CHAPTER 4 - DESIGN AND CONSTRUCTION Section 1. GENERAL [RESERVED] Section 2. CONTROL SURFACES [RESERVED] Section 3. CONTROL SYSTEMS
34. GENERAL - § 25.671 .
a. Explanation. This material deals with the all-engines out case of § 25.671(d). The intent of this rule is to assure that in the event of failure of all engines, the airplane will be controllable and an approach and landing flare possible. This may be done by analysis where the method is considered reliable.
b. Procedures . The airplane should be evaluated to determine that:
(1) It is controllable following the failure of all engines in the climb, cruise, descent, approach, and holding configurations and can be flared to a landing attitude from a reasonable approach speed. The airplane must be controllable when all engines fail in each of the specified configurations and in any specific configuration that is to be selected and maintained following the failure of the engines in accordance with the AFM emergency operating procedures.
(2) The effectiveness of the emergency power to drive the airplane control system, whether generated from a windmilling engine or an auxiliary power supply, should be demonstrated in flight.
(3) For airplanes with fully powered or electronic flight control systems, the emergency procedures section of the approved Airplane Flight Manual should contain the appropriate operating procedures and a statement similar to the following:
"The airplane has a fully powered (or electronic) control system that is dependent upon engine windmill RPM, or an auxiliary power supply, to provide the necessary source of control system power in the event all engines fail in flight. A minimum airspeed of XXX knots IAS will provide adequate hydraulic or electrical power for airplane controllability in this emergency condition."
35. - 45. [RESERVED]
46. FLAP AND SLAT INTERCONNECTIONS - § 25.701 .
a. Explanation. In accordance with § 25.701(a), if the wing flaps are not mechanically interconnected, tests or analysis should be conducted to simulate appropriate flap malfunctioning during takeoffs, approaches, and landings to demonstrate that the airplane is safe under these conditions. This demonstration should not require exceptional pilot skills or strength.
b. Reference. See paragraph 20a(4) of this AC and Advisory Circular 25-14, "High Lift and Drag Devices," dated May 4, 1988, for additional guidance.
c. Procedures . As appropriate to the installation.
47. TAKEOFF WARNING SYSTEM - § 25.703 . [RESERVED]
Section 4. LANDING GEAR
48. - 51. [RESERVED]
52. RETRACTING MECHANISM - § 25.729 .
a. Explanation. None.
b. Procedures .
(1) In accordance with the provisions of § 25.729, flig ht tests should be conducted to demonstrate the ability of the landing gear and associated components, in their heaviest configuration, to properly extend and retract at:
(i) VLO (the placard airspeed) in the cruise configuration at near 1g flight and normal yaw angles;
and
(ii) airspeeds and flap settings corresponding to typical landings. The landing gear operating placard airspeed, VLO, established in accordance with § 25.1515(a), should not exceed the 1.6 V S1 design value of § 25.729(a)(1)(ii).
NOTE: “Normal” yaw angles are those associated with engine-out flight and counteracting crosswinds of up to 20 knots.
(2) The alternate extend system should be demonstrated at airspeeds up to V LO at near 1g flight and normal yaw angles. An envelope of emergency extension capability should be established and presented in the emergency operating procedures section of the AFM. (Refer to NOTE paragraph above.)
(3) Operation test.- § 25.729(d).
(i) The engine-out gear retraction time should be d etermined from flight tests with one engine at idle power and the operating engine(s) adjusted to provide the lowest thrust-to-weight ratio to be certificated. The hydraulic system should be in the critical configuration corresponding to an actual engine failure condition. The airplane should be stabilized on a steady heading before gear retraction is initiated. The resulting gear retraction time will be used in developing AFM takeoff flight path performance information in accordance with § 25.111.
(ii) Gear retraction time is the time from landing gear lever movement to the “UP” position until the last landing gear, including doors, is in the retracted configuration. Allowance should be made for any delays associated with the landing gear indication system.
(4) Position indication and aural warning, § 25.729(e): It should be confirmed that the actual landing gear position agrees with the position indicated on the landing gear indicator. Landing gear aural warning should meet the intent of §§ 25.729(e)(2) through (e)(4). A combination of flight tests, ground tests, and analysis may be used to show compliance with these requirements.
53. WHEELS - § 25.731.
a. References .
(1) Technical Standard Order (TSO) C26c, "Aircraft Wheels and Wheel-B rake Assemblies, with Addendum I," dated May 18,1984, for additional guidance.
(2) Paragraph 55b(4)vi of this AC, Wheel Fuse Plugs.
(3) Paragraph 55c(7) of this AC, Wheel Fuse Plug Design.
(4) Advisory Circular 21-29A, " Detecting and Reporting Su spected Unapproved Parts," dated July 16, 1992.
b. Explanation.
(1) Background .
