2 Thermodynamic interaction (i.e., conduction. radiation, and convection between adjacent tires, wheels, and brakes, between the tire and the runway surface, and the effects of the air).
3 Airplane/System dynamics (i.e., the interaction between the airplane and supporting gear and systems, control systems and so forth).
(d) Adequate means of simulation could probably be developed with new test criteria (if comparisons are to be made) and new techniques. The end result would most likely be a test fixture on a track with a complete landing system including supporting structure, systems duplicating the number of landing gear, and so on, which would be capable of intricate simulations of takeoff, landing and braking dynamics. However, it would probably be less expensive to test the airplane as is currently the practice.
(e) Another problem is establishing the adequacy of the tire relative to the TSO data and to additional tests. The airframe manufacturer may request that the tire, wheel, and brake manufacturers conduct specific tests beyond the scope of the TSO. Side loading and overloading are good examples of additional tests. We can never be sure after completing the airplane tests that the tire (wheel or brake also) reliability is dependent totally on the TSO tests or on the TSO test and the additional tests specified. We are aware of at least one case where two tires manufactured by two different companies were installed on the same airplane and operated within the same TSO envelopes, yet one tire type failed while the other passed. The successful tire type had additional side loads specified which were not part of the TSO testing. As a repetitive statement, the FAA position is that TSO data is not a satisfactory substitute for tire wheel or brake tests on the airplane.
(f) If the radial ply tire is heavier than the bias tire, it must be determined whether that is significant. Lighter tire weight can affect some emergency extension systems as some of these systems use the landing gear weight to overcome the unlocking residual forces to provide enough inertia to extend the gear down, over center, and to lock the gear in place. Heavier weight may have an effect on retraction times. Radial tires may weigh as much as 1/4 less than the bias tires they would replace.
(g) Rejected takeoff tests are not only to prove the capability of the brakes, but also to prove the landing gear system and its interaction with associated components as well.
These tests are to evaluate performance, dynamics, and system adequacy. As stated earlier, the dynamometer tests of the TSO are not an acceptable equivalent or substitute for the airplane tests. We also note that an applicant has stated that: "The radial tire exhibits less drag, and would therefore result in a slightly longer stopping distance at the same brake pressure (i.e., brake torque)." The maximum RTO is conducted at the maximum brake pressure, therefore, the braking distance would appear to be longer, based on an applicant's comment. In addition to other considerations, the maximum RTO and other performance tests should help establish how much longer the braking distance is going to be.
(h) Note that for the same bias tire dynamometer test energy, the brake energy increases with a decrease in energy absorbed by the radial tire; therefore, the heat being delivered to the fuseplug through the wheel will probably increase. It is not known how much or how significant that increase will be. How the fuseplugs are going to act (no-melt/melt) should be established on the airplane prior to certification.
(i) A sufficient number of flight tests should be conducted on wet and dry runways to establish that the anti-skid and the landing gear system will function compatibly and acceptably with the radial tires.
(j) It should be established prior to certification that the airplane tire will behave structurally and dynamically on the airplane as well as it did on the dynamometer at high speed conditions. Because dynamic stability may be more of a function of gear dynamics coupled to the tire dynamics, airplane tests should be conducted, as the dynamometer is insufficient in simulating airplane and landing gear system dynamics.
(6) Critical Conditions & Maximum Ramp Weight Definitions. The following is extracted from an FAA telegraphic message dated January 26, 1981, which addresses definitions of "critical conditions" and "maximum ramp weight" as they relate to § 25.733.
(a) Section 25.733(a) Critical Conditions. The critical conditions are those conditions where the takeoff or landing speed extremes that have been established by evaluating takeoff or landing performance for temperature, altitude, aircraft weight, aircraft configuration, runway conditions, engine performance, etc., may approach the tire speed rating. The critical conditions in this case are the takeoff or landing performance conditions that establish maximum tire speed.
(b) Section 25.733(a)(1); (b)(1); (b)(3);Maximum ramp weight. The maximum ramp weight is the maximum weight allowed on the airplane (includes airplane weight, fuel, oil, baggage, etc.). This definition is used interchangeably with maximum taxi weight.
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 to this AC.
AC 20-97A, High-Speed Tire Maintenance and Operational Practices.
AC 43.13-1A, Acceptable Methods, Techniques and Practices - Aircraft Inspection and Repair, chap. 8, L. G. Equipment.
AC 145-4, Inspection, Retread, Repair and Alterations of Tires.
