(e) If it has not been established which engine is critical, conduct a separate test for each engine. If it has been determined there isn't a critical engine for a twin engine airplane, a test with either engine at takeoff power will suffice.
(5) Worn Brake Requirements for Non-Original-Equipment Parts. The following is extracted from an FAA letter dated February 18, 1992, which addresses an original equipment manufacturer (OEM) inquiring whether a non-OEM was required to follow the same procedures and conduct the same tests as the OEM. The FAA response was affirmative and explained below.
(a) An FAA issued AD proposes brake wear limits for specific series airplanes.
This proposal required that all landing gear brakes be inspected for wear and be replaced if the wear limits prescribed in this proposal are not met, and that the new wear limits be incorporated into the FAA-approved maintenance inspection program. An original equipment manufacturer asked that the AD require the following for non-original equipment (NOE) certified parts:
1 NOE manufacturers submit worn brake RTO test results for all brake assemblies using their parts.
2 Brake assemblies containing NOE parts be given new part numbers and TSO markings.
3 Installation approval and notification of all operators and overhaul shops be required concerning these approvals, and
4 Compliance with the terms of the AD be required for anyone using or furnishing parts for brakes.
(b) The FAA position was that regulations, orders and advisory material provide guidance concerning changes to approved parts, components and assemblies and that events 1, 2, 3, and 4 occur. As an example, a brake assembly was approved with NOE rotors under a supplemental type certificate (STC) project. The Aircraft Certification Office, which issued the STC, required the company manufacturing the brake assembly meet all applicable worn brake RTO requirements.
(6) Determination of Allowable Worn Brake Limits. The following is extracted from an FAA memorandum dated February 23, 1990, which provides guidance regarding worn brake capability determination. A policy letter dated March 2, 1990, contained the same information.
Issue papers were used to administer the same policy for new certification programs until Amendment 25-92 was issued in February 1998.
(a) This memorandum established criteria for worn brake testing for in-service transport category airplanes. Worn-brake accountability determination was intended to validate brake wear limits with respect to brake energy capacity only, and was not meant to account for 41
any reduction in brake force due solely to the wear state of the brake. This criteria is intended for transport category airplanes used in part 121 operation, and which have a certificated maximum takeoff weight in excess of 75,000 pounds.
(b) It should be noted that this worn-brake accountability determination will validate brake wear limits with respect to brake energy capacity only, and is not meant to account for any reduction in brake force due solely to the wear state of the brake. Any reduction in brake force (or torque) that may develop over time as a result of brake wear is to be evaluated and accounted for as part of the rulemaking project described in (c) below. The accident that prompted this investigation into brake wear is believed to have been caused by inadequate brake energy-absorbing capability. The issue of loss of brake effectiveness with brake wear, and the resulting increase in runway lengths required for a RTO is addressed under Amendment 25-92.
(c) The following dynamometer test guidelines are the result of a series of meetings. These test procedures may be used for the determination of acceptable airplane brake wear limits.
1 Acceptable Test Brakes.
(aa) Either airplane-worn or mechanically-worn brakes may be used.
Mechanically-worn is defined as not being airplane-worn, e.g., machined or dynamometer-worn.
If mechanically-worn brakes are used, it must be shown that they can be expected to provide similar results to airplane-worn brakes.
(bb) Each test brake shall be subjected to a sufficient number and type of stops to ensure that the brake's performance is representative of in-service use.
2 Wear State of the Test Brake.
(aa) Degree of Wear. The degree of wear of the test brake shall be 100 percent. One hundred percent worn is defined as that degree of wear which the applicant intends to allow before the brake is to be removed for overhaul. At the overhaul limit the brake will not be fully worn out, but will contain sufficient braking capability to meet the energy absorption requirements discussed below. The chosen test brake shall be such that the wear-in conditions produce a brake ready to perform the RTO test at the correct wear setting. If a brake to be tested is worn less than 100 percent, an acceptable method of extrapolation to the fully worn state must be provided prior to test.
(bb) Definition of Degree of Wear. The degree of wear shall be defined in terms of the linear, axial direction dimension relating to the allowable wear of the brake as commonly determined by noting wear pin extension.
(cc) Distribution of Wear (Applicable only to mechanically-worn brakes).
The proportioning of the wear through the brake for the various friction pairs shall be based on 42
either: Service experience on the test brake or an appropriate equivalent brake, or Dynamometer wear test data.