(i) Original guidelines for wheels in § 4b.335 of the Civil Air Regulations (CAR) were superseded by TSO-C26 and subsequent revisions. Early versions of TSO-C26 referred to minimum standard requirements in versions of Society of Automotive Engineers (SAE) Aeronautical Standard (AS) 227. Minimum standards were subsequently specified in TSO-C26b and later revisions. For braked wheels, wheels and brakes must be approved as an assembly in recognition of design and safety interdependencies associated with thermal control, vibration control, structural stresses, etc. References (2) and 3) of paragraph a, above, provide insight into the criticality of proper fuse function to support airworthiness. As demands increased for longer life wheels and more robust designs, changes were introduced such as the addition of combined vertical and side loads for a portion of roll testing; an increase in roll miles at maximum static load from 1,000 to 2,000 miles; and addition of a roll-on-rim requirement to increase robustness in wheel flange areas.
(ii) In most cases, wheels are usually removed from service based upon condition. Typically, inspection frequency will increase as life on a particular wheel is accumulated in accordance with the wheel and brake suppliers Component Maintenance Manual (CMM). If wheel failures occur on the airplane, they typically occur when the airplane is on the ground with wheel and tire assembly loaded.
(iii) The trend in certain airplane manufacturer specifications is to place additional longevity and safety focused test requirements on wheel and brake suppliers. Added requirements, such as fail safe design verification(s), testing to failure of uncorroded and corroded wheels, and mandated use of overpressurization protection devices (in addition to fuses) have been incorporated. With the introduction of longer life tires, the demands on wheels and components, such as wheel bearings, have further intensified as landings between inspections at tire overhauls have increased. In addition, airplane manufacturers should be involved in wheel design, test, and manufacturer approvals to ensure that airplane specific needs have been addressed. For example, at least one airplane manufacturer specifies a missing wheel tie bolt requirement so that Minimum Equipment List (MEL) dispatch relief can be provided for a limited number of cycles. As a second example, individual airplane manufacturers may specify intensified wheel load and/or test requirements to account for various tire failure modes on multi-wheeled landing gear truck and other airplane landing gear configurations. Therefore, continued airworthiness of a particular wheel/tire or wheel/brake/tire assembly is usually demonstrated by compliance with airplane manufacturer requirements and not TSO minimum standards.
(iv) Separate wheel applicable TSO and ( if applicable) airplane manufacturer tests are performed with radial and bias tires due to different tire-to-wheel interface loading and resultant wheel stress pattern and deflection/clearance differences. For braked wheels, separate wheel and brake assembly tests are performed with radial and bias tires due to differences in tire energy absorption that may be encountered. Some significant differences in wheel life have been reported by one wheel and brake supplier in one case using bias tires from different tire manufacturers. Typically, however, wheel/tire, or wheel/brake/tire assembly tests using different manufacturer bias tires have not been shown to be necessary.
(v) Wheel bearings in transport category airplane wheels should be qualified as part of the wheel assembly. Industry experience indicates that qualification of a specific manufacturer's bearing(s) in a given wheel assembly is required to assure proper performance and airworthiness . Standard part bearing assemblies are not acceptable unless performance can be demonstrated during wheel qualification testing. In response to demands for more robust wheel bearing systems, due in part to longer life tires and increased numbers of landings between wheel and bearing inspections, at least one airplane manufacturer has required qualification testing of each wheel and specific wheel bearing supplier assembly to test requirements exceeding those in TSO-C26c and typical airplane manufacturer specifications. This has required suppliers of larger wheel and brake assemblies to develop special test capabilities for qualification of specific wheel bearing assembly systems (i.e.: bearing manufacturer, grease, seals and other means of grease retention). It has been reported that roller end scoring is the most common mechanism leading to bearing failures on the airplane.
(vi) Improved wheel beari ng grease and bearing preload retention means have also been introduced on some recent airplanes to increase wheel bearing longevity in the severe landing gear system environments and account for longer tire lives/ increased intervals between tire changes and inspections. Bearing wheel grease recommendations are usually specified in wheel and brake supplier CMMs. Intermixing of bearing cups/rollers/cones from different bearing manufacturers is not recommended on some large transport category airplane wheels due to different roller end scoring resistance capabilities and other often subtle differences.
c. Procedures/Method of Compliance . Due to the unique and critical nature of wheel, and wheel and brake designs, and historical airplane and personnel safety problems that have been experienced, compliance should only be approved upon successful completion of applicable TSO and tests any additional airplane manufacturer tests.
Guidance should also be solicited from the original wheel and brake supplier(s)/TSO holders on any replacement part(s) to assure that continued airworthiness is not degraded.
54. TIRES - § 25.733 [RESERVED]
55. BRAKES - § 25.735
a. References .
(1) Technical Standard Order (TSO)- C26c, "Aircraft Wheels and Wheel-Brake Assemblies , with Addendum I," dated May 18, 1984.
