157 landing gear system environments and account for longer tire lives/ increased intervals between tire changes and inspections. Bearing wheel grease recommendations are usually specified in wheel and brake supplier CMMs. Intermixing of bearing cups/rollers/cones from different bearing manufacturers is not recommended on some large transport category airplane wheels due to different roller end scoring resistance capabilities and other often subtle differences.
c. Procedures/Method of Compliance . Due to the unique and critical nature of wheel, and wheel and brake designs, and historical airplane and personnel safety problems that have been experienced, compliance should only be approved upon successful completion of applicable TSO tests and any additional airplane manufacturer tests. Guidance should also be solicited from the original wheel and brake supplier(s)/TSO holders on any replacement part(s) to assure that continued airworthiness is not degraded.
54. Tires - § 25.733 [Reserved]
55. Brakes - § 25.735
a. References .
(1) TSO-C135a “Transport Airplane Wheels and Wheel and Brake Assemblies,” dated July 1, 2009.
(2) AC 21-29C, Change 1, “Detecting and Reporting Suspected Unapproved Parts,” dated July 22, 2008.
b. Explanation .
(1) Background .
(a) The original objective of § 25.735 (formerly § 4b.337) developed from a study to define a reasonable brake life for operational landings. This element is still retained in current § 25.735(f), which requires substantiation that the brakes have the ability to absorb the energy resulting from “operational landings at maximum landing weight,” and is directly related to the brake energy qualification tests of TSO-C135a . The two methods for compliance, stated in § 25.735(f), are also used in TSO-C135a for the determination of KEDL, the design landing kinetic energy rating for 100 stops at a deceleration rate of not less than 10 ft/sec2. The initial wear state of the brakes for the testing used to determine KEDL may be selected by the applicant.
Any condition representative of service use, including new, and that satisfies TSO-C135a (or acceptable equivalent), may be used. Section 25.735(f) makes reference to “operational landings” and “in the landing configuration,” which, in the context of modern transport airplane designs, could encompass a number of trailing edge flap positions and related landing speeds.
The actual “landing configuration” for showing compliance with § 25.735(f) is left to the option of the applicant, but should be selected on the basis of its being the predominant landing configuration expected in operational service.
(b) Over the years, as experience was gained in establishing takeoff and landing field lengths, it became evident that the rejected takeoff was critical in determining overall brake capability and could dictate aircraft maximum gross weight for dispatch.
(c) Investigation of an RTO overrun accident, in which 80 percent of the brakes on the subject airplane were at or very near their completely worn state, brought about the need to consider the effect of brake wear state on: 1) energy absorption capability, and 2) stopping capability. As a result, the FAA issued a series of specific airworthiness directives for the existing fleet of transport category airplanes to establish brake wear limits such that the brakes would be capable of absorbing a maximum energy absorption RTO in the fully worn state. The FAA also initiated rulemaking to address energy absorption capability and stopping distance with fully worn brake for future airplane types. The resulting rule, amendment 25-92, added:
1 A requirement in § 25.735(h) for the maximum rejected takeoff kinetic energy capacity rating of the aircraft brakes to be determined with the brakes at 100 percent of the allowable wear limit;
2 A new requirement in § 25.109(i) for the maximum kinetic energy rejected takeoff flight test demonstration to be conducted using brakes that have not more than 10 percent of their allowable wear range remaining; and
3 A general performance requirement under § 25.101(i) that requires the accelerate-stop and landing distances of §§ 25.109 and 25.125, respectively, to be determined with all wheel brake assemblies at the fully worn limit of their allowable wear range.
(2) Approval (§ 25.735(a)) . In accordance with § 21.305, a wheel and brake assembly may be approved under a TSO issued in accordance with subpart O to 14 CFR part 21; in conjunction with the type certification procedures for the airplane; or in any other manner approved by the Administrator, all of which are normally subject to flight test evaluation. (See TSO-C135a for additional guidance.)
