(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 (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.
(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 brak e 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, 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 (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 (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).
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 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
174 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.
(d) 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, 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.
(e) 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.
(f) 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.
(7) Wheel Fuse Plug Integrity.
(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 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 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 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 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 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.
a. Explanation. If certification with ditching provisions is requested, § 25.801 requires investigation of the probable behavior of the airplane in a water landing. As stated in the regulation, this investigation can be accomplished by model testing or by comparison with airplanes of similar configuration for which the ditching characteristics are known. Applicants should also demonstrate that their ditching parameters used to show compliance with § 25.801 can be attained without the use of exceptional piloting skill, alertness, or strength.
b Procedures. None.
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. If there is a significant effect, this effect should be accounted for.
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. AC 25-20, “Pressurization, Ventilation, and Oxygen Systems Assessment for Subsonic Flight Including High Altitude Operations,” dated September 10, 1996, provides guidance for methods of showing compliance with the ventilation requirements.
Reference should also be made to paragraphs 137 and 165 of this AC dealing with Exhaust Systems, § 25.1121, and Fire Extinguishing Agents, § 25.1197, respectively.
(2) The objective of the inflight smoke evacuation test is to confirm that the flightcrew emergency procedures and the ventilation system are capable of handling heavy smoke, and to show that when using the emergency procedures, the smoke will dissipate at a reasonable rate.
This is a quantitative and qualitative evaluation.
b. Procedures.
(1) Flight testing should be conducted to ensure the amount of ventilation air provided meets the requirements specified and the flightcrew is able to accomplish their duties without undue fatigue and discomfort. Ventilation system controls in the flight deck should be demonstrated to perform their intended function.
(2) The passenger and crew compartment should be monitored for the presence of carbon monoxide. Various flight and equipment configurations should be tested. A carbon monoxide test kit is normally used for this evaluation.
(3) Inflight smoke evacuation testing should be conducted in accordance with AC 25- 9A, “Smoke Detection, Penetration, and Evacuation Tests and Related Flight Manual Emergency Procedures,” dated January 6, 1994.
85. Cabin Ozone Concentration - § 25.832. [Reserved]
86. Combustion Heating Systems - § 25.833. [Reserved]
Section 9. Pressurization
87. Pressurized Cabins - § 25.841.
a. Explanation.
(1) Section 25.841(a) specifies cabin pressure altitude limits, as a function of the external pressure altitude, for cabins and compartments intended to be occupied. AC 25-20 provides additional guidance for pressurized cabins. These cabin pressure altitude limits, to be demonstrated by flight testing, are:
(a) Not more than 8,000 ft., at the maximum operating altitude of the airplane, for normal operation of the pressurization system.
(b) For airplanes to be operated above 25,000 ft., the airplane must be able to maintain a cabin pressure altitude of not more than 15,000 ft. in the event of any reasonably probable failure or malfunction in the pressurization system.
(2) The 8,000 foot limit on cabin pressure altitude applies during normal operations at all altitudes, up to the maximum operating altitude of the airplane. For airplanes operating at airports with altitudes higher than 8,000 feet (see paragraph 87a(3)), an equivalent level of safety finding must be made in accordance with § 21.21(b)(1) to allow the higher than 8,000 foot cabin pressure altitude needed for these operations.
(3) Though not addressed by § 25.841, airplanes may incorporate a “high altitude mode” to permit takeoff and landing at airports above 8,000 ft. For takeoff and landing altitudes above 8,000 ft., the cabin pressure limits to be demonstrated by flight testing are:
(a) Prior to beginning a descent into a high altitude airport, not more than 8,000 ft.
(b) During a descent into or climb out of a high altitude airport, not more than 15,000 ft.
(c) After a takeoff from a high altitude airport and after the cabin pressure altitude decreases to 8,000 ft., not more than 8,000 ft.
(4) Section 25.841(b)(6) requires an aural or visual warning to the flightcrew when the cabin pressure altitude exceeds 10,000 ft. The intent of this requirement is to warn the crew when a safe or preset cabin pressure altitude limit is exceeded. However, complying with this regulation when operating at airports with altitudes higher than 10,000 feet (see paragraph 87a(3)) would result in nuisance cabin pressure warnings. For systems that set the cabin pressure altitude warning limit above 10,000 feet for operations at high altitude airports, an equivalent level of safety finding must be made in accordance with § 21.21(b)(1).
b. Procedures. It is recommended that the test airplane have the maximum allowable leakage rate permitted by the type design specifications. If an airplane does not meet this criterion, it will be necessary to substantiate compliance by additional testing or analysis for the maximum leakage rate allowed by the type design.
(1) Normal Operating Conditions - 8,000 Ft. Cabin Pressure Altitude.
(a) With the pressurization system operating in its normal mode, verify that the cabin pressure altitude does not exceed 8,000 feet during climb, cruise, or descent at any altitude up to and including the maximum operating altitude for which the airplane is to be certificated.
(b) A stable condition should be held long enough to record any cyclic fluctuations in cabin pressure due to relief valve operation.
(2) Failure Conditions - 15,000 Ft. Cabin Pressure Altitude.
