23.685 Control system details
No policy available as of December 31, 2007.
The corresponding rule in CAR 3 is CAR 3.344.
The corresponding airworthiness criteria is section 4.20, Airship Design Criteria, FAA-P-8110-2, Change 2.
23.687 Spring devices
This rule was adopted on February 1, 1965 as a recodification of CAR 3.347.
The corresponding airworthiness criteria is section 4.21, Airship Design Criteria, FAA-P-8110-2, Change 2.
Compliance may be shown by flight tests with the spring disconnected to show that failure of the spring will not cause flutter or unsafe flight characteristics, or by performing a reliability analysis.
The flight test is preferred since this approach clearly addresses the safety issue and is historically a minimal test burden.
An applicant who chooses reliability analysis must accept that a failure of the spring could create unsafe flight characteristics and, therefore, meet the corresponding level of reliability.
This may involve the use of redundant design such as dual springs and demonstration of flight characteristics with one removed.
A positive determination of spring reliability requires that an applicant show the spring will perform its intended function for a specified interval under operational and environmental conditions appropriate for the proposed airplane. Although not normally used for structural substantiation, the reliability assessment methods for § 23.1309 may contain some concepts that would be helpful in showing the reliability of the spring device. The reliability assessment should consider, but not necessarily be limited to, fatigue failures, failures because of corrosive environments, and any in-service changes in the spring characteristics, the spring constant. The reliability assessment of a spring device used in any airplane flight control system must consider airworthiness standards other than § 23.687. These include, but are not limited to, flutter characteristics and handling qualities.
In addition, § 23.687 is clear in that the applicant must show reliability of the spring with tests that simulate service conditions. Tests are the required substantiation method; however, an applicant may show compliance with tests supported by analysis. Also the spring device testing requirement, 14 CFR, part 23, § 23.601, General, requires testing for any design detail or part that has an important bearing on safety of flight. An applicant should also show compliance with 14 CFR, part 23, § 23.305 (a) and (b). Finally, when a single spring is required for flutter, handling qualities, or any other regulatory reason, the spring should be considered a single path critical structure and meet the A-Basis requirements of § 23.613(b)(1).
23.689 Cable systems
The corresponding rule in CAR 3 is CAR 3.345.
The corresponding airworthiness criteria is section 4.22, Airship Design Criteria, FAA-P-8110-2, Change 2.
Original Issue and Subsequent
If tabs are installed with cable less than 1/8th-inch diameter, the airplane should be safely controllable with the tabs in the most adverse position as if from a failed cable. Using emergency procedures, the pilot should be able to return and land safely. Airplane configurations, such as flaps, landing gear, and power are permissible devices to use in relieving control forces. The temporary control forces of part 23, § 23.143, are applicable until the force reduction procedures are completed.
Smaller diameter cables (no less than 1/16-inch diameter) may be used for rudder pedal interconnections (used for pulling one pedal back when the other is pushed forward. They are not used to drive the control surface), if the failure of this interconnection will not affect rudder operation.
This rule was put in the original issue of part 23 from CAR 3.345, and it was intended to apply to airframe control cables not engine controls cables, which are certified under part 23, Subpart E.
Final Rule 8083 revised this section by Amendment 23-7. NPRM 67-14 stated as follows:
“Pulley specifications (Sec. 23.689). Section 23.689(b) was amended by striking out the words "as specified in the pulley specifications" after the words "is used. Explanation.
Correspondence of pulley and cable is prescribed in the first sentence of Sec. 23.689(b).
Correspondence may be verified in several ways. The reference to pulley specifications adds nothing.”
23.691 Artificial stall barrier system
There are no corresponding rule or airworthiness criteria in CAR 3 or the Airship Design Criteria.
Amendment 23-49 and Subsequent
This amendment adopted a proposed revision in NPRM 94-21 that stated: “This proposed new section would provide standards for stall barrier systems if a stall barrier is necessary to show compliance with Sec. 23.201(c).
The requirements of Sec. 23.201(c) provide criteria for the in-flight demonstration of wings level stall. The requirements also specify the means of identifying when a stall has occurred.
