESPONSE CHARACTERISTICS Quasi-Modulating System ### Page 45 Chg. 1 Chap 2 80- 12 (8) When properly tuned, fully modulating systems are characterized by much smaller variations in brake pressure around a fairly high average value. These systems can respond quickly to developing skids, and are capable of modulating brake pressure to reduce the frequency and depth of skidding. As a result, the average wheel speed remains much closer to the synchronous wheel speed. Figure 11-5 illustrates an example of the response characteristics of a fully modulating system on dry and wet runways. ] ### Page 46 Chg. 1 Chap 2 80- 13 FIGURE 11-5. ANTI-SKID SYSTEM RESPONSE CHARACTERISTICS Fully Modulating System ### Page 47 Chg. 1 Chap 2 80- 14 (B) Demonstration of Anti-Skid System Operation When Using the Anti-Skid Efficiency Values Specified in § 25.109(c)(2). (1) If the applicant elects to use one of the anti-skid efficiency values specified in § 25.109(c)(2), a limited amount of flight testing must still be conducted to verify that the anti-skid system operates in a manner consistent with the type of anti-skid system declared by the applicant. This testing should also demonstrate that the anti-skid system has been properly tuned for operation on wet runways. (2) A minimum of one complete stop, or equivalent segmented stops, should be conducted on a smooth (i.e., not grooved or porous friction course) wet runway at an appropriate speed and energy to cover the critical operating mode of the anti-skid system. Since the objective of the test is to observe the operation (i.e., cycling) of the anti-skid system, this test will normally be conducted at an energy well below the maximum brake energy condition. (3) The section of the runway used for braking should be well soaked (i.e., not just damp), but not flooded. The runway test section should be wet enough to result in a number of cycles of anti-skid activity, but should not cause hydroplaning. (4) Before taxi and with cold tires, the tire pressure should be set to the highest value appropriate to the takeoff weight for which approval is being sought. (5) The tires and brakes should not be new, but need not be in the fully worn condition. They should be in a condition considered representative of typical in-service operations. (6) Sufficient data should be obtained to determine whether the system operates in a manner consistent with the type of anti-skid system declared by the applicant, provide evidence that full brake pressure is being applied upstream of the anti-skid valve during the flight test demonstration, determine whether the anti-skid valve is performing as intended, and show that the anti-skid system has been properly tuned for a wet runway. Typically, the following parameters should be plotted versus time: (i) The speed of a representative number of wheels. (ii) The hydraulic pressure at each brake (i.e., the hydraulic pressure downstream of the anti-skid valve or the electrical input to each anti-skid valve). (iii) The hydraulic pressure at each brake metering valve (i.e., upstream of the anti-skid valve). (7) A qualitative assessment of anti-skid system response and airplane controllability should be made by the test pilot(s). In particular, pilot observations should confirm that: (i) Anti-skid releases are neither excessively deep nor prolonged; (ii) The landing gear is free of unusual dynamics; and (iii) The airplane tracks essentially straight, even though runway seams, water puddles, and wetter patches may not be uniformly distributed in location or extent. (C) Determination of a Specific Wet Runway Anti-Skid System Efficiency . (1) If the applicant elects to derive the anti-skid system efficiency from flight test demonstrations, sufficient flight testing, with adequate instrumentation, must be conducted to ensure confidence in the value obtained. An anti-skid efficiency of 92 percent (i.e., a factor of 0.92) is considered to be the maximum efficiency on a wet runway normally achievable with fully modulating digital anti-skid systems. ### Page 48 Chg. 1 Chap 2 80- 15 (2) A minimum of three complete stops, or equivalent segmented stops, should be conducted on a wet runway at appropriate speeds and energies to cover the critical operating modes of the anti-skid system. Alternatively, if the operation and efficiency of the anti-skid system on a wet runway can be predicted by laboratory simulation data and validated by flight test demonstrations, a lesser number of stops may be acceptable. In this case, as many complete stops, or equivalent segmented stops, as necessary to present six independent antiskid efficiency calculations should be conducted on a wet runway at appropriate speeds and energies to cover the critical operating modes of the anti-skid system. An independent anti-skid efficiency calculation can be presented for each stop for each independently controlled wheel, or set of wheels. (3) Since the objective of the test is to determine the efficiency of the anti-skid system, these tests will normally be conducted at energies well below the maximum brake energy condition. A sufficient range of speeds should be covered to investigate any variation of the anti-skid efficiency with speed. (4) The testing should be conducted on a smooth (i.e., not grooved or porous friction course) runway. If the applicant chooses to determine accelerate-stop distances for grooved and porous friction course (PFC) surfaces under § 25.109(d)(2), testing should also be conducted on a grooved or porous friction course runway to determine the anti-skid efficiency value applicable to those surfaces. Other means for determining the anti-skid efficiency value for grooved and PFC surfaces may also be acceptable, such as using the efficiency value previously determined for smooth runways, if that value is shown to also be representative of or conservative for grooved and PFC runways. (5) The section of the runway