AMC 25.735 Brakes and Braking Systems Certification Tests and Analysis

ED Decision 2016/010/R

1.       PURPOSE  

This AMC (Acceptable Means of Compliance) which is similar to the FAA Advisory Circular AC 25.735-1 provides guidance material for use as an acceptable means, although not the only means, of demonstrating compliance with the requirements of CS 25.731 and CS 25.735. It also identifies other paragraphs of the EASA Certification Specifications (CS) that contain related requirements and other related and complementary documents.

2.       RELATED REGULATORY MATERIAL AND COMPLEMENTARY DOCUMENTS

a.       Related EASA Certification Specifications

PART-21 and CS-25 paragraphs (and their associated AMC material where applicable) that prescribe requirements related to the design substantiation and certification of brakes and braking systems include:

Additional Part-21 and CS-25 paragraphs (and their associated AMC material where applicable) that prescribe requirements which can have a significant impact on the overall design and configuration of brakes and braking systems are, but are not limited to:

b.       Complementary Documents

Documents that provide appropriate standards for the design substantiation and certification of Brakes and Braking Systems are, but are not limited to:

(i)      European Technical Standard Orders (ETSO)

(ii)     Advisory Circulars/Acceptable Means of Compliance

(iii)     Society of Automotive Engineers (SAE) Documents

(iv)     International Organisation for Standardisation (ISO) Documents

(v)      US Military Documents

(vi)     The European Organisation for Civil Aviation Equipment Documents

3.       RESERVED

4.       DISCUSSION

a.       Ref. CS 25.735(a) Approval

(1)     CS 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 tyre type and size, where appropriate, should be shown to be capable of meeting the minimum standards and capabilities detailed in the applicable European Technical Standard Order (E)TSO, in conjunction with the type certification procedure for the aeroplane, or by any other means approved by the Agency. This applies equally to replacement, modified, and 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 aeroplane 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 aeroplane certification, any additional wheel and brake assemblies should meet the applicable airworthiness requirements specified in 21 A.101(a) and (b) to eliminate situations that may have adverse consequences on aeroplane braking control and performance. This includes the possibility of the use of modified brakes either alone (i.e., as a ship set) or alongside the OEM’s brakes and the mixing of separately approved assemblies.

(2)     Respecting brake energy qualification limits

The ETSO standard for wheels and wheel and brake assemblies includes an ‘Accelerate-Stop Test’ and a ‘Most Severe Landing Stop Test’ (if applicable), which establish the kinetic energy (KE) absorption capability of the brake assembly. The ETSO tests demonstrate the KE absorption capability of the brake with that brake at a predetermined (threshold) start temperature. Both of these tests are required to be performed on (new and worn) brakes with threshold temperatures that must ‘as closely as practicable, be representative of a typical in-service condition’.

Two methods are permitted and accepted by the Agency to calculate the energy required to bring the heat pack to this representative thermal condition:

(a)     by a rational analysis; or

(b)     by the addition of a percentage of the KERT Wheel/Brake Rated Accelerate-Stop Energy: 10 % for ‘Accelerate-Stop Test’ or 5 % for ‘Most Severe Landing Stop Test’.

A brake with an initial temperature higher than the threshold temperature has less KE absorption capability than it has at the threshold temperature. This could lead to the brake being unable to generate the required torque to stop the aeroplane in the available distance, or being unable to safely dissipate the additional thermal energy generated during the stop (hence, a risk of fire). Therefore, the applicant should ensure that the demonstrated brake KE absorption capability is not exceeded when the brake is installed on the aeroplane.

It should be demonstrated how the temperature thresholds, determined for the brake qualification testing, will not be exceeded.

Acceptable methods of demonstrating this include, but are not limited to, the following:

(a)     use of brake temperature monitoring: by allowing the crew to check the brake temperature prior to a take-off, it can be ensured that that the brake temperature does not exceed thetemperature threshold of the demonstrated brake qualification testing, or

(b)     use of brake cool-down charts: by establishing the cool-down rate of the brake heat sink, an estimate can be made that relates the energy absorbed by the brake to its temperature and also to the appropriate cool-down time.

