Appendix 2AC 23-17A 61
(1) Replacement Wheel, Wheel/Brake Parts or Assemblies
(a) Brake-Anti-skid Compatibility Replacement part or assembly changes, or both, defined under paragraph 3(d)(2) titled “Minor Design Changes,” are considered to be minor whether they are proposed by the original wheel and brake manufacturer who holds the TSO authorization or by another manufacturer seeking to produce a replacement part or assembly. While such changes are not expected to affect braking performance, functional landings may be required as a minimum to verify airplane/pilot/brake/anti-skid combination compatibility (reference part 23, § 23.735(d)). Normally five (5) non-instrument, functional landings are necessary to verify this compatibility.
(b) Brake Rotors/Stators In general, changes to the friction surfaces of the aircraft brake, including the stator and rotor, are considered to represent a major change per 3(d)(1), titled “Major Design Changes,” unless it can be shown that the change cannot affect the airplane stopping performance, brake energy absorption characteristics, or continued airworthiness (reference part 23, § 23.735(a)/(e)). In addition, if changes in heat sink friction components are proposed, certain provisions of 4(b)(2), titled “Modified Wheel, Wheel/Brake Parts, or Assemblies,” may also be applicable. Changes to continuing airworthiness, such as thermal control, vibration control, etc., should also be considered for the major/minor determination. In this regard, the original manufacturer of the wheel or wheel/brake assembly who holds the TSO authorization may possess data sufficient to show that such changes could be considered minor (e.g., airplane performance would not be affected). On the other hand, a manufacturer other than the original manufacturer who may wish to produce replacement rotors and stators may not have data sufficient to show that performance would not be affected. In this case, the major/minor status would be determined by applicable dynamometer tests per TSO-C26c and some functional airplane tests as a minimum.
(c) Brake Performance Equivalency It may be difficult to determine identicalness but a finding of equivalency can be shown by additional design, analysis, and dynamometer tests as applicable. A change to an approved part that is determined to be minor can be validated on the dynamometer by a controlled test at the maximum certified kinetic energy capacity of the original brake assemblies from TSOA, or the dynamometer testing may be done to the design landing and accelerate-stop kinetic energy levels appropriate to the aircraft [Reference part 23, § 23.735 (a) through (e)]. The following dynamometer test protocol is acceptable to validate replacement rotors/stators proposed by an applicant other than the original TSO holder:
1. Use of new stator or rotor parts in the replacement manufacturer's brakes for each dynamometer 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 may be questionable. Suspect tests would be carefully scrutinized by the FAA, and retesting may then be necessary. Test methods, test hardware (including the tire size, ply and condition), and test procedures should be the same to ensure proper comparative evaluations. If brake friction materials are being compared, the heat sinks to be used for maximum certified kinetic energy (KE) testing should not have been subjected to test energies higher than design landing energy.
2. The maximum certified kinetic energies approved under TSOA for the original manufacturer are proprietary data. Therefore, a PMA applicant that desires to maintain the TSOA status of a modified assembly will have to do the testing in this paragraph without knowing the kinetic energy levels the OEM tested for the TSOA.
A series of tests may be necessary for a replacement manufacturer to reach the maximum certified level of the original manufacturer's brake. For each succeeding run, the KE will be increased by at least 5 percent over the previous run until the maximum certified KE level is reached. The initial KE level for this series of tests will be at the discretion of the applicant. If maintaining the TSOA is not desired, the PMA applicant may perform dynamometer tests at the airplane derived kinetic energy levels.
3. Maximum braking force pressure, derived from the airplane maximum brake pressure capability, is to be applied during the tests.
4. Fuse plugs may be released or the tire deflated after each test run to reduce the risk to test personnel.
5. A minimum of five (5) functional landings for anti-skid equipped airplanes and a minimum of three (3) functional landings for non-anti-skid equipped airplanes, as described above, are needed.
(d) Worn Brakes [optional] While there are no provisions under part 23 to require the evaluation of brake performance using worn brakes, there have been rejected takeoff accidents in which the brakes on subject airplanes were at or very near their completely worn state of energy absorption capability and stopping capability. Therefore, as an optional test to a replacement brake performance evaluation (when there is question concerning variances in worn brake performance), it is recommended that such an assessment on the dynamometer be undertaken to support compliance with maximum Rejected Takeoff (RTO) performance in the AFM. Dynamometer tests simulating a maximum energy RTO should be performed on the replacement brake assemblies with individual brakes within 10 percent of their wear limit (e.g., at least 90 percent worn). The tests, used to verify the safety of a replacement brake system and to determine the maximum energy
Appendix 2AC 23-17A 63 absorption capability of brakes in their fully worn state, should be substantiated as being representative of actual airplane and runway conditions.