FAA Order 8000.54, Process Specifications for Retreading Tires and Inspection Procedures Manuals for Tire Retreaders.
FAA Order 8000.64, Qualification of Aircraft Radial Tires for Use on Aircraft and for Retreading.
FAA's Tire Approval Process Video, MTS 422/422.1, 42:15 Minutes.
TSO-C62c, of § 37.167 14 CFR. This TSO was removed from regulations (§ 37.167, Amendment 25-52, 1980), and became a voluntary standard.
TSO-C62c, With Addendum I. This TSO was removed from regulations (§ 37.167, Amendment 25-52, 1980), and became a voluntary standard.
TSO-C62d, Tires.
SAE AIR-1904 - Tire Spray Suppression - Airplane, Design Consideration and Testing.
SAE ARP-1322 - Overpressurization Release Devices.
SAE ARP-4834 - Recommended Practice for Retreaded Aircraft Tires - Radial and Bias.
SAE AS-707B - Thermal Sensitive Inflation Pressure Release Devices for Tubeless Aircraft Wheels.
SAE-AS 1188 - Aircraft Tire Inflation-Deflation Equipment.
SAE AS-4833 - Aircraft New Tire Standard- Bias and Radial.
14. SECTION 25.735 - BRAKES.
a. Rule Text.
(a) Each brake must be approved.
(b) The brake system and associated systems must be designed and constructed so that 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, it is possible to bring the airplane to rest under conditions specified in 25.125, with a mean deceleration during the landing roll of at least 50 percent of that obtained in determining the landing distance prescribed in that section. Subcomponents within the brake assembly, such as brake drum, shoes, and actuators (or their equivalents), shall be considered as connecting or transmitting elements, unless it is shown that leakage of hydraulic fluid resulting from failure of the sealing elements in these subcomponents within the brake assembly would not reduce the braking effectiveness below that specified in this paragraph.
(c) Brake controls may not require excessive control force in their operation.
(d) The airplane must have a parking control that, when set by the pilot, will without further attention, prevent the airplane from rolling on a paved, level runway with takeoff power on the critical engine.
(e) If antiskid devices are installed, the devices and associated systems must be designed so that no single probable malfunction will result in a hazardous loss of braking ability or directional control of the airplane.
(f) The design landing brake kinetic energy capacity rating of each main wheelbrake assembly shall be used during qualification testing of the brake to the applicable Technical Standard Order (TSO) or an acceptable equivalent. This kinetic energy rating may not be less than the kinetic energy absorption requirements determined under either of the following methods:
(1) The brake kinetic energy absorption requirements must be based on a rational analysis of the sequence of events expected during operational landings at maximum landing weight. This analysis must include conservative values of airplane speed at which the brakes are applied, braking coefficient of friction between tires and runway, aerodynamic drag, propeller drag or power-plant forward thrust, and (if more critical) the most adverse single engine or propeller malfunction.
(2) Instead of a rational analysis, the kinetic energy absorption requirements for each main wheel-brake assembly may be derived from the following formula, which must be modified in cases of designed unequal braking distributions.
KE = 0.0443 WV2 N where- KE = Kinetic energy per wheel (ft.-lb.);
W = Design landing weight (lb.);
V = Airplane speed in knots. V must be not less than VS0 , the power off stalling speed of the airplane at sea level, at the design landing weight, and in the landing configuration; and N = Number of main wheels with brakes.
(g) The minimum stalling speed rating of each main wheel-brake assembly (that is, the initial speed used in the dynamometer tests) may not be more than the V used in the determination of kinetic energy in accordance with paragraph (f) of this section, assuming that the test procedures for wheel-brake assemblies involve a specified rate of deceleration, and, therefore, for the same amount of kinetic energy, the rate of energy absorption (the power absorbing ability of the brake) varies inversely with the initial speed.
(h) The rejected takeoff brake kinetic energy capacity rating of each main wheelbrake assembly that is at the fully worn limit of its allowable wear range shall be used during qualification testing of the brake to the applicable Technical Standard Order (TSO) or an acceptable equivalent. This kinetic energy rating may not be less than the kinetic energy absorption requirements determined under either of the following methods:
(1) The brake kinetic energy absorption requirements must be based on a rational analysis of the sequence of events expected during an accelerate-stop maneuver.
This analysis must include conservative values of airplane speed at which the brakes are applied, braking coefficient of friction between tires and runway, aerodynamic drag, propeller drag or powerplant forward thrust, and (if more critical) the most adverse single engine or propeller malfunction.
