(vii) Under limit load, no signs of jamming, or of any permanent set of any connection, bracket, attachment, and so forth, may be present.
(viii) The control system should operate freely without excessive friction.
NOTE 4: The tests described in section (3) above are normally performed using a complete airplane. As a minimum, they must be completed using an airframe/control system that represents the final product from the cockpit controls to the control surface.
Regardless of the amount of travel of a control surface when tested , the airplane must have adequate flight characteristics as specified in § 23.141. Any airplane that is a close derivative of a previous type certificated airplane need not exceed the control surface travel of the original airplane; however, the flight characteristics should be tested to assure compliance.
EASA AMC 23.683 is acceptable for FAA certification.
23.685 Control system details
No policy available as of September 30, 2003.
The corresponding rule in CAR 3 is CAR 3.344.
The corresponding rule in the Airship Design Criteria, FAA-P-8110-2, Change 2, is section 4.20.
23.687 Spring devices
This rule was adopted on February 1, 1965 as a recodification of CAR 3.