Amendment No. 23-45 (58 FR 42136, August 6, 1993) revised Sec. 23.201(c) by adding the activation of an artificial stall barrier as an acceptable means of identifying when a stall has occurred.
As the technology of airplane designs improved and engines with increased power became available, airplanes were developed that did not meet the older wings level stall requirement of Sec. 23.201. Consequently, these airplanes were equipped with an artificial stall barrier that moved the airplane elevator controls and caused a nose down pitching motion similar to the pitching motion of airplanes that meet the wings level stall requirement of Sec. 23.201.
The manufacturer selected the airspeed where this pitching motion occurred and flight testing established compliance with the other flight regulations at airspeeds above the speed selected for the push. These stall barrier systems are commonly called "stick pushers." Such systems have been accepted for compliance with Sec. 23.201 under the equivalent safety provisions of Sec. 21.21(b)(1), since they provide a pitch motion that is equivalent to that experienced during stalls of airplanes that meet the stall requirements of Sec. 23.201.
Appropriate compliance with other applicable requirements of part 23 has been established by other design characteristics of the stall barrier system.
The provisions of the proposed new section are based on system design characteristics necessary to ensure the safe operation of previously approved stall barrier systems. The proposed section also requires such systems to include provisions to prevent unwanted activation of the stall barrier systems. This is necessary to ensure that such systems do not cause downward pitching motions at higher airspeeds when such pitching could be unsafe.
The proposed sections would basically codify those provisions that have been found necessary for approving stick pusher systems under the equivalent safety requirements of Sec.
21.21(b)(1). Therefore, in effect, no new requirements would be added by this proposed amendment. The proposed new section would be applicable only to airplanes with flight characteristics that need an artificial stall barrier system to ensure safe operation of that airplane. Including provision for the installation of an optional stick pusher system would relieve the manufacturer of the financial burden that would be needed to redesign the airplane so that it would meet the wings level stall requirements.”
33 The proposed rule was changed by Final Rule, Docket 27806 as follows: “In the course of the FAAs review, however, the FAA noted that the word "necessary" in the introductory paragraph of Sec. 23.691 should be changed to "used," to make it clear that the equipment requirements of this section are applicable if a stick pusher system is used in the airplane to show compliance with Sec. 23.201(c).”
Section 23.201(b), Amendment 23-45, added the activation of an artificial stall barrier as an acceptable means of identifying when a stall has occurred. A stall barrier is a device that prevents an actual stall (that is, a stick pusher) while a stall warning is a device that alerts a pilot of an impending stall (that is, a stick shaker). Of course, the stall should not occur before activation of the stall barrier. This amendment provided the standards for an artificial stall barrier system when it is used to show compliance with § 23.201(b).
For § 23.201, a stick shaker is a "stall avoidance" device. In the NPRM for Amendment 23- 45, an applicant may identify a stall as the speed at which a stick shaker activates. This is an acceptable means of compliance to the rules. Stick shaker activation is then identified as a stall for a pilot by Aircraft Flight Manual (AFM.) However, from an engineering perspective and considering the "stall avoidance" in § 23.201, this is an acceptable means of compliance not an aerodynamic stall. The difference is important in certification since the aerodynamic stall determines whether a stick shaker is required equipment.
A stick pusher system would be a critical system for an airplane with stall recovery that is undetermined, marginal, or unacceptable. Failure of the system is then required to be unlikely. The FAA does not consider the chance of entering a stall environment as a factor in developing system reliability. The exception would be developing specific system component reliability where that component would be active only when the airplane is in a stall environment. The FAA does not give credit toward developing reliability for the use of a “Go/No Go” preflight system check, although the FAA does recommend that preflight procedures for all essential/critical systems be provided for pilot use. (Service experience has shown that some part 23 airplane pilots do not have the discipline to conduct the prescribed preflight checks.) To develop normal/abnormal/emergency procedures is not a factor in determining system reliability; however, such procedures are desirable, as well as required by § 23.1581. These factors may be considered when exercising engineering judgment in approval of the overall system. Stall can be identified by stick shaker/pusher operation, uncontrollable downward pitching, or the elevator control reaching the stop (see AC 23-8C) - whichever occurs first in any particular flight regime is acceptable. An airplane may be approved if it has stick shaker/pusher operation in one configuration, such as power on, and it has acceptable stall characteristics for the remaining configurations.
