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Chapter 4. OPERATING LIMITATIONS AND INFORMATION
Section 1. OPERATING LIMITATIONS
72. Section 25.1529 Instructions for continued airworthiness 169 H25.4 airworthiness limitations section 169
a. Rule Text 169
b. Intent of Rule 169
c. Background 169
d. Policy/Compliance 170
e. Reference 170
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APPENDICES
Appendix Subject
1. Cross reference for selected sections of part 25, 171 Section 4b of the civil air regulations (CAR) and Section 4b of the civil aeronautics manual (CAM)
2. Glossary of acronyms 172
3. Index of advisory circular references 173
4. Index of incorporated guidance material and references 178
(1) FAA memorandums and letters 178
(2) FAA technical standard orders (TSO) 181
(3) FAA orders 182
(4) FAA videos 182
(5) Society of Automotive Engineers (SAE) documents 183
(6) Miscellaneous references 186
(7) Cross reference to other AC handbooks 189
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Chapter 1. DESIGN AND CONSTRUCTION
Section 1. CONTROL SYSTEMS
1. SECTION 25.671 - GENERAL.
a. Rule Text.
(a) Each control and control system must operate with the ease, smoothness, and positiveness appropriate to its function.
(b) Each element of each flight control system must be designed, or distinctively and permanently marked, to minimize the probability of incorrect assembly that could result in the malfunctioning of the system.
(c) The airplane must be shown by analysis, tests, or both, to be capable of continued safe flight and landing after any of the following failures or jamming in the flight control system and surfaces (including trim, lift, drag, and feel systems), within the normal flight envelope, without requiring exceptional piloting skill or strength. Probable malfunctions must have only minor effects on control system operation and must be capable of being readily counteracted by the pilot.
(1) Any single failure, excluding jamming (for example, disconnection or failure of mechanical elements, or structural failure of hydraulic components, such as actuators, control spool housing, and valves).
(2) Any combination of failures not shown to be extremely improbable, excluding jamming (for example, dual electrical or hydraulic system failures, or any single failure in combination with any probable hydraulic or electrical failure).
(3) Any jam in a control position normally encountered during takeoff, climb, cruise, normal turns, descent, and landing unless the jam is shown to be extremely improbable, or can be alleviated. A runaway of a flight control to an adverse position and jam must be accounted for if such runaway and subsequent jamming is not extremely improbable.
(d) The airplane must be designed so that it is controllable if all engines fail.
Compliance with this requirement may be shown by analysis where that method has been shown to be reliable.
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as amended by Amdt. 25-23, 35 FR 5674, Apr. 8, 1970]
NOTE: This regulation is the subject of a Federal Aviation Regulations/Joint Aviation Requirements (FAR/JAR) harmonization effort under the Aviation Rulemaking Advisory Committee (ARAC). The ARAC working group may recommend revisions to §§ 25.671 and 25.672. The ARAC working group is developing Advisory Circular (AC) 25.671-1.
2. SECTION 25.672 - STABILITY AUGMENTATION AND AUTOMATIC AND POWER-OPERATED SYSTEMS.
a. Rule Text.
If the functioning of stability augmentation or other automatic or power-operated systems is necessary to show compliance with the flight characteristics requirements of this part, such systems must comply with § 25.671 and the following:
(a) A warning which is clearly distinguishable to the pilot under expected flight conditions without requiring his attention must be provided for any failure in the stability augmentation system or in any other automatic or power-operated system which could result in an unsafe condition if the pilot were not aware of the failure. Warning systems must not activate the control systems.
(b) The design of the stability augmentation system or of any other automatic or power-operated system must permit initial counteraction of failures of the type specified in § 25.671(c) without requiring exceptional pilot skill or strength, by either the deactivation of the system, or a failed portion thereof, or by overriding the failure by movement of the flight controls in the normal sense.
(c) It must be shown that after any single failure of the stability augmentation system or any other automatic or power-operated system-
(1) The airplane is safely controllable when the failure or malfunction occurs at any speed or altitude within the approved operating limitations that is critical for the type of failure being considered;
(2) The controllability and maneuverability requirements of this part are met within a practical operational flight envelope (for example, speed, altitude, normal acceleration, and airplane configurations) which is described in the Airplane Flight Manual; and
(3) The trim, stability, and stall characteristics are not impaired below a level needed to permit continued safe flight and landing.
[Amdt. 25-23, 35 FR 5675, Apr. 8, 1970]
NOTE: This regulation is the subject of a Federal Aviation Regulations/Joint Aviation Requirements (FAR/JAR) harmonization effort under the Aviation Rulemaking Advisory Committee (ARAC). The ARAC working group may recommend revisions to §§ 25.671 and 25.672. The ARAC working group is developing AC 25.671-1.
