d. P olicy/Compliance Methods. For policy and guidance on compliance with this requirement, refer to Advisory Circular (AC) 25-14, High Lift and Drag Devices, incorporated into this AC in § 25.699 paragraph d.
e. Reference. The address for ordering the latest revision of the advisory circular listed below can be found in the Appendix to this AC.
AC 25.703-1, Takeoff Configuration Warning Systems.
6-10. [RESERVED] 9
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Section 2. LANDING GEAR
11. SECTION 25.729 - RETRACTING MECHANISM.
a. Rule Text.
(a) General. For airplanes with retractable landing gear, the following apply:
(1) The landing gear retracting mechanism, wheel well doors, and supporting structure, must be designed for-
(i) The loads occurring in the flight conditions when the gear is in the retracted position,
(ii) The combination of friction loads, inertia loads, brake torque loads, air loads, and gyroscopic loads resulting from the wheels rotating at a peripheral speed equal to 1.3 VS (with the flaps in takeoff position at design takeoff weight), occurring during retraction and extension at any airspeed up to 1.6 VS1 (with the flaps in the approach position at design landing weight), and
(iii) Any load factor up to those specified in § 25.345(a) for the flaps extended condition.
(2) Unless there are other means to decelerate the airplane in flight at this speed, the landing gear, the retracting mechanism, and the airplane structure (including wheel well doors) must be designed to withstand the flight loads occurring with the landing gear in the extended position at any speed up to 0.67 VC .
(3) Landing gear doors, their operating mechanism, and their supporting structures must be designed for the yawing maneuvers prescribed for the airplane in addition to the conditions of airspeed and load factor prescribed in paragraphs
(a)(1) and (2) of this section.
(b) Landing gear lock. There must be positive means to keep the landing gear extended, in flight and on the ground.
(c) Emergency operation. There must be an emergency means for extending the landing gear in the event of-
(1) Any reasonably probable failure in the normal retraction system; or
(2) The failure of any single source of hydraulic, electric, or equivalent energy supply.
(d) Operation test. The proper functioning of the retracting mechanism must be shown by operation tests.
(e) Position indicator and warning device. If a retractable landing gear is used, there must be a landing gear position indicator (as well as necessary switches to actuate the indicator) or other means to inform the pilot that the gear is secured 11
in the extended (or retracted) position. This means must be designed as follows:
(1) If switches are used, they must be located and coupled to the landing gear mechanical systems in a manner that prevents an erroneous indication of ''down and locked'' if the landing gear is not in a fully extended position, or of ''up and locked'' if the landing gear is not in the fully retracted position. The switches may be located where they are operated by the actual landing gear locking latch or device.
(2) The flightcrew must be given an aural warning that functions continuously, or is periodically repeated, if a landing is attempted when the landing gear is not locked down.
(3) The warning must be given in sufficient time to allow the landing gear to be locked down or a go-around to be made.
(4) There must not be a manual shutoff means readily available to the flightcrew for the warning required by paragraph (e)(2) of this section such that it could be operated instinctively, inadvertently, or by habitual reflexive action.
(5) The system used to generate the aural warning must be designed to eliminate false or inappropriate alerts.
(6) Failures of systems used to inhibit the landing gear aural warning, that would prevent the warning system from operating, must be improbable.
(f) Protection of equipment in wheel wells. Equipment that is essential to safe operation of the airplane and that is located in wheel wells must be protected from the damaging effects of-
(1) A bursting tire, unless it is shown that a tire cannot burst from overheat; and
(2) A loose tire tread, unless it is shown that a loose tire tread cannot cause damage.
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as amended by Amdt. 25-23, 35 FR 5676, Apr. 8, 1970; Amdt. 25-42, 43 FR 2323, Jan. 16, 1978; Amdt. 25-72, 55 FR 29777, Jul. 20, 1990; Amdt. 25-75, 56 FR 63762, Dec. 5, 1991]
b. Intent of Rule. This rule provides minimum design and certification requirements for landing gear actuation systems to address:
(1) Structural integrity for the nose and main landing gear, retracting mechanism(s), doors, gear supporting structure for actuation loads, maneuvering loads, and yawing flight condition loads.
