(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
approved performance levels on each type design would be required. The actual content of the test program would depend on many factors, including the similarity of the old and new designs, the relative performance envelopes of the demonstrated brake and candidate airplane, effect of the new brake design on fuse plug performance (if applicable), presence or absence of antiskid systems, and possible effects of the change on ground handling. It is also possible that the addition of these new brakes to certain existing airplanes could be considered to be a minor change, in which case AC 25-7A (formerly Order 8110.8, paragraph 89(b)) would apply, and only uninstrumented functional landings would be required. It was recognized that it may be impossible to duplicate test data that may have been created over 20 years ago, especially if the airplane in question is not equipped with an antiskid system. In this case, it may be appropriate to apply a suitable performance penalty to allow for this uncertainty. In any event, engineering judgment should be applied to each individual situation so that the intent of the test program discussed above will be followed.
(d) While the performance of the new brake must be no less than that of the originally approved brake, it also should not be greater; e.g., a higher brake torque is desirable for reducing stopping distance but may be undesirable for structural integrity of other components of the landing gear. Original landing gear designs are based on structural analysis, which could be adversely affected by a brake system change.
(11) Certification Requirements for Brake Components - Performance. The following is extracted from an FAA letter dated October 26, 1987, which addresses "major" and "minor" changes to aircraft brake components and explains FAA policy on this subject.
(a) Measured accelerate-stop distance, landing distance, and functional flight tests may be required depending upon an evaluation of the individual characteristics of each brake system change and in order to establish aircraft landing and RTO certification performance levels for a replacement or modified brake. The type and magnitude of flight tests required will depend on whether or not a requested change to the brake involves a change in heat sink capacity or torque requirements of the original certificated brake. In addition, such tests will also depend on whether or not an increase in the FAA certificated performance level is desired by the applicant, and whether the proposed change to the brake would affect the landing gear static or dynamic loads.
(b) The flight test procedures applicable to brake certification, among others, are contained in AC 25-7A, Flight Test Guide for Transport Category Airplanes (formerly FAA Order 8110.8). The testing requirements in AC 25-7A (formerly 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. The FAA concurs that a change to either the rotors or stators in a previously approved brake could meet this definition, and be considered a major change. In order to adequately demonstrate the performance of a major change to the brakes or brake 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 53
substantiation tests (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. Order 8110.8 has been canceled. Relevant portions of Order 8110.8 and additional information have been included in AC 25-7A, chapter 4, section 4, paragraph 55.
(c) Minor brake changes that cannot 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. A change to thicker friction material, heavier heat sink elements, or other structural improvements generally fall into this minor change category, and the effect of the weight change on landing gear performance and other airplane effects would be evaluated. If the change to the brake resulted in a lighter weight heat sink, the effects of this change would be carefully evaluated to ensure that the thermal characteristics of the brake, wheel, axle, fuse plugs, or other landing gear structure is not adversely affected.
(d) Technical Standard Order (TSO) approval of a modification to a brake is only the first step towards approval for the brake to be installed on an airplane. In each case, the modification must be shown to have no adverse effect on airplane performance. Depending on the magnitude of the proposed change, this demonstration may consist of functional tests only, a full test program, or a selection of the various test procedures described in AC 25-7A (formerly Order 8110.8). Enough flexibility exists in FAA procedures to allow for a thorough evaluation of a proposed brake change without causing undue burden on the applicant. In any event, if there is a question whether or not a given modification to a brake will result in a change to the certificated performance of the airplane, FAA policy has been to demonstrate the performance change on the airplane, or, at the very least, to impose a suitable performance penalty so as to cover any eventuality. There are no plans to relax this requirement.
(12) Approval of Brake Components Utilizing New Brake Friction Material. The following is extracted from an FAA memorandum dated July 27, 1987, which addresses guidance regarding certification of brake components utilizing new brake friction material.
(a) This is in response to questions regarding an approval for new replacement brake pads. For the purpose of approving a replacement brake pad, the FAA may issue a design approval by either a Parts Manufacturer Approval (PMA) or a Technical Standard Order Authorization (TSOA) followed by a Supplemental Type Certificate (STC) for installation, or a procedure using an STC followed by a PMA for production. Each of these approval methods is discussed below. Once a PMA approval is given, the applicant could then mark the brake assembly with his own nameplate, indicating the method of approval (STC number), without altering the existing TSO nameplate.