3 Brake Energy Level to Set 100 Percent Wear Limits.
(aa) It will be necessary to conduct the dynamometer test with an initial energy value prior to the RTO test that is analogous to that generated during the airplane certification flight test of that brake (e.g., energy due to taxi stops).
(bb) The dynamometer RTO maximum energy level may be based on the Airplane Flight Manual limiting conditions, including, if desired by the applicant, the effects of:
Engine reverse thrust, following normal procedures for power setting, cutback speed, and The recommended number of reversers to be used with a critical engine inoperative, and the demonstrated transition times achieved in or equivalent to an RTO flight test.
(cc) The dynamometer test should be conducted to duplicate the airplane's speed versus time performance as closely as practicable.
(dd) The use of reverse thrust shall be limited to a determination of the energy to be absorbed by the brake during the dynamometer test. No thrust reverser credit shall be given for the purpose of determining runway distance requirements or airplane stopping performance.
4 Power Level. The test shall be conducted at either of the conditions below, provided that the test is conducted at the condition which more closely represents the actual braking conditions obtainable on the airplane. The intent of these procedures is to simulate actual airplane conditions as closely as possible:
(aa) The maximum brake pressure, or
(bb) The maximum tire drag or brake torque consistent with the airplane's hydraulic system and any antiskid and/or torque limiter pressure limitations that would occur on the airplane during an equivalent RTO operation.
(cc) In order to assure that the test results are accurate, repeatable, and simulate actual airplane conditions as closely as possible, the dynamometer tests must be conducted on an inertial-wheel type dynamometer, using a suitable tire/wheel/brake assembly. If use of a shaft-type dynamometer is desired, it must be shown prior to any testing, that the shafttype dynamometer will provide results equivalent to that provided by the inertial-wheel type dynamometer.
5 Final Condition Definition.
(aa) A full-stop demonstration is not required for the worn-brake RTO test. The test brake pressure may be released at a dynamometer speed of up to 20 knots to facilitate a detailed post-test inspection of the brake. The dynamometer test may be started at a slightly higher speed so that the test may be terminated at 20 knots or less, provided that the data submitted for each test show that the energy absorbed by the brake during a test that is terminated at 20 knots or less, is equal to the energy that would have been absorbed if the test had been started at the proper speed and continued to zero ground speed.
(bb) There shall be no wheel burst as a result of this test.
6 Data Requirements.
(aa) As a minimum, the following technical data shall be obtained for each dynamometer test conducted:
Brake torque (or force).
Brake pressure.
T ime.
Road wheel speed.
Road wheel distance.
Dyna mometer inertia equivalent.
(bb) The absorbed dynamometer kinetic energy shall be computed based on measured data. Additional data may also be obtained to aid in interpolating and extrapolating test results.
(cc) A test report shall be prepared which, as a minimum, shall include:
A detailed description of the test article (e.g., component part numbers, individual disk measurements, wear pin measurements, etc.), The test procedures, The test results, and If possible, photographs of brake rotors/stators before and after the dynamometer test.
(dd) In order to receive credit for the effects of (N-1) thrust reversers, where N is the number of reversers available at dispatch, thrust reverser effectiveness must be 44
substantiated. Any existing validated reverser performance data are to be submitted for FAA review and approval. This is required for each engine family for each airplane model.
7 Interpretation of Data. Any adjustment of energy levels or allowable wear from the dynamometer test shall be based on a review of the test data, inspection of brake hardware after test, and subsequent analysis.
(aa) An extrapolation of wear data or energy data up to 5 percent of the test values shall be permissible.
(bb) An interpolation of data up to 20 percent of the test values shall be permissible to establish energy levels from multiple dynamometer tests which are within this range from the target condition.
8 Acceptance of Prior Tests.
(aa) Worn-brake RTO tests which have been conducted successfully prior to the adoption of these procedures may be acceptable. These tests need not be repeated solely to gather test data specified here.
(bb) These worn-brake dynamometer test guidelines are a recommended test procedure, and, as such, represent one means, but not necessarily the only means, of determining acceptable maximum brake wear. The FAA would consider deviations from these guidelines if it can be shown that the proposed procedure is appropriate and would produce equivalent results.
