(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:
1) they are proposed by the original wheel and brake manufacturer who holds the TSO authorization or 2) 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 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 by 3(d)(1), titled “Major Design Changes,” unless it can be shown the change cannot affect: 1) the airplane stopping performance, 2) brake energy absorption characteristics, or 3) 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, and vibration control, should also be considered for the major/minor determination. The original manufacturer of the wheel or wheel/brake assembly who holds the TSO authorization may possess sufficient data to show that such changes could be considered minor (for example, 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 enough data to show that performance would not be affected. In this case, the major/minor status would be determined by applicable dynamometer tests by 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: 1) a controlled test at the maximum certified kinetic energy capacity of the original brake assemblies from TSOA, or 2) 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
75 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 manufacturers brakes for each dynamometer test will be required to minimize test configuration variables. If rebuilt or in-service components other than these fail during testing, it should be realized the results may be questionable. Suspect tests would be inspected 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 KEs approved under TSOA for the original manufacturer are proprietary data. Therefore, a PMA applicant that wants 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 manufacturers brake. For each succeeding run, the KE will be increased by at least five 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 functional landings for anti-skid equipped airplanes and a minimum of three functional landings for non-anti-skid equipped airplanes, as, are needed.
(d) Worn Brakes [optional]
While there are no provisions under part 23 to require evaluating brake performance using worn brakes, there have been rejected takeoff accidents in which the brakes on subject airplanes were at or near their worn state of energy absorption capability and stopping capability. Therefore, as an optional test to a replacement brake performance evaluation (when there is question about variances in worn brake performance), it is recommended that such an assessment on the dynamometer be undertaken to support compliance with
76 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 (for example, at least 90 percent worn). The tests, used to verify the safety of a replacement brake system and to determine the maximum energy absorption capability of brakes in their fully worn state, should be substantiated as being representative of airplane and runway conditions.
(2) Modified Wheel, Wheel/Brake Parts, or Assemblies
(a) Modified Brake Design This laboratory and airplane test requirement applies to adding 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:
1. For improved performance (aa) Applicable dynamometer tests under TSO-C26c.
(bb) Instrument flight tests to include six takeoffs and six 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 that may be applicable to the test.
(cc) Additional tests may be necessary for each airplane configuration change (for example, takeoff flaps, landing flaps, nose wheel brakes, anti-skid devices inoperative, and deactivation of wheel/brakes).
(dd) Brake system response evaluation.
(ee) Parking brake adequacy. Tires are allowed to skid during maximum power engine checks.
(ff) Alternate braking system stops.
77 (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.
(jj) At least two 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 7: Improved performance implies an increase in the friction coefficient (mu) versus energy level for the desired operation(s) and may be requested for landing, RTOs, 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 AFM 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 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 8: Equivalent performance implies that enough data will be obtained to verify the performance level for the change is equal to or better than the existing performance levels.
The change may be to change the CG 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).
78 NOTE 9: Extended performance implies 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).
(b) Modified Anti-skid System
This airplane test requirement applies to adding a new anti-skid system or changes to an existing anti-skid system that may affect airplane performance (for example, 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. If 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 adding 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 the fuse plugs would 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 acceptable for this test.
NOTE 10: Wheel fuse plug integrity should be confirmed during braking tests where the energy level simulates the maximum landing energy. It should be shown 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 that 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).
79 (dd) Conduct accelerate-stop test at maximum landing energy, keeping 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).
(ff) Park in an area to minimize wind effects until it is assured 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, a landing and quick turnaround may be performed. Caution should be exercised to prevent jeopardizing the safety of the flight crew and airplane if the wheel plugs release 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, keeping 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. Go 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 assured that fuse plug temperatures have peaked and that no plugs have released. Fuse plug protection of wheels and tires should show the fuse plugs would 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 to
80 certify 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.”
(e) Other Substantial Airplane Tests
Depending on the extent of wheel, wheel/brake part, or assembly modifications that may be involved, there will be several airplane tests that should be considered as well as 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 Kinetic Energy (KE) Absorption Tests Verify the brake KE absorption test determined by the laboratory test meets the TSO requirements and the airplane manufacturers 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 that may be applicable to the test.
2. Brake Pressure Test Verify brake pressure tests conducted under the TSO are satisfactory 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 manufacturers 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 assure 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 the landing performance is adequate to the previously approved performance data shown in the AFM. If the AFM performance data is not available because it is not required by the airplane certification basis, the manufacturers data (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 enough static torque when parked and during appropriate engine run up conditions [reference part 23, § 23.735(b)].
8. Brake Response Tests During the 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 relevant to endurance of landing gear system components.
5. INSTRUCTIONS FOR CONTINUED AIRWORTHINESS A PMA applicant may be required to provide 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 acceptable for the applicants 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 people 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.
82 For a part based on an STC, identifying installation eligible type certified products should include reference to the STC. Under 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 11: Because of 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.
b. Part Numbering The PMA holders' part should be numbered so it is sufficiently different from the OEM holders' part number to be distinguishable. The OEM holders' part number with a prefix/suffix is enough 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 holders product line. The FAA-PMA letter should show the type approved part number with which the PMA holders' 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 with 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
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.
83 Amendment 23-7 and Subsequent
Amendment 7 was a major change in the standard. The following explanation in NPRM 67- 14 is stated as follows: “Part 23 does not provide for the determination of the energy absorption requirements of brakes. The present requirement in Sec. 23.735 (b) may not ensure the design of wheel brake combinations adequate for safe operation of the airplane.
Proposed Sec. 23.735 (a) would therefore provide standards similar to the energy absorption determination requirements for transport category airplanes (Sec. 25.735 (f)). Proposed Sec.
23.735(b) would reflect the fact that, for aircraft with high power-to-weight ratios, it may be impossible to prevent motion, with locked wheels, when takeoff power is applied to the critical engine. Some question has arisen as to whether such motion constitutes "rolling" of the airplane within the meaning of present (a). It is not the intent of present (a) to prevent this motion, but only to ensure that the brakes can prevent the wheels themselves from rolling. The proposed energy absorption requirements reflect current industry practice in the design of small airplanes.”
Amendment 23-42 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 (that is, brakes, autopilots, and ice protection systems).
In the past, the Small Airplane Directorate has not considered brake failures catastrophic or severe-major because of mitigating factors, which include: 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)):
In the following circumstances, the brake system and associated systems must be designed and constructed so 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), should be considered as connecting or transmitting elements unless it is shown
84 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 airplanes.
Amendment 23-49 and Subsequent
The intent of the proposed revision to NPRM 94-21 is given as follows: “Section 23.735(a) would be revised to state plainly that wheel brakes must be provided. A proposed new Sec. 23.735(c) would require the brake system to be designed so that the brake manufacturer's specified brake pressures are not exceeded during the landing distance determination required by Sec. 23.75. Proposed new Sec. 23.735(e), applicable to commuter category airplanes, would require establishing the minimum rejected takeoff brake kinetic energy capacity rating of each main wheel brake assembly. Section 23.45 provides that the determination of the accelerate-stop distance for commuter category airplanes be made in accordance with the applicant's procedures for operation in service. The proposed requirement is needed to ensure that the brakes will perform safely under accelerate-stop conditions.”
EASA AMC 23.735(c) is acceptable for FAA certification.
23.737 Skis
The corresponding rule in CAR 3 is CAR 3.364.
There is no corresponding rule in the Airship Design Criteria.
Amendment 23-45 and Subsequent
See § 23.505, Supplementary conditions for ski-planes, for additional guidance about aircraft skis.