(2) Advisory Circular 21-29A, "Detecting and Reporting Suspected Unapproved Parts," dated July 16, 1992.
b. Explanation.
(1) Background .
* (i) The original objective of § 25.735 (formerly CAR 4b.337) evo lved from a study to define a reasonable brake life for operational landings. This element is still retained in the current § 25.735(f), which requires substantiation that the brakes have the ability to absorb the energy resulting from “operational landings at maximum landing weight,” and is directly related to the brake energy qualification tests of TSO-C26c. The two methods for compliance, stated in § 25.735(f), are also used in TSO -C26c for the determination of KE DL, the design landing kinetic energy rating for 100 stops at a deceleration rate of not less than 10 ft/sec 2. The initial wear state of the brakes for the testing used to determine KE DL may be selected by the applicant. Any condition representative of service use, including new, and that satisfies TSO -C26c (or acceptable equivalent), may be used. Section 25.735(f) makes reference to “operational landings” and “in the landing configuration,” which, in the context of modern transport airplane designs, could encompass a number of trailing edge flap positions and related landing speeds. The landing configuration for showing compliance with § 25.735(f) is left to the option of the applicant, but should be selected on the basis of its being the predominant landing configuration expected in operational service.
(ii) Over the years, as experience was gained in establishing takeoff and landing field lengths, it became evident that the refused takeoff was critical in determining overall brake capability and could dictate aircraft maximum gross weight for dispatch.
* (iii) Investigation of an RTO overrun accident, in which 80 percent of the brakes on the subject aircraft were at or very near their completely worn state, brought about the need to consider the effect of brake wear state on: 1) energy absorption capability, and 2) stopping capability. As a result, the FAA issued a series of specific airworthiness directives for theexisting fleet of transport category airplanes to establish brake wear limits such that the brakes would be capable of absorbing a maximum energy absorption RTO in the fully worn state. The FAA also initiated rulemaking to address energy absorption capability and stopping distance with fully worn brakes for future airplane types. The resulting rule, amendment 25-92, added:
(A) A requirement in § 25.735(h) for the maximum rejected takeoff kinetic energy capacity rating of the aircraft brakes to be determined with the brakes at 100 percent of the allowable wear limit;
(B) A new requirement in § 25.109( i) for the maximum kinetic energy rejected takeoff flight test demonstration to be conducted using brakes that have not more than 10 percent of their allowable wear range remaining; and (C) A general performance requirement under § 25.101(i) that requires the accelerate-stop and landing distances of §§ 25.109 and 25.125, respectively, to be determined with all wheel brake assemblies at the fully worn limit of their allowable wear range.
(2) Approval (§ 25.735(a)) . In accordance with § 21.305, a wheel and brake assembly may be approved under a Technical Standard Order (TSO) issued in accordance with Subpart O to 14 CFR part 21; in conjunction with the type certification procedures for the airplane; or in any other manner approved by the Administrator, all of which are normally subject to flight test evaluation. (See TSO-C26c for additional guidance.)
(3) Brake tests (§ 25.735(b)) . If the applicant desires to make the maximum possible use of the brakes in establishing the landing distance, and if the contribution of the brakes to the total deceleration is relatively large, the brake system should be designed to permit the application of slightly less than half the braking deceleration developed under the conditions specified in this paragraph. The following dual system is recommended: dual wheel elements (drums or disc units), transmitting elements, power sources, master cylinders, etc., connected to a single pedal on each rudder pedal, such that the failure of any single one of these would leave half the total braking capacity symmetrically disposed about the plane of symmetry of the airplane. With such a system, it should be possible to show compliance with § 25.735(b) by means of calculations based upon the test data necessary to establish the landing distance, plus the brake data calculated by the airplane manufacturer.
(4) If the system is designed so that , under conditions herein specified, appreciably less than half the total braking capacity remains, or if the remaining capacity is asymmetrically disposed, tests should be conducted to determine that half the mean deceleration may, in fact, be developed, and/or that the airplane may be safely controlled directionally while doing so. In order to obtain a minimum landing distance under § 25.125 and at the same time meet the deceleration requirements of § 25.735(b) in the event of failure of the normal brake system, it is common practice to provide an alternate brake system. When hydraulic (or pneumatic) brakes are used in the normal brake system, this alternate means usually consists of a duplicate hydraulic or pneumatic brake system and is commonly referred to as the "emergency brake system." The following items should be considered in the design of such systems:
(i) Relationship between normal and emergency brake systems . The systems for actuating the normal brake and the emergency brake should be separated so that a failure (i.e., hydraulic leakage of fluid) from one system will not render the other system inoperative. A hydraulic brake assembly may be common to both the normal and emergency brake systems if it is shown that the leakage of hydraulic fluid resulting from failure of the sealing elements in the brake assembly would not reduce the airplane's braking effectiveness below that specified in § 25.735(b).
(ii) Brake controls (§ 25.735(c)) . General brake control force and operation should be noted throughout the flight test program to determine that they are satisfactory.