(a) The critical probable system failure condition should be identified. The cabin pressure altitude warning system should be set to the high altitude side of its tolerance band or additional testing or analysis may be necessary for compliance. If more than one system failure mode is determined to meet the “probable failure condition” criterion, the flight test should be conducted for each failure mode identified.
(b) The airplane should be stabilized in the cruise configuration at the maximum operating altitude it is to be certificated to, with the pressurization system operating in its normal mode.
(c) After initiating the critical failure to allow the cabin pressure altitude to increase, the flight test crew should immediately don their oxygen masks, but take no further corrective action until 17 seconds after the 10,000 ft. cabin pressure altitude warning activates.
Emergency descent procedures should then be initiated and the descent continued to an altitude below 15,000 ft.
(d) The cabin pressure altitude should not exceed 15,000 ft. at any time during the test.
(3) High Altitude Takeoff Conditions (Greater than 8,000 Ft.).
(a) The pressurization system should be placed in its high altitude mode. It may be necessary to configure the oxygen system to prevent deployment of the oxygen masks at 10,000 ft., if the system is so designed. AFM procedures may need to be developed. The same applies to high altitude landings above 10,000 ft.
(b) The cabin pressure altitude should not exceed the maximum allowable takeoff and landing altitude (within an acceptable tolerance) during an actual climb-out from a high altitude airport to the maximum operating altitude the airplane is to be certificated for, or during an inflight simulation of such a climb-out. The simulation should commence the climb, from a starting altitude of 8,000 ft. or more, with the airplane unpressurized.
(c) The cabin pressure altitude should eventually decrease to 8,000 ft. and then not exceed 8,000 ft. for the duration of the test.
(4) High Altitude Landing Conditions (Greater than 8,000 Ft).
(a) The airplane should be stabilized in the cruise configuration prior to beginning a descent.
(b) The pressurization system should be placed in its high altitude mode.
(c) The cabin pressure altitude should not exceed the maximum allowable takeoff and landing altitude (within an acceptable tolerance) during the descent to an actual, or simulated, landing at a high altitude airport.
88. Tests For Pressurized Cabins - § 25.843.
a. Explanation.
(1) Section 25.843(b)(3) requires flight testing to evaluate the performance of the pressurization system and all related sub-systems at maximum altitude and under the dynamic conditions of climbing and descending flight. This testing substantiates the ability of the pressurization system to function correctly in stable and dynamic external pressure conditions.
(2) Section 25.843(b)(4) requires an investigation of the functionality of all doors and emergency exits after the flight test required by § 25.843(b)(3). The concern is the potential for jamming caused by the variable relative positions of doors and fuselage structure during the pressurization/depressurization cycle.
b. Procedures. The following tests may be initiated from an airport at any altitude within the airplane’s proposed takeoff limitations. The pressurization system should be operated in its normal mode. MMEL dispatch may be taken into consideration and additional testing required to verify MMEL configuration.
(1) Steady Climb/Descent. The steady climb/descent pressurization system tests should be performed under conditions (i.e., weight, altitude, temperature, and configuration) that
181 will result in rates of climb/descent corresponding to the maximum attainable within the operating limitations of the airplane.
(a) After takeoff, maintain a stable, continuous climb to the maximum operating altitude for which the airplane will be certificated.
(b) Maintain that altitude until the cabin pressure altitude has stabilized.
(c) Initiate a steady, maximum rate of descent within the operating limitations of the airplane down to the airport.
(2) Stepped Climb/Descent. The stepped climb/descent pressurization system tests should be performed under conditions (i.e., weight, altitude, temperature, and configuration) that will result in rates of climb/descent corresponding to the maximum attainable within the operating limitations of the airplane.
(a) After takeoff, initiate a stepped climb to the maximum operating altitude the airplane will be certificated to. Step increments should be 5,000 to 7,500 ft.
(b) Maintain each level-off altitude long enough for the cabin pressure altitude to stabilize.
(c) Initiate a stepped descent from the maximum altitude allowing the cabin pressure altitude to stabilize at each level-off altitude. Step increments should be 7,500 to 10,000 ft.
(3) Positive Pressure Relief. If two valves are provided, one should be deactivated for this test.
(a) After takeoff, climb to the operating altitude that provides maximum cabin differential pressure.
(b) Manually close the outflow valve, allowing the cabin differential pressure to increase.
(c) Verify the cabin pressure differential pressure warning functions properly.
(d) Verify that the relief valve functions and the maximum cabin differential pressure is not exceeded.
(4) Negative Pressure Relief/Emergency Descent. If two valves are provided, deactivate one for this test.
(a) At cruise altitude, perform an emergency descent with the airplane in the critical condition for negative pressure on the fuselage.
(b) Verify that the maximum negative differential pressure for the fuselage is not exceeded.
(5) Manual Cabin Pressure Control. If manual means for pressure control are provided, these means should be evaluated under normal and emergency operations of the airplane and flight envelope.
(6) Testing of Doors and Exits.
(a) Prior to flight, all doors and exits should be checked for proper operation.
(b) After returning from a flight where the airplane is subjected to the maximum certificated altitude and cabin differential pressure, and immediately upon landing, all passenger doors and emergency exits should be opened. There should be no change in the operating characteristics of any door or emergency exit relative to its pre-flight operation.
Section 10. Fire Protection [Reserved]
Section 11. Miscellaneous [Reserved]