(b) The airplane should be shown to have safe flight characteristics with any combination of extreme positions of individual movable surfaces (mechanically interconnected surfaces are to be considered as a single surface).
During a recent review of this new requirement, it was noted that the new § 23.701(b), particularly the parenthetical portion of that paragraph, could be improperly interpreted and applied. It is possible that this misinterpretation could result in the use of differing terminology (i.e., "mechanical interconnection" and "mechanically interconnected") in paragraphs (a)(1) and (b). These terms mean the same thing; direct positive mechanical interconnection between separate flap surfaces that are isolated from the flap control or actuation system.
Novel and unusual design features, such as an interconnection of the leading and trailing edge flap systems or an interconnection of flaps and ailerons, would require special conditions.
Equivalent Level of Safety Findings Several findings have been accepted for the mechanical interconnection requirement.
a. Synchronized by a Mechanical Interconnection These words appeared in 14 CFR parts 23 and 25 and in CAR 03 and 04b since they were first issued. The synchronization requirement for the motion of the flaps by a mechanical interconnection is applicable to airplanes not having safe flight characteristics under asymmetrical flap operations. For these cases, there would be a hazardous condition when the flaps are retracted on one side and extended on the other side.
b. Mechanical Interconnection Requirement of § 23.70l(a)(l) This requirement is to ensure against hazardous asymmetrical operation of the flaps after any probable single or probable combination of failures of the flap actuating system. A probable combination of failures should be considered when the first failure would not be detected during normal operation of the system, including periodic checks, or when the first failure would inevitably lead to other failures. (Systems where a probable combination of failures may occur include the electrical and hydraulic systems.) The airplane also should be shown to be capable of continued safe flight and landing without requiring exceptional pilot skill or strength following these failures. To demonstrate that the airplane is safe under these conditions, tests should be conducted with the flaps being retracted on one side and extended on the other during takeoffs, approaches, and landing. If there is a probable hazardous condition, a separate positive connection that is not part of the flap actuation system is required.
ACCEPTABLE MEANS OF COMPLIANCE An acceptable means of compliance with the airworthiness requirements for the flap's mechanical interconnections of § 23.701(a)(1) are described below:
a. Reliability Reliability of the mechanical interconnections is generally shown either by load analysis or load tests, or both, not by numerical probabilistic analysis. The mechanical interconnection should be designed for the loads resulting when interconnected flap 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. It should also be designed to account for the asymmetrical loads resulting from flight with the engines on one side of the plane of symmetry inoperative and the remaining engines at takeoff power. For single engine airplanes and multiengine airplanes with no slipstream effects on the flaps, it may be assumed that 100 percent of the critical air loads acts on one side and 70 percent on the other. The flight loads from § 23.345 acting on the surfaces should be considered in combination with the actuating system loads (including system inertia loads). Critical air load conditions should consider flap retraction and flap extension, including go-around. These conditions are considered limit loads. If there are no hazardous conditions when the flaps are asymmetrical, the jam or maximum load conditions could be considered an ultimate load.
b. Friction Loads It may be necessary to consider friction loads in the actuating system that may be reasonably expected to occur in service. Each design should be evaluated to determine its susceptibility to friction in the mechanism and any loads with such resistance.
c. Equivalent Means by Use of the Mechanical Actuation System The mechanical actuating system for the flaps may be considered the mechanical interconnection, if all elements are mechanically interconnected from the actuator source to the flaps. These mechanical elements may include structures, interconnection linkages, and drive system components. When the mechanical interconnection is through the actuating system, and it is the only means to prevent an unsafe asymmetrical condition, the loads associated with the jam conditions are considered limit loads. A
1.5 factor of safety is required if a failure as a result of the jam condition would cause a hazardous flap asymmetrical operation. A mechanical actuating system having a 1.5 factor of safety may not need to be evaluated for probable failure conditions. Also, if the drive system is designed so that a hazardous flap asymmetrical operation would not occur after a jam condition, the 1.5 factor of safety should not be required.