(2) Emergency means to extend gear under certain failure conditions.
(3) Downlock and uplock design.
(4) Gear up-and-locked and down-and-locked position indications and aural warning.
(5) Protection of essential equipment located in the wheel well from a bursting tire or loose tire tread.
(6) Function demonstration.
c. Background. Effective February 1, 1965, part 25 was added to Title 14, Code of Federal Regulations (14 CFR) to replace part 4b of the Civil Air Regulations (CAR). Sections 4b.334 and 4b.334-2 of the CAR, became § 25.729 of 14 CFR for landing gear retracting mechanism.
(1) Amendment 25-23 (April 8, 1970) added a wheel rotational speed based on a factored takeoff speed of 1.3 Vs to be used for load computations under § 25.729(a)(1)(ii) and changed the reference from § 25.345 to § 25.345(a) under § 25.729(a)(1)(iii).
(2) Amendment 25-42 (January 16, 1978) clarified the rule and made minor editorial changes to § 25.729(e)(3).
(3) Amendment 25-72 (July 20, 1990) amended the rule. It made editorial changes and deleted reference to § 25.67(e) under § 25.729(e)(4), since § 25.67 no longer existed.
(4) Amendment 25-75 (December 5, 1991) revised § 25.729(e)(2) through (e)(6) to state objectives without stating how the requirements were to be met; thus allowing manufacturers to use their ingenuity in designing systems to minimize the occurrence of nuisance and inappropriate aural warnings.
d. Policy/Compliance Methods. Guidance addressing flight testing used to demonstrate compliance with this section may be found in Advisory Circular (AC) 25-7A, Flight Test Guide for Transport Category Airplanes, chapter 4, section 4, paragraph 52, issued March 31, 1998.
Also see AC 25.963-1, Fuel Tank Access Covers, section 4, for compliance to § 25.729(f)(2)
(1) Protection of Equipment in wheel wells, § 25.729(f)(1). The following is extracted from an FAA memorandum dated December 4, 1997, which addresses whether wheel/tire assemblies containing thermal fuse plugs are sufficient to comply with the requirements of § 25.729(f)(1).
(a) The intent of the regulation is to protect essential equipment from the effects of a tire burst, regardless of the cause of the burst; overheat is just one way of causing a tire burst.
Other ways a tire burst may be caused are:
1 foreign object damage.
2 under-inflation.
3 over-inflation.
4 overload.
5 an abnormal wheel component such as a melted/defective fuse plug.
Additionally, compliance with § 25.1309(b) is required for a continued safe flight and landing.
A tire burst must be considered regardless of its probability of occurrence.
(b) As cited in the preamble to Amendment 25-78: “A tire burst, as referred to in § 25.729(f), is a sudden, sometimes violent, venting of the pressure from within a tire, usually associated with a flaw in the tire, foreign object damage, or tire overheat/overload. The FAA assumes that tire bursts will occasionally occur, given the severe operating environment of airplane tires, and the fact that certain tire damage may go undetected until tire failure. With this in mind, equipment installed in wheel wells is evaluated at the time of certification to determine its ability to withstand the effects of a bursting tire. Analyses and laboratory tests are performed to identify critical areas, and design changes are often made to ensure that a single tire burst will not cause loss of critical functions.”
(2) Landing Gear Position Indication System - "Backup Requirement" (Section
25.729(e)). The following is extracted from an FAA memorandum dated July 12, 1988, which addresses whether a backup gear position system is always required.
(a) The failure of the landing gear position indicating system does not prevent the continued safe flight and landing of the airplane. Further, service history has shown that gear-up landings on transport category airplanes are not catastrophic. Compliance with the requirements of § 25.729 may be accomplished by a straightforward engineering assessment of the design. A numerical probability analysis is not required.