(iii) Brake control valves . In the normal brake systems of all airplanes, the brake valves should be of a type such that the pilots may exercise variable control of the pressure to the brakes. The foregoing provision need not necessarily apply to the emergency brake system, although obviously such a provision would be desirable.
Flight tests should be conducted to determine that the normal and emergency brake systems fulfill the requirements of § 25.231.
(iv) Parking brake control (§ 25.735(d)) . A demonstration should be made to determine that sufficient braking is provided with the parking brake to prevent the airplane from rolling on a paved, dry, level runway (or any suitable level hard surface) while maximum takeoff power is applied on the most critical engine, with the airplane loaded to maximum ramp weight at aft center of gravity (or the airplane loaded to a weight- c.g.
combination that prevents the wheels from sliding). In the case of propeller-driven airplanes, the effects of propeller wash and engine/propeller torque should also be considered in determining the critical engine. Because the resultant thrust vector can be at an angle to a propeller's axis of rotation, one engine/propeller may be more critical than its counterpart on the opposite wing, particularly if all propellers turn in the same direction.
(v) Antiskid devices (§ 25.735(e)) .
(A) In addition to meeting acceptable industry specifications, the installation of the antiskid device should comply with the requirements specified in paragraphs (B) and (C) below. The antiskid device and its installation will be approved for use on civil airplanes when the tests specified in the procedures section (see paragraph 55c of this AC) have been satisfactorily demonstrated.
(B) Data required.
(1) An engineering evaluation of the antiskid installation as installed on the airplane, including all necessary components, should be conducted. This analysis and complete descriptive data should be submitted to the FAA. The data should include hydraulic and electrical schematic diagrams of the installation, assembly drawings of antiskid system units, functional hazard analyses, software and hardware substantiations for electronic components, test results or stress analysis substantiating structural strength of attachments and modification of the axle or other structural members, installation drawings, recommended instructions pertaining to installation, maintenance and operation, and analysis of flight test data and results. Schematic drawings should refer to all units in the normal and emergency brake systems. The engineering evaluation should also assure that the antiskid system does not cause undesirable yaw characteristics.
(2) The engineering evaluation should account for a bounce condition wherein the wheels may leave the runway after the brakes have been applied, for a condition wherein the wheels stay on the runway but the oleos are extended (if the system utilizes landing gear oleo compression in its operation), and for a condition in which the wheels of one main gear may not be in contact with the runway for a considerable time while the wheels of the other main gear are firmly on the runway. If the antiskid installation incorporates the "landing with brake pedals depressed" feature, then this type of operation should also be considered.
(3) It should be shown that the brake cycling frequency imposed by the antiskid installation will not result in excessive loads on the landing gear.
(4) The antiskid equipment should ensure satisfactory braking action on wet and snow/ice-covered runways, as well as on dry, hard-surfaced runways, without additional antiskid adjustments.
(C) Systems requirements . The entire brake system (including both the basic brake system and the antiskid system) should conform to § 25.735. The single failure criterion of § 25.735 should be extended to include the antiskid system. The following should be considered for approval of antiskid systems:
(1) In the event of a probable malfunction within the antiskid system that would result in loss of the antiskid feature on one or more brake units, those brake units affected should automatically revert to normal braking.
(2) A means should be provided so that the pilot or copilot can readily deactivate the antiskid system to prevent loss of all braking capability due to antiskid system malfunctions.
(3) A Failure Mode and Effects Analysis (FMEA) should be provided to evaluate the effect of failures of the antiskid and associated systems (tires, wheels, brakes, etc.) on airplane safety and braking performance. For antiskid systems utilizing "locked wheel protection" circuits, this analysis should include a determination that assures mechanical faults, which could permit the affected axle to continue rotation (e.g., blown tires, broken wheel rims), will not significantly reduce the braking effectiveness of any cross-coupled wheel positions with intact wheels and tires. A tire or wheel failure that results in the loss of additional tires or wheels, either from shrapnel impingement or ensuing overload, should be considered a single failure event.
(vi) Wheel Fuse Plugs .
(A) The hazardous condition of exploding tires and wheels associated with high energy emergency stopping conditions has been greatly alleviated by the installation of wheel fuse plugs. These plugs
relieve the tire pressure when the wheel temperatures approach a dangerous limit. The effectiveness of these plugs in preventing hazardous tire blowouts must be demonstrated during a refused takeoff (RTO) test where the brake energy to be absorbed exceeds the maximum landing energy, but not the RTO energy, and the fuse plug must release, thereby deflating the tire(s) before blowout.
(B) An improperly designed blowout plug that allows premature or unwanted rel ease of tire pressure during takeoff or landing could also constitute a hazardous condition. Such a situation would most probably arise during a takeoff from a quick turn-around type of airline operation. Fuse plug integrity should be demonstrated by conducting a maximum landing brake energy (which then becomes the quick turn-around chart limit for the AFM) test, which must not result in a fuse plug release.