(2) Instead of a rational analysis, the kinetic energy absorption requirements for each main wheel brake assembly may be derived from the following formula, which must be modified in cases of designed unequal braking distributions:
KE = 0.0443 WV2 N where- KE = Kinetic energy per wheel (ft.-lb.);
W = Design landing weight (lb.);
V = Airplane speed (knots);
N = Number of main wheels with brakes; and W and V are the most critical combination of takeoff weight and ground speed obtained in a rejected takeoff.
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as amended by Amdt. 25-23, 35 FR 5676, Apr. 8, 1970; Amdt. 25-48, 44 FR 68742, Nov. 29, 1979; Amdt. 25- 72, 55 FR 29777, Jul. 20, 1990, as amended by Amdt. 25-78, 58 FR 11781, Feb. 26, 1993; Amdt. 25-92, 63 FR 8320, Feb. 18, 1998]
b. Intent of Rule.
(1) This rule requires use of approved brakes and wheel assemblies, either approved under an applicable technical standard order (TSO) e.g., TSO-C26c, or approved under the type certificate for the airplane. The existence of TSO approval with the article displaying required markings does not automatically constitute the authority to install and use the article on an airplane.
(2) It is the responsibility of those desiring to install this article to determine that the aircraft operating conditions are within the TSO standards. Additional requirements may be imposed based on airplane specifications, wheel and brake design, and quality control specifications. The airframe manufacturer/user must conduct the applicable airplane certification tests and receive FAA approval.
(3) In addition to brake performance and safety requirements, the rule provides standards for systems and equipment associated with brakes, e.g., control mechanisms and antiskid systems. The brake and wheel assembly must have proper energy and load ratings for taxi, takeoff, refused takeoff, and landing. The braking system must have acceptable pilot-control forces, adequate parking brake capability, emergency braking capacity and directional control, and protection against overpressure, overtemperature, and fire.
c. Background. Effective February 1, 1965, part 25 was added to Title 14, Code of Federal Regulations (14 CFR) to replace part 4b of the Civil Air Regulations (CAR). Sections 4b.337
(a)(1), 4b.337(a)(2) and 4b.337(a)(3), 4b.337(b), 4b.337(c), 4b.337(d), 4b.335(c), and 4b.335(d) of the CAR, respectively became §§ 25.735(a), 25.735(b), 25.735(c), 25.735(d), 25.735(e),
25.735(f) and 25.735(g) of 14 CFR, for brakes/braking systems.
(1) Amendment 25-23 (April 8, 1970) deleted reference to a military specification (MIL-B-8075) as the means of compliance for antiskid devices under § 25.735(e) to allow any other acceptable means of compliance. In addition, proper units of "knots" were added to stall speed under § 25.735(f)(2).
(2) Amendment 25-48 (November 29, 1979) revised the technical standard order TSO-C26b for aircraft wheels and wheel-brake assemblies and related type certification requirements for airplane brakes. The revised standards in § 37.172 - TSO-C26c incorporated an updated and improved minimum performance standard for the design and construction of aircraft wheels and brakes. Under § 25.735(b), the incorrect reference to § 25.75 was replaced by a correct reference to § 25.125. Under § 25.735(f)(2), the numerical constant 0.0442 was corrected as 0.0443, and the letter "N" was appropriately redefined as the number of main wheels with brakes. Under § 25.735(f)(2), the term Vso in the formula was replaced with "V" such that V must not be less than Vso. Under § 25.735(g), the term Vso was replaced by V to be consistent with terminology used under § 25.735(f)(2).
(3) Amendment 25-52 (June 9, 1980) eliminated § 37.172 Aircraft wheels and brakes - TSO-C26c from the regulations. TSO-C26c was made available at FAA Headquarters in the Office of Airworthiness.
(4) Amendment 25-72 (July 20, 1990) revised the text of the last sentence in § 25.735(b) to clarify the intent.
(5) Amendment 25-92 (February 18, 1998) added § 25.735(h) requiring that the maximum rejected takeoff kinetic energy capacity rating of the aircraft brakes be determined with the brake at the fully worn limit of its allowable wear range. The requirement evolved from an National Transportation Safety Board (NTSB) recommendation as follows: In 1988, a large transport category airplane experienced an 86% maximum kinetic energy (KE) rejected takeoff (RTO) in a dispatch configuration in which eight of the ten brakes were worn close to the maintenance limits. The eight brakes failed early in the braking run and the airplane overran the runway. As a result, the FAA reviewed the methodology used in the determination of allowable brake wear limits for transport category airplanes. It was determined that brake wear limits should be established during certification to ensure that fully worn brakes will function properly during a maximum KE RTO. A series of airplane specific airworthiness directives were issued between 1989 and 1994 to establish brake wear limits using the new criteria. These criteria were applied to transport category airplanes that exceeded a gross weight of 75000 pounds. The test criteria to establish brake wear limits are discussed later under Policy/Compliance item d.(6).
