___________________________________________

Subject BRAKES AND BRAKING

SYSTEMS CERTIFICATION

TESTS AND ANALYSIS

Date: 4/10/02 AC No.: 25.735-1

Initiated by: ANM-100 Change

_________________________

1. PURPOSE. This Advisory Circular (AC) provides guidance material for use as an

acceptable means, although not the only means, of demonstrating compliance with the

braking system requirements of Title 14, Code of Federal Regulations (14 CFR) part 25

for transport category airplanes. Like all AC material, this AC is not, in itself, mandatory

and does not constitute a regulation. Terms used in this AC, such as “shall” or “must,”

are used only in the sense of ensuring applicability of this particular method of

compliance when the acceptable method of compliance described herein is used. While

these guidelines are not mandatory, they are derived from extensive Federal Aviation

Administration (FAA) and industry experience in determining compliance with the

pertinent CFR. This AC does not change, create any additional, authorize changes in, or

permit deviations from, regulatory requirements.

2. RELATED DOCUMENTS.

a. Related Federal Aviation Regulations. Sections 25.731 and 25.735 of 14 CFR, as

amended through Amendment 25-107, and other sections relating to brakes and braking

system installations. Sections that prescribe requirements for the design, substantiation,

and certification of braking systems include:

§ 21.303 Replacement and modification parts

§ 25.101 General

§ 25.109 Accelerate-stop distance

§ 25.125 Landing

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AC 25.735-1 4/10/02

2

§ 25.301 Loads

§ 25.303 Factor of safety

§ 25.729 Retracting mechanism

§ 25.733 Tires

§ 25.1301 Function and installation.

§ 25.1309 Equipment, systems and installations.

§ 25.1322 Warning, caution and advisory lights.

§ 25.1501 General: Systems and equipment limitations (JAR25x1524)

§ 25.1541 Markings and Placards

14 CFR part 21, Subpart O

b. Additional sections (and their associated advisory circulars where applicable) that

prescribe requirements which can have a significant impact on the overall design and

configuration of braking systems include, but are not limited to:

§ 21.101 Designation of applicable regulations

§ 25.863 Flammable fluid fire protection

§ 25.943 Negative acceleration (JAR 25x1315)

§ 25.1001 Fuel jettisoning system

§ 25.1183 Flammable fluid-carrying components

§ 25.1185 Flammable fluids

c. Advisory Circulars (AC's).

AC 25.1309-1A System Design and Analysis

AC 25-7A Flight Test Guide for Certification of Transport Category

Airplanes

AC 91-6A Water, Slush, and Snow on the Runway (AMJ 25x1591

Supplementary Performance Information for Takeoff from Wet Runways and for

Operation on Runways Contaminated by Standing Water, Slush, Loose Snow,

Compacted Snow, or Ice)

d. Technical Standard Orders (TSO's).

TSO-C26c Aircraft Wheels and Wheel-Brake Assemblies with

Addendum I

TSO-C135 Transport Airplane Wheel and Wheel and Brake Assemblies

TSO-C62d Tires

TSO-C75 Hydraulic Hose Assemblies

e. Federal Aviation Administration Orders.

Order 8110.4A Type Certification Process

Order 8110.8 Engineering Flight Test Guide For Transport Category

Airplanes
4/10/02 AC 25.735-1

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f. Advisory Circulars, TSO’s, and FAA Orders can be obtained from the U.S.

Department of Transportation, Subsequent Distribution Office, SVC-121.23, Ardmore

East Business Center, 3341 Q 75th Avenue, Landover, MD 20785, or an electronic copy

may be downloaded using the Internet at the following address:

http://www.faa.gov/certification/aircraft/air_index.htm

g. Society of Automotive Engineers (SAE) Documents.