(cc) The dynamometer guidelines listed above indicate that credit for reverse thrust, using the recommended number of thrust reversers with a critical engine inoperative, is to be allowed in the determination of the maximum brake energy on the dynamometer. The effect of inoperative thrust reversers due to Minimum Equipment List (MEL) dispatch must also be accounted for.
(dd) If a particular brake is affected by the Airworthiness Directive (AD), and is later tested on the dynamometer and shown to require a different wear pin cut-back than that specified in the AD, then the AD should be amended at that time to reflect the correct wear pin length. The continued airworthiness aspects of this statement apply to all brakes whether or not an AD was issued.
(ee) On any given airplane model there may be many different brake configurations that are quite similar, and identified by different dash numbers. It may be acceptable for the manufacturer to demonstrate acceptable brake wear limits on one brake representative of a brake family, and extend that brake wear limit to others of the same family by analysis. Each case will have to be evaluated on its own merits.
(ff) There may be situations in which a brake modification or brake installation is proposed by someone other than the airframe manufacturer or the original brake manufacturer. In these cases, if the brake modification is judged to be a major change involving the friction couple (rotor/stator), or in any way could affect brake energy capacity, a determination of the effects of brake wear in accordance with these procedures must be accomplished. The applicant may claim the same brake wear limit as the original brake manufacturer, and this may be acceptable, but this would have to be demonstrated by analysis or test.
(gg) A project to amend part 25 to include a determination of the effects of brake wear on brake force has been initiated. At this time it has not been determined what the format for this determination will be. For the present time, any certification project involving new brakes or a substantial redesign of existing brakes (major change) should be conducted as described in AC 25-7A (formerly Order 8110.8), i.e., airplane flight tests should be conducted using new brakes. An adjustment to the allowable brake wear limits should then be made, if applicable, using the procedures described above.
(7) Credit for Reverse Thrust in Worn Brake Testing. The following is extracted from an FAA memorandum dated April 28, 1989, which established the acceptability of reverse thrust credit for the purposes of establishing worn brake limits only.
(a) During certification of aircraft brake systems, a demonstration of stopping performance at the maximum takeoff weight and speed expected in service is conducted. This RTO test shows that the brake has the capacity to absorb the required kinetic energy, and may limit certain performance information in the Airplane Flight Manual. This test is conducted with new brakes and without reverse thrust or other deceleration means other than spoilers.
(b) Accident data has shown that the brake-wear limits and brake design in general may be inadequate to provide the required stopping performance at maximum RTO energy levels with worn brakes.
(c) An adopted rule AD that reduces allowable brake wear prior to overhaul on the XYZ model airplane has been issued. This AD was based on dynamometer testing, and provides for a brake overhaul interval such that sufficient brake mass remains at overhaul to absorb the certificated RTO energy and allow the airplane to stop on the runway.
(d) FAA/Industry meetings were held to develop a dynamometer test protocol prior to any testing, so as to minimize costs and delays. A major issue, which arose during these meetings, is credit for the effects of reverse thrust on the kinetic energy that must be absorbed by the brakes. Because of the way brake energy for the initial dynamometer tests was calculated, 46
the initial AD included credit for (N-1) thrust reversers, i.e., 2-engine reverser credit for the 3-engine aircraft. As you know, the FAA has been reluctant in the past to grant credit for reverse thrust relating to braking performance or brake kinetic energy determination. We remain unconvinced that reverse thrust credit should be granted in the case of airplane braking performance, because reverse thrust has not been shown to be sufficiently reliable for this purpose. However, the FAA has no valid reason to deny the use of reverse thrust in the determination of brake kinetic energy for brake wear limit tests. Reverse thrust will certainly be used in any RTO (if available), and the probability of a high-energy, field-length-limited RTO combined with an inoperative reversers is sufficiently low to allow relief in this area. It will be acceptable to allow credit for the effects of (N-1) thrust reversers in the determination of allowable wear limits in dynamometer tests discussed above.
(e) It should be noted that this use of reverse thrust applies to kinetic energy determination for the purpose of establishing worn brake limits only. The FAA will not consider allowing credit for reverse thrust in the determination of accelerate-stop distances or braking performance.
(8) Certification of Brake Replacement Components. The following is extracted from FAA letters and memorandums dated March 16, 1989, December 27, 1988, and March 16, 1988, which address the certification data/testing required for obtaining FAA approval for replacement parts of an existing FAA approved brake installation.