(3) Brake tests (§ 25.735(b)) . If the applicant desires to make the maximum possible use of the brakes in establishing the landing distance, and if the contribution of the brakes to the total deceleration is relatively large, the brake system should be designed to permit the application of slightly less than half the braking deceleration developed under the conditions specified in this paragraph. The following dual system is recommended: dual wheel elements (drums or disc units), transmitting elements, power sources, master cylinders, etc., connected to a single pedal on each rudder pedal, such that the failure of any single one of these would leave half the total braking capacity symmetrically disposed about the plane of symmetry of the airplane. With such a system, it should be possible to show compliance with § 25.735(b) by means of calculations based upon the test data necessary to establish the landing distance, plus the brake data calculated by the airplane manufacturer.
(4) If the system is designed so that , under conditions herein specified, appreciably less than half the total braking capacity remains, or if the remaining capacity is asymmetrically disposed, tests should be conducted to determine that half the mean deceleration may, in fact, be
159 developed, and/or that the airplane may be safely controlled directionally while doing so. In order to obtain a minimum landing distance under § 25.125 and at the same time meet the deceleration requirements of § 25.735(b) in the event of failure of the normal brake system, it is common practice to provide an alternate brake system. When hydraulic (or pneumatic) brakes are used in the normal brake system, this alternate means usually consists of a duplicate hydraulic or pneumatic brake system and is commonly referred to as the “emergency brake system.” The following items should be considered in the design of such systems:
(a) Relationship between normal and emergency brake systems . The systems for actuating the normal brake and the emergency brake should be separated so that a failure (i.e., hydraulic leakage of fluid) from one system will not render the other system inoperative. A hydraulic brake assembly may be common to both the normal and emergency brake systems if it is shown that the leakage of hydraulic fluid resulting from failure of the sealing elements in the brake assembly would not reduce the airplane’s braking effectiveness below that specified in § 25.735(b).
(b) Brake controls (§ 25.735(c)) . General brake control force and operation should be noted throughout the flight test program to determine that they are satisfactory.
(c) Brake control valves . In the normal brake systems of all airplanes, the brake valves should be of a type such that the pilots may exercise variable control of the pressure to the brakes. The foregoing provision need not necessarily apply to the emergency brake system, although obviously such a provision would be desirable. Flight tests should be conducted to determine that the normal and emergency brake systems fulfill the requirements of § 25.231.
(d) Parking brake control (§ 25.735(d)) . A demonstration should be made to determine that sufficient braking is provided with the parking brake to prevent the airplane from rolling on a paved, dry, level runway (or any suitable level hard surface) while maximum takeoff power is applied on the most critical engine, with the airplane loaded to maximum ramp weight at aft center of gravity (or the airplane loaded to a weight-c.g. combination that prevents the wheels from sliding). In the case of propeller-driven airplanes, the effects of propeller wash and engine/propeller torque should also be considered in determining the critical engine. Because the resultant thrust vector can be at an angle to a propeller’s axis of rotation, one engine/propeller may be more critical than its counterpart on the opposite wing, particularly if all propellers turn in the same direction.
(e) Antiskid devices (§ 25.735(e)) .
1 In addition to meeting acceptable industry specifications, the installation of the antiskid device should comply with the requirements specified in paragraphs (2) and (3) below. The antiskid device and its installation will be approved for use on transport airplanes when the tests specified in the procedures section (see paragraph 55c of this AC) have been satisfactorily demonstrated.
2 Data required.
160 (aa) An engineering evaluation of the antiskid installation as installed on the airplane, including all necessary components, should be conducted. This analysis and complete descriptive data should be submitted to the FAA. The data should include hydraulic and electrical schematic diagrams of the installation, assembly drawings of antiskid system units, functional hazard analyses, software and hardware substantiations for electronic components, test results or stress analysis substantiating structural strength of attachments and modification of the axle or other structural members, installation drawings, recommended instructions pertaining to installation, maintenance and operation, and analysis of flight test data and results. Schematic drawings should refer to all units in the normal and emergency brake systems. The engineering evaluation should also assure that the antiskid system does not cause undesirable yaw characteristics.