ARP 597C Wheels and Brakes, Supplementary Criteria for Design

Endurance-– Civil Transport Aircraft

ARP 813A Maintainability Recommendations for Aircraft Wheels and Brakes

AIR 1064B Brake Dynamics

ARP 1070B Design and Testing of Antiskid Brake Control Systems for Total

Aircraft Compatibility

AS 1145A Aircraft Brake Temperature Monitor System (BTMS)

ARP 1619 Replacement and Modified Brakes and Wheels

AIR 1739 Information on Antiskid Systems

ARP 1907 Automatic Braking System Requirements

AIR 1934 Use of Carbon Heat Sink Brakes on Aircraft

ARP 4102/2 Automatic Braking System (ABS)

ISO 7137 Environmental Conditions and Test Procedures for Airborne

Equipment (not an SAE document but is available from the SAE)

h. These documents can be obtained from the Society of Automotive Engineers,

Inc., 400 Commonwealth Drive, Warrendale, Pennsylvania, 15096.

i. RTCA Documents.

RTCA/DO-160D Conditions and Test Procedures for Airborne equipment,

Issued July 12, 1996.

RTCA/DO-178B Software Considerations in Airborne Systems and Equipment

Certification, Issued December 1, 1992

j. Copies of RTCA documents may be purchased from the RTCA Inc., 1140

Connecticut Avenue NW, Suite 1020, Washington, D.C. 20036.

k. Military Documents.

MIL-STD-810 Environmental Test Methods and Engineering Guidelines

l. This document can be obtained from the Department of Defense, DODSSP,

Standardization Document Order Desk, 700 Robbins Avenue, Building 4D, Philadelphia,

PA 19111-5094.
AC 25.735-1 4/10/02

4

3. BACKGROUND.

a. Effective February 1, 1965, part 25 was added to 14 CFR to replace part 4b of

the Civil Air Regulations (CAR). For wheels, CAR 4b.335(a) and (b), became

§§ 25.731(a) and (b) respectively, of 14 CFR. For brakes/braking systems, CAR 4b.337

(a)(1), 4b.337(a)(2) and (a)(3), 4b.337(b), 4b.337(c), 4b.337(d), 4b.335(c), and 4b.335(d),

became §§ 25.735(a), 25.735(b), 25.735(c), 25.735(d), 25.735(e), 25.735(f), and

25.735(g) respectively, of 14 CFR. Since then, § 25.735 has been revised by Amendment

25-23 (1970), Amendment 25-48 (1979), Amendment 25-52 (1980), Amendment 25-72

(1990), Amendment 25-92, Improved Standards for Determining Rejected Takeoff and

Landing Performance (1998), and Amendment 25-107, Revision of Braking Systems

Airworthiness Standards to Harmonize with European Airworthiness Standards for

Transport Category Airplanes. These amendments were adopted to make the regulations

more comprehensive and to delete redundancies.

(1) Amendment 25-23 deleted reference to military specification (MIL-B-8075)

to show compliance for antiskid devices under § 25.735(e), and to allow any other

acceptable means of compliance. In addition, proper units of "knots" were added to the

stall speed under § 25.735(f)(2).

(2) Amendment 25-48 revised the technical standard order TSO-C26b for

aircraft wheels and wheel-brake assemblies and related type certification requirements for

airplane brakes (§ 25.735). The revised standard TSO-C26c incorporated updated and

improved minimum performance standards for the design and construction of aircraft

wheels and brakes. The amendment also changed § 25.735 as follows:

(a) Under § 25.735(b), the incorrect reference to § 25.75 was replaced by a

correct reference to § 25.125.

(b) 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.

(c) 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 definition.

(d) Under § 25.735(g), the term Vso was replaced by V to be consistent

with terminology used under § 25.735(f)(2).