(2) Modified Wheel, Wheel/Brake Parts, or Assemblies
(a) Modified Brake Design This laboratory and airplane test requirement applies to the addition of a major change brake design to an existing airplane for which FAA approved braking performance test data exists. Testing may be performed either for performance credit or to the existing performance level of the aircraft. As provided under examples of 3(d)(1), a modified brake is one that contains new or modified parts that may cause a significant variance in the kinetic energy absorption characteristics, AFM stopping distances and continuing airworthiness of the brake [Reference 14 CFR § 23.735 (a) through (e)]. Substantiating laboratory and airplane flight testing required for approval of a major changed brake will include the following:
1. For improved performance (aa) Applicable dynamometer tests under TSO-C26c.
(bb) Instrument flight tests to include six (6) takeoffs and six (6) landings. The six landings are to be conducted on the same wheels, tires, and brakes. All tests should be conducted with engines trimmed to the high side of the normal idle range, if applicable. The engine idle schedule may include a flight-idle schedule which may be applicable to the test.
(cc) Additional tests may be necessary for each airplane configuration change (e.g., takeoff flaps, landing flaps, nose wheel brakes, anti-skid devices inoperative, deactivation of wheel/brakes, etc.).
(dd) Brake system response evaluation.
(ee) Parking brake adequacy. Tires are allowed to skid during maximum power engine checks.
(ff) Alternate braking system stops.
(gg) Fuse plug evaluation.
(hh) Anti-skid compatibility on wet runway.
(ii) Taxi tests, to ensure that ground handling, maneuvering, and brake sensitivity are satisfactory, should be conducted.
64 (jj) At least two (2) braking stops, one at maximum takeoff weight and one at minimum landing weight, should be conducted on a wet runway to verify brake and anti-skid compatibility.
[Note: Improved performance implies an increase in the friction coefficient (mu) versus energy level for the desired operation(s) and may be requested for landing, RTO's, or a specific configuration such as anti-skid "on" only.]
2. For equivalent performance (aa) Applicable dynamometer tests under TSO-C26c.
(bb) A sufficient number of conditions to verify the existing approved performance levels (RTO and landing for either TSOA levels or Airplane Flight Manual levels). Consideration should be given to verification of fuse plugs, performance verification at appropriate energy levels, and configuration differences, including anti-skid on and off.
(cc) Taxi tests, to ensure that ground handling, maneuvering, and brake sensitivity are satisfactory, should be conducted.
(dd) At least two (2) braking stops, one at maximum takeoff weight and one at minimum landing weight, should be conducted on a wet runway to verify brake and anti-skid compatibility.
[Note: Equivalent performance implies that sufficient data will be obtained to verify that the performance level for the 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. For extended performance (aa) Applicable dynamometer tests under TSO-C26c. Consideration should be given to the items in Section 4b(2)(a)(2).
(bb) A sufficient number of conditions to define the extended life and determine equivalency to the existing performance levels. Consideration should be given to the items in Section 4b(2)(a)(2).
[Note: Extended performance implies that the existing certification µ versus energy line will extend to establish the braking force level for a proposed change, such as gross weight or the desired maximum energy level, and may be applied to a specific operation (such as landing only).]
Appendix 2AC 23-17A 65
(b) Modified Anti-skid System This airplane test requirement applies to the addition of a new anti-skid system or changes to an existing anti-skid system that may affect airplane performance (e.g., new anti-skid system, or a change from couple to individual wheel control). A sufficient number of either airplane performance tests or functional tests, or both, should be conducted to verify existing approved performance anti-skid "on" levels. In the event an increase of braking performance is desired, full airplane performance testing will be required [reference part 23, § 23.735 (d)].
(c) Modified Fuse Plugs/Wheels This item covers the addition of a significant modification to any portion of the existing wheel design on an airplane (change of wheel design, redesign, or relocation of fuse plugs). The following airplane tests can be performed when such changes are made:
1. One airplane braking test should be conducted to show that the fuse plugs will release when excessive energies are absorbed.
2. One airplane braking test should be conducted to verify the maximum kinetic energy at which fuse plugs will not release (fuse plug substantiation). Dynamometer tests are not adequate for this test.
[Note: Wheel fuse plug integrity should be substantiated during braking tests where the energy level simulates the maximum landing energy. It should be demonstrated that the wheel fuse plugs will remain intact and that unwanted releases do not occur. One acceptable method to determine this is as follows:] (aa) Set engine idle thrust at the maximum value specified (if applicable). The engine idle schedule may include a flight-idle schedule which may be applicable to the test.
