162 tailwind values), it is not considered necessary to incorporate the chase-around chart and pertinent statement in the limitations section of the AFM. Where restrictions are necessary, a pertinent statement and reference to the chase-around chart could be included in the limitations section of the AFM. The chart may be in Section 4, or in an appendix to the AFM.
(g) Replacement and Modified Brakes .
1 In order to establish aircraft landing and RTO certification performance levels for a replacement brake or a modified brake, measured accelerate-stop tests, and functional flight tests (landing distance), may be required, depending upon an evaluation of the individual merits of each brake system change. The type and magnitude of flight tests required will depend on whether or not a requested change involves a corresponding change of heat sink and/or torque requirements of the original certificated brake. A review of the change by the cognizant aircraft certification office (ACO) for the type certificate holder is necessary, since original landing gear designs are based on structural analysis, which could be adversely affected by a brake system change. In addition, such tests will also depend on whether or not an increase in the FAA certificated performance level is desired by the applicant.
2 Changes to the friction couple elements (rotors and stators) are generally considered to be a major change, requiring the testing described in paragraph 55c(1), unless it can be shown that the change cannot affect the airplane stopping performance, brake energy absorption characteristics, or continued airworthiness. Historically, continued airworthiness considerations include such items as landing gear system/airplane vibration control, braking feel, landing gear system compatibilities etc.
3 Changes to a brake by a manufacturer other than the original TSO holder might be considered to be a minor change, as long as the changes are not to the friction couple elements, and the proposed change(s) cannot affect the airplane stopping performance, brake energy absorption, vibration, and/or thermal control characteristics, and continued airworthiness of the airplane. In certain circumstances, the change to a steel rotor by a manufacturer other than the original TSO holder may be considered to be a minor change, as discussed in paragraph 55c(5)(b).
c. Procedures . The extent of the flight test requirements, except new airplane certification, may vary depending upon an evaluation of the individual merits of each airplane brake system change, and whether or not an increase in the FAA certificated performance level is desired by the applicant. Past experience has proven that dynamometer tests alone are not considered adequate in determining compliance with this requirement. Flight test procedures for new, replacement, and modified brakes are categorized as follows:
(1) Basic new airplane certification -- complete new design where no airplane performance data exist.
(a) Tests required.
163 1 For complete analysis, at least six rejected takeoffs and six landings will normally be necessary.
(aa) Six landings should be conducted on the same wheels, tires, and brakes.
(bb) All tests should be conducted with engines trimmed to the high side of the normal idle range (if applicable).
(cc) For airplanes whose certification basis includes amendment 25-92, § 25.101(i) requires the stopping distance portions of the accelerate-stop and landing distances to be determined with all the aircraft brake assemblies in the fully worn state. An acceptable means of compliance with this requirement is to accomplish the flight test braking tests with less than fully worn brakes, and then correct the test results using dynamometer test data determined with fully worn brakes. It should be substantiated that the dynamometer test methodology employed, and analytical modeling of the airplane/runway system, are representative of actual conditions.
2 Additional tests may be necessary for each airplane configuration change
(i.e., takeoff and/or landing flaps, nose wheel brakes, antiskid devices inoperative, deactivation of wheel brakes, etc.).
3 Brake system response evaluation including braking during taxiing (see paragraph 30b(2)(i).
4 Parking brake adequacy (see paragraph 55c(1)(c)).
5 Alternate braking system stops.
6 Wheel fuse plug evaluation (see paragraph 55c(7)).
7 Antiskid compatibility on a wet runway.
8 Automatic gear retraction braking system on airplanes so equipped.
(b) Maximum RTO energy will be established by conducting an RTO at the maximum brake energy level for which the airplane will be certified. Fires on or around the landing gear are acceptable if the fires can be allowed to burn during the first 5 minutes after the airplane comes to a stop, before extinguishers are required to maintain the safety of the airplane.
The condition of the tires, wheels, and brakes can be such that the airplane would require maintenance prior to removal from the runway. A deceleration rate should be maintained during this test that is consistent with the values used by performance scheduling. Tire or wheel explosions are not acceptable. Tire fuse plug releases may occur late in the RTO run, provided directional control is not compromised. The resulting distance (with fuse plugs blown during the RTO run) is to be included in the data used to establish AFM performance, only if it is longer than the data obtained with normal full braking configuration.