(b) Issuance of a PMA. If the applicant intends to show identicality with existing TSO-authorized parts, he must consider not only the form and fit of the parts, but also the chemical and/or metallurgical characteristics (including microstructure, surface and cross- 54
sectional hardness, and material spectrum analysis), and process and manufacturing specifications. Previous experience with various brake manufacturers has shown that it is extremely difficult to duplicate the physical and chemical characteristics of brake lining materials. For example, two linings with similar hardness or other measurable characteristics could have been produced using different process specifications. If the applicant satisfactorily shows that his parts are indeed identical to the originals, the change could then be considered to be a "minor change," as discussed in AC 25-7A (formerly Order 8110.8, paragraphs 89(b, (c)).
Five non-instrumented landings would be sufficient to demonstrate compatibility between the airplane, pilot, brake, and anti-skid systems on the airplane. With respect to the TSO, a statement of conformance and submittal of the data showing identicality would be sufficient.
Order 8110.8 has been canceled. Relevant portions of Order 8110.8 and additional information have been included in AC 25-7A, chapter 4, section 4, paragraph 55.
(c) Issuance of a TSOA+STC or an STC+PMA. If the applicant is unable to demonstrate identicality, the change must then be considered to be a major change, since the coefficient of friction and wear rates may be different, and the new parts could, therefore, cause a significant variance in the heat sink capacity, the torque characteristics, or the kinetic energy absorption characteristics of the brake. It would be necessary, therefore, for the applicant to demonstrate compliance with the pertinent parts of TSO-C26c, including the 100 landing stops and 1 accelerate-stop described in Table III of § 4.2 of the TSO.
(d) In addition to the TSO requirements, a maximum kinetic energy Rejected Takeoff (RTO), as described in AC 25-7A (formerly Order 8110.8), must be conducted in the airplane, as well as six functional takeoffs and landings. In addition, fuse plug substantiation tests (melt and no-melt) must be conducted, unless the applicant can demonstrate that heat transfer from the brake stack to the wheel is identical to the original. Also, antiskid-on and antiskid-off tests, qualitative taxi tests, and wet runway tests (antiskid-on) must be conducted.
These test procedures are described in AC 25-7A (formerly Order 8110.8, paragraph 89), and are required to adequately demonstrate the performance of the new lining material .
(e) If the applicant successfully demonstrates that the performance of the brake with his parts installed is identical to the original, he may mark the brake as described above. If the demonstrated performance is greater than that of the original, however, an FAA evaluation by the cognizant ACO is essential, since original landing gear designs are based on structural analysis which could be adversely affected by a brake system change.
(13) Maximum Quick Turnaround Times. The following is extracted from an FAA letter dated August 19, 1983, which addresses a request to intermix brakes from different manufacturers for testing of the fuse plug integrity and receive certification credit for each brake/wheel system.
(a) It is fundamental FAA policy to test the complete configuration of an airplane system that is to be certificated for transportation service. In this case the applicant proposed to certificate two unmixed configurations, the Original Equipment Manufacturer 1(OEM1) and the Original Equipment Manufacturer 2 (OEM2) wheel/brake systems. In general, the FAA does not 55
certificate unmixed configurations based solely on a test of mixed configurations. Further, certification of the mixed configuration requires extensive testing, beyond the test outlined in AC 25-7A (formerly FAA Order 8110.8). Order 8110.8 has been canceled. However, the relevant portions of the Order and additional information have been included in AC 25-7A, chapter 4, section 4, paragraph 55.
(b) Precedent tests previously accepted by the FAA have been cited. These cases were judged on their merits, as must be done here. Approval of the eutectic melt temperature reduction was based on dynamometer tests and an analysis. In that case, wherein the wheel/brake configuration was fixed and test date/analysis were provided, the FAA judged equivalency in a single system element (fuse plugs). The FAA did not consider it appropriate to compare the equivalency finding in the foregoing case to that of mixing wheel/brake configurations for establishing new turnaround limitations.