(3) Under Amendment 25-52, § 37.172, Aircraft wheels and brakes, TSO-C26c

was removed from the regulations, previously published as Subpart B of 14 CFR part 37,

and made available to the public. The TSO-C26c is available through the FAA Office of

Airworthiness, Aircraft Engineering Division, Systems Branch (AWS-130) at FAA

Headquarters in Washington, D.C., and at all regional Flight Standards Engineering and

Manufacturing Offices. Subpart A of 14 CFR part 37 was included in Subpart O of

14 CFR part 21. Part 37 of 14 CFR was revoked.
4/10/02 AC 25.735-1

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(4) Under Amendment 25-72, the text of the last sentence in existing

§ 25.735(b) was changed to clarify the intent. In addition, § 25.731 was amended to

become compatible with § 25.25, which had been amended to provide for weights that

are in excess of takeoff weight, such as ramp weights, provided that compliance with the

applicable structural requirements, including wheel strength, is demonstrated at the

higher weights.

(5) Under Amendment 25-92, the regulations were updated to add the brake

wear limits determination requirements. On May 21, 1988, an American Airlines DC-10

experienced an 86 percent 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 in the early portion of 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. The FAA issued a

series of airplane specific airworthiness directives between 1989 and 1994 to establish

brake wear limits using the new criteria.

(6) Although part 25 and JAR-25 are very similar, they are not identical.

Differences between part 25 and the JAR can result in substantial additional costs when

airplanes are type certificated to both standards. Starting In 1992, the harmonization

effort for various systems-related airworthiness requirements was undertaken by the

Aviation Rulemaking Advisory Committee (ARAC). A working group of industry and

government braking systems specialists from Europe, the United States, and Canada was

chartered by notice in the Federal Register (59 FR 30080, June 10, 1994). The working

group was tasked to develop harmonized standards and any collateral documents, such as

advisory circulars, concerning new or revised requirements for braking systems, and the

associated test conditions for braking systems, installed in transport category airplanes

(§§ 25.731 and 25.735). The advisory material contained in this AC was developed by

the Braking Systems Harmonization Working Group to ensure consistent application of

the standards revised under Amendment 25-107, Revision of Braking Systems

Airworthiness Standards to Harmonize with European Airworthiness Standards for

Transport Category Airplanes, and the corresponding new TSO-C135.

4. DISCUSSION.

a. Approval.

(1) Section § 25.735(a) states that each assembly consisting of a wheel(s) and

brake(s) must be approved. Each wheel and brake assembly fitted with each designated

and approved tire type and size, where appropriate, should be shown to be capable of

meeting the minimum standards and capabilities detailed in the applicable TSO, in

combination with the type certification procedures for the airplane, or by any other means

approved by the Administrator. This applies equally to replacement, modified, and
AC 25.735-1 4/10/02

6

refurbished wheel and brake assemblies or components, whether the changes are made by

the Original Equipment Manufacturer (OEM) or others. Additionally, the components of

the wheels, brakes, and braking systems should be designed to:

(a) Withstand all pressures and loads, applied separately and in

conjunction, to which they may be subjected in all operating conditions for which the

airplane is certificated.

(b) Withstand simultaneous applications of normal and emergency braking

functions, unless adequate design measures have been taken to prevent such a

contingency.

(c) Meet the energy absorption requirements without auxiliary cooling

devices (such as cooling fans).

(d) Not induce unacceptable vibrations at any likely ground speed and

condition or any operating condition (such as retraction or extension).

(e) Protect against the ingress or effects of foreign bodies or materials

(water, mud, oil, and other products) that may adversely affect their satisfactory

performance. Following initial airplane certification, any additional wheel and brake

assemblies should meet the applicable airworthiness requirements specified in

§§ 21.101(a) and (b) to eliminate situations that may have adverse consequences on

airplane braking control and performance. This includes the possibility of the use of

modified brakes either alone (i.e., as a shipset) or alongside the OEM’s brakes and the

mixing of separately approved assemblies.

(2) Refurbished and Overhauled Equipment. Refurbished and overhauled

equipment is equipment overhauled and maintained by the applicable OEM or its

designee in accordance with the OEM’s Component Maintenance Manual (CMM) and

associated documents. It is necessary to demonstrate compliance of all refurbished

configurations with the applicable TSO and airplane manufacturer’s specifications. It is

also necessary to verify that performances are compatible for any combination of mixed

brake configurations, including refurbished/overhauled and new brakes. It is essential to

assure that Airplane Flight Manual braking performance and landing gear and airplane

structural integrity are not adversely altered.