164 1 For airplanes whose certification basis includes amendment 25-92, the maximum brake energy absorption level must be determined, per § 25.101(i), for an airplane with all wheel brake assemblies in the fully worn state. In accordance with § 25.109(i), the flight test maximum energy RTO demonstration must be accomplished with all brake assemblies within 10 percent of their allowable wear limit (i.e., at least 90 percent worn). Dynamometer testing, when used to extend the flight test results to determine the maximum energy absorption capability of the brakes in the fully worn state (i.e., 100 percent worn), should be substantiated as being representative of actual airplane and runway conditions. The fully worn limit is defined as the amount of wear allowed before the brake needs to be removed from the airplane for overhaul.
The allowable wear should be defined in terms of a linear dimension in the axial direction, which is typically determined by measuring the wear pin extension.
2 The maximum energy RTO demonstration should be preceded by at least a three mile taxi, with at least three intermediate full stops, using normal braking and with all engines operating.
3 Landings are not an acceptable means to conduct maximum energy RTO demonstrations. Though permitted in the past, service experience has shown that methods utilized to predict brake and tire temperature increases that would have occurred during taxi and acceleration were not able to account accurately for the associated energy increments.
(c) The ability of the parking brake to prevent the airplane from rolling should be demonstrated on a paved, dry, level runway (or any suitable level hard surface) with takeoff power applied on the critical engine using the following test procedure:
1 The airplane should be loaded to its maximum takeoff weight (or the airplane loaded to a weight-c.g. combination that prevents the wheels from sliding) with the tires inflated to the normal pressure for that weight, the flaps should be retracted, the control surfaces centered, and the parking brake set.
2 Apply takeoff power to the critical engine with the other engine(s) at idle.
3 Compliance with the requirements of § 25.735(d) is shown if the wheels do not rotate; this is best observed by painting a white radial stripe(s) on the wheels. The airplane may skip, tire tread may shear, or the tire may slip on the wheel, but the parking brake must prevent the wheels from rotating. Skidding of the tires is acceptable.
(2) Addition of New or Modified Brake Design .
(a) This item concerns the addition of a new or highly modified brake design to an existing type certificated airplane for which FAA-approved braking performance test data exists, either for performance credit, or to the existing performance level. A highly modified brake is defined as one that contains new or modified parts that may cause a significant variance in brake kinetic energy absorption characteristics, airplane stopping performance, or continued airworthiness of the airplane. Examples are: significant change in rotor and/or stator lining compound or area, number of stages, piston area, reduction in heat sink weight, changes in total number of friction faces and elements, change in brake geometry (friction, radius, friction area), fuse plug relocation or change in release temperature, heat shield changes that would affect the temperature profile of the wheel and/or fuse plugs, or seal changes.
(b) Tests Required.
1 For improved performance credit, all applicable portions of paragraph 55c(1).
2 For equivalent performance, a sufficient number of conditions to verify the existing approved performance levels (RTO and landing). Consideration should be given to verification of fuse plugs, performance verification at appropriate energy levels, and configuration differences, including antiskid on and off. Taxi tests to ensure that ground handling, maneuvering, and brake sensitivity are satisfactory should be conducted. At least two braking stops, one at heavy weight and one at light weight, should be conducted on a wet runway to verify brake and antiskid system compatibility.
3 For extended performance, a sufficient number of conditions to define the extended line and determine equivalency to the existing performance levels. Consideration should be given to the items in paragraph 2, above.
(c) Definitions.
1 Improved performance implies an increase in the mu versus energy level for the desired operation(s) and may be requested for landing, RTO’s, or a specific configuration such as antiskid “on” only.
2 Equivalent performance implies that sufficient data will be obtained to verify that the performance level for the desired change is equal to or better than the existing performance levels. The change may be for the purpose of changing the c.g. envelope, or for airplane configuration changes (such as flap angles), and may apply to specific operations (such as landings).
3 Extended performance implies that the existing certification mu versus energy line will be extended to establish the braking force level for a proposed change, such as
166 gross weight or the maximum desired energy level, and may be applied to a specific operation (such as landing only).