NOTE: For policy and guidance on compliance with this requirement, refer to AC 25.672-1, Active Flight Controls.
3. SECTION 25.699 - LIFT AND DRAG DEVICE INDICATOR.
a. Rule Text.
(a) There must be means to indicate to the pilots the position of each lift or drag device having a separate control in the cockpit to adjust its position. In addition, an indication of unsymmetrical operation or other malfunction in the lift or drag device systems must be provided when such indication is necessary to enable the pilots to prevent or counteract an unsafe flight or ground condition, considering the effects on flight characteristics and performance.
(b) There must be means to indicate to the pilots the takeoff, en route, approach, and landing lift device positions.
(c) If any extension of the lift and drag devices beyond the landing position is possible, the controls must be clearly marked to identify this range of extension.
[Amdt. 25-23, 35 FR 5675, Apr. 8, 1970]
b. Intent of Rule. This rule prescribes standards for providing visual indication to the pilot of the high lift and drag device(s) surface positions for the takeoff, enroute, approach, and landing conditions.
c. Background. Effective February 1, 1965, part 25 was added to Title 14, Code of Federal Regulations (14 CFR), to replace part 4b of the Civil Air Regulations (CAR). Sections 4b.323(e) and 4b.323(f) of the CAR basically covered trailing edge devices. These sections became §§ 25.699(a) and 25.699(b) of 14 CFR for wing flap position indicator.
(1) Amendment 25-23 (April 8, 1970) expanded the rule to specifically cover all high drag devices or high lift devices (such as slots, spoilers, and slats), and added the requirement for position indication for all lift and drag devices with separate cockpit controls. The title was revised accordingly.
d. Policy/Compliance Methods. Indicators that show the position of the lift or drag devices must be provided. If asymmetric extension of lift or drag devices could result in an unsafe condition, an indication of each individual control surface is required. The indication must clearly identify each position setting. The controls should be designed so that inadvertent extension beyond the landing position is not possible. For guidance on compliance with this requirement, refer to AC 25-14, dated 5/4/88, which is incorporated below.
AC No.: AC 25-14 HIGH LIFT AND DRAG DEVICES
Initiated by: ANM-110 Date: 5/4/88
1. PURPOSE. This advisory circular (AC) sets forth an acceptable means of compliance with the provisions of part 25 of the Federal Aviation Regulations (FAR) dealing with the certification requirements for high lift and drag devices. Guidance information is provided for showing compliance with structural and functional safety standards for high lift and drag devices and their operating systems. The intent of the requirements and some acceptable means of compliance are discussed. Other means are acceptable if they meet the intent of the regulations.
2. RELATED FAR SECTIONS. The contents of this AC are considered by the FAA in determining compliance of flaps, slats and drag devices with part 25. Related sections are
25.301, 25.303, 25.333(e), 25.345, 25.457, 25.571, 25.671, 25.672, 25.697, 25.699, 25.701,
25.703, and 25.1309.
3. BACKGROUND. For several years, special consideration has been given to high lift and drag devices to ensure that malfunction or failure will not result in an unsafe condition.
These considerations are consolidated and incorporated in this AC.
4. STRUCTURAL REQUIREMENTS. The structure of high lift and drag devices must be designed to comply with the damage tolerance requirements of § 25.571, Amendment 25-45, of the FAR. The design should incorporate features which would provide a high probability of detection of any damage before the damage causes loss of the surface from the airplane. High lift and drag components to be evaluated under the requirements of § 25.571 typically include all structure which contributes significantly in reacting applied flight and actuation loads. Examples of such structure are the flap or slat surfaces, support linkages or tracks, hinges, fittings and attachments.
5. CONTROL SYSTEM REQUIREMENTS. The control system for high lift and drag devices must be designed to comply with the requirements of § 25.671. For the purpose of compliance with § 25.671, the control system ends where the control surface attaches to fixed structure such as the wing or fuselage. Examples of elements to be evaluated under the requirements of § 25.671 are linkages, hinges, cables, pulleys, quadrants, valves, actuator components, track rollers, movable tracks, bearings, and hydraulic or electrical systems. In accordance with § 25.671, the airplane must be shown to be capable of continued safe flight and landing without requiring exceptional pilot skill or strength following the failure of any single mechanical element or any combination of failures not shown to be extremely improbable, excluding jamming. Following this failure or combination of failures, the remaining structure 4
must be able to withstand the loads defined by §§ 25.333 and 25.345. These are considered ultimate loads for this condition. If the surfaces are automatically or power operated, the control system must also be designed to meet the requirements of § 25.672.