(C) Most turbojet transport airplanes have been able to demonstrate wheel blowout plug integrity at a maximum energy level in accordance with the procedures outlined in this section. More restrictive operational limitations (e.g., runway slope and tailwind values) have been imposed to stay within this maximum energy level demonstrated for wheel blowout plug integrity. With the advent of requests to increase the maximum landing weights, and to eliminate these restrictive operational limits, it has been considered acceptable to remove the pertinent restrictions and operational limitations and substitute in their place a chase-around chart as a limitation in the FAA-approved Airplane Flight Manual (AFM). This chart will permit determining whether or not a critical energy level has been exceeded for the operating conditions of altitude, temperature, runway slope, tailwind, and landing weight. When the critical value is exceeded, a statement in the limitations section of the AFM will require that following a landing under such operating conditions, the airplane must remain on the ground a certain length of time prior to taxiing out for takeoff. This length of time will be the time to reach peak wheel temperatures (appropriate to the blowout plug location), plus 15 minutes. In lieu of the AFM fuse plug limitation chart, an alternate method for determining limit operational landing energy, such as a brake temperature limit, can be considered for approval.
* (D) The wear level of the brakes used for the chase-around chart discussed in the preceding paragraph should be selected by the applicant. Service experience indicates that the conservatism contained in the method of determining the turnaround time limits is adequate to allow these limits to be determined using new brakes* (E) In the case where it is possible to demonstrate wheel blowout pl ug integrity at a maximum energy level in accordance with the procedures in this section and without imposing certain restrictions on the operational limitations (e.g., runway slope and tailwind values), it is not considered necessary to incorporate the chase-around chart and pertinent statement in the limitations section of the AFM. Where restrictions are necessary, a pertinent statement and reference to the chase-around chart could be included in the limitations section of the AFM. The chart may be in Section 4, or in an appendix to the AFM.
(vii) Replacement and Modified Brakes .
(A) In order to establish aircraft landing and RTO certification performance levels for a replacement brake or a modified brake, measured accelerate-stop tests, and functional flight tests (landing distance), may be required, depending upon an evaluation of the individual merits of each brake system change. The type and magnitude of flight tests required will depend on whether or not a requested change involves a corresponding change of heat sink and/or torque requirements of the original certificated brake. A review of the change by the cognizant Aircraft Certification Office (ACO) for the type certificate holder is necessary, since original landing gear designs are based on structural analysis, which could be adversely affected by a brake system change. In addition, such tests will also depend on whether or not an increase in the FAA certificated performance level is desired by the applicant.
(B) Changes to the f riction couple elements (rotors and stators) are generally considered to be a major change, requiring the testing described in paragraph 55c(1), unless it can be shown that the change cannot affect the airplane stopping performance, brake energy absorption characteristics, or continued airworthiness.
Historically, continued airworthiness considerations include such items as landing gear system/airplane vibration control, braking feel, landing gear system compatibilities etc.
(C) Changes to a brake by a manufacturer other than the original TSO holder might be considered to be a minor change, as long as the changes are not to the friction couple elements, and the proposed
change(s) cannot affect the airplane stopping performance, brake energy absorption, vibration, and/or thermal control characteristics, and continued airworthiness of the airplane. In certain circumstances, the change to a steel rotor by a manufacturer other than the original TSO holder may be considered to be a minor change, as discussed in paragraph 55c(5)(ii).
c. Procedures . The extent of the flight test requirements, except new airplane certification, may vary depending upon an evaluation of the individual merits of each airplane brake system change, and whether or not an increase in the FAA certificated performance level is desired by the applicant. Past experience has proven that dynamometer tests alone are not considered adequate in determining compliance with this requirement. Flight test procedures for new, replacement, and modified brakes are categorized as follows:
(1) Basic new airplane certification -- complete new design where no airplane performance data exist.
(i) Tests required.
(A) For complete analysis, at least six rejected takeoffs and six landings wi ll normally be necessary.
(1) Six landings must be conducted on the same wheels, tires, and brakes.
(2) All tests should be conducted with engines trimmed to the high side of the normal idle range (if applicable).
* (3) For airplanes whose certification basis includes Amendment 25-92, § 25.101( i) requires the stopping distance portions of the accelerate-stop and landing distances to be determined with all the aircraft brake assemblies in the fully worn state. An acceptable means of compliance with this requirement is to accomplish the flight test braking tests with less than fully worn brakes, and then correct the test results using dynamometer test data determined with fully worn brakes. It should be substantiated that the dynamometer test methodology employed, and analytical modeling of the airplane/runway system, are representative of actual conditions. * (B) Additional tests may be necessary for each airplane configuration change (i.e., takeoff and/or landing flaps, nose wheel brakes, antiskid devices inoperative, deactivation of wheel brakes, etc.).