(b) If a review of the certification basis of the airplane reveals that a second method of landing gear position indication (e.g., a viewing port) was required due to the characteristics of the design, a replacement for the now-unusable viewing ports would have to be provided. The replacement system could be of any type (electrical, mechanical, etc.), as long as it provided a back-up with reliability and functionality not less than that of the system approved under the original type certification basis.
(c) If the back-up indication system was installed at the manufacturer's option, and was not required by the type certification basis, removing the system now could result in crew confusion or other operational problems. Although it is not recommended to remove an optional backup system (to avoid confusion in the cockpit), it would be acceptable provided the airplane continues to meet the original type certification basis requirements. Replacing the viewing ports with a system that would be similar in function to the original is encouraged, i.e., a redundant 14
means of determining that the landing gear is indeed down if there is any uncertainty from using the primary system. The determination of the reliability of the system need not be as rigorous as in (b), above, as long as the replacement system can be shown to perform its intended function.
(d) Service difficulty reports relating to the landing gear indication system should be reviewed, and if the primary electrical system has required the use of the back-up viewing system to an inordinate degree, the back-up system would then become more important to the overall reliability of the indication system, and would be considered to be necessary under the provisions of § 21.21(b)(2).
(3) Flap System/Landing Gear Warning System Tie-In. The following is extracted from an FAA memorandum dated December 19, 1983. The memo addressed whether it was an acceptable design to provide two approach flap settings and two landing flap positions without including the “landing gear-up” warning required by § 25.729(e)(4), redesignated as § 25.729(e)(2), effective at Amendment level 25-72.
(a) The FAA originally determined that providing no warning in this configuration was unacceptable. However, it was noted that there were transport category aircraft already certificated with approved alternate "selectable" means of changing the gear-up warning onset for alternate approach/landing flap configurations and that "uniformity" of certification warranted that this concept be approved if the proposed alternate means was judged acceptable by the cognizant Aircraft Certification Office. The concept was the use of a guarded, manually operated selection switch (location not specified) and the associated proposed Airplane Flight Manual procedures for the use of this system be provided.
(b) FAA flight test personnel evaluated this proposed alternate design and found it acceptable. Based on these events, it was determined that the proposed installation is acceptable, meets the intent of § 25.729(e)(4), and provides an acceptable equivalent level of safety.
(4) Landing Gear Position Indication System. The following policy is extracted from an FAA memorandum dated June 3, 1983, which addresses whether other regulations need to be considered when finding compliance with § 25.729(e) (e.g., §§ 25.1301 and 25.1309).
(a) Section 25.729(e) provides standards for landing gear position indicating systems. This section is necessary, but not sufficient, for certification of these systems. This section only establishes the requirement for such a system; however, it is necessary to consider the requirements of § 25.1301 (that requires the system to be of a kind and design appropriate to its intended function and function properly when installed), and §§ 25.1309(a) and 25.1309(b) (that require failures which reduce the capability of the aircraft or crew to cope with adverse operating conditions to be "improbable" and those that prevent the continued safe flight and landing of the airplane to be "extremely improbable"). In summary, all sections of part 25 contained in the certification basis should be reviewed for applicability.
(b) In addition, failures that result in presenting hazardously misleading indication to the crew must be improbable. An example of a hazardously misleading failure condition 15
would be if the gear indicating portions of the landing gear system indicate that all gear are down/locked when in actuality, all gear are not fully down and locked.
(c) If the failure evaluation process of the landing gear system (including the gear position indication) shows that a gear-up landing will have catastrophic consequences, then the occurrence of this condition should be shown to be extremely improbable. However, if the evaluation shows that a gear up landing would only result in a hazardous condition, then the occurrences of this condition would fall in the improbable category.
(d) Reviews of transport category accident/incident reports from the FAA files covering the five years prior to this memorandum have not listed a catastrophic result due to failures in the landing gear and/or indicating system. Further, providing a second independent landing gear position indicating system may not improve the overall safety of' the aircraft and may, in some instances, lead to a more confused condition on the part of the crew in trying to sort out which indicating system should be used.
