must be able to withstand the loads defined by §§ 25.333 and 25.345. These are considered ultimate loads for this condition. If the surfaces are automatically or power operated, the control system must also be designed to meet the requirements of § 25.672.
6. DETAIL DESIGN CONSIDERATIONS.
a. Unless the airplane has safe flight characteristics with the functionally related high lift or drag devices retracted on one side and extended on the other, the motion of the devices opposite sides of the plane of symmetry must be synchronized by a mechanical interconnection or approved equivalent means as required by § 25.701. The criteria of § 25.701 are considered equally applicable to high lift and drag devices.
(1) The surface interconnection must be designed for the loads resulting when the surfaces on one side of the plane of symmetry are jammed and immovable while the surfaces on the other side are free to move and the full power of the surface actuating system is applied. The flight loads from § 25.345 acting on the surface must be considered in combination with the actuating system loads (including system inertia loads). This is considered a limit load condition.
(2) In showing compliance with the interconnection requirements of § 25.701, all possible jam locations in the drive and support system should be considered. The surface mechanical interconnection must be able to withstand the jam condition and preclude any unsafe asymmetrical condition. The interconnection system is comprised of all elements which react the drive output from the actuator source to the jam point. These elements may include structures, interconnection linkages, and drive system components. When the interconnection is the only means to prevent an unsafe asymmetrical condition, the loads associated with the jam conditions are considered limit loads and require a 1.5 factor of safety. A factor of safety less than 1.5 may be used when a reliable (i.e., a probability of failure of 10-3 or less) and independent means is used, in addition to the mechanical interconnection, to prevent unsafe asymmetry of a high lift system. The alternate system should detect the jam and shut down the drive system before the loads from any jam condition are reacted by the mechanical interconnection. The factor of safety may be as low as 1.25 if the probability of failure of the alternate system is 10-5 or less; however, it should not be less than 1.25 unless the alternate system is found to be equivalent to a mechanical interconnection. When a torque limiter is used, the torque tolerance limit should be used to react the required load rather than the nominal or set torque. A torque limiter should not be located in the drive system in a position where the limiter itself would allow an unsymmetrical configuration if a jam occurred.
(3) An equivalent means of compliance with the requirements for a mechanical interconnection system may be substantiated using a systems safety analysis. Guidelines for performing a systems safety analysis are given in AC 25.1309-1, System Design Analysis.
b. Where failures in the drive system can result in uncommanded extensions or retractions of the high lift or drag devices, a positive means should be provided to limit the movement of the affected surfaces. This may be accomplished through irreversible drive actuators, no-back devices, redundancy in the drive system, or other equivalent means.
c. In determining loads on high lift devices during actuation, it may be necessary to consider friction loads in the actuating system, which may be reasonably expected to occur in service. Flap tracks and rollers for instance, are often subjected to ice and slush which may offer high resistance to flap actuation. Each design should be evaluated to determine its susceptibility to friction in the mechanism and any loads associated with such resistance should be accounted for and applied in combination with normal operating loads.
d. In evaluating the effects of failures or jamming of high lift surfaces, the effects of skewed surfaces on the operation of adjacent surfaces should be evaluated. Damage to adjacent structures and systems due to skewing of the surface should also be evaluated.
7. INDICATING AND WARNING SYSTEMS.
a. Indicating systems for high lift and drag devices must provide visual indication to the pilot of the surface positions for the takeoff, enroute, approach, and landing conditions.
The position sensors should be located such that they show a direct indication of failure conditions. There should be independent monitoring of each functionally related set of surfaces
(i.e., a set of surfaces on each side of the plane of symmetry that is driven by a common actuator, or is synchronized by some other means to ensure symmetric actuation) for which a failure will require an action or procedural change by the flightcrew. For instance, a functionally related flap set which is in an unsymmetrical configuration about the fuselage centerline would require an indication to the pilot of the unsymmetrical condition before takeoff. The indication to the flightcrew need not indicate the specific surface which has failed, but must clearly reflect the abnormal configuration (§ 25.699). The cockpit surface position display must also clearly distinguish a fault which was caused by a high lift "asymmetrical" deployment from a high lift "disagree" condition. A "disagree" condition exists when the high lift surface is stopped at a position different than the position commanded-by the pilot through the flap selection switch or handle. This distinction will aid the pilot in using proper procedures to further deploy, retract, or leave the high lift or drag devices for continued flight.
b. The takeoff warning system required by § 25.703 should sense the position of each functionally related set of high lift devices (symmetric about the airplane centerline) and provide aural warning during the initial portion of the takeoff roll if any set is not in an approved takeoff position.