(3) Addition of new, or changes to, antiskid systems that may affect airplane performance (e.g., new antiskid system, or a change from coupled to individual wheel control).
A sufficient number of airplane performance tests and/or functional tests should be conducted to verify existing approved performance antiskid “on” levels. In the event an increase of braking performance is desired, full airplane performance testing is required.
(4) Fuse plug modification.
(a) Airplane tests for changes to the fuse plugs should be evaluated on a case-bycase basis. While airplane tests are required to establish the initial fuse-plug-no-melt energy, minor changes to fuse plugs or wheel designs may be validated by a back-to-back dynamometer comparison of old versus new designs, provided it is acceptable to the cognizant FAA ACO.
(b) Airplane tests should be conducted when a significant change of wheel design and/or redesign or relocation of thermal or pressure fuse plugs is made.
1 One airplane test should be conducted to show that the fuse plugs will release when excessive energies are absorbed.
2 Another airplane test should be conducted to verify the maximum kinetic energy at which fuse plugs will not release (fuse plug substantiation). Dynamometer tests are not considered adequate for this test.
(5) Minor/Major Changes.
(a) Minor brake changes that do not affect airplane braking performance may require functional landings. This may be required to verify airplane-pilot-brake-antiskid combination compatibility. Normally, five non-instrumented, functional landings are considered sufficient to verify this compatibility. Examples of minor changes might include structural improvements (increased fatigue life), adjuster/retractor modifications, material and process specifications changes for structural components, and modified heat-sink relief slots (steel brakes). Examples of other minor changes that do not require functional landings are paint/corrosion changes, changes to bleed ports or lube fittings, revised over inflation devices, metal repair, and salvage procedure. Such changes could be considered to be minor whether they are proposed by the original manufacturer who holds the TSO authorization, or by another manufacturer seeking to produce replacement parts.
(b) Changes to heat sink friction couple elements are to be considered major changes, unless the applicant can provide evidence that changes are minor. Based upon experience, thicker friction material or heavier heatsink elements are usually acceptable as minor changes in steel brakes. Thicker or heavier heat sink elements in carbon brakes may require additional laboratory and/or airplane testing to assess brake performances and continued airworthiness. Major changes are subject to extensive airplane testing, unless it can be shown
167 that the change cannot affect the airplane stopping performance, brake energy absorption characteristics, and continued airworthiness. In this regard, the original manufacturer of the wheel/brake assembly who holds the TSO authorization, and the type certificate holder who is knowledgeable with respect to such items as landing gear design assumptions and airplane braking system history, may possess data sufficient to show that such changes could be considered to be minor (i.e., performance would not be affected). In contrast, an applicant other than the original manufacturer who wishes to produce replacement rotors or stators may not have access to or have established developmental or other test data required to show that performance, braking energy capacities, braking system compatibilities, or overall continued airworthiness safeguards have been addressed. Due to the complex nature of the friction surfaces and airplane braking system interfaces, proposed replacement stators/rotors by an applicant other than the original manufacturer(s) should always be considered a major change.
(c) It is considered very difficult to determine 100 percent identicality. This is particularly true for brake friction rubbing components (e.g., linings in cups, linings sintered to plates, steels used in steel brakes, and carbon discs in carbon brakes). A finding of equivalence based upon physical documentation and dynamometer testing may not be possible or practical due to friction material complexities and/or the extent of dynamometer testing required.
(d) Due to the complexities associated with aircraft brake friction couples, industry and authorities have generally discouraged mixing of friction components from various suppliers within the same brake or mixing wheel and brake assemblies from different manufacturers on the same airplane. Typically, the manufacturers of large transport category airplanes have confined the use of specific wheel and brake supplier’s assemblies to specific airplanes through approved equipment lists. While multiple wheel and brake suppliers (i.e., multiple original TSO holders) are often selected to provide wheel and brake assemblies on a specific airplane model, intermixing of wheel and brake assemblies has been discouraged to avoid potential problems such as unequal energy sharing; unfavorable dynamic cross coupling between brakes, landing gear, and the airplane; degradation of vibration and/or thermal controls;
unique brake control system tuning requirements for each wheel and brake assembly, etc.