Inadvertent stick pusher operation should be investigated and shown not to be hazardous and to be recoverable, or that inadvertent operation is extremely unlikely.
23.693 Joints
No policy available as of December 31, 2007.
This rule was adopted on February 1, 1965 as a recodification of CAR 3.346.
The corresponding airworthiness criteria is section 4.23, Airship Design Criteria, FAA-P-8110-2, Change 2.
23.697 Wing flap controls
The corresponding rule in CAR 3 is CAR 3.338.
There is no corresponding airworthiness criteria in the Airship Design Criteria.
Amendment 23-49 and Subsequent
A revision in NPRM 94-21 proposed § 23.697(c) as follows: “Proposed new Sec. 23.697(c) would provide safety standards for the wing flap control lever designs installed in airplanes that use wing flap settings other than fully retracted when showing compliance with Sec.
23.145. This revision is needed to ensure that the flap settings, which establish the safe operation of the airplane, can be positively selected.”
23.699 Wing flap position indicator
No policy available as of December 31, 2007.
This rule was adopted on February 1, 1965 as a recodification of CAR 3.338.
There is no corresponding airworthiness criteria in the Airship Design Criteria.
23.701 Flap interconnection
The corresponding rule in CAR 3 is CAR 3.339.
There is no corresponding airworthiness criteria in the Airship Design Criteria.
Original Issue
The flaps should be synchronized by a mechanical interconnection unless the airplane has safe flight characteristics with the flaps retracted on one side and fully extended on the other side. The safe flight demonstration with asymmetry should be shown throughout the airspeed range allowed for flap extension. The control forces should not exceed those shown for temporary application in the table in § 23.143(c). However, they may not exceed the force that can be shown as safe with one hand on the control wheel/stick (other hand needed to retrim, pull circuit breaker, operate flap control). If the forces of asymmetry cannot be alleviated in a reasonable period of time, the remaining forces should not exceed those specified for prolonged application in § 23.143(c).
After showing the airplane has safe flight characteristics with the flaps in their most adverse position, it is permissible to readjust the remaining flap surfaces after a malfunction occurs.
Amendment 23-42
This amendment adopted a proposed revision in NPRM 89-5 that stated: “This proposal would update the regulations to include provisions for airplanes with a flap configuration other than one flap on each wing. It would also address the failure of any single element in the flap control system and allow for an alternate equivalent means to the mechanical interconnection required by the present rule. Airplanes are currently being manufactured with two flaps on each side of the airplane and are being designed with flaps on canards and tandem wings. On an airplane with four flaps, there is a possibility that only one flap may be asymmetric with respect to the other three and this issue needs to be addressed in the airworthiness standards, as proposed in paragraphs (a) and (b).”
Amendment 23-42 was not intended to change the requirement that “The main wing flaps and related movable surfaces as a system must be synchronized by mechanical connection.” The main purpose of this change was to add the following requirement that would maintain synchronization so the event of an unsafe condition has been shown to be unlikely. This requirement includes provisions for synchronization of the flaps other than by mechanical interconnection of the flap. These reliability requirements by numerical probability analysis for other synchronization methods should not be applied to mechanical interconnection. It is difficult to assess the reliability of mechanical interconnections by examples of different types of mechanisms. The complete system needs to be analyzed and tested.
Section 23.701, as amended by Amendment 23-42, in part, states the following:
a. The main wing flaps and related movable surfaces, as a system must:
(1) Be synchronized by mechanical connection; or
(2) Maintain synchronization so the event of an unsafe condition has been shown to be unlikely; or
b. The airplane should be shown to have safe f light 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 the new § 23.701(b), particularly the parenthetical portion of that paragraph, could be improperly interpreted and applied. It could be possible that this misinterpretation could result in the use of differing terminology (that is, "mechanical interconnection" and "mechanically interconnected") in § 23.701(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 (ELOS) 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 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.
39 Amendment 23-49 and Subsequent
This amendment adopted a proposed revision in NPRM 94-21 that 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) is 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
40 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.