(3) Replacement and Modified Equipment. Replacement and modified

equipment includes changes to any approved wheel and brake assemblies not addressed

under paragraph 4a(2) of this AC. Consultation with the airplane manufacturer on the

extent of testing is recommended. Particular attention should be paid to potential

differences in the primary brake system parameters (e.g., brake torque, energy capacity,

vibration, brake sensitivity, dynamic response, structural strength, and wear state). If

comparisons are made to previously approved equipment, the test articles (other than the

proposed parts to be changed) and conditions should be comparable, as well as the test

procedures and equipment on which comparative tests are to be conducted. For wheel
4/10/02 AC 25.735-1

7

and brake assembly tests, the tire size, manufacturer, and ply rating used for the test

should be the same and the tire condition should be comparable. For changes of any heat

sink component parts, structural parts (including the wheel), and friction elements, it is

necessary for the applicant to provide evidence of acceptable performance and

compatibility with the airplane and its systems.

(a) Minor Changes. Changes to a brake might be considered as a minor

change, as long as the changes are not to the friction elements. The proposed change

cannot affect the airplane stopping performance, brake energy absorption characteristics,

and/or continued airworthiness of the airplane or wheel and brake assembly (e.g.,

vibration and/or thermal control, and brake retraction integrity). It is incumbent on the

applicant to provide technical evidence justifying a minor change.

(b) Major Changes. Changes to a wheel assembly outside the limits

allowed by the OEM’s CMM should be considered a major change due to potential

airworthiness issues.

(c) Past history with friction elements has indicated the necessity of ongoing monitoring (by dynamometer test) of frictional and energy absorption capabilities

to assure that they are maintained over the life of the airplane program. These monitoring

plans have complemented the detection and correction of unacceptable deviations. The

applicant should submit a monitoring plan to the cognizant FAA Aircraft Certification

Office to ensure continued airworthiness of the product.

(d) Intermixing of wheel and brake assemblies from different suppliers is

generally not acceptable due to complexities experienced with different friction elements,

specific brake control tuning, and other factors.

b. Brake System Capability.

(1) The system should be designed so that no single failure of the system

degrades the airplane stopping performance beyond doubling the braked roll stopping

distance (refer to § 25.735(b)(1)). Failures are considered to be fracture, leakage, or

jamming of a component in the system, or loss of an energy source. Components of the

system include all parts that contribute to transmitting the pilot's braking command to the

actual generation of braking force. Multiple failures resulting from a single cause should

be considered a single failure (e.g., fracture of two or more hydraulic lines as a result of a

single tire failure). Sub-components within the brake assembly, such as brake discs and

actuators (or their equivalents), should 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 sub-components within the brake assembly would not reduce

the braking effectiveness below that specified in § 25.735(b)(1).

(a) In order to meet the stopping distance requirements of § 25.735(b)(1)

in the event of failure of the normal brake system, it is common practice to provide an

alternate brake system. The normal and alternate braking systems should be independent,
AC 25.735-1 4/10/02

8

being supplied by separate power sources. Following a failure of the normal system, the

changeover to a second system (whether manually or by automatic means) and the

functioning of a secondary power source should be effected rapidly and safely. The

changeover should not involve risk of wheel locking, whether the brakes are applied or

not at the time of changeover.

(b) The brake systems and components should be separated or

appropriately shielded so that complete failure of the braking system(s) as a result of a

single cause is minimized.

(2) Compliance with § 25.735(b)(2) may be achieved by:

(a) Showing that fluid released would not impinge on the brake, or any part

of the assembly that might cause the fluid to ignite;

(b) Showing that the fluid will not ignite; or

(c) Showing that the maximum amount of fluid released is not sufficient to

sustain a fire.