6. DETAIL DESIGN CONSIDERATIONS.
a. Unless the airplane has safe flight characteristics with the functionally related high lift or drag devices retracted on one side and extended on the other, the motion of the devices opposite sides of the plane of symmetry must be synchronized by a mechanical interconnection or approved equivalent means as required by § 25.701. The criteria of § 25.701 are considered equally applicable to high lift and drag devices.
(1) The surface interconnection must be designed for the loads resulting when the surfaces on one side of the plane of symmetry are jammed and immovable while the surfaces on the other side are free to move and the full power of the surface actuating system is applied. The flight loads from § 25.345 acting on the surface must be considered in combination with the actuating system loads (including system inertia loads). This is considered a limit load condition.
(2) In showing compliance with the interconnection requirements of § 25.701, all possible jam locations in the drive and support system should be considered. The surface mechanical interconnection must be able to withstand the jam condition and preclude any unsafe asymmetrical condition. The interconnection system is comprised of all elements which react the drive output from the actuator source to the jam point. These elements may include structures, interconnection linkages, and drive system components. When the interconnection is the only means to prevent an unsafe asymmetrical condition, the loads associated with the jam conditions are considered limit loads and require a 1.5 factor of safety. A factor of safety less than 1.5 may be used when a reliable (i.e., a probability of failure of 10-3 or less) and independent means is used, in addition to the mechanical interconnection, to prevent unsafe asymmetry of a high lift system. The alternate system should detect the jam and shut down the drive system before the loads from any jam condition are reacted by the mechanical interconnection. The factor of safety may be as low as 1.25 if the probability of failure of the alternate system is 10-5 or less; however, it should not be less than 1.25 unless the alternate system is found to be equivalent to a mechanical interconnection. When a torque limiter is used, the torque tolerance limit should be used to react the required load rather than the nominal or set torque. A torque limiter should not be located in the drive system in a position where the limiter itself would allow an unsymmetrical configuration if a jam occurred.
(3) An equivalent means of compliance with the requirements for a mechanical interconnection system may be substantiated using a systems safety analysis. Guidelines for performing a systems safety analysis are given in AC 25.1309-1, System Design Analysis.
b. Where failures in the drive system can result in uncommanded extensions or retractions of the high lift or drag devices, a positive means should be provided to limit the movement of the affected surfaces. This may be accomplished through irreversible drive actuators, no-back devices, redundancy in the drive system, or other equivalent means.
c. In determining loads on high lift devices during actuation, it may be necessary to consider friction loads in the actuating system, which may be reasonably expected to occur in service. Flap tracks and rollers for instance, are often subjected to ice and slush which may offer high resistance to flap actuation. Each design should be evaluated to determine its susceptibility to friction in the mechanism and any loads associated with such resistance should be accounted for and applied in combination with normal operating loads.
d. In evaluating the effects of failures or jamming of high lift surfaces, the effects of skewed surfaces on the operation of adjacent surfaces should be evaluated. Damage to adjacent structures and systems due to skewing of the surface should also be evaluated.
7. INDICATING AND WARNING SYSTEMS.
a. Indicating systems for high lift and drag devices must provide visual indication to the pilot of the surface positions for the takeoff, enroute, approach, and landing conditions.
The position sensors should be located such that they show a direct indication of failure conditions. There should be independent monitoring of each functionally related set of surfaces
(i.e., a set of surfaces on each side of the plane of symmetry that is driven by a common actuator, or is synchronized by some other means to ensure symmetric actuation) for which a failure will require an action or procedural change by the flightcrew. For instance, a functionally related flap set which is in an unsymmetrical configuration about the fuselage centerline would require an indication to the pilot of the unsymmetrical condition before takeoff. The indication to the flightcrew need not indicate the specific surface which has failed, but must clearly reflect the abnormal configuration (§ 25.699). The cockpit surface position display must also clearly distinguish a fault which was caused by a high lift "asymmetrical" deployment from a high lift "disagree" condition. A "disagree" condition exists when the high lift surface is stopped at a position different than the position commanded-by the pilot through the flap selection switch or handle. This distinction will aid the pilot in using proper procedures to further deploy, retract, or leave the high lift or drag devices for continued flight.
b. The takeoff warning system required by § 25.703 should sense the position of each functionally related set of high lift devices (symmetric about the airplane centerline) and provide aural warning during the initial portion of the takeoff roll if any set is not in an approved takeoff position.
8. FLIGHT LOADS MEASUREMENT. Notwithstanding the advancements in analytical methods used in predicting loads on airplane structures, accurate prediction of loads on wing leading edge and trailing edge high lift devices continues to be a problem. It is, therefore, advisable to verify the loads on these surfaces by conducting flight loads surveys regardless of the level of confidence in the overall loads program.
