15.4.4.2 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.
15.4.4.2.1 For airplanes whose certification basis includes amendment 25-92, the maximum brake energy absorption level must be determined, according to § 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.
15.4.4.2.2 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.
15.4.4.2.3 Landings are not an acceptable means to conduct maximum energy RTO demonstrations. 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 were not able to account accurately for the associated energy increments.
15.4.4.3 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 or thrust applied on the critical engine and up to maximum ground idle power or thrust is applied to all other engines using the following test procedure:
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15.4.4.3.1 The airplane should be loaded to its maximum takeoff weight (or a weight-CG 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.
15.4.4.3.2 Apply takeoff power or thrust to the critical engine with the other engine(s) at maximum ground idle power or thrust.
15.4.4.3.3 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.
15.4.5 Addition of New or Modified Brake Design.
15.4.5.1 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.
15.4.5.2 Required Tests.
15.4.5.2.1 For improved performance, all applicable portions of paragraph 15.4.4.
15.4.5.2.2 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 antis-kid 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 anti-skid system compatibility.
15.4.5.2.3 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 15.4.5.2.2 above.
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15.4.5.3 Definitions.
15.4.5.3.1 Improved performance implies an increase in the mu versus energy level for the desired operation(s) and may be requested for landing, RTOs, or a specific configuration such as anti-skid “on” only.
15.4.5.3.2 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 CG envelope, or for airplane configuration changes (such as flap angles) and may apply to specific operations (such as landings).
15.4.5.3.3 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 gross weight or the maximum desired energy level and may be applied to a specific operation (such as landing only).
15.4.6 Addition of New, or Changes to, Anti-skid Systems that may Affect Airplane Performance.
(For example, new anti-skid 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 anti-skid “on” levels. In the event an increase of braking performance is desired, full airplane performance testing is required.
15.4.7 Fuse Plug Modification.
15.4.7.1 Airplane tests for changes to the fuse plugs should be evaluated on a case-by-case 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.
15.4.7.2 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. One airplane test should be conducted to show that the fuse plugs will release when excessive energies are absorbed. 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.
15.4.8 Minor/Major Changes.
15.4.8.1 Minor brake changes that do not affect airplane braking performance may require functional landings. This may be required to verify airplane-pilot-brake-anti-skid combination compatibility. Normally, five 15-11
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.
15.4.8.2 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 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.
15.4.8.3 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.
15.4.8.4 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 15-12
specific airplanes through approved equipment lists. While multiple wheel and brake suppliers (e.g., 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.
15.4.8.5 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.
15.4.8.6 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).
15.4.8.7 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.
15.4.8.8 The dynamometer test plan, and, if applicable, the overall wear and wear pattern test plan, should include:
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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 the airplane manufacturer’s specified requirements, or the alternate procedures specified in paragraph 15.4.8.9.
15.4.8.9 Alternate Procedures (Steel Rotors Only).
15.4.8.9.1 Energy and Torque Capacity Tests.
A series of tests (not only one) may be necessary to demonstrate, in backto-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 (pound-force per square inch gauge per second) (psig/sec), maximum pressures, initial brakes-on-speeds, 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.
15.4.8.9.2 The initial kinetic energy (KE) 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 (one-half 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:
1. 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
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releases in any tests should demonstrate safe release of approved nitrogen-air mixtures.
2. 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.
15.4.8.9.3 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 15.4.4 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 15.4.4.2.1 of this AC.
15.4.8.9.4 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).
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.
15.4.8.9.5 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 15-15
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.
15.4.8.9.6 As a minimum, the five functional landings described in paragraph 15.4.8.1 above are also a required part of this approval procedure.
15.4.8.9.7 Continued Airworthiness.
Past history 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.
15.4.9 Auto-Braking.
The following are required for auto-braking installations based on function, non-hazard, and non-interference on airplanes for which performance without auto-braking has been determined:
15.4.9.1 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.
15.4.9.2 Positive indication of whether the system is operative or inoperative should be provided.
15.4.9.3 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.
15.4.9.4 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 anti-skid activity is expected to occur throughout the stop, and a single lower setting, where anti-skid activity is expected to occur for only a portion of the stop. AFM 15-16
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, to assess the effect of the use of the automatic braking system on the runway margin provided by the factored field length.
15.4.9.5 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.
15.4.9.6 Procedures describing how the automatic braking system was used during the FAA evaluation and in determining the landing ground roll distance of paragraph 15.4.9.3 should be presented in the AFM.
15.4.10 Wheel Fuse Plug Integrity.
15.4.10.1 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 power or 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 to minimize wind effects until it is assured that fuse plug temperatures have peaked and that no plugs have released.
15.4.10.2 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 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:
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1. Set engine idle power or 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 to minimize wind effects until it is assured that fuse plug temperatures have peaked and that no plugs have released.
15.4.10.3 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.
15.5 Skis—§ 25.737. [Reserved] 15-18
CHAPTER 16. DESIGN AND CONSTRUCTION: FLOATS AND HULLS [RESERVED] 16-1
CHAPTER 17. DESIGN AND CONSTRUCTION: PERSONNEL AND CARGO
ACCOMMODATIONS
17.1 Pilot Compartment View—§ 25.773.
17.1.1 Explanation.
[Reserved.]
17.1.2 Procedures.
For detailed guidance on complying with pilot compartment view requirements, see AC 25.773-1.
17.2 [Reserved.] 17-1