(3) Additionally, in the case of a fire, the applicant may show that the fire is not

hazardous, taking into consideration such factors as landing gear geometry, location of

fire sensitive (susceptibility) equipment and installations, system status, flight mode, etc.

If more than one fluid is allowed for the hydraulic system, compliance should be

addressed for all fluids.

c. Brake Controls.

(1) The braking force should increase or decrease progressively as the force or

movement applied to the brake control is increased or decreased (refer to § 25.735(c)(1)).

The braking force should respond to the control as quickly as is necessary for safe and

satisfactory operation. A brake control intended only for parking need not operate

progressively. There should be no requirement to select the parking brake “off” in order

to achieve a higher braking force with manual braking.

(2) When an automatic braking system is installed (refer to § 25.735(c)(2))

such that various levels of braking (e.g., low, medium, high) may be preselected to occur

automatically following a touchdown, the pilot(s) should be provided with a means that is

separate from other brake controls to arm and/or disarm the system prior to the

touchdown.

(3) The automatic braking system design should be evaluated for integrity and

non-hazard, including the probability and consequence of insidious failure of critical

components, and noninterference with the non-automatic braking system. Single failures

in the automatic braking system should not compromise non-automatic braking of the

airplane. Automatic braking systems that are to be approved for use in the event of a
4/10/02 AC 25.735-1

9

rejected takeoff should have a single selector position, set prior to takeoff, enabling this

operating mode.

d. Parking Brake. It should be demonstrated that the parking brake has sufficient

capability in all allowable operating conditions (Master Minimum Equipment List

(MMEL) to be able to prevent the rotation of braked wheels. This demonstration is to be

accomplished with the stated engine power settings, and with the airplane configuration

(i.e., ground weight, c.g., position and nosewheel (or tailwheel) angle) least likely to

result in skidding on a dry, level runway surface (refer to § 25.735(d)). Use of ground

idle thrust on the “other” engine is not mandatory, higher thrust levels may be used to

prevent airplane motion due to the asymmetric engine thrust. Where reliable test data are

available, substantiation by means other than airplane testing may be acceptable.

(1) For compliance with the requirement for indication that the parking brake is

not fully released, the indication means should be associated, as closely as is practical,

with actual application of the brake rather than the selector (control). The intent is to

minimize the possibility of false indication due to failures between the brake and the

point at which the parking brake state is sensed. This requirement is separate from, and

in addition, to the parking brake requirements associated with JAR 25.703(a)(3), Take-off

warning systems.

(2) The parking brake control, whether or not it is independent of the

emergency brake control, should be marked with the words "Parking Brake" and should

be constructed in such a way that, once operated, it can remain in the selected position

without further flightcrew attention. It should be located where inadvertent operation is

unlikely, or be protected by suitable means against inadvertent operation.

e. Antiskid System.

(1) If an antiskid system is installed (refer to § 25.735(e)), then no single failure

in the antiskid system should result in the brakes being applied, unless braking is being

commanded by the pilot. In the event of an antiskid system failure, means should be

available to allow continued braking without antiskid. These means may be automatic,

pilot controlled, or both.

(2) Compliance with §§ 25.735(e)(1) and (e)(2)) may be achieved by:

(a) Failures that render the system ineffective should not prevent manual

braking control by the pilot(s) and should normally be indicated. Failure of wheels,

brakes, or tires should not inhibit the function of the antiskid system for unaffected

wheel, brake, and tire assemblies.

(b) The antiskid system should be capable of giving a satisfactory braking

performance over the full range of tire to runway friction coefficients and surface

conditions, without the need for pre-flight or pre-landing adjustments or selections. The
AC 25.735-1 4/10/02

10

range of friction coefficients should encompass those appropriate to dry, wet, and

contaminated surfaces and for both grooved and ungrooved runways.