8. FLIGHT LOADS MEASUREMENT. Notwithstanding the advancements in analytical methods used in predicting loads on airplane structures, accurate prediction of loads on wing leading edge and trailing edge high lift devices continues to be a problem. It is, therefore, advisable to verify the loads on these surfaces by conducting flight loads surveys regardless of the level of confidence in the overall loads program.
9. AIRPLANE CONTROLLABILITY. It should be shown by analysis, and where necessary by ground, simulation or flight tests, that the airplane has adequate stall margins and controllability to sustain the failure conditions addressed in paragraphs 4, 5, and 6 of this AC without requiring exceptional flightcrew skill or strength. It should also be demonstrated that no hazardous change in altitude or attitude will develop during transition to the unsymmetric condition considering likely transition rates.
END OF ADVISORY CIRCULAR 25-14
e. Reference. None.
4. SECTION 25.701 - FLAP INTERCONNECTION.
a. Rule Text.
(a) Unless the airplane has safe flight characteristics with the flaps or slats retracted on one side and extended on the other, the motion of flaps or slats on opposite sides of the plane of symmetry must be synchronized by a mechanical interconnection or approved equivalent means.
(b) If a wing flap or slat interconnection or equivalent means is used, it must be designed to account for the applicable unsymmetrical loads, including those resulting from flight with the engines on one side of the plane of symmetry inoperative and the remaining engines at takeoff power.
(c) For airplanes with flaps or slats that are not subjected to slipstream conditions, the structure must be designed for the loads imposed when the wing flaps or slats on one side are carrying the most severe load occurring in the prescribed symmetrical conditions and those on the other side are carrying not more than 80 percent of that load.
(d) The interconnection must be designed for the loads resulting when interconnected flap or slat surfaces on one side of the plane of symmetry are jammed and immovable while the surfaces on the other side are free to move and the full power of the surface actuating system is applied.
[Amdt. 25-72, 55 FR 29777, July 20, 1990]
NOTE: 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.
5. SECTION 25.703 - TAKE OFF WARNING SYSTEM.
a. Rule Text.
A takeoff warning system must be installed and must meet the following requirements:
(a) The system must provide to the pilots an aural warning that is automatically activated during the initial portion of the takeoff roll if the airplane is in a configuration, including any of the following, that would not allow a safe takeoff:
(1) The wing flaps or leading edge devices are not within the approved range of takeoff positions.
(2) Wing spoilers (except lateral control spoilers meeting the requirements of § 25.671), speed brakes, or longitudinal trim devices are in a position that would not allow a safe takeoff.
(b) The warning required by paragraph (a) of this section must continue until-
(1) The configuration is changed to allow a safe takeoff;
(2) Action is taken by the pilot to terminate the takeoff roll;
(3) The airplane is rotated for takeoff; or
(4) The warning is manually deactivated by the pilot.
(c) The means used to activate the system must function properly throughout the ranges of takeoff weights, altitudes, and temperatures for which certification is requested.
[Amdt. 25-42, 43 FR 2323, Jan. 16, 1978]
b. Intent of Rule. This rule prescribes a requirement for a takeoff warning system to warn the pilots during the initial portion of the takeoff roll if the airplane is in a configuration that would prevent successful completion of the takeoff.
c. Background. This section was not addressed in either part 4b of the Civil Air Regulations (CAR) or part 25 of Title 14, Code of Federal Regulations (14 CFR) when it was originally codified. This rule was introduced with Amendment 25-42, dated January 16, 1978.
It was realized that wing flaps and associated leading edge devices posed a special problem because some airplanes have take off flap settings that vary with weight, altitude, temperature, and runway length. A warning system that accounts for these variables would be extremely complex, and would still require the pilot to enter the proper data. In the interest of reliability, the rule requires the system to give a warning when the flaps or leading edge devices are not within the approved range of takeoff positions, e.g., when the pilot has not placed the flaps in an approved takeoff position or has retracted the flaps inadvertently, or if the flaps fail to move from the retracted position in response to a control input. Additionally, longitudinal trim devices (such as movable stabilizers) and drag devices (such as ground and flight spoilers) may be positioned inadvertently or due to failure conditions in a configuration that may prevent successful completion of a takeoff. The regulation applies to the stabilizer, speed brake systems, and rudder trim as well.
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.