b. Mechanical Interconnection Requirement of § 23.70l(a)(l) This requirement is to assure 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 show 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.
Amendment 23-49 and Subsequent
A proposed revision to NPRM 94-21 revised this section as follows: “Section 23.701
(a)(1) and (a)(2) would be revised to clarify the requirements for flap systems installed on part 23 airplanes. Following the revision of Sec. 23.701, as adopted by Amendment No. 23-42 (56 FR 353, January 3, 1991), the FAA discovered that the new requirements could be interpreted in a way that was not intended and that this interpretation could result in approval of airplanes with unsafe flight characteristics in the event of flap failure. To clarify the intent of the requirements, the FAA issued on March 14, 1991, a policy letter to all aircraft certification offices that provided guidance for the correct application of the requirements.
Since then, the FAA has reexamined the requirements and determined that Sec.
23.701(a)(1) and (a)(2) need to be revised to ensure that a failure of the flap system would not create an asymmetric flap configuration that could result in an unsafe flight condition. Therefore, Sec. 23.701 (a)(1) and (a)(2) would be revised to clarify that one of the following would apply:
(1) The moveable flap surfaces must be synchronized by a mechanical interconnection or by an approved equivalent means that is independent of the flap drive system.
(2) The wing flap system must be designed so that any failures of the flap system that would result in an unsafe flight characteristic of the airplane, such as flap asymmetry, are extremely improbable.
These revisions would ensure that a failure of the flap drive systems will not result in a flap asymmetry configuration.”
ACCEPTABLE MEANS OF COMPLIANCE An acceptable means of compliance with the airworthiness requirements for the flaps mechanical interconnections of § 23.701(a)(1) are described as follows:
a. Reliability Reliability of the mechanical interconnections is 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 because 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 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 (that is, 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 on design safety assessments, environmental and atmospheric conditions, and software assessment.
23.703 Takeoff warning system
There is no corresponding rule in CAR 3 or the Airship Design Criteria.
Amendment 23-49 and Subsequent
A revision to NPRM 94-21 proposed this rule as follows: “This proposed new section would require a takeoff warning system on some commuter category airplanes. The requirement would be applicable if the flight evaluation showed that an unsafe takeoff condition would result if lift devices or longitudinal trim devices are set to any position outside the approved takeoff range. If the evaluation shows that no unsafe condition would result at any setting of these devices, a takeoff warning system would not be required. For those airplanes on which a warning system would not be required. For those airplanes on which a warning system must be installed, the proposal would provide requirements for the installation of the system.”
04/12/05
AC 23-17B
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.
There is no corresponding rule in CAR 3 or the Airship Design Criteria.
23.723 Shock absorption tests
The corresponding rules in CAR 3 are CAR 3.351 and 3.352.
The corresponding rule in the Airship Design Criteria, FAA-P-8110-2, Change 2, is section 4.24.
Original Issue
This regulation requires shock absorption tests be performed for certification.
Amendment 23-23 and Subsequent
This amendment allows 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 (for example, structure, wheel tire, shock absorber) should be included in determining the 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
The corresponding rules in CAR 3 are CAR 3.353 and 3.354.
There is no corresponding rule in the Airship Design Criteria.
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 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 September 30, 2003.
There is no corresponding rule in CAR 3 or the Airship Design Criteria.
23.727 Reserve energy absorption drop test
The corresponding rule in CAR 3 is CAR 3.355.
There is no corresponding rule in the Airship Design Criteria.
Amendment 23-7 and Subsequent
Paragraph (b) in § 23.727 requires 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
The corresponding rules in CAR 3 are CAR 3.356, 3.357, 3.358 and 3.359.
The corresponding rule in the Airship Design Criteria, FAA-P-8110-2, Change 2, is section 4.25.
Original Issue
14 CFR, part 23, does not define landplanes, seaplanes and amphibians. These terms passed unchanged from CARs 3 and 4. As we use them, landplanes can only land on the ground, seaplanes (both boat and float types) can only land on water, and amphibians can land on both water and on the ground.
Therefore, where a rule such as § 23.729(c) states "landplanes" it does not apply to seaplanes or amphibians. When the rule states "airplanes" such as § 23.729(a) it applies to any aircraft that incorporates that equipment. Thus, § 23.729(a) can apply to landplanes and amphibians, but not seaplanes since they don't have retractable land gear.
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 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 suggest 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,. This logic may minimize nuisance warnings and may provide the equivalent safety intended by the rule.
Because part 23 is not specific about 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 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.
Amendment 23-21, NPRM 75-25, changed this section to require a warning when any throttle is closed. The NPRM explanation states as follows: “The present rule requires that a manual shutoff for the aural landing gear warning device be installed so that reopening the throttle will reset the device. However, when an engine has been shut down in flight, its throttle may not be reopened before landing. The proposal would require the aural warning to be activated when any throttle is subsequently retarded to or beyond the position for a normal landing approach, thus requiring the warning intended by paragraph (e)(2) regardless of the position of any other throttle and the prior deactivation.”
Amendment 23-45, NPRM 90-18, changed this section again. The NPRM states as follows: “This proposal revises § 23.729(f)(1) and (2) by changing the power and flap settings necessary to activate the device that warns the pilot that the landing gear is not fully extended and locked. The power setting necessary to activate the warning device is changed from when one or more "throttles are closed" to when one or more "throttle are closed beyond the power settings normally used for landing approach." The flap setting necessary to activate the warning device is changed from "flaps are extended to or beyond the approach flap position" to " flaps are extended beyond the approach flap position."
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 for 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 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 15 degrees. 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 so 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 intent of the proposed revision to NPRM 94-21 is given as follows: “This proposal would also add a new Sec. 23.729(g) requiring that if the landing gear bay is used as the location for equipment other than landing gear, the equipment must be designed and installed to minimize damage. On larger airplanes, such as the commuter category, a 50