CHAPTER 12. DESIGN AND CONSTRUCTION: GENERAL [RESERVED] 12-1
CHAPTER 13. DESIGN AND CONSTRUCTION: CONTROL SURFACES [RESERVED] 13-1
CHAPTER 14. DESIGN AND CONSTRUCTION: CONTROL SYSTEMS
14.1 General—§ 25.671.
14.1.1 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.
14.1.2 Procedures.
14.1.2.1 In accordance with § 25.671(d), the airplane must be controllable when all engines fail. Compliance should be shown for each approved configuration. The airplane should remain controllable following the failure of the engines. Reconfiguration, if possible with all engines failed, is permitted. Any such reconfiguration should be included in the AFM operating procedures for an all engines failed condition.
14.1.2.2 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.
14.1.2.3 Past approaches to showing compliance with § 25.671(d) have been to show that the airplane 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.
14.1.2.4 For airplanes with fully powered or electronic flight control systems, the non-normal procedures section of the approved AFM should contain 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.”
14.2 Flap and Slat Interconnections—§ 25.701.
14.2.1 Explanation.
In accordance with § 25.701(a), if the wing flaps are not mechanically interconnected, tests or analysis should be conducted to show that the airplane has safe flight characteristics with asymmetric flap or slat deployment.
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14.2.2 Reference.
See AC 25-22 for additional guidance.
14.2.3 Procedures.
Simulate appropriate flap and slat malfunctioning during takeoffs, approaches, and landings to demonstrate that the airplane is safe under these conditions. To be considered safe, adequate stall margins and controllability should be retained without requiring exceptional piloting skill or strength. Additionally, there should be no hazardous change in altitude or attitude during transition to the asymmetric condition considering likely transition rates.
14.3 Takeoff Warning System—§ 25.703. [Reserved] 14-2
CHAPTER 15. DESIGN AND CONSTRUCTION: LANDING GEAR
15.1 Retracting Mechanism—§ 25.729.
15.1.1 Explanation.
None.
15.1.2 Procedures.
15.1.2.1 In accordance with the provisions of § 25.729, flight 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:
15.1.2.1.1 VLO (the placard airspeed) in the cruise configuration at near 1 g flight and normal yaw angles; and
15.1.2.1.2 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 VS1 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.
15.1.2.2 The alternate extend system should be demonstrated at airspeeds up to VLO at near 1 g 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 the note under paragraph 15.1.2.1.2 above.)
15.1.2.3 Operation Test—§ 25.729(d).
15.1.2.3.1 The engine-out gear retraction time should be determined from flight tests with one engine at idle power or thrust 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.
15.1.2.3.2 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.
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15.1.2.4 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.
15.2 Wheels—§ 25.731.
15.2.1 References.
TSO-C135a, Transport Airplane Wheels and Wheel and Brake Assemblies, dated July 1, 2009.
Paragraph 15.4.2.5 of this AC, Wheel Fuse Plugs.
Paragraph 15.4.10 of this AC, Wheel Fuse Plug Integrity.
AC 21-29D.
15.2.2 Explanation.
15.2.2.1 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 under § 25.735(a) as an assembly in recognition of design and safety interdependencies associated with thermal control, vibration control, structural stresses, etc. Paragraphs 15.4.2.5 and 15.4.10 of this AC 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 rollon-rim requirement to increase robustness in wheel flange areas.
15.2.2.2 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 supplier’s 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.
15.2.2.3 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 15-2
verification(s), testing to failure of uncorroded and corroded wheels, and mandated use of over pressurization 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.
15.2.2.4 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.
15.2.2.5 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. It has been reported that roller end scoring is the most common mechanism leading to bearing failures on the airplane.
15.2.2.6 Improved wheel bearing 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 15-3
transport category airplane wheels due to different roller end scoring resistance capabilities and other often subtle differences.
15.2.3 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 all applicable tests.
We recommend guidance be solicited from the original wheel and brake supplier(s)
(e.g., technical standard order authorization (TSOA) or letter of design approval (LODA) holder) on any replacement part(s) to assure that continued airworthiness is not degraded.
15.3 Tires—§ 25.733.
[Reserved]
15.4 Brakes—§ 25.735.
15.4.1 References.
TSO-C135a, Transport Airplane Wheels and Wheel and Brake Assemblies, dated July 1, 2009.