(a) In early 1989, the FAA issued an STC to provide replacement brake parts for certain part number brakes on a specific airplane. Throughout the STC review process, the FAA was acutely aware that granting this STC would be precedent-setting. Brakes to date had been designed and approved as a system, involving the wheel/brake assembly, the tire, and the airplane landing gear and brake systems. This approval involved a component of a previously approved system, and as such, the approval method for this modification was necessarily different from that applied to the original manufacturer.
(b) The key issue to be addressed was the amount of product development and testing that must be conducted in order to demonstrate that the proposed parts are satisfactory for their intended use. An original brake manufacturer must perform considerable research and development in order to design an aircraft brake with the best compromise of low weight, durability, useful life, and performance. The interface between the airplane structure and systems and the proposed brake must be investigated, and this sometimes requires much dynamometer and airplane flight testing. Early in-service experience with the new brake may reveal the need for minor "fine tuning" in the brake design.
(c) If another manufacturer later desires to produce any of the components of an approved assembly under an STC or a Parts Manufacturer Approval (PMA), he or she must show that the airplane, as modified to incorporate the new components, continues to comply with the regulations incorporated by reference in the type certificate for that airplane. This does not necessarily entail following the same approval path as that of the original manufacturer. In many cases, he or she need only duplicate the part in question, and if the part can be shown to be 47
"identical" to the original part, FAA approval can be granted on that basis. This method of approval of replacement parts has a long, successful history and will continue to be used when appropriate. The original manufacturer of a TSO-authorized part does enjoy certain advantages over another manufacturer that seeks to modify that part. He may make modifications to the part based on the experience gained during the product development without extensive testing, and have the change approved as a "minor" change. The same change proposed by another applicant might be judged to be a "major" change, which would necessitate considerably more testing.
(d) In this case, the STC applicant originally sought approval of their part based on identicality with the original part. A finding of identicality could not be made, however, due to some small differences between the parts. The differences existed and could not be ignored, but were judged to be so insignificant as to obviate the need for the full spectrum of ground and flight tests discussed in AC 25-7A, Flight Test Guide for Transport Category Airplanes (formerly Order 8110.8), chapter 4, section 4, paragraph 55. It was agreed by the FAA and the STC applicant that a back-to-back comparison of the original brake and the company's modified brake on a flywheel-type dynamometer at the ultimate energy capacity of the original brake would be an acceptable test to show equivalent performance. New OEM brakes would be tested at successively higher energies until brake failure occurred, or was imminent; the applicant's modified brake would then be tested at this energy, with a second modified brake tested for a demonstration of repeatability.
(e) Although, by its very nature this dynamometer test alone could not support a "major" brake change (as discussed in AC 25-7A, formerly Order 8110.8), it would demonstrate that the ultimate kinetic energy (KE) capacity of the replacement brake part and the OEM brake part were equivalent under identical laboratory test conditions. This, together with the near identicality of the two components, was the basis for the FAA approval. No attempt was made to duplicate any conditions relating to on-airplane brake testing. The well-documented differences between dynamometer testing and airplane testing prevented any such comparison.
It is possible that this series of dynamometer tests was a more severe test than a single RTO, since the brake was tested to destruction in the laboratory under controlled conditions, and its behavior could be more closely monitored than on the airplane. The FAA was aware of the torque and deceleration window mentioned by the OEM, and the replacement part was shown to perform within that envelope satisfactorily.
(f) The OEM was concerned that the friction couple was a key factor in the determination of the performance of the brake, and the FAA concurred. In future applications for similar approvals, either by the STC applicant or other manufacturers, the FAA expects to apply the test regimen discussed in FAA Order 8110.8. The dynamometer test used in this case would only be used if the situation were essentially the same as this case ; i.e., a part that is so nearly identical with the original that there is little question that the part would perform the same as the original, and the FAA was satisfied with a demonstration of manufacturing competence by the STC applicant.
(g) Another issue raised by the OEM was the unilateral FAA approval of the replacement part without the concurrence of the airframe manufacturer. This is the normal 48
method of FAA approval for STC applications. Data supplied to the FAA by the STC applicant is, in most cases, proprietary to the applicant, and the FAA is obliged not to reveal details of the application to another manufacturer. The FAA is in a position to have access to pertinent data from the original airframe manufacturer, and questions of compatibility can be answered without involving the original manufacturer. It is unreasonable to expect assistance from a manufacturer if that assistance would benefit a competitor or interfere with their own interests. There are numerous examples of major modifications to airplanes that have been completed without the assistance or cooperation of the original airframe manufacturer. In this particular case, the replacement part manufacturer unsuccessfully attempted to obtain technical data from both the airframe and brake manufacturer, as well as from the FAA, under the Freedom of Information Act.