(bb) Set tire pressures to the minimum value appropriate for the airplane test weight.
(cc) Taxi at least three miles (normal braking, at least three intermediate stops, and all engines operating).
(dd) Conduct accelerate-stop test at maximum landing energy, maintaining the deceleration rate consistent with the values used to determine performance distance.
(ee) Taxi at least three miles (normal braking, at least three intermediate stops, and all engines operating).
66 (ff) Park in an area to minimize wind effects until it is ensured that fuse plug temperatures have peaked and that no plugs have released.
Instead of simulating the maximum kinetic energy landing during an accelerate-stop test, an actual landing and quick turnaround may be performed; however, caution should be exercised in order to prevent jeopardizing the safety of the flight crew and airplane if the wheel plugs release right after liftoff, requiring a landing to be made with some flat tires.
The following elements should be included in the tests:
(aa) Set engine idle thrust at maximum value specified (if applicable).
(bb) Set tire pressures to the minimum value appropriate to the airplane test weight.
(cc) Conduct a landing stop at maximum landing energy, maintaining the acceleration rate consistent with the values used to determine performance distance.
(dd) Taxi to ramp (three miles minimum with normal braking, at least three intermediate stops, and all engines operating).
(ee) Stop at the ramp. Proceed immediately to taxi for takeoff.
(ff) Taxi for takeoff (three miles minimum with normal braking, at least three intermediate stops, and all engines operating).
(gg) Park in an area to minimize wind effects until it is ensured that fuse plug temperatures have peaked and that no plugs have released. Fuse plug protection of wheels and tires should be demonstrated to show that the fuse plugs will release when excessive energies are absorbed. Normally, this will occur during RTO performance tests.
(d) Accelerate Stop Tests Accelerate-stop tests for commuter category airplanes and other airplanes are defined under part 23, §§ 23.55 and 23.735 (e). Accelerate-stop tests should be conducted for all modified wheel, wheel/brake parts or assemblies involving a major design change when this testing was performed for the certification of the original brake assembly. Such tests should include substantiation of the critical maximum brake energy stop (highest ground speed based on the V1 speed applicable to the maximum altitude and temperature the airplane is certified for according to the FAA approved AFM). On airplanes with wheel fuse plugs, a satisfactory demonstration of fuse plug compatibility should be conducted as stated under Item (2)(c) titled, “Modified Fuse Plugs/Wheels.”
Appendix 2AC 23-17A 67
(e) Other Substantial Airplane Tests Depending upon the extent of wheel, wheel/brake part, or assembly modifications that may be involved, there will be a number of airplane tests that should be considered in addition to those above. As applicable to specific changes and to the type of part 23 airplane involved, the following are tests that may be appropriate and required by the FAA for approval:
1. Brake KE Absorption Tests Verify that the brake kinetic energy absorption test determined by the laboratory test meets the TSO requirements and the airplane manufacturer's requirements (to be identified by the FAA) [reference part 23, § 23.735(a) through (e)]. The engine idle schedule may include a flight-idle schedule which may be applicable to the test.
2. Brake Pressure Test Verify brake pressure tests conducted under the TSO are adequate for the brake system pressure on the airplane, as determined by the manufacturer's brake system pressure data. Conduct a brake pressure test on the airplane if manufacturer's brake system data is not available to verify the adequacy of the TSO test [reference part 23, § 23.735(c)].
3. Taxi Ground Handling Tests Perform taxi tests to ensure that ground handling, controllability, maneuverability, and brake sensitivity are satisfactory. Use normal braking, intermediate stops, with all engines operative [see 4b(2)(a), Modified Brake Design].
4. Wet Running Tests Perform brake stops on a wet runway to verify brake and, if applicable, anti-skid system compatibility [see 4b(2)(a), Modified Brake Design].
5. Function Reliability Tests Perform function reliability landing stops. Normally six maximum brake landings should be satisfactorily conducted on the same set of wheels, tires, and brakes [see 4b(2)(a), Modified Brake Design].
6. Landing Performance Tests Determine that the landing performance is adequate to the previously approved performance data shown in the AFM. If the AFM performance data is not available because that it is not required by the airplane certification basis, the manufacturer's data
68 (if available) provided to the pilot should be used as a basis for comparison [see 4b
(2)(a) Modified Brake Design].
7. Static Torque Tests Determine whether there is adequate static torque when parked and during appropriate engine run up conditions [reference part 23, § 23.735(b)].
8. Brake Response Tests During the aforementioned tests, brake response characteristics should be monitored for unacceptable vibrations, squeal, fade, grabbing, and chatter. These characteristics may have a destructive effect on the brake assembly components and may be pertinent to endurance of landing gear system components.