(bb) The engineering evaluation should account for a bounce condition wherein the wheels may leave the runway after the brakes have been applied, for a condition wherein the wheels stay on the runway but the oleos are extended (if the system utilizes landing gear oleo compression in its operation), and for a condition in which the wheels of one main gear may not be in contact with the runway for a considerable time while the wheels of the other main gear are firmly on the runway. If the antiskid installation incorporates the “landing with brake pedals depressed” feature, then this type of operation should also be considered.
(cc) It should be shown that the brake cycling frequency imposed by the antiskid installation will not result in excessive loads on the landing gear.
(dd) The antiskid equipment should ensure satisfactory braking action on wet and snow/ice-covered runways, as well as on dry, hard-surfaced runways, without additional antiskid adjustments.
3 Systems requirements . The entire brake system (including both the basic brake system and the antiskid system) should conform to § 25.735. The single failure criterion of § 25.735 should be extended to include the antiskid system. The following should be considered for approval of antiskid systems:
(aa) In the event of a probable malfunction within the antiskid system that would result in loss of the antiskid feature on one or more brake units, those brake units affected should automatically revert to normal braking.
(bb) A means should be provided so that the pilot or copilot can readily deactivate the antiskid system to prevent loss of all braking capability due to antiskid system malfunctions.
(cc) A failure mode and effects analysis (FMEA) should be provided to evaluate the effect of failures of the antiskid and associated systems (tires, wheels, brakes, etc.)
on airplane safety and braking performance. For antiskid systems utilizing “locked wheel protection” circuits, this analysis should include a determination that assures mechanical faults, which could permit the affected axle to continue rotation (e.g., blown tires, broken wheel rims), will not significantly reduce the braking effectiveness of any cross-coupled wheel positions with
161 intact wheels and tires. A tire or wheel failure that results in the loss of additional tires or wheels, either from shrapnel impingement or ensuing overload, should be considered a single failure event.
(f) Wheel Fuse Plugs .
1 The hazardous condition of exploding tires and wheels associated with high energy emergency stopping conditions has been greatly alleviated by the installation of wheel fuse plugs. These plugs relieve the tire pressure when the wheel temperatures approach a dangerous limit. The effectiveness of these plugs in preventing hazardous tire blowouts should be demonstrated during an RTO test where the brake energy to be absorbed exceeds the maximum landing energy, but not the RTO energy, and the fuse plug releases, thereby deflating the tire(s) before blowout.
2 An improperly designed blowout plug that allows premature or unwanted release of tire pressure during takeoff or landing could also constitute a hazardous condition.
Such a situation would most probably arise during a takeoff from a quick turn-around type of airline operation. Fuse plug integrity should be demonstrated by conducting a maximum landing brake energy (which then becomes the quick turn-around chart limit for the AFM) test, which should not result in a fuse plug release.
3 Most turbojet transport airplanes have been able to demonstrate wheel blowout plug integrity at a maximum energy level in accordance with the procedures outlined in this section. More restrictive operational limitations (e.g., runway slope and tailwind values) have been imposed to stay within this maximum energy level demonstrated for wheel blowout plug integrity. With the advent of requests to increase the maximum landing weights, and to eliminate these restrictive operational limits, it has been considered acceptable to remove the pertinent restrictions and operational limitations and substitute in their place a chase-around chart as a limitation in the AFM. This chart will permit determining whether or not a critical energy level has been exceeded for the operating conditions of altitude, temperature, runway slope, tailwind, and landing weight. When the critical value is exceeded, a statement should be placed in the limitations section of the AFM to require that following a landing under such operating conditions, the airplane must remain on the ground a certain length of time prior to taxiing out for takeoff. This length of time will be the time to reach peak wheel temperatures (appropriate to the blowout plug location), plus 15 minutes. In lieu of the AFM fuse plug limitation chart, an alternate method for determining limit operational landing energy, such as a brake temperature limit, can be considered for approval.
4 The wear level of the brakes used for the chase-around chart discussed in the preceding paragraph should be selected by the applicant. Service experience indicates that the conservatism contained in the method of determining the turnaround time limits is adequate to allow these limits to be determined using new brakes.
5 In the case where it is possible to demonstrate wheel blowout plug integrity at a maximum energy level in accordance with the procedures in this section and without imposing certain restrictions on the operational limitations (e.g., runway slope and
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,