Appropriate limitations have to be specified in the Aeroplane Flight Manual (AFM)

(3)     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 (E)TSO and aeroplane 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 Aeroplane Flight Manual braking performance and landing gear and aeroplane structural integrity are not adversely altered.

(4)     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 AMC. Consultation with the aeroplane 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 and brake assembly tests, the tyre size, manufacturer, and ply rating used for the test should be the same and the tyre condition should be comparable. For changes of any heat sink component parts, structural parts (including the wheel), and friction elements, it is necessary to provide evidence of acceptable performance and compatibility with the aeroplane 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 aeroplane stopping performance, brake energy absorption characteristics, and/or continued airworthiness of the aeroplane or wheel and brake assembly (e.g., vibration and/or thermal control, and brake retraction integrity). Technical evidence justifying a minor change should be provided.

(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 aeroplane program. These monitoring plans have complemented the detection and correction of unacceptable deviations. A monitoring plan should be submitted to the cognisant 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.       Ref. CS 25.735(b) Brake System Capability

(1)     The system should be designed so that no single failure of the system degrades the aeroplane stopping performance beyond doubling the braked roll stopping distance (refer to CS 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 tyre 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 CS 25.735(b)(1).

(a)     In order to meet the stopping distance requirements of CS 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, 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 minimised.

(2)     Compliance with CS 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, it may be shown 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.       Ref. CS 25.735(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 CS 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 CS 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 non interference with the non-automatic braking system. Single failures in the automatic braking system should not compromise non-automatic braking of the aeroplane. Automatic braking systems that are to be approved for use in the event of a rejected take-off should have a single selector position, set prior to take-off, enabling this operating mode.

d.       Ref. CS 25.735(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 aeroplane configuration (i.e., ground weight, c.g., position and nose-wheel (or tail-wheel) angle) least likely to result in skidding on a dry, level runway surface (refer to CS 25.735(d)). Use of ground idle thrust on the “other” engine is not mandatory, higher thrust levels may be used to prevent aeroplane motion due to the asymmetric engine thrust. Where reliable test data are available, substantiation by means other than aeroplane 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 minimise 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 CS 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 flight crew attention. It should be located where inadvertent operation is unlikely, or be protected by suitable means against inadvertent operation.

e.       Ref. CS 25.735(e) Anti-skid System

(1)     If an anti-skid system is installed (refer to CS 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 anti-skid system failure, means should be available to allow continued braking without anti-skid. These means may be automatic, pilot controlled, or both.

(2)     Compliance with CS 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 tyres should not inhibit the function of the anti-skid system for unaffected wheel, brake, and tyre assemblies.

(b)     The anti-skid system should be capable of giving a satisfactory braking performance over the full range of tyre to runway friction coefficients and surface conditions, without the need for preflight or pre-landing adjustments or selections. The 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 optimised for operation over the full range of expected conditions for which the aeroplane 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 prelanding selection for a landing on a contaminated low µ (friction level) runway, following a take-off 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 anti-skid installation will not result in excessive loads on the landing gear. Anti-skid 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 tyre sizes and type combinations permitted and for all allowable wear states of the brakes and tyres. 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 anti-skid function must be able to reduce braking for a wheel/tyre that is going into a skid, whether the braking level is commanded by the pilot or an auto-brake system if installed.

f.       Ref. CS 25.735(f) Kinetic Energy Capacity

The kinetic energy capacity of each tyre, wheel, and brake assembly should be at least equal to that part of the total aeroplane 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 CS 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:

(i)      A conservative rational analysis of the sequence of events expected during the braking manoeuvre; or

(ii)      A direct calculation based on the aeroplane kinetic energy at the commencement of the braking manoeuvre.

(b)     When determining the tyre, wheel, and brake assembly kinetic energy absorption requirement using the rational analysis method, the analysis should use conservative values of the aeroplane speed at which the brakes are first applied, the range of the expected coefficient of friction between the tyres 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 tyre, 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  =   Aeroplane Weight (lb.)

V   =   Aeroplane 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  =   Aeroplane Mass (kg.)