d. Equivalent Means by Use of a Warning and Prevention System A second equivalent means is the use of a warning and prevention system. This system monitors the symmetrical condition of the flaps and warns the pilot when an unsymmetrical flap condition occurs, but the asymmetry is still kept within safe limits. It prevents further movement of the flaps from exceeding safe limits. The warning and prevention system should be independent for each functionally related set of surfaces
(i.e., a set of flaps on each side of a plane of symmetry that is driven by a common actuator). Again, the airplane should be shown to have safe flight characteristics without requiring exceptional piloting skill or strength at the extreme limits of the asymmetrical condition where the flaps are stopped. Tests should be conducted to simulate flap malfunctioning at the most severe case in the static asymmetrical condition of the flaps during takeoffs, approaches, and landings. The warning and prevention system should provide a pilot with a selectable or automatic test mode that exercises the system to an appropriate depth, so the pilot can determine proper operation of this system.
e. Electrical/Electronic Flap Interconnection System When Amendment 23-42 was adopted, § 23.701 was amended to include provisions for airplanes with a flap configuration other than a mechanical interconnection. This amendment added the following requirement in § 23.701(a)(2): “Maintain synchronization so that the occurrence of an unsafe condition has been shown to be extremely improbable." This requirement is applicable for electrical/electronic flap interconnection systems, such as airplanes that have additional flaps and tandem wings.
Guidelines for performing a design safety assessment by application of § 23.1309(b), as adopted by Amendment 23-41, are given in AC 23.1309-lC. This AC also provides guidance regarding design safety assessments, environmental and atmospheric conditions, and software assessment.
23.703 Takeoff warning system No policy available as of June 30, 2001.
LANDING GEAR
23.721 General The Small Airplane Directorate has no criteria or policy for taxi over rough surfaces for small airplane landing gear design. Manufacturers are free to develop and substantiate their own criteria.
23.723 Shock absorption tests Original Issue This regulation requires shock absorption tests be performed for certification.
Amendment 23-23 and Subsequent This amendment permits an analysis rather than a shock absorption test, but to do so the applicant should have a landing gear system with identical (similar is not acceptable) energy absorption characteristics. The energy absorption characteristics of the landing gear system (e.g., structure, wheel tire, shock absorber) should be included in determining the dynamic response of the landing gear system. The tests should cover a range of energy absorption characteristics and weights over which the analysis is shown to be valid. If these conditions are not met, drop tests will be required to substantiate maximum takeoff and landing weight increases. It is acceptable to modify individual gear drop test data by adapting the results to the complete aircraft analytically, accounting for the aircraft flexibility.
23.725 Limit drop tests Original Issue This rule gives requirements for limit load drop tests if the applicant uses free drop tests to meet the requirements of § 23.723(a). The applicant should make ten drops from limit height for each basic design condition. The applicant should make one drop from the height (maximum is 2.25 times the limit drop height) needed to develop 1.5 times the limit load using the limit drop weight. It is acceptable to modify individual gear drop test data by adapting the results to the complete aircraft analytically, accounting for the aircraft flexibility.
Amendment 23-7 and Subsequent This amendment requires that the limit inertial load factor be determined in a rational and conservative manner during the drop test using a landing gear unit attitude and applied drag loads that represent the landing conditions. It is acceptable to modify individual gear drop test data by adapting the results to the complete aircraft analytically, accounting for the aircraft flexibility.
23.726 Ground load dynamic tests No policy available as of June 30, 2001.
23.727 Reserve energy absorption drop test Amendment 23-7 and Subsequent Paragraph (b) in § 23.727 requires that the effect of wing lift be provided for in reserve energy drop tests. You should also use the applicable drag loads, as specified in § 23.725(c).
23.729 Landing gear extension and retraction system Original Issue No policy available as of June 30, 2001.
Amendment 23-7 and Subsequent A warning device with no manual shutoff is required when the flaps are “to or beyond” the approach flap setting if the landing gear is not down and locked. The “to or beyond” phrase in relation to using a normal landing procedure is intended to provide for differences in design, as follows:
a. For airplanes whose normal procedures only prescribe landings with flaps extended past the approach setting, only the “beyond” aspect of this rule is appropriate.
Operating information for these airplanes should convey that landings with approach flaps, or less, are not normal and will not activate the flap/landing gear aural warning.
b. For airplanes whose normal procedures include landings with a flap setting at the approach setting, the “to and beyond” aspect of this rule is appropriate. Designers may choose to include additional logic in the flap/landing gear warning system, such as airspeed, thrust/throttle position, etc. This logic may tend to minimize nuisance warnings and may provide the equivalent safety intended by the rule.