(5) Landing Gear Slush Tests. The following policy is extracted from an FAA memorandum dated April 22, 1983. This memo addresses the need for slush tests to ensure that the landing gear can be extended for an aircraft with a single hydraulic system and no auxiliary power source.
(a) There is no specific certification requirement for slush tests to evaluate landing gear retraction/extension systems during wet, freezing conditions. Service history indicates this can be a problem, even on large transport category airplanes with multiple hydraulic systems.
The FAA has not made such tests a certification requirement because there is no practical means of preventing accumulation of slush in the wheel well area, short of prohibiting operation during slushy conditions. Slush tests on a new airplane maintaining original design tolerances and properly lubricated joints would be of questionable value since the problem is often associated with poor maintenance practices.
(b) In several reported instances of failure of the landing gear to extend, even with hydraulic pressure applied, the problems were usually corrected by lubrication of rotating joints.
During slushy runway operation, water can enter rotating joints and freeze after cold soaking at altitude. The problem is usually reported during cold winter months and during low surface temperatures.
(c) The design should be evaluated for possible accumulation of ice on the uplock mechanism and rotating joints, which could prevent manual operation of the emergency extension system. If, after careful review of a particular landing gear design, it appears that slush testing is necessary, such tests could be required under §§ 21.21(b)(2) and 25.729(d). If tests are deemed necessary, the tests should be conducted with little or no lubrication in all rotating joints.
e. References. The addresses for ordering the latest revision of advisory circulars and other referenced documents listed below can be found in the Appendix to this AC.
AC 20-34D, Prevention of Retractable Landing Gear Failures.
AC 23.729-1, Landing Gear Doors and Retraction Mechanism. (For information only).
AC 25-7A, Flight Test Guide for Transport Category Airplanes.
AC 43.13-1A, Acceptable Methods, Techniques and Practices - Aircraft Inspection and Repair.
AC 25.963-1, Fuel Tank Access Covers.
SAE AIR-4566 - Crashworthiness Landing Gear Design.
SAE ARP-1311A - Landing Gear- Aircraft.
f. Definitions. For definitions of VS, VS1, and VC, see 14 CFR part 1, section 1.2, titled Abbreviations and symbols.
12. SECTION 25.731 - WHEELS.
a. Rule Text.
(a) Each main and nose wheel must be approved.
(b) The maximum static load rating of each wheel may not be less than the corresponding static ground reaction with-
(1) Design maximum weight; and
(2) Critical center of gravity.
(c) The maximum limit load rating of each wheel must equal or exceed the maximum radial limit load determined under the applicable ground load requirements of this part.
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as amended by Amdt. 25-72, 55 FR 29777, Jul. 20, 1990]
b. Intent of Rule. This rule requires use of approved wheels, either approved under an applicable Technical Standard Order (TSO) e.g., TSO-C26c, or approved as part of the type design for the airplane. Wheels must satisfy both a design static (1g) load and design limit landing or taxiing load determined under the applicable ground load requirements (§§ 25.471 through 25.511). Standards for a tire installed on a wheel are contained in § 25.733. Standards for a brake installed on a wheel are contained in § 25.735. A TSO approval is not an approval to install wheels on the airplane. If the airframe manufacturer decides to install equipment approved under a TSO, they must conduct the applicable airplane certification tests and obtain FAA approval for the installation.
c. Background. Effective February 1, 1965, part 25 was added to Title 14, Code of Federal Regulations (14 CFR) to replace part 4b of the Civil Air Regulations (CAR). Sections 4b.335(a) and 4b.335(b) of the CAR became §§ 25.731(a) and 25.335(b) of 14 CFR for wheels.
(1) Amendment 25-72 (July 20, 1990) amended the rule to become compatible with § 25.25 (which had been amended). This amendment provides for weights that are in excess of takeoff weight, such as ramp weights, provided that the compliance with applicable structural requirements, including wheel strength, is demonstrated at the higher weights.