(c) The use of the phrase “without external adjustment” is intended to

imply that once the antiskid system has been optimized for operation over the full range

of expected conditions for which the airplane is to be type certificated, pre-flight or

pre-landing adjustments made to the equipment to enable the expected capabilities to

be achieved are not acceptable. For example, a specific pre-landing selection for a

landing on a contaminated, low μ (friction level) runway, following a takeoff from a dry,

high μ runway, should not be necessary for satisfactory braking performance to be

achieved.

(d) It should be shown that the brake cycling frequency imposed by the

antiskid installation will not result in excessive loads on the landing gear. Antiskid

installations should not cause surge pressures in the brake hydraulic system that would be

detrimental to either the normal or emergency brake system and components.

(e) The system should be compatible with all tire sizes and type

combinations permitted and for all allowable wear states of the brakes and tires. Where

brakes of different types or manufacture are permitted, compatibility should be

demonstrated or appropriate means should be employed to ensure that undesirable

combinations are precluded.

(f) The antiskid function must be able to reduce braking for a wheel/tire

that is going into a skid, whether the braking level is commanded by the pilot or an

autobrake system if installed.

f. Kinetic Energy Capacity. The kinetic energy capacity of each tire, wheel, and

brake assembly should be at least equal to that part of the total airplane energy that the

assembly will absorb during a stop, with the heat sink at a defined condition at the

commencement of the stop (Refer to § 25.735(f)).

(1) Calculation of Stop Kinetic Energy.

(a) The design landing stop, the maximum kinetic energy accelerate-stop,

and the most severe landing stop brake kinetic energy absorption requirements of each

wheel and brake assembly should be determined using either of the following methods:

(1) A conservative rational analysis of the sequence of events

expected during the braking maneuver; or

(2) A direct calculation based on the airplane kinetic energy at the

commencement of the braking maneuver.

(b) When determining the tire, wheel, and brake assembly kinetic energy

absorption requirement using the rational analysis method, the analysis should use
4/10/02 AC 25.735-1

11

conservative values of the airplane speed at which the brakes are first applied, the range

of the expected coefficient of friction between the tires and runway, aerodynamic and

propeller drag, powerplant forward thrust, and, if more critical, the most adverse single

engine or propeller malfunction.

(c) When determining the tire, wheel, and brake assembly energy

absorption requirement using the direct calculation method, the following formula, which

needs to be modified in cases of designed unequal braking distribution, should be used:

KE = 0.0443 WV2/N (ft-lb.)

 where KE = Kinetic Energy per wheel (ft-lb.)

 N = Number of main wheels with brakes

 W = Airplane Weight (lb.)

 V = Airplane Speed (knots)

or if SI (Metric) units are used:

KE = 1/2 mV2

/N (Joule)

 where KE = Kinetic Energy per wheel (J)

 N = Number of main wheels with brakes

 m = Airplane Mass (kg.)

 V = Airplane Speed (m/s)

(d) For all cases, V is the ground speed and takes into account the

prevailing operational conditions. All approved landing flap conditions should be

considered when determining the design landing stop energy.

(e) These calculations should account for cases of designed unequal

braking distributions. “Designed unequal braking distribution” refers to unequal braking

loads between wheels that result directly from the design of the airplane. An example

would be the use of both mainwheel and nosewheel brakes, or the use of brakes on a

centerline landing gear supporting lower vertical loads per braked wheel than the main

landing gear braked wheels. It is intended that this term should account for effects such

as runway crown. Crosswind effects need not be considered.

(f) For the design landing case, the airplane speed should not be less than

VREF/1.3, where VREF is the airplane steady landing approach speed at the maximum

design landing weight and in the landing configuration at sea level. Alternatively, the

airplane speed should not be less than VSO, the poweroff stall speed of the airplane at sea

level, at the design landing weight, and in the landing configuration.

(g) For the most severe landing case, the applicant should address effects

and consequences of typical single and multiple failure conditions that are foreseeable

events and can necessitate landings at abnormal speeds and weights. The critical landing

weight for this condition is the maximum takeoff weight, less fuel burned and jettisoned

during a return to the departure airfield. A 30-minute flight should be assumed, with

15 minutes of active fuel jettisoning if equipped with a fuel jettisoning system.
AC 25.735-1 4/10/02

12

(2) Heat Sink Condition at Commencement of the Stop.