The regulation was updated to reflect this new criteria.
(6) Harmonization. This regulation is the subject of a Federal Aviation Regulations/Joint Aviation Requirements (FAR/JAR) harmonization effort under Aviation Rulemaking Advisory Committee (ARAC). The ARAC working group has recommended revisions to §§ 25.731 and 25.735, and has developed a new advisory circular (AC) 25.735-1. A new TSO-C135 for transport category airplanes, replacing the applicable parts of the existing TSO-C26c, was also developed. The proposed rule change, the proposed AC, and the proposed TSO package were published in the Federal Register, August 10, 1999, inviting public comments. The proposed harmonized AC 25.735-1 will not, however, replace all policy matter contained in this AC because several issues are not addressed in AC 25.735-1.
d. Policy/Compliance Methods. Flight and ground testing procedures for demonstrating compliance with the requirements of § 25.735 may be found in AC 25-7A, Flight Test Guide for Transport Category Airplanes.
(1) Carbon Brake Refurbishment (redensification). The following is extracted from an FAA memorandum, dated April 11, 1996, which provides guidance regarding approval of an alternate process for redensification of carbon brake disks during brake overhaul. The redensification process involves a 2-for-1 brake disk refurbishment. The term 2-for-1 is associated with a re-building of the brake where two worn rotors are resurfaced, placed in an oven to have the carbon "redensified" and riveted together to form a new rotor, which is then placed in the brake as part of the re-building of the assembly. An approved repair shop that overhauls brakes in accordance with the original equipment manufacturer (OEM) overhaul procedures, requested the FAA to approve, as a minor change, the option of using a different carbon brake redensification process developed by the shop and its associates. The only redensification option presently available is to return the brake components to the OEM. The proposed process was initially applied to carbon brake disks used on a military aircraft.
(a) Due to continued in-service problems associated with carbon brakes and their complex processing techniques, the FAA contacted major U.S. manufacturers to solicit background information regarding carbon brake overhaul procedures including the 2-for-1 refurbishment and redensification process. All brake manufacturers consider carbon brake redensification a complex and proprietary process which can affect brake performance.
Experience at three (3) brake manufacturers of original equipment has shown that a very minor carbon processing change (heat treat operation) can have a significant effect on brake performance and airplane structural compatibility. By the criteria established in AC 25-7A, Flight Test Guide for Transport Category Airplanes, paragraph 55, it has been determined that redensification is a major change.
(b) For additional guidance on compliance with this requirement, refer to the latest revision of AC 25-7A, chapter 4, section 4, paragraph 55.
(c) The typical dynamometer test program conducted by an OEM for an overhauled brake configuration can be quite substantial. An airplane manufacturer usually requires that the OEM conduct testing to obtain FAA TSO-C26c approval and re-qualify the brake before airline introduction and retrofit. This full scale brake requalification effort depends 35
on the type and degree of modification(s) done to the brake to obtain the overhauled configuration.
(d) An airplane manufacturer pays particular attention to potential differences in brake torque, energy capacity, vibration, brake sensitivity, dynamic response, and structural strength. A brake requalification test program including the conditions below would be accomplished using a wheel/brake assembly in the most adverse service condition.
1 Design Landing Stops (Normal and Overload Energy levels).
2 Wet Brake Landing Stops (effect of moisture content in carbon).
3 New Brake Rejected Takeoffs (RTO's) at 60%, 80%, and 100% of maximum brake kinetic energy).
4 Worn Brake RTO's.
5 Wet Brake RTO's.
6 High Power/Torque Repeatability RTO.
7 Torque Sensitivity Testing.
8 Service Landing Wear Testing.
9 Fuse Plug No-Melt RTO series to evaluate disk warpage and thermal characteristics.
1 0 Brake Rolling Drag.
1 1 Brake Peak Torque Survey.
1 2 Static Low Speed Torque.
1 3 Vibration Testing.
1 4 Frequency Response.
1 5 Torque Recovery.