(2) Harmonization. This regulation is the subject of a Federal Aviation Regulation/Joint Aviation Requirements (FAR/JAR) harmonization effort under the Aviation Rulemaking Advisory Committee (ARAC). The ARAC working group is revising §§ 25.731 and 25.735, and is developing a new advisory circular AC 25.735-1, and a new TSO-C135 for transport category airplanes, replacing the applicable parts of the existing TSO-C26c.
d. Policy/Compliance Methods. For guidance on compliance, the following material is extracted from Advisory Circulars (AC) 27-1, Certification of Normal Category Rotorcraft, and AC 29-2B, Certification of Transport Category Rotorcraft. It is applicable to certification of transport category airplanes. While not specifically prepared for transport category airplanes, this policy material has been used for demonstrating compliance with § 25.731. For additional guidance, see AC 25-7A, Flight Test Guide for Transport Category Airplanes, chapter 4, section 4, par 53, issued March 31, 1998.
(1) The structural design loads data shall contain both a static load and a landing and taxiing load for each wheel. These loads are determined by virtue of compliance with the standards of §§ 25.731(b) and 25.731(c). The ratings of the wheel shall not be exceeded.
TSO-C26c contains minimum performance standards for TSO approval of aircraft wheels and wheel-brake assemblies. Ratings are assigned in accordance with this performance standard.
(2) If a wheel selected for an aircraft design has TSO-C26c approval, the wheel manufacturer will supply the rating to the aircraft manufacturer. Each wheel shall be marked as prescribed which includes a listing of the TSO number. Even though a wheel is TSO approved, the application on the aircraft (loads imposed on the wheel) requires proof that the rating is not exceeded. Wheel rating must not be less than airplane maximum radial load limits.
(3) If a wheel selected for an aircraft design is not approved under TSO-C26, the necessary data, both detail design and assembly drawings and qualification tests and test report data, will be required to comply with the standards contained in part 25. Design control and inspections will be accomplished as a part of the aircraft type design. Structural substantiation and any appropriate qualification tests shall be accomplished. See §§ 25.471 through 25.497 for the ground load conditions.
(4) The Tire and Rim Association Inc., issues a yearbook listing aircraft tire and rim sizes and ratings. The dimensions and contours for aircraft wheel rims are contained in the yearbook.
e. References. The addresses for ordering the latest revision of advisory circulars, technical standard orders, and other referenced documents listed below can be found in the Appendix to this AC.
Section 25.731 Wheels and § 25.735 Brakes, part 25 of 14 CFR.
AC 25-7A, Flight Test Guide for Transport Category Airplanes.
AC 27-1, Certification of Normal Category Rotorcraft.
AC 29-2A, Certification of Transport Category Rotorcraft.
AC 43.13-1A, Acceptable Methods, Techniques and Practices-Aircraft Inspection and Repair, Chap. 8, L. G. Equipment.
TSO-C26, was an acceptable standard to Civil Aeronautics Administrator, but was not part of the regulations.
TSO-C26a, was a part of the regulations, under part 514 of the Regulations of the Administrator, as § 514.72.
TSO-C26b, was part of the regulations as § 37.172 of 14 CFR.
TSO-C26c, was part of the regulations as § 37.172 of 14 CFR.
TSO-C26c with Addendum I. The TSO was removed from regulations (§ 37.172, Amendment 25-52, 1980) and became a voluntary standard.
SAE AIR-811B, Disposition of Wheels which have been Overheated.
SAE ARP-1322, Overpressurization Release Devices.
SAE ARP-1786, Wheel Roll on Rim Criteria for Aircraft Application.
SAE AS-707B, Thermal Sensitive Inflation Pressure Release Devices for Tubeless Aircraft Wheels.
13. SECTION 25.733 - TIRES.
a. Rule Text.