(h) During a meeting between the OEM and the FAA, the issues of product support, configuration management, and product maintenance were discussed at some length.
Product improvements are a normal part of any manufactured product, and we expect the OEM brake that was the basis for the STC approval will continue to evolve as the market changes and service history dictates. In each case, the burden will be on the STC holder to demonstrate that their STC is satisfactory as a replacement for any new OEM brake part numbers, if they choose to make such an application, and that applicability must be demonstrated, either by the dynamometer test method discussed above, or other test requirements dictated by the particular situation under consideration. It is possible that future changes to the OEM brake under consideration here may make the replacement part STC obsolete.
(i) We concur with the OEM’s statement that the performance of the proposed replacement brake component must necessarily be identical to the original. In many other areas, the FAA will accept a demonstration of performance that is "equal to or greater than" the original, since many of our requirements are intended to be minimum performance standards. In this case, improved brake performance may actually be a detriment, because landing gear structure, and brake and antiskid systems are often tuned to brake performance, and a change in braking µ or other brake characteristics may have an adverse affect on the airplane itself. If the performance of the replacement part is shown to be different in any way from the original, the applicant would be required to show that the effect on the airplane would be acceptable.
(j) In 1987 the FAA convened a special technical review team to examine the facts surrounding the original application for approval of replacement brake components. The team reviewed the brake approval procedures that had been applied in the past, including TSO approvals, dynamometer testing, and airplane flight testing. The team also considered information presented by the STC applicant. The consensus of the team is as follows:
1 The distinction between “major” and “minor” changes to brakes and brake systems, as discussed in AC 25-7A, chapter 4, section 4, paragraph 55, (formerly FAA Order 8110.8), is valid, and should continue to be followed. The discussion and examples given in paragraph 55 are generally adequate to determine the proper course of testing to be followed in the certification of brake changes.
2 The applicant had failed to establish identicality of the proposed replacement part with the originally approved brake component. There was also the possibility that the change could impact the airplane stopping performance, or the kinetic energy absorption characteristics of the brake. Such a design change would normally lead to a classification as a major change requiring a demonstration of brake performance on the airplane, including a maximum energy RTO.
3 The review team concluded, however, that the proposed replacement brake part could be properly considered a minor change, on the basis of demonstrated equivalency, as supported by additional analysis and tests. This means that the testing requirements of a major change need not apply, if functional airplane landings and comparative dynamometer tests could be successfully completed. Given the close similarity of the replacement part and the original, the inherent simplicity of the brake part (as compared to the stator with its friction linings, for example), and the applicant's long experience in producing acceptable brake rotors for the U. S. Air Force, it was determined that the procedure proposed by the FAA for dynamometer testing of the applicants part was acceptable. This procedure contains the following items:
(aa) Use of new stator and rotor parts in both the applicant's and OEM’s brakes for each test will be required, in order to minimize test configuration variables. If rebuilt or in-service components other than these fail during testing, it should be realized that the results of the test may be questionable. Suspect tests would be carefully scrutinized by the FAA and retesting may then be necessary.
(bb) A series of tests may be necessary to find the ultimate KE level of the OEM brake, as agreed upon in the original test plan. For each succeeding run, the KE will be increased by 5% over the previous run until the ultimate KE level is determined, i.e., brake failure has occurred, or is imminent. The initial kinetic energy level for this series of tests will be at the discretion of the applicant. After each test run, the brake will be examined and a determination will be made whether additional tests at higher energies are necessary.
(cc) A minimum of two test runs at this ultimate energy level must be conducted on the OEM brakes for validation of this level prior to conducting the applicant's comparison tests. These two runs must show similar results. This provides confidence similar to that obtained from multiple brake assemblies on actual airplane tests.
(dd) At least one run at this ultimate energy level must then be run on the applicant's brake assembly, i.e., a back-to-back demonstration of the two brakes. The need for additional test runs on the applicant's brake will be determined after the first test. If the applicant's brake shows good correlation to the OEM brake, additional tests will not be necessary.