(a) For the maximum kinetic energy accelerate-stop case, the calculation

should account for

(b) The brake temperature following a previous typical landing,

(c ) The effects of braking during taxi-in, the temperature change while

parked,

(d) The effects of braking during taxi-out, and

(e) The additional temperature change during the takeoff acceleration

phase, up to the time of brake application.

(f) The analysis may not take account of auxiliary cooling devices.

Assessment of ambient conditions within the operational limits established by the

applicant and the typical time the airplane will be on the ground should be used.

(g) For the most severe landing stop case, the same temperature conditions

and changes used for the maximum kinetic energy accelerate-stop case should be

assumed, except that further temperature change during the additional flight phase may

be considered.

(h) The brake temperature at the commencement of the braking maneuver

should be determined using the rational analysis method. However, in the absence of

such analysis, an arbitrary heat sink temperature should be used equal to the normal

ambient temperature, increased by the amount that would result from a 10 percent

maximum kinetic energy accelerate-stop for the accelerate-stop case and from a 5 percent

maximum kinetic energy accelerate-stop for landing cases.

(3) Substantiation.

(a) Substantiation is required to show that the wheel and brake assembly is

capable of absorbing the determined levels of kinetic energy at all permitted wear states

up to and including the declared fully worn limits. The term “wear state" is used to

clarify that consideration should be given to possible inconsistencies or irregularities in

brake wear in some circumstances, such as greater wear at one end of the heat sink than

the other end. Qualification related to equally distributed heat sink wear may not be

considered adequate. If in-service wear distribution is significantly different from wear

distribution used during qualification testing, additional substantiation and/or corrective

action may be necessary.

(b) The minimum initial brakes-on speed used in the dynamometer tests

should not be more than the velocity (V) used in the determination of the kinetic energy

requirements of § 25.735(f). This assumes that the test procedure involved a specific rate
4/10/02 AC 25.735-1

13

of deceleration and, therefore, for the same amount of kinetic energy, a higher initial

brakes-on speed would result in a lower rate of energy absorption. Such a situation is

recognized and is similarly stated in TSO-C135, which provides an acceptable means for

brake approval under § 25.735(a).

(c) For certification purposes, a brake having a higher initial brakes-on

speed is acceptable if the dynamometer test showed that both the energy absorbed and the

energy absorption rates required by § 25.735(f) had been achieved.

(d) Brake qualification tests are not intended as a means of determining

expected airplane stopping performance, but may be used as an indicator for the most

critical brake wear state for airplane braking performance measurements.

g. Brake condition after high kinetic energy dynamometer stop(s). (Reference

§ 25.735(g)).

(1) Following the high kinetic energy stop(s), the parking brake should be

capable of restraining further movement of the airplane and should maintain this

capability for the period during which the need for an evacuation of the airplane can be

determined and then fully accomplished. It should be demonstrated that, with a parking

brake application within a period not exceeding 20 seconds of achieving a full stop, or

within 20 seconds from the time that the speed is retarded to 20 knots (or lower), in the

event that the brakes are released prior to achieving a full stop (as permitted by

TSO-C135), the parking brake can be applied normally and that it remains functional for

at least 3 minutes.

(2) Practical difficulties associated with dynamometer design may preclude

directly demonstrating the effectiveness of the parking brake in the period immediately

following the high energy dynamometer stop(s). Where such difficulties prevail, it

should be shown that, for the 3-minute period, no structural failure or other condition of

the brake components occurs that would significantly impair the parking brake function.

(3) Regarding the initiation of a fire, it should be demonstrated that no

continuous or sustained fire, extending above the level of the highest point of the tire,

occurs before the 5-minute period has elapsed. Neither should any other condition arise

during this same period or during the stop, either separately or in conjunction with a fire,

that could be reasonably judged to prejudice the safe and complete airplane evacuation.