1 6 Hysteres is.
1 7 Static Structural Torque.
1 8 Dynamic Structural Torque Test.
1 9 Static Brake Torque to evaluate park brake hold capability.
2 0 Carbon Disk Oxidation Resistance.
2 1 Heat Sink Contamination Testing.
2 2 Intermix of New and Overhauled disks.
2 3 Brake Thermal Modeling.
(e) The test results are then compared to the original new brake qualification test results to determine equivalence of the overhauled brake configuration. When changes include different heat sink material processing or are done by a supplier other than the OEM, it may be 36
necessary to perform airplane/brake system flight testing. This will validate the Airplane Flight Manual performance and the brake control system compatibility for the overhauled brake configuration.
(f) Some additional considerations are as follows:
1 The FAA does not accept military airplane equipment test data for compliance with transport category airplane regulations.
2 The applicant has not conducted any tests whatsoever on the modified brakes.
3 The applicant may need to apply for a new TSO and possibly a supplemental type certificate (STC).
(2) FAA/JAA certification requirements: steel/carbon brakes. The following is extracted from an FAA letter dated April 9, 1996, which addresses obtaining parts manufacturer approval (PMA) for manufacturers located outside of the United States, TSO procedures, and brake certification requirements for steel and carbon brakes.
(a) There are no differences in the certification requirements between carbon and steel brakes. The TSO-C26c and AC 25-7A, Flight Test Guide for Certification of Transport Category Airplanes, are still applicable. In addition, each airframe manufacturer has specific vibration control criteria in their wheel and brake specification. This criteria is crucial to the brake dynamics, the particular airplane installation, and structural integrity of the airplane.
(b) There are no known differences on the carbon brake certification requirements between the FAA and the Joint Aviation Authorities (JAA). You are, however, advised to verify this with the JAA.
(3) Certification of Replacement Brakes. The following is extracted from an FAA letter dated May 19, 1992, which addresses policy on aircraft brake replacement. The information was requested by a foreign airworthiness authority and is provided in the advisory circular format.
ADVISORY INFORMATION FOR BRAKE REPLACEMENT
(a) Purpose. To provide advisory information and guidance for a non-U.S.
manufacturer concerning an acceptable means of showing compliance with Title 14, Code of Federal Regulations (14 CFR), applicable to the replacement of brakes for part 25 airplanes.
(b) Related Regulations and Documents.
1 Sections. The related sections of 14 CFR, part 25, are as follows:
§ 21.305 Approval of materials, parts, process, and appliances.
§ 21.502 Approval of materials, parts, and appliances.
§ 21.607 General rules governing holders of TSO authorizations.
§ 21.609 Approval for deviation.
§ 21.617 Issue of letters of TSO design approval: import appliances.
§ 25.109 Accelerate-stop distance.
§ 25.125 Landing.
§ 25.143 Controllability and maneuverability.
§ 25.231 Longitudinal stability and control.
§ 25.233 Direction stability and control.
§ 25.493 Braked roll conditions.
§ 25.735 Brakes.
§ 25.1301 Function and Installation.
§ 25.1309 Equipment, systems, and installations.
2 Advisory Circulars (AC's). The addresses for ordering the latest revision of the advisory circulars and technical standard orders listed below can be found in the Appendix to this AC.
AC 21-23, Airworthiness Certification of Civil Aircraft Engine, Propellers, and Related Products.
AC 25-7A, Flight Test Guide for Certification of Transport Category Airplanes.
TSO-C26c, Aircraft Wheels and Wheel-Brake Assemblies with Addendum I.
3 Industry Documents. These documents provide additional information, guidance, and/or standards. The address to order the documents listed below can be found in the Appendix to this AC.
SAE ARP-597C, Wheels and Brakes, Supplementary Criteria for Design Endurance - Civil Transport Aircraft.
SAE ARP-813A, Maintainability Recommendations for Aircraft Wheels and Brakes.
SAE ARP-1064B, Brake Dynamics.
SAE AS-1145A, Aircraft Brake Temperature Monitor System (BTMS).
SAE ARP-1619, Replacement and Modified Brakes and Wheels.
(c) Airworthiness Requirements.
1 Applicable Requirements. Replacement of brakes must be evaluated against the applicable part 25 airworthiness requirements. They are §§ 25.109, 25.125, 25.143,
25.231, 25.233, 25.493, 25.735, 25.1301, 25.1309, of Title 14 Code of Federal Regulations (14 CFR), and TSO-C26c.