(ee) The correlation factor between the OEM and the applicant's tests must be such that the results of the applicant's tests can be shown to be equivalent to or better than the OEM tests.
(ff) Constant maximum braking force pressure appropriate to the airplane braking system must be applied during the tests.
(gg) Fuse plugs may be released or the tire deflated after each test run, to reduce the risk to test personnel.
(hh) The functional landings already completed were also a required part of this approval.
4 This additional dynamometer testing in lieu of actual airplane testing was applied in this case due to the unusual circumstances relating to the applicant's application. The applicant had considerable manufacturing experience with brake parts, indicated by long and trouble-free service history in the Air Force inventory. While this Air Force service history was not directly applicable to commercial operations, and would not in itself be a basis for FAA approval, it did indicate that the applicant's parts performed adequately in service. This gave a measure of confidence that the ultimate energy dynamometer testing to be performed would be successfully completed.
5 It should not be inferred from this action taken on this specific brake part proposed by this applicant that this dynamometer test procedure will be applicable to any other application, either from this applicant or other manufacturers. Each request for approval should be judged on its own merits.
(9) Ground Equipment - Brake Cooling Unit. The following is extracted from an FAA letter dated December 27, 1988, which addresses certification of an aircraft wheel/brake cooling unit.
(a) This type of equipment is not approved as part of the aircraft type certificate.
The FAA does not impose any technical standards on most ground-support equipment that is used to support airline operations. Ground service trucks, baggage carts, and the device proposed by the applicant fall into this category. The only exception to this is if the particular item of ground equipment performs a critical function, or could cause damage to the airplane if used 51
incorrectly. Examples of the latter include portions of jacking equipment which must mate with jacking fittings on the airplane, towing bars that attach to the nose gear, and fuel systems on fuel trucks to ensure cleanliness of the fuel.
(b) Airlines that choose to use the wheel/brake cooling device proposed by the applicant would be responsible for showing that the device is appropriate for its intended use in their operations and would not pose a hazard to their personnel or the airplane. In addition, performance credit for reduced ground turn-around time (required time for brake cooling prior to the next takeoff) cannot be granted without an evaluation of the effectiveness of the device by the appropriate FAA Aircraft Certification office.
(10) Certification Requirements for Asbestos-Free Brake Linings. The following is extracted from an FAA letter dated December 16, 1987, which addresses guidance for certification of replacement brakes with asbestos-free linings, based only on dynamometer and TSO testing.
(a) The testing requirements in AC 25-7A, Flight Test Guide for Transport Category Airplanes (formerly FAA Order 8110.8, paragraph 89(b)), relate to the addition of a highly modified brake design to an existing type certificated airplane. A highly modified brake is defined in this section as one which contains new or modified parts which in turn will cause a significant variance in the kinetic energy absorption characteristics of the brake. It has been our experience that airplane performance cannot be adequately predicted from the dynamometer testing specified in the TSO. While it may be satisfactory to assign some performance penalty to compensate for this, there remains the question of the effect of the change in friction materials on fuse plug performance, ground handling, and brake sensitivity. AC 25-7A, chapter 4, section 4, paragraph 55, issued March 31, 1998, contains relevant portions of canceled FAA Order 8110.8, and additional information.
(b) In addition, the coefficient of friction wear rates of the new materials may prove to be different from the original. In order to adequately demonstrate the performance of a major change to the brakes or brakes control system, a maximum kinetic energy RTO, as described in AC 25-7A (formerly Order 8110.8), would normally be required, as well as six functional takeoffs and landings. In addition, fuse plug substantiation test (melt and no-melt) would be required (if applicable), unless it can be demonstrated that heat transfer from the brake stack to the wheel is identical to the original. Also, antiskid-on and antiskid-off tests (if applicable), qualitative taxi tests, and wet runway tests (antiskid-on, if applicable) would be required. These test procedures are described in AC 25-7A (formerly Order 8110.8, paragraph 89(f)).
(c) It is noted that numerous older airplanes may use these new asbestos-free brakes. The intent of the test program discussed above is to validate the brake change, as it affects each type design under consideration. It is important to note that each airplane type which uses these brakes provides a different operating environment, considering landing gear geometry, dynamic response, weight, rate of energy absorption, etc. As stated in AC 25-7A (formerly Order 8110.8, paragraph 89(b)), a sufficient number of conditions to verify the existing 52