Fire of a limited extent and of a temporary nature (e.g., those involving wheel bearing

lubricant or minor oil spillage) is acceptable. For this demonstration, neither firefighting

means nor coolants may be applied.

h. Stored energy systems.

(1) Stored energy systems use a self-contained source of power, such as a

pressurized hydraulic accumulator or a charged battery (refer to § 25.735(h)). This

requirement is not applicable for those airplanes that provide a number of independent
AC 25.735-1 4/10/02

14

braking systems, including a stored energy system, but are not "reliant" on the stored

energy system for the demonstration of compliance with § 25.735(b).

(2) The indication of usable stored energy should show:

(a) The minimum energy level necessary to meet the requirements of

§§ 25.735 (b)(1) and (h) (i.e., the acceptable level for dispatch of the airplane);

(b) The remaining energy level; and

(c) The energy level below which further brake application may not be

possible.

(3) If a gas pressurized hydraulic accumulator is to be used as the energy

storage means, indication of accumulator pressure alone is not considered adequate

means to indicate available stored energy, unless verification can be made of the correct

precharge pressure with the hydraulic system pressure off and the correct fluid volume

with the hydraulic system pressure on. Furthermore, additional safeguards may be

necessary to ensure that sufficient energy will be available at the end of the flight.

Similar considerations should be made if other stored energy systems are used.

(4) A full brake application cycle is defined as an application from brakes fully

released to brakes fully applied, and back to fully released.

i. Brake wear indicators. The indication means should be located such that no

special tool or illumination (except in darkness) is required. Expert interpretation of the

indication should not be necessary (refer to § 25.735(i)).

j. Overtemperature and overpressure burst prevention. Generally, two separate

types of protection should be provided: one specifically to release the tire pressure

should the wheel temperature increase to an unacceptable level, and the other to release

the tire pressure should the pressure become unacceptably high, particularly during the

inflation process. The temperature sensitive devices are required in braked wheels only,

but the pressure sensitive devices are required in all wheels (refer to §§ 25.735(j) and

25.731(d)).

(1) The temperature sensitive devices (e.g., fuse or fusible plugs) should be

sufficient in number and appropriately located to reduce the tire pressure to a safe level

before any part of the wheel becomes unacceptably hot, irrespective of the wheel

orientation. The devices should be designed and installed so that once operated (or

triggered) their continued operation is not impaired by the releasing gas. The

effectiveness of these devices in preventing hazardous tire blowout or wheel failure

should be demonstrated. It should also be demonstrated that the devices will not release

the tire pressure prematurely during takeoff and landing, including during “quick

turnaround” types of operation.
4/10/02 AC 25.735-1

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(2) It should be shown that the overpressurization devices, or the devices in

conjunction with the tire inflation means permanently installed in the wheel, would not

permit the tire pressure to reach an unsafe level regardless of the capacity of the inflation

source.

(3) Both types of devices should normally be located within the structure of the

wheel in positions that minimize the risk of damage or tampering during normal

maintenance.

k. Compatibility. Compliance with § 25.735(k) may be achieved by the following:

(1) As part of the overall substantiation of safe and anomaly free operation, it is

necessary to show that no unsafe conditions arise from incompatibilities between the

brakes and brake system with other airplane systems and structures. Areas that should be

explored include antiskid tuning, landing gear dynamics, tire type and size, brake

combinations, brake characteristics, brake and landing gear vibrations, etc. Similarly,

wheel and tire compatibility should be addressed. These issues should be readdressed

when the equipment is modified.

(2) During brake qualification testing, sufficient dynamometer testing over the

ranges of permissible brake wear states, energy levels, brake pressures, brake

temperatures, and speeds should be undertaken to provide information necessary for

systems integration.

/s/ Ali Bahrami

Ali Bahrami

Acting Manager

Transport Airplane Directorate

Aircraft Certification Service