(e) The FAA has, however, approved the use of replacement steel rotors. The following protocol for steel rotor equivalency findings has been updated to include brake wear and wear pattern assessments to assure that the worn brake capability of the original manufacturer’s wheel and brake assembly is not degraded by a replacement steel rotor(s). The following criteria and evaluations represent protocol for replacement steel rotors to be considered as a minor change:
1 A very close correlation between the original part and the proposed replacement part;
2 Considerable and satisfactory prior manufacturing and in-service experience with a similar replacement part;
3 A reasonable plan of test for completion of the dynamometer portion of the test program;
4 Successful completion of the dynamometer testing; and
5 As a minimum, successful completion of a series of functional landings on the airplane.
(f) The necessity of conducting maximum energy RTO testing and other brake system tests on the airplane will depend upon the outcome of the above evaluations and worn brake RTO airplane test experiences (if applicable).
(g) If intermixing of replacement steel rotors with the original manufacturer’s steel rotors is proposed, the applicant should propose an airplane test evaluation plan to the FAA that provides data to guide a worn brake rejected-takeoff equivalency assessment. If the applicant cannot provide evidence on similar overall wear and wear patterns from the new to worn condition for the proposed mixing configuration(s), intermixing will not be permitted in order to assure that the worn brake rejected-takeoff rating and approved wear limit of the original TSO holders wheel and brake assembly is not jeopardized.
(h) The dynamometer test plan, and, if applicable, the overall wear and wear pattern test plan, should include:
1 TSO Minimum Standard Performance Demonstration. The wheel and brake assembly containing the applicant’s proposed steel rotor configuration should successfully demonstrate compliance with the braking and structural tests of the applicable TSO, and
2 Compliance with:
(aa) the airplane-manufacturer-specified requirements, or
(bb) the alternate procedures specified below:
(i) Alternate Procedures (steel rotors only).
1 Energy and Torque Capacity Tests. A series of tests (not only one) may be necessary to demonstrate, in back-to-back tests, that brake energy absorption and torque and pressure vs. time profiles are equivalent. All friction components and structures should be in the new condition to obtain credit for this test. If rebuilt or in-service components other than these fail during testing, it should be realized that the results of the test(s) may be questionable.
Suspect tests will be carefully reviewed by the FAA, and may require retesting. Prior to test, the applicant should carefully document wheel hardness and wheel drive, torque tube spline, piston/bushing assembly conditions to assure comparable test articles are being used. The same tire size and ply rating, manufacturer, tire condition, radial load, and rolling radius should be used in each test. Test machines and test conditions should be consistent from test to test, including test brake, wheel and tire break-in stop histories, brake pressure onset rates (poundforce per square inch gauge per second) (psig/sec), maximum pressures, initial brakes-on-speeds,
169 flywheel inertias, etc., to assure consistent test control. Artificial cooling is not permitted during or subsequent to the test until wheel and piston housing temperatures have peaked.
2 The initial kinetic energy (IKE) level for this series of tests will be at the discretion of the applicant. For each succeeding run, the KE will be increased by approximately 5 percent over the previous run, until the ultimate KE level is determined (i.e., points at which pistons are about to exit bores or flywheel deceleration falls below 3 ft/sec2 (½ of the TSO RTO minimum average deceleration requirement)). The deceleration reported by the applicant should be based upon distance (and not time) in accordance with the following formula:
Distance Averaged Deceleration =
((Initial brakes-on speed) 2 - (Final brakes-on-speed)2 )/2(braked flywheel distance).
(aa) A minimum of two runs at this ultimate energy level should be conducted on the original manufacturer’s wheel and brake assembly for baselines. These test runs should show similar results. Maximum braking force pressure should be applied during the tests. Fuse plug releases in any tests should demonstrate safe release of approved nitrogen-air mixtures.
(bb) A minimum of two test runs at the ultimate energy level should then be conducted on the applicant’s proposed wheel and brake assembly (i.e., a back-to-back demonstration of the two manufacturers’ brakes). Tests should show similar brake energy absorption, torque, and thermal performance capabilities, and torque and pressure versus time profiles, while demonstrating sealing and structural integrity comparable to the original manufacturer’s wheel and brake assembly.