(C) Brake system response evaluation including braking during taxiing (see paragraph 30b(2)(i).
(D) Parking brake adequacy (see paragraph 55c(1)(iii)).
(E) Alternate braking system stops.
(F) Wheel fuse plug evaluation (see paragraph 55c(7)).
(G) Antiskid compatibility on a wet runway.
(H) Automatic gear retraction braking system on airplanes so equipped.
(ii) Maximum rejected takeoff (RTO) energy will be established by conducting an RTO at the maximum brake energy level for which the airplane will be certified. Fires on or around the landing gear are acceptable if the fires can be allowed to burn during the first 5 minutes after the airplane comes to a stop, before extinguishers are required to maintain the safety of the airplane. The condition of the tires, wheels, and brakes can be such that the airplane would require maintenance prior to removal from the runway. A deceleration rate must be maintained during this test that is consistent with the values used by performance scheduling. Tire or wheel explosions are not acceptable. Tire fuse plug releases may occur late in the RTO run, provided directional control is not compromised. The resulting distance (with fuse plugs blown during the RTO run) is to be included in the data used to establish AFM performance, only if it is longer than the data obtained with normal full braking configuration.
* (A) For airplanes whose certification basis include s Amendment 25-92, the maximum brake energy absorption level must be determined for an airplane with all wheel brake assemblies in the fully worn state.
In accordance with § 25.109( i), the flight test maximum energy RTO demonstration must be accomplished with all brake assemblies within 10 percent of their allowable wear limit (i.e., at least 90 percent worn). Dynamometer testing, when used to extend the flight test results to determine the maximum energy absorption capability of the brakes in the fully worn state (i.e., 100 percent worn), should be substantiated as being representative of actual airplane and runway conditions. The fully worn limit is defined as the amount of wear allowed before the brake must be removed from the airplane for overhaul. The allowable wear should be defined in terms of a linear dimension in the axial direction, which is typically determined by measuring the wear pin extension.
* (B) The maximum energy RTO demonst ration should be preceded by at least a three mile taxi, with at least three intermediate full stops, using normal braking and with all engines operating.
(C) Landings are not an acceptable means to conduct maximum energy RTO demonstrations.
Though permitted in the past, service experience has shown that methods utilized to predict brake and tire temperature increases that would have occurred during taxi and acceleration were not able to accurately account for the associated energy increments.
(iii) The ability of the parking brake to prevent the airplane from rolling should be demonstrated on a paved, dry, level runway (or any suitable level hard surface) with takeoff power applied on the critical engine using the following test procedure:
(A) The airplane should be loaded to its maximum takeoff weight (or the airplane loaded to a weight-c.g. combination that prevents the wheels from sliding) with the tires inflated to the normal pressure for that weight, the flaps should be retracted, the control surfaces centered, and the parking brake set.
(B) Apply takeoff power to the critical engine with the other engine(s) at idle.
(C) Compliance with the requirements of § 25.735(d) is shown if the wheels do not rotate;
this is best observed by painting a white radial stripe(s) on the wheels. The airplane may skip, tire tread may shear, or the tire may slip on the wheel, but the parking brake must prevent the wheels from rotating. Skidding of the tires is acceptable.
(2) Addition of New or Modified Brake Design .
(i) This item concerns the addition of a new or highly modified brake design to an existing type certificated airplane for which FAA-approved braking performance test data exists, either for performance credit, or to the existing performance level. A highly modified brake is defined as one that contains new or modified parts that may cause a significant variance in brake kinetic energy absorption characteristics, airplane stopping performance, or continued airworthiness of the airplane . Examples are: significant change in rotor and/or stator lining compound or area, number of stages, piston area, reduction in heat sink weight, changes in total number of friction faces and elements, change in brake geometry (friction, radius, friction area), fuse plug relocation or change in release temperature, heat shield changes that would affect the temperature profile of the wheel and/or fuse plugs, or seal changes.
(ii) Tests Required.
(A) For improved performance credit, all appli cable portions of paragraph 55c(1).
(B) For equivalent performance, a sufficient number of conditions to verify the existing approved performance levels (RTO and landing). Consideration should be given to verification of fuse plugs, performance verification at appropriate energy levels, and configuration differences, including antiskid on and off.
Taxi tests to ensure that ground handling, maneuvering, and brake sensitivity are satisfactory should be conducted.
At least two braking stops, one at heavy weight and one at light weight, should be conducted on a wet runway to verify brake and antiskid system compatibility.
(C) For extended performance, a sufficient number of conditions to define the extended line and determine equivalency to the existing performance levels. Consideration should be given to the items in paragraph (B), above.
(iii) Definitions.
(A) Improved performance implies an increase in the mu versus energy level for the desired operation(s) and may be requested for landing, RTO's, or a specific configuration such as antiskid "on" only.