(c) The OEM1 and OEM2 wheel/brake configurations are “similar in design and performance" was cited as justification to allow mixed wheel/brake fuse plug turnaround testing.
The FAA noted that the wheel brake designs were intended to mount on the same axle using the same hydraulic and antiskid/autobrake systems and brake torque reaction rods. Both wheel/ brake assemblies have double heat shields and the tolerance of the OEM1 fuse plug is inside the tolerance of the OEM2 fuse plug. The nominal difference is not considered significant.
(d) The structural arrangement in detail, however, is different in a number of places, including in and around the fuse plugs. The FAA must assume the variations in metal mass and configuration will affect heat transfer. Also, there are differences in the hydraulic actuators. The brakes themselves are made up of proprietary materials that will exhibit different thermal reactions. The wheels are similar in appearance because they have to be interchangeable; however, it has not been established that they are similar relative to the fuse plug performance.
(e) "Significant margin exists for higher certificated no melt energy levels" is also proposed. It is the FAA's understanding that the applicant intends to use this margin, derived from dynamometer tests minus airplane test data available, to increase the fuse plug "no melt" energy. The FAA has determined this increase is a valid reason to substantiate by test both configurations, i.e., OEM1 and OEM2, with the wheels and brakes in the unmixed state.
(f) The applicant proposed that twice as many fuses be instrumented as in the basic certification. This would give useful empirical data, although the FAA does not require fuse plug instrumentation. In the final analysis, the success of the test is judged by whether the fuse plug integrity is maintained.
(g) In light of the foregoing, the FAA has determined that each wheel/brake configuration should be tested separately for the "no melt" condition to provide a technically defensible minimum wheel/brake fuse plug sample size. This decision is consistent with FAA policy published in AC 25-7A, Flight Test Guide for Transport Category Airplanes, chapter 4, section 4, paragraph 55. This information was in Order 8110.8, paragraphs 89(a), (b), and (c), 56
which has been canceled. However, the relevant portions of the Order and additional information have been included in AC 25-7A,
(h) It has been suggested that the proposed intermix and test procedure ensures minimal data scatter between brakes. If this statement means that the test data scatter between the two tire/wheel configurations will be minimized because all the data will be collected on one test run versus two, which may occur at separate times of the day or different days under varying conditions, then we can concur. However, this does not lend credence to the selection of an inadequate sample size. The proposal does not address what occurs if differences in brake performance are observed; or how to establish a significant difference (which could affect turnaround times), when comparing one brake system to another.
e. References. The addresses for ordering the latest revision of advisory circulars, technical standard orders, and other referenced documents listed below can be found in the Appendix 4 to this AC.
TSO-C26, was an acceptable standard to CAA but not part of the regulations.
TSO-C26a, was part of regulations as § 514.72 of the CAR.
TSO-C26b, was part of regulations as § 37.172.
TSO-C26c, was part of regulations as § 37.172 of 14 CFR.
TSO-C26c with Addendum I, was removed from regulations (§ 37.172, Amendment 25-52, 1980) and became a voluntary standard.
AC 25-7A, Flight Test Guide for Transport Category Airplanes (formerly FAA Order 8110.8).
AC 43.13-1A, Acceptable Methods, Techniques and Practices- Aircraft Inspection and Repair; L. G. Equipment.
FAA Letter, Dynamometer Test Guidelines to Establish Brake wear Limits.
FAA's Wheels and Brakes Approval Process Videos, parts 1 and 2.
SAE AS-1145A - Aircraft Brake Temperature Monitor System.
SAE ARP-1907 - Automatic Braking Systems Requirements.
SAE AIR-1064C - Brake Dynamics.
SAE ARP-1070B - Design and Testing of Antiskid Brake Control.
SAE AIR-1739 - Information on Antiskid Systems.
SAE ARP-813B - Maintainability Recommendations for Aircraft Wheels and Brakes.
SAE ARP-1619 - Replacement and Modified Brakes and Wheels.
SAE AS-483A - Skid Control Equipment.
SAE ARP-862A - Skid Control Performance.
SAE AIR-1934A - Use of Carbon Heat Sink Brakes on Aircraft.
SAE ARP-597C - Wheels and Brakes, Supplementary Criteria for Design Endurance.