(a) When a landing gear axle is fitted with a single wheel and tire assembly, the wheel must be fitted with a suitable tire of proper fit with a speed rating approved by the Administrator that is not exceeded under critical conditions and with a load rating approved by the Administrator that is not exceeded under-
(1) The loads on the main wheel tire, corresponding to the most critical combination of airplane weight (up to maximum weight) and center of gravity position, and
(2) The loads corresponding to the ground reactions in paragraph (b) of this section, on the nose wheel tire, except as provided in paragraphs (b)(2) and (b)(3) of this section.
(b) The applicable ground reactions for nose wheel tires are as follows:
(1) The static ground reaction for the tire corresponding to the most critical combination of airplane weight (up to maximum ramp weight) and center of gravity position with a force of 1.0g acting downward at the center of gravity.
This load may not exceed the load rating of the tire.
(2) The ground reaction of the tire corresponding to the most critical combination of airplane weight (up to maximum landing weight) and center of gravity position combined with forces of 1.0g downward and 0.31g forward acting at the center of gravity. The reactions in this case must be distributed to the nose and main 19
wheels by the principles of statics with a drag reaction equal to 0.31 times the vertical load at each wheel with brakes capable of producing this ground reaction. This nose tire load may not exceed 1.5 times the load rating of the tire.
(3) The ground reaction of the tire corresponding to the most critical combination of airplane weight (up to maximum ramp weight) and center of gravity position combined with forces of 1.0g downward and 0.20g forward acting at the center of gravity. The reactions in this case must be distributed to the nose and main wheels by the principles of statics with a drag reaction equal to 0.20 times the vertical load at each wheel with brakes capable of producing this ground reaction. This nose tire load may not exceed 1.5 times the load rating of the tire.
(c) When a landing gear axle is fitted with more than one wheel and tire assembly, such as dual or dual-tandem, each wheel must be fitted with a suitable tire of proper fit with a speed rating approved by the Administrator that is not exceeded under critical conditions, and with a load rating approved by the Administrator that is not exceeded by-
(1) The loads on each main wheel tire, corresponding to the most critical combination of airplane weight (up to maximum weight) and center of gravity position, when multiplied by a factor of 1.07; and
(2) Loads specified in paragraphs (a)(2), (b)(1), (b)(2), and (b)(3) of this section on each nose wheel tire.
(d) Each tire installed on a retractable landing gear system must, at the maximum size of the tire type expected in service, have a clearance to surrounding structure and systems that is adequate to prevent unintended contact between the tire and any part of the structure or systems.
(e) For an airplane with a maximum certificated takeoff weight of more than 75,000 pounds, tires mounted on braked wheels must be inflated with dry nitrogen or other gases shown to be inert so that the gas mixture in the tire does not contain oxygen in excess of 5 percent by volume, unless it can be shown that the tire liner material will not produce a volatile gas when heated or that means are provided to prevent tire temperatures from reaching unsafe levels.
[Amdt. 25-23, 35 FR 5676, Apr. 8, 1970; Amdt. 25-38, 41 FR 55467, Dec. 20, 1976; Amdt. 25-49, 44 FR 68745, Nov. 29, 1979; Amdt. 25-72, 55 FR 29777, Jul 20, 1990, as amended by Amdt 25-78, Mar 29, 1993]
b. Intent of Rule. This rule specifies type certification requirements for both design and performance of tires used on transport category airplanes. The tire must be of proper fit and have approved speed and load ratings for a particular airplane application. The maximum static ground reaction for the condition specified must not exceed the maximum static load rating of each tire. Retractable gear system tires must have adequate clearance from surrounding structure and systems. The tire inflation medium is to be an inert gas to avoid explosions. Tires installed on landing gear axles with multi-wheels (main wheel tires only), must have a 7% load margin included in their rating. Tire performance standards are contained in Technical Standard Order (TSO) TSO-C62. A TSO approval is not approval to install tires on the airplane. The airframe manufacturer/user must conduct the applicable airplane certification tests and receive FAA approval for installation.