V   =   Aeroplane 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 aeroplane. An example would be the use of both main-wheel and nosewheel brakes, or the use of brakes on a centreline 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 aeroplane speed should not be less than V_REF/1.3, where V_REF is the aeroplane steady landing approach speed at the maximum design landing weight and in the landing configuration at sea level. Alternatively, the aeroplane speed should not be less than V_SO, the power-off stall speed of the aeroplane at sea level, at the design landing weight, and in the landing configuration.

(g)     For the most severe landing case, the effects and consequences of typical single and multiple failure conditions that are foreseeable events and can necessitate landings at abnormal speeds and weights should be addressed. The critical landing weight for this condition is the maximum take-off 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.

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

(a)     For the maximum kinetic energy accelerate-stop case, the calculation should account for:

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

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

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

(iv)     The additional temperature change during the take-off acceleration phase, up to the time of brake application.

(b)     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 aeroplane will be on the ground should be used.

(c)      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.

(d)     The brake temperature at the commencement of the braking manoeuvre 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 acceleratestop case and from a 5 percent maximum kinetic energy accelerate-stop for landing cases. The temperature determined for the beginning of the test becomes the highest allowable temperature at commencement of the take-off run unless another test is performed at a higher temperature.

(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 CS 25.735(f). This assumes that the test procedure involved a specific rate 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 recognised and is similarly stated in (E)TSO-C135, which provides an acceptable means for brake approval under CS 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 CS 25.735(f) had been achieved.

(d)     Brake qualification tests are not intended as a means of determining expected aeroplane stopping performance, but may be used as an indicator for the most critical brake wear state for aeroplane braking performance measurements.

g.       Ref. CS 25.735(g) Brake Condition after High Kinetic Energy Dynamometer Stop(s)

(1)     Following the high kinetic energy stop(s), the parking brake should be capable of restraining further movement of the aeroplane and should maintain this capability for the period during which the need for an evacuation of the aeroplane 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 37 km/h (20 knots) (or lower), in the event that the brakes are released prior to achieving a full stop (as permitted by (E)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 tyre, 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 aeroplane 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 fire-fighting means nor coolants may be applied.

h.       Ref. CS 25.735(h) Stored energy systems

(1)     Stored energy systems use a self-contained source of power, such as a pressurised hydraulic accumulator or a charged battery (refer to CS 25.735(h)). This requirement is not applicable for those aeroplanes that provide a number of independent braking systems, including a stored energy system, but are not "reliant" on the stored energy system for the demonstration of compliance with CS 25.735(b).

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

(a)     The minimum energy level necessary to meet the requirements of CS 25.735(b)(1) and (h) (i.e., the acceptable level for dispatch of the aeroplane);

(b)     The remaining energy level; and

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

(3)     If a gas pressurised 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 pre-charge 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.        Ref. CS 25.735(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 CS 25.735(i)).

j.        Ref. CS 25.731(d) and CS 25.735(j) Over-temperature and Over-pressure Burst Prevention

Over-temperature and over-pressure burst prevention. Generally, two separate types of protection should be provided: one specifically to release the tyre pressure should the wheel temperature increase to an unacceptable level, and the other to release the tyre 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 CS 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 tyre 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 tyre blow-out or wheel failure should be demonstrated. It should also be demonstrated that the devices will not release the tyre pressure prematurely during take-off and landing, including during “quick turnaround” types of operation.

(2)     It should be shown that the over-pressurisation devices, or the devices in conjunction with the tyre inflation means permanently installed in the wheel, would not permit the tyre 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 minimise the risk of damage or tampering during normal maintenance.

k.       Ref. CS 25.735(k) Compatibility

Compliance with CS 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 aeroplane systems and structures. Areas that should be explored include anti-skid tuning, landing gear dynamics, tyre type and size, brake combinations, brake characteristics, brake and landing gear vibrations, etc. Similarly, wheel and tyre 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.

l.        Ref. CS 25.735(l) Wheel brake temperature.

The use of fusible plugs in the wheels is not a complete safeguard against damage due to tyre burst. Where brake overheating could be damaging to the structure of, or equipment in, the wheel wells, an indication of brake temperature should be provided to warn the pilot.

[Amdt No: 25/2]

[Amdt No: 25/8]

[Amdt No: 25/12]

[Amdt No: 25/14]

[Amdt No: 25/18]