(B) Equivalent performance implies that sufficient data will be obtained to verify that the performance level for the desired change is equal to or better than the existing performance levels. The change may be for the purpose of changing the c.g. envelope, or for airplane configuration changes (such as flap angles), and may apply to specific operations (such as landings).
(C) Extended performance implies that the existing certifi cation mu versus energy line will be extended to establish the braking force level for a proposed change, such as gross weight or the maximum desired energy level, and may be applied to a specific operation (such as landing only).
(3) Addition of new, o r changes to, antiskid systems that may affect airplane performance (e.g., new antiskid system, or a change from coupled to individual wheel control). A sufficient number of airplane performance tests and/or functional tests should be conducted to verify existing approved performance antiskid "on" levels. In the event an increase of braking performance is desired, full airplane performance testing is required.
(4) Fuse plug modification.
(i) Airplane tests for changes to the fuse plugs should be e valuated on a case-by-case basis. While airplane tests are required to establish the initial fuse-plug-no-melt energy, minor changes to fuse plugs or wheel designs may be validated by a back-to-back dynamometer comparison of old versus new designs, provided it is acceptable to the cognizant FAA ACO.
(ii) Airplane tests should be required when a significant change of wheel design and/or redesign or relocation of thermal or pressure fuse plugs is made.
(A) One airplane test should be conducted to sh ow that the fuse plugs will release when excessive energies are absorbed.
(B) Another airplane test is required to verify the maximum kinetic energy at which fuse plugs will not release (fuse plug substantiation). Dynamometer tests are not adequate for this test.
(5) Minor/Major Changes.
(i) Minor brake changes that do not affect airplane braking performance may require functional landings. This may be required to verify airplane-pilot-brake- antiskid combination compatibility. Normally, five non-instrumented, functional landings are considered sufficient to verify this compatibility. Examples of minor changes might include structural improvements (increased fatigue life), adjuster/retractor modifications, material and process specifications changes for structural components, and modified heat-sink relief slots (steel brakes).
Examples of other minor changes that do not require functional landings are paint/corrosion changes, changes to bleed ports or lube fittings, revised over inflation devices, metal repair, and salvage procedure. Such changes could be considered to be minor whether they are proposed by the original manufacturer who holds the TSO authorization, or by another manufacturer seeking to produce replacement parts.
(ii) Changes to heat sink friction couple elements are to be considered major changes, unless the applicant can provide evidence that changes are minor. Based upon experience, thicker friction material or heavier heatsink elements are usually acceptable as minor changes in steel brakes. Thicker or heavier heat sink elements in carbon brakes may require additional laboratory and/or airplane testing to assess brake performances and continued airworthiness. Major changes are subject to extensive airplane testing, unless it can be shown that the change cannot affect the airplane stopping performance, brake energy absorption characteristics, and continued airworthiness. In this regard, the original manufacturer of the wheel/brake assembly who holds the TSO authorization, and the Type
Certificate holder who is knowledgeable with respect to such items as landing gear design assumptions and airplane braking system history, may possess data sufficient to show that such changes could be considered to be minor (i.e., performance would not be affected). In contrast, an applicant other than the original manufacturer who wishes to produce replacement rotors or stators may not have access to or have established developmental or other test data required to show that performance, braking energy capacities, braking system compatibilities, or overall continued airworthiness safeguards have been addressed. Due to the complex nature of the friction surfaces and airplane braking system interfaces, proposed replacement stators/rotors by an applicant other than the original manufacturer(s) should always be considered a major change.
(iii) It is considered very difficult to determine 100 percent identicality. This is particularly true for brake friction rubbing components (e.g., linings in cups, linings sintered to plates, steels used in steel brakes, and carbon discs in carbon brakes). A finding of equivalence based upon physical documentation and dynamometer testing may not be possible or practical due to friction material complexities and/or the extent of dynamometer testing required.
(iv) Due to the complexities associated with aircraft brake friction couples, industry and authorities have generally discouraged mixing of friction components from various suppliers within the same brake or mixing wheel and brake assemblies from different manufacturers on the same airplane. Typically, the manufacturers of large transport category airplanes have confined the use of specific wheel and brake suppliers assemblies to specific airplanes through approved equipment lists. While multiple wheel and brake suppliers (i.e., multiple original TSO holders) are often selected to provide wheel and brake assemblies on a specific airplane model, intermixing of wheel and brake assemblies has been discouraged to avoid potential problems such as unequal energy sharing; unfavorable dynamic cross coupling between brakes, landing gear, and the airplane; degradation of vibration and/or thermal controls; unique brake control system tuning requirements for each wheel and brake assembly, etc.