5. INSTRUCTIONS FOR CONTINUED AIRWORTHINESS A PMA applicant may be required to furnish instructions for continued airworthiness if the article on which the part is eligible for installation has an existing set of instructions for continued airworthiness that are not considered adequate for the applicant's PMA part (reference part 23, § 23.1529).
6. IDENTIFICATION OF PMA PARTS
a. General Under part 45, § 45.15, parts produced under a PMA should be permanently and legibly marked in a manner that will enable persons to identify the following:
• It is a PMA part.
• The manufacturer.
• The part number.
• The type certificated product(s) or TSOA article(s) on which it may be installed.
For a part based on an STC, the identification of installation eligible type certified products should include reference to the STC. In accordance with part 45, § 45.14, parts that have been identified as critical components should be marked with a part number, or equivalent, and serial number or equivalent. If the TC or TSOA holder applies serial numbers to a critical part, the PMA holder should also "permanently mark" their parts with serial numbers.
Note: Due to the harsh environments that wheels and brakes experience, decals or adhesive backed "metalcals" are not considered permanent forms of marking. Metal stamping, etching or permanently affixing a data plate with rivets or drive screws in a non-critical area is satisfactory.
Laser marking is also acceptable if it can be read under 2X magnification. Ink stamping is allowed only if more permanent means are not possible.
Appendix 2AC 23-17A 69
b. Part Numbering The PMA holder's part should be numbered such that it is sufficiently different from the OEM holder's part number to be distinguishable. The OEM holder's part number with a prefix/suffix is sufficient for this purpose. The requirements of part 45, § 45.15(a)(2), to mark with name, trademark, or symbol of the PMA holder may be satisfied by the prefix/suffix if the prefix/suffix is done consistently across the PMA holder's product line. The FAA-PMA letter should show the type approved part number with which the PMA holder's part is interchangeable.
c. Parts Manufactured Under License When the PMA is issued by showing evidence of a license agreement or equivalent, the PMA part number may be identical to that on the type certificated part providing the PMA holder also meets the requirements of part 45, § 45.15(a)(1) and (2) to permanently mark the part with the letters "FAA-PMA" and the name, trademark, or symbol of the PMA holder.
d. Parts that are Impractical to Mark In all cases where the part is found by the FAA to be too small (or to have other characteristics that make it impractical) to mark all (or any) of the information on the part, the information not marked on the part should be put on the tag that is attached to the part or marked on the container for the part. If the number of certificated products or TSOA articles on which the part is eligible for installation is too long to be practicable to include with the part, the tag or container may refer to a readily available manual or catalog made available by the PMA holder for part eligibility information.
Amendment 23-34 and Subsequent Policy for 14 CFR part 23, § 23.735, Brakes, and Specific Sections versus § 23.1309; Equipment, Systems and Installations Brake systems are approved based on compliance to the specific standard, § 23.735, not the general standard, § 23.1309. This is true for any installations where there are specific standards in 14 CFR part 23 (i.e., brakes, autopilots, ice protection systems, etc.).
In the past, the Small Airplane Directorate has not considered brake failures catastrophic or severe-major due to mitigating factors, which include the following: low stall speeds, minimum field lengths for landing, and propeller feathering or reverse thrust. For commuter category airplanes where loss of the brakes is catastrophic or severe-major, we will accept an emergency brake system or a single failure in the primary system that meets the following requirements (based on § 25.735(b)):
70 In the following circumstances, the brake system and associated systems must be designed and constructed so that the airplane may be brought to rest under conditions specified in § 23.75:
- If any electrical, pneumatic, hydraulic, or mechanical connecting or transmitting element (excluding the operating pedal or handle) fails, or
- If any single source of hydraulic or other brake operating energy supply is lost.
The airplane can be brought to rest with a mean deceleration during the landing roll of at least 50 percent of that obtained in determining the landing distance as prescribed in that section.
Subcomponents in a brake assembly such as brake drums, shoes and actuators (or their equivalents), shall be considered as connecting or transmitting elements unless it is shown that leakage of hydraulic fluid from a failure of sealing elements in these subcomponents would not reduce the braking effectiveness below that specified in this paragraph.
We agree that requiring an emergency brake system is not desirable for small single-engine airplanes. We cannot agree that brake system failure for commuter category and turbojet powered airplanes can automatically be classified as severe-major. Mitigating factors must always be considered for any part 23 airplane.
JAA ACJ 23.735(c) is acceptable for FAA certification.
23.737 Skis Amendment 23-45 and Subsequent See § 23.505, Supplementary conditions for ski-planes, for additional guidance about aircraft skis.
23.745 Nose/tail wheel steering No policy available as of June 30, 2001.