Because part 23 is not specific with regard to flap positions used, we cannot specify the flap position that actuates the warning device. This rule provides a basis for the FAA and the applicant to mutually agree on the set point for the warning device.
Although not defined in part 23, most airplanes do have a “normal landing procedure” and an “approach flap position.” The flap position will vary among models, but it is this position that should be used to show compliance.
This information is applicable to the structural substantiation to the loads resulting only from all yawing conditions for the landing gear doors and retraction mechanism of small airplanes per part 23, § 23.729(a)(2).
Section 23.729(a)(2) requires the landing gear doors and retraction mechanism to be substantiated for the loads resulting from all yawing conditions. Attempts have been made to meet these requirements by flight testing to dive speed with some yaw or by flight testing at full yaw at a lower speed. These procedures normally do not result in
37 a test that substantiates a 1.5 factor of safety. If substantiation by flight testing is desired, the landing gear doors and retraction mechanism should be subjected to 1.5 times the limit "q" loading. The limit “q” loading is the “q” at VLE or VLO, whichever is greater.
The higher of the above speeds at which Vq is to be computed is designated as VLG.
ACCEPTABLE MEANS OF COMPLIANCE One method, but not the only method, for showing compliance with the structural requirements of § 23.729(a)(2) for the loads resulting from all yawing conditions for the landing gear doors and retraction mechanism is as follows:
a. Substantiation may be accomplished by flight testing at a speed of Vq and the yaw angle determined in paragraph a(3) below, unless this will exceed the structural limitation as determined by analysis, static test, or a combination of both, where:
(1) Vq = square root of (VLG squared times 1.5).
(2) VLG = The greater of VLO or VLE.
(3) For the yawed condition, the limit “q” load will be at VLG with the airplane at the yaw angle determined by § 23.441. This angle need not exceed 15o. Substantiation should be to 1.5 “q”.
(4) If Vq is equal to or less than VA, substantiation by flight test may be accomplished.
(5) If Vq is greater than VA, the yaw necessary to produce 1.5 “q” could result in overloading other airplane structures, and the maneuver should not be performed.
(6) VLG may be reduced by imposing limitations on the airplane such that Vq is less than VA.
(7) The definitions of the terms used above are equivalent airspeeds, as follows:
VA = Design maneuvering speed VD = Design diving speed VLG = Landing gear speed used in the calculation of Vq VLE = Maximum landing gear extended speed VLO = Maximum landing gear operating speed Vq = Speed which results in 1.5 times limit "q" loading
b. If the condition of Item a.(5) above exists, substantiation of the landing gear doors and retraction mechanism may be accomplished by static tests, analyses, or a combination of both.
Amendment 23-49 and Subsequent This guidance provides clarification of the requirements for tire burst as related to landing gear and 14 CFR part 23, §§ 23.729(g) and 23.1309 compliance.
The requirement in § 23.729(g) states: “If the landing gear bay is used as the location for equipment other than the landing gear….” As used within this requirement, the term landing gear applies to all parts and systems contained in the “LANDING GEAR” section in Subpart D (§§ 23.721 through 23.745) that are specific to the landing gear.
For example, wheels, brakes, wheel steering mechanism, and structural portions of the landing gear that are within the gear are all considered part of the “landing gear” for the purposes of § 23.729(g) compliance. Systems that act upon the landing gear, e.g., extension and retraction systems, hydraulic systems, etc., are not considered part of the landing gear itself and should be addressed in § 23.729(g) compliance. Compliance to this section can be by isolation of non-landing gear equipment from the landing gear bay or by protection/shielding of this equipment from the effects of tire burst/loose tread and external forces. Using either of these means of compliance to § 23.729(g) also shows compliance to the § 23.1309 Particular Risk Analysis for non-landing gear equipment.
The effects of tire burst must still be evaluated for § 23.1309 compliance. When showing compliance with § 23.1309 for tire burst, in accordance with § 23.1309(f)(2) and Advisory Circular 23.1309-1C, only the structural portions of the landing gear are exempt from the § 23.1309 requirements. Addressing external effects as part of the