(v) The FAA has, however, approved the use of replacement steel rotors. The following protocol for steel rotor equivalency findings has been updated to include brake wear and wear pattern assessments to assure that the worn brake capability of the original manufacturer's wheel and brake assembly is not degraded by a replacement steel rotor(s). The following criteria and evaluations represent protocol for replacement steel rotors to be considered as a minor change:
(A) a very close correlation between the original part and the proposed replacement part;
(B) considerable and satisfactory prior manufacturing and in-service experience with a similar replacement part;
(C) a reasonable plan of test for completion of the dynamometer portion of the test program;
(D) successful completion of the dynamometer testing; and (E) as a minimum, successful completion of a series of functional landings on the airplane.
(vi) The necessity of conductin g maximum energy RTO testing and other brake system tests on the airplane will depend upon the outcome of the above evaluations and worn brake RTO airplane test experiences (if applicable).
(vii) If intermixing of replacement steel rotors with the orig inal manufacturer’s steel rotors is proposed, the applicant must propose an airplane test evaluation plan to the FAA that provides data to guide a worn brake rejected-take-off equivalency assessment. If the applicant cannot provide evidence on similar overall wear and wear patterns from the new to worn condition for the proposed mixing configuration(s), intermixing will not be permitted in order to assure that the worn brake rejected-take-off rating and approved wear limit of the original TSO holders wheel and brake assembly is not jeopardized.
(viii) The dynamometer test plan, and, if applicable, the overall wear and wear pattern test plan, must include:
(A) TSO Minimum Standard Performance Demonstration. The wheel and brake assembly containing the applicant's proposed steel rotor configuration must successfully demonstrate compliance with the braking and structural tests of the applicable TSO, and (B) Compliance with:
(1) the airplane-manufacturer-specified requirements, or
(2) the alternate procedures specified below:
(ix) Alternate Procedures (steel rotors only).
(A) Energy and Torque Capacity Tests. A series of tests (not only one) may be necessary to demonstrate, in back-to-back tests, that brake energy absorption and torque and pressure vs. time profiles are equivalent. All friction components and structures should be in the new condition to obtain credit for this test. If rebuilt or in-service components other than these fail during testing, it should be realized that the results of the test(s) may be questionable. Suspect tests will be carefully reviewed by the FAA, and may require retesting. Prior to test, the applicant should carefully document wheel hardness and wheel drive, torque tube spline, piston/bushing assembly conditions to assure comparable test articles are being used. The same tire size and ply rating, manufacturer, tire condition, radial load, and rolling radius should be used in each test. Test machines and test conditions must be consistent from test to test, including test brake, wheel and tire break-in stop histories, brake pressure onset rates ( psig/sec), maximum pressures, initial brakes-on-speeds, flywheel inertias, etc., to assure consistent test control. Artificial cooling is not permitted during or subsequent to the test until wheel and piston housing temperatures have peaked.
(B) The initial kinetic energy level for this series of tests will be at the discretion of the applicant. For each succeeding run, the KE will be increased by approximately 5 percent over the previous run, until the ultimate KE level is determined (i.e., points at which pistons are about to exit bores or flywheel deceleration falls below 3 ft/sec 2 (½ of the TSO RTO minimum average deceleration requirement)). The deceleration reported by the applicant should be based upon distance (and not time) in accordance with the following formula:
Distance Averaged Deceleration = ((Initial brakes-on speed) 2 - (Final brakes-on-speed) 2 )/2(braked flywheel distance).
(1) A minimum of two runs at this ultimate energy level must be conducted on the original manufacturer's wheel and brake assembly for baselines. These test runs must show similar results.
Maximum braking force pressure must be applied during the tests. Fuse plug releases in any tests must demonstrate safe release of approved nitrogen-air mixtures.
(2) A minimum of two test runs at the ultimate energy level must then be conducted on the applicant's proposed wheel and brake assembly (i.e., a back-to-back demonstration of the two manufacturers brakes). Tests must show similar brake energy absorption, torque, and thermal performance capabilities, and torque and pressure versus time profiles, while demonstrating sealing and structural integrity comparable to the original manufacturer's wheel and brake assembly.
(C) Worn Brake RTO Capability. Worn brake RTO capability for the proposed wheel and brake assembly with steel rotor replacement configuration(s), and at the wear pin limit(s) proposed by the applicant, must be established during dynamometer test(s). The test brake energy absorption criteria, torque performance , and pass/fail requirements should be requested of the airplane manufacturer to provide supporting evidence that the worn brake RTO capability is equivalent to that achieved with the original TSO holder’s wheel and brake assembly( ies).
If unavailable, the applicant should propose a worn brake RTO test plan similar to that in paragraph 55c(1) for new brakes. The applicant should also propose to the FAA the method which will be followed by the applicant to verify that the worn brake RTO capability of in-service worn brakes with replacement rotors is equivalent to the capability established in initial dynamometer test(s) in accordance with paragraph 55(c)(1)(ii)(A) of this AC.
(D) Intermixing of steel rotor assemblies produced by two manufacturers will not be allowed until it can be demonstrated that wear patterns of the intermixed assembly( ies), through a determined number of in-