9. AIRPLANE CONTROLLABILITY. It should be shown by analysis, and where necessary by ground, simulation or flight tests, that the airplane has adequate stall margins and controllability to sustain the failure conditions addressed in paragraphs 4, 5, and 6 of this AC without requiring exceptional flightcrew skill or strength. It should also be demonstrated that no hazardous change in altitude or attitude will develop during transition to the unsymmetric condition considering likely transition rates.
END OF ADVISORY CIRCULAR 25-14
e. Reference. None.
4. SECTION 25.701 - FLAP INTERCONNECTION.
a. Rule Text.
(a) Unless the airplane has safe flight characteristics with the flaps or slats retracted on one side and extended on the other, the motion of flaps or slats on opposite sides of the plane of symmetry must be synchronized by a mechanical interconnection or approved equivalent means.
(b) If a wing flap or slat interconnection or equivalent means is used, it must be designed to account for the applicable unsymmetrical loads, including those resulting from flight with the engines on one side of the plane of symmetry inoperative and the remaining engines at takeoff power.
(c) For airplanes with flaps or slats that are not subjected to slipstream conditions, the structure must be designed for the loads imposed when the wing flaps or slats on one side are carrying the most severe load occurring in the prescribed symmetrical conditions and those on the other side are carrying not more than 80 percent of that load.
(d) The interconnection must be designed for the loads resulting when interconnected flap or slat surfaces on one side of the plane of symmetry are jammed and immovable while the surfaces on the other side are free to move and the full power of the surface actuating system is applied.
[Amdt. 25-72, 55 FR 29777, July 20, 1990]
NOTE: For policy and guidance on compliance with this requirement, refer to Advisory Circular (AC) 25-14, High Lift and Drag Devices, incorporated into this AC in § 25.699 paragraph d.
5. SECTION 25.703 - TAKE OFF WARNING SYSTEM.
a. Rule Text.
A takeoff warning system must be installed and must meet the following requirements:
(a) The system must provide to the pilots an aural warning that is automatically activated during the initial portion of the takeoff roll if the airplane is in a configuration, including any of the following, that would not allow a safe takeoff:
(1) The wing flaps or leading edge devices are not within the approved range of takeoff positions.
(2) Wing spoilers (except lateral control spoilers meeting the requirements of § 25.671), speed brakes, or longitudinal trim devices are in a position that would not allow a safe takeoff.
(b) The warning required by paragraph (a) of this section must continue until-
(1) The configuration is changed to allow a safe takeoff;
(2) Action is taken by the pilot to terminate the takeoff roll;
(3) The airplane is rotated for takeoff; or
(4) The warning is manually deactivated by the pilot.
(c) The means used to activate the system must function properly throughout the ranges of takeoff weights, altitudes, and temperatures for which certification is requested.
[Amdt. 25-42, 43 FR 2323, Jan. 16, 1978]
b. Intent of Rule. This rule prescribes a requirement for a takeoff warning system to warn the pilots during the initial portion of the takeoff roll if the airplane is in a configuration that would prevent successful completion of the takeoff.
c. Background. This section was not addressed in either part 4b of the Civil Air Regulations (CAR) or part 25 of Title 14, Code of Federal Regulations (14 CFR) when it was originally codified. This rule was introduced with Amendment 25-42, dated January 16, 1978.
It was realized that wing flaps and associated leading edge devices posed a special problem because some airplanes have take off flap settings that vary with weight, altitude, temperature, and runway length. A warning system that accounts for these variables would be extremely complex, and would still require the pilot to enter the proper data. In the interest of reliability, the rule requires the system to give a warning when the flaps or leading edge devices are not within the approved range of takeoff positions, e.g., when the pilot has not placed the flaps in an approved takeoff position or has retracted the flaps inadvertently, or if the flaps fail to move from the retracted position in response to a control input. Additionally, longitudinal trim devices (such as movable stabilizers) and drag devices (such as ground and flight spoilers) may be positioned inadvertently or due to failure conditions in a configuration that may prevent successful completion of a takeoff. The regulation applies to the stabilizer, speed brake systems, and rudder trim as well.
d. P olicy/Compliance Methods. For policy and guidance on compliance with this requirement, refer to Advisory Circular (AC) 25-14, High Lift and Drag Devices, incorporated into this AC in § 25.699 paragraph d.
e. Reference. The address for ordering the latest revision of the advisory circular listed below can be found in the Appendix to this AC.
AC 25.703-1, Takeoff Configuration Warning Systems.
6-10. [RESERVED] 9