used for braking should be well soaked (i.e., not just damp), but not flooded. The runway test section should be wet enough to result in a number of cycles of anti-skid activity, but should not cause hydroplaning. (6) Before taxi and with cold tires, the tire pressure should be set to the highest value appropriate to the takeoff weight for which approval is being sought. (7) The tires and brakes should not be new, but need not be in the fully worn condition. They should be in a condition considered representative of typical in-service operations. (8) A qualitative assessment of anti-skid system response and airplane controllability should be made by the test pilot(s). In particular, pilot observations should confirm that: (i) The landing gear is free of unusual dynamics; and (ii) The airplane tracks essentially straight, even though runway seams, water puddles, and wetter patches may not be uniformly distributed in location or extent. (9) Two acceptable methods, referred to as the torque method and the wheel slip method, for determining the wet runway anti-skid efficiency value from wet runway stopping tests are described below. Other methods may also be acceptable if they can be shown to give equivalent results. The test instrumentation and data collection should be consistent with the method used. (i) Torque Method : (a) Under the torque method, the anti-skid system efficiency is determined by comparing the energy absorbed by the brake during an actual wet runway stop to the energy that is determined by integrating, over the stopping distance, a curve defined by connecting the peaks of the instantaneous brake force curve (see Figure 11-6). The energy absorbed by the brake during the actual wet runway stop is determined by integrating the curve of instantaneous brake force over the stopping distance. **FIGURE 11-6. INSTANTANEOUS BRAKE FORCE AND PEAK BRAKE FORCE** ### Page 49 Chg. 1 Chap 2 80- 16 Time Instantaneous Brake Force Peak Brake Force Time Intervals Brake Force (b) Using data obtained from the wet runway stopping tests of paragraph 11c(4)(ii)(C), instantaneous brake force can be calculated from the following relationship: ( ) tire b b R ITF a+= where: Fb = brake force Tb = brake torque α = wheel acceleration I = wheel and tire moment of inertia and Rtire = tire radius. (c) For brake installations where measuring brake torque directly is impractical, torque may be determined from other parameters (e.g., brake pressure) if a suitable correlation is available. Wheel acceleration is obtained from the first derivative of wheel speed. Instrumentation recording rates and data analysis techniques for wheel speed and torque data should be well matched to the anti-skid response characteristics to avoid introducing noise and other artifacts of the instrumentation system into the data. (d) Since the derivative of wheel speed is used in calculating brake force, smoothing of the wheel speed data is usually necessary to give good results. The smoothing algorithm should be carefully designed as it can affect the resulting efficiency calculation. Filtering or smoothing of the brake torque or brake force data should not normally be done. If conditioning is applied, it should be done in a conservative manner (i.e., result in a lower efficiency value) and should not misrepresent actual airplane/system dynamics. (e) Both the instantaneous brake force and the peak brake force should be integrated over the stopping distance. The anti-skid efficiency value for determining the wet runway accelerate-stop distance is the ratio of the instantaneous brake force integral to the peak brake force integral: ∫ ∫ ⋅ ⋅ dsforce brakepeak dsforce brake ousinstantane =efficiency skid-anti where s = stopping distance (f) The stopping distance is defined as the distance traveled during the specific wet runway stopping demonstration, beginning when the full braking configuration is obtained and ending at the lowest speed at which anti-skid cycling occurs (i.e., the brakes are not torque-limited), except that this speed need not be less than 10 knots. Any variation in the anti-skid efficiency with speed should also be investigated, which can be accomplished by determining the efficiency over segments of the total stopping distance. If significant variations are noted, this variation should be reflected in the braking force used to determine the accelerate-stop distances (either by using a variable efficiency or by using a conservative single value). ### Page 50 Chg. 1 Chap 2 80- 17 (ii) Wheel Slip Method : (a) At brake application, the tire begins to slip with respect to the runway surface (i.e., the wheel speed slows down with respect to the airplane’s ground speed). As the amount of tire slip increases, the brake force also increases until an optimal slip is reached. If the amount of slip continues to increase past the optimal slip, the braking force will decrease. (b) Using the wheel slip method, the anti-skid efficiency is determined by comparing the actual wheel slip measured during a wet runway stop to the optimal slip. Since the wheel slip varies significantly during the stop, sufficient wheel and ground speed data must be obtained to determine the variation of both the actual wheel slip and the optimal wheel slip over the length of the stop. A sampling rate of at least 16 samples per second for both wheel speed and ground speed has been found to yield acceptable fidelity. (c) For each wheel and ground speed data point, the instantaneous anti-skid efficiency value should be determined from the relationship shown in Figure 11-7. FIGURE 11-7. ANTI-SKID EFFICIENCY - WHEEL SLIP RELATIONSHIP Wheel Slip Ratio0 free rolling 1 locked wheel 1.0 0 Optimal Slip 0.5 Anti-Skid Efficiency ( ) ( ) ratio slip optimal theis OPS and speed Ground speed Wheel1=ratio slip wheelthe= WSRwhere OPS1 WSR1+10.5=Efficiency OPS>WSR 1.0=Efficiency OPS=WSR OPS WSR5.0 OPS WSR5.1=Efficiency OPS