3 Worn Brake RTO Capability. Worn brake RTO capability for the proposed wheel and brake assembly with steel rotor replacement configuration(s), and at the wear pin limit(s) proposed by the applicant, should be established during dynamometer test(s).
The test brake energy absorption criteria, torque performance, and pass/fail requirements should be requested of the airplane manufacturer to provide supporting evidence that the worn brake RTO capability is equivalent to that achieved with the original TSO holder’s wheel and brake assembly(ies). If unavailable, the applicant should propose a worn brake RTO test plan similar to that in paragraph 55c(1) for new brakes. The applicant should also propose to the FAA the method which will be followed by the applicant to verify that the worn brake RTO capability of in-service worn brakes with replacement rotors is equivalent to the capability established in initial dynamometer test(s) in accordance with paragraph 55c(1)(b)1 of this AC.
4 Intermixing of steel rotor assemblies produced by two manufacturers will not be allowed until it can be demonstrated that wear patterns of the intermixed assembly(ies), through a determined number of in-service tours, does not jeopardize the worn brake RTO capabilities of the original or the replacement wheel and brake assemblies. Since dynamometer testing is generally impractical, the applicant should forward to the FAA an in-service plan to survey wear and wear patterns from a sampling of worn in-service brakes containing steel rollers from the original manufacturer only, from the applicant only, and from an intermix brake(s).
170 Since the original manufacturer’s brake often contains second, and possibly third tour reground steel rotors, at least two tours of in-service evaluation with replacement steel rotors may be required to assess equivalence. This data will provide guidance for approved worn brake RTO wear limits for brakes containing replacement rotors.
5 Torque/pressure ratio-profiles. A torque/pressure ratio test plan and tests are required to demonstrate equivalent gain performances over a range of test speeds and test pressures. The test article conditions, break-in conditions and procedures, test speed range, and test pressure matrix, used to evaluate both the original and applicant wheel and brake assemblies, should be the same with tests conducted in the same order. The results of these tests will provide guidance for braking system control compatibility assessments.
6 As a minimum, the five functional landings described in paragraph 55c(5)(a), above, are also a required part of this approval procedure.
7 Continued Airworthiness. Past hist ory with friction material couples has indicated the necessity of ongoing monitoring (by dynamometer test) of RTO capability to assure that the AFM limitations are not exceeded over the life of airplane programs. For larger transport category commercial airplanes, it has been shown that these monitoring plans have complemented the detection and correction of unacceptable deviations. The applicant should provide the FAA with a quality plan to demonstrate that the RTO capability of the friction couple is maintained with replacement steel rotors over time.
(6) Auto-braking. The following are required for auto-braking installations based on function, non-hazard, and non-interference on airplanes for which performance without autobraking has been determined:
(a) The system design should be evaluated for integrity and non-hazard, including the probability and consequence of insidious failure of critical components. No single failure may compromise non-automatic braking of the airplane.
(b) Positive indication of whether the system is operative or inoperative should be provided.
(c) For each auto-brake setting for which approval is desired, the ground roll distance from touchdown to stop should be determined for the landing weights and altitudes within the envelope for which approval is desired. In determining ground roll distance, the performance should be established without reverse thrust, and any adverse effect upon performance associated with the use of reverse thrust should be established and accounted for.
Repeatability of initial application should be shown by comparing the onset of braking for each of the range of settings. Landing ground roll distance data determined as prescribed herein should be presented in the performance information section of the AFM as guidance material.
If the auto-braking system is to be approved for wet runways, auto-brake compatibility on a wet runway should be demonstrated. These tests may be limited to the highest auto-brake setting, where antiskid activity is expected to occur throughout the stop, and a single lower setting,
171 where antiskid activity is expected to occur for only a portion of the stop. AFM stopping distances for other settings can be computed based on predicted wet-runway friction coefficients and do not require demonstrations on wet runways of all auto-brake settings. Landing ground roll distance data determined on a wet runway should also be presented in the AFM for all operating modes of the system. This information is considered necessary so that the pilot can readily compare the automatic brake stopping distance and the actual runway length available, so as to assess the effect of the use of the automatic braking system on the runway margin provided by the factored field length.