2 Substantiation Procedures:
(aa) The design and its design deviations should be substantiated by conducting the necessary analytical investigations, laboratory testing, and airplane testing to ensure that the change can be made without impairing airplane braking or rolling performance.
The recommended substantiation procedures for replacement brakes are based on parts approval and their impact on the prior certification. Depending upon the type and extent of change and the certification basis of the airplane, engineering judgment will be used to determine which of the substantiation procedures discussed in AC 25-7A, Flight Test Guide for Certification of Transport Category Airplanes, chapter 4, section 4, paragraph 55, should be considered for replacement brake installations in addition to the laboratory tests required by the TSO.
(bb) Aircraft brakes are approved under the Technical Standard Order (TSO) system (TSO-C26c). A TSO-C26c authorization indicates that the brake and its associated wheel have met certain requirements relating to strength, energy absorption, durability, construction, performance, and manufacture. In addition, the holder of a TSO authorization has demonstrated certain quality control procedures and the ability to produce duplicate parts from approved drawings.
(cc) A TSO authorization, by itself, is generally considered adequate to show that the wheel/brake combination is satisfactory for flight testing on an airplane, and would be safe for use during subsequent certification testing. However, it is not sufficient to show satisfactory performance within the operational envelope of the airplane, due to the welldocumented differences (load distribution on the landing gear, heat dissipation, brake efficiency, runway crown, etc.) between laboratory dynamometer testing and full-scale airplane testing.
(d) Continued Airworthiness/Brake Wear Limits.
1 . During previous certification activity, the determination of airplane performance and the demonstration of maximum brake kinetic energy capability was based on testing brakes at or near their new state. In 1988, a transport category airplane experienced a rejected takeoff (RTO) at 86% of the brake's maximum kinetic energy capability. The airplane went off the end of the runway. An investigation determined that eight of the ten brakes were near the maximum allowable wear limits before the RTO and were unable to absorb the required energy, thus contributing to the accident.
2 Since this accident, the FAA has reviewed the methodology used in the determination of allowable wear limits and airplane performance for transport category airplanes.
The FAA has issued airworthiness directives (AD's) that require more conservative brake wear limits on various models of US and foreign transport category airplanes having a U.S. type certificate.
(4) Parking Brake Testing. The following is extracted from an FAA message dated March 20, 1992, which addresses the intent and application of § 25.735(d).
(a) The parking brake requirement was added to the CAR under § 4b.397 of the CAR on September 1, 1949, and changed to § 4b.337(c) on July 20, 1950. Section 4b.337(c) of the CAR became § 25.735(d) of Title 14, Code of Federal Regulations (14 CFR). The guidance material in § 4b.337-1 indicated this was to be a demonstration to determine that sufficient braking was maintained to prevent the airplane from rolling on a paved runway while takeoff power was applied on the most critical engine.
(b) The FAA certification offices agree that the test is intended to demonstrate that the parking brake will hold with the power of the critical engine at takeoff power and the other engine(s) at idle. The effects of propeller wash or propeller/engine torque, in addition to asymmetric thrust and aircraft weight, should be used in determining the critical engine and vertical loads on the main landing gear. The vectored thrust point can be outside the propellers axis of rotation. Therefore, propeller rotation and engine inclination are a factor in determining the critical engine.
(c) To test the maximum torque for the parking brake, the airplane should be configured at its highest gross weight with the flaps retracted. The critical engine for a four engine airplane is usually one of the outboard engines. For a two engine executive jet airplane with near centerline engines, or a twin engine propeller airplane with propellers rotating the opposite direction, it may not make a difference which is the critical engine. For some executive jets, where there wasn't considered to be a critical engine relative to § 25.735(d), the manufacturers tested by putting both engines at takeoff power. This is not being tested according to the rule, but it should lead to the same conclusion.
(d) For consistency, the following test procedure is recommended:
Paint a radial stripe(s) on the wheels. With the airplane sitting on dry pavement at the airplane's maximum takeoff weight, the tires pressurized normally for this weight, the flaps retracted and control surfaces centered (neutral), conduct the parking brake test as follows:
1 Set the parking brake.
2 Apply takeoff power to the critical engine (critical relative to § 25.735(d)) leaving the other engine(s) at idle.
3 An external observer, looking at the white stripe(s) on the wheels, verifies the wheels are not turning, i.e., the brakes are locked. The airplane may skip or shear tire tread or the tire may slip around the wheel, but the wheels must not rotate.