AC 25-22.
AC 25.735-1.
15.4.2 Explanation.
15.4.2.1 Background.
15.4.2.1.1 The original objective of § 25.735 (formerly § 4b.337) developed from a study to define a reasonable brake life for operational landings. This element is still retained in current § 25.735(f)(1), which requires substantiation that the wheel, brake, and tire assembly have the ability to absorb the energy resulting from an operational landing stop at maximum landing weight. Compliance with this requirement is shown through dynamometer testing.
15.4.2.1.2 It later became evident that a rejected takeoff could be critical in determining overall brake capability, and the maximum RTO energy absorption capability by the brakes could limit the aircraft maximum allowable gross weight for dispatch.
15.4.2.1.3 Investigation of an RTO overrun accident, in which 80 percent of the brakes on the subject airplane were at or very near their completely worn state, brought about the need to consider the effect of brake wear state on energy absorption capability, and stopping capability. As a result, the FAA 15-4
issued a series of specific airworthiness directives for the existing 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 brake for future airplane types. The resulting rule, amendment 25-92, added:
1. A requirement 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;
2. A new requirement 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
3. 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.
15.4.2.1.4 ACs 25-22 and 25.735-1 provide guidance and policy on how to show compliance with § 25.735. Additional information is provided in the following paragraphs pertaining to flight test evaluations performed in connection with showing compliance.
15.4.2.2 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.
15.4.2.3 Brake Control Valves.
The brake valves in the normal brake system should allow the pilots to modulate pressure to the brakes. The foregoing provision need not necessarily apply to an alternate or emergency brake system, although obviously such a provision would be desirable. Flight tests should be conducted to determine that the normal and alternate/emergency brake systems fulfill the requirements of § 25.231.
15.4.2.4 Parking Brake—§ 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 or thrust is applied on the most critical engine and up to maximum ground idle power or thrust is applied to all other engines. The airplane should be loaded to the maximum ramp weight at aft CG (or a weight-CG 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 15-5
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.
15.4.2.5 Wheel Fuse Plugs.
15.4.2.5.1 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 should be demonstrated during an RTO test where the brake energy to be absorbed exceeds the maximum landing energy, but not the RTO energy, and the fuse plug releases, thereby deflating the tire(s) before blowout.
15.4.2.5.2 An improperly designed blowout plug that allows premature or unwanted release 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 test, which should not result in a fuse plug release.
15.4.2.5.3 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 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 should be placed in the limitations section of the AFM to 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.
15.4.2.5.4 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 15-6
determining the turnaround time limits is adequate to allow these limits to be determined using new brakes.
15.4.2.5.5 In the case where it is possible to demonstrate wheel blowout plug 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.
15.4.2.6 Replacement and Modified Brakes.
15.4.2.6.1 In order to establish aircraft landing and RTO certification performance levels for a replacement brake or a modified brake, measured acceleratestop 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 improvement in the FAA certificated performance is desired by the applicant.
15.4.2.6.2 Changes to the friction couple elements (rotors and stators) are generally considered to be a major change, requiring the testing described in paragraph 15.4.4, 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.
15.4.2.6.3 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 15.4.8.2.
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15.4.3 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 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:
15.4.4 New Airplane Certification.
This is a completely new design where no airplane performance data exist.
15.4.4.1 Required Tests.
15.4.4.1.1 For complete analysis, at least six rejected takeoffs and six landings will normally be necessary.
1. Landings should 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.
15.4.4.1.2 Additional tests may be necessary for each airplane configuration change
(i.e., takeoff and/or landing flaps, nose wheel brakes, anti-skid devices inoperative, deactivation of wheel brakes, etc.).
15.4.4.1.3 Brake system response evaluation including braking during taxiing (see paragraph 9.2.2.1.)
15.4.4.1.4 Parking brake adequacy. (See paragraph 15.4.4.3.)
15.4.4.1.5 Alternate braking system stops.
15.4.4.1.6 Wheel fuse plug evaluation. (See paragraph 15.4.10.)
15.4.4.1.7 Anti-skid compatibility on a wet runway.
15.4.4.1.8 Automatic gear retraction braking system on airplanes so equipped.
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