(d) Automatic braking systems that are to be approved for use during rejected takeoff conditions should provide only a single brake setting that provides maximum braking. In the event that automatic brakes result in a longer rejected takeoff distance than manual brakes, the AFM should present the longer rejected takeoff distance.
(e) Procedures describing how the automatic braking system was used during the FAA evaluation and in determining the landing ground roll distance of paragraph 55c(6)(c) should be presented in the AFM.
(f) Compliance with operational requirements regarding landing runway length should continue to be based on landing distance data established using the requirements of § 25.125 for non-automatic braking.
(7) Wheel Fuse Plug Design.
(a) Wheel fuse plug integrity should be substantiated during braking tests where the energy level simulates the maximum landing energy. It should be demonstrated that the wheel fuse plugs will remain intact, and that unwanted releases do not occur. One acceptable method to determine this is as follows:
1 Set engine idle thrust at the maximum value specified (if applicable).
2 Taxi at least three miles (normal braking, at least three intermediate stops, and all engines operating).
3 Conduct accelerate-stop test at maximum landing energy, maintaining the deceleration rate consistent with the values used to determine performance distance.
4 Taxi at least three miles (normal braking, at least three intermediate stops, and all engines operating).
5 Park in an area so as to minimize wind effects until it is assured that fuse plug temperatures have peaked and that no plugs have released.
(b) In lieu of simulating the maximum kinetic energy landing during an accelerate-stop test, an actual landing and quick turn-around may be performed; however, caution should be exercised in order to prevent jeopardizing the safety of the flightcrew and
172 airplane if the wheel plugs release right after liftoff, requiring a landing to be made with some flat tires. The following elements should be included in the tests:
1 Set engine idle thrust at maximum value specified (if applicable).
2 Conduct a landing stop at maximum landing energy, maintaining the deceleration rate consistent with the values used to determine stopping performance distance.
3 Taxi to the ramp (three miles minimum with normal braking, at least three intermediate stops, and all engines operating).
4 Stop at the ramp. Proceed immediately to taxi for takeoff.
5 Taxi for takeoff (three miles minimum with normal braking, at least three intermediate stops, and all engines operating).
6 Park in an area so as to minimize wind effects until it is assured that fuse plug temperatures have peaked and that no plugs have released.
(c) Fuse plug protection of wheels and tires should be demonstrated to show that the fuse plugs will release when excessive energies are absorbed. Normally, this will occur during RTO performance tests.
56. Skis - § 25.737 [Reserved]
Section 5. Floats and Hulls
57. - 59. [Reserved]
Section 6. Personnel and Cargo Accommodations
60. - 61. [Reserved]
62. Pilot Compartment View - § 25.773.
a. Explanation . (Reserved.)
b. Procedures . For detailed guidance on complying with pilot compartment view requirements, refer to AC 25.773-1, “Pilot Compartment View for Transport Category Airplanes,” dated January 8, 1993.
63. - 72. [Reserved]
Section 7. Emergency Provisions
73. Ditching - § 25.801 [Reserved]
74. Emergency Egress Assist Means and Escape Routes - § 25.810.
a. Explanation .
(1) Installation of slip-resistant escape route. (See §25.810(c), formerly § 25.803(e).)
See AC 25-17A, “Transport Airplane Cabin Interiors Crashworthiness Handbook,” dated May 18, 2009, § 25.803(e), for guidance regarding slip resistant material.
(2) The effect of the slip-resistant surfaces on airplane performance, flight characteristics, and buffet should be evaluated and should, if necessary, account for significant drag increments.
b. Procedures . None.
75. - 83. [Reserved]
Section 8. Ventilation and Heating
84. Ventilation - § 25.831.
a. Explanation .
(1) This requirement deals with minimum ventilation requirements for each occupant of the airplane, control of the ventilating air, accumulation and evacuation of smoke and harmful or hazardous concentrations of gases or vapors, and failure conditions of the ventilation system.
Specific quantities of fresh air along with carbon monoxide and carbon dioxide concentration limits are specified in the rule. Reference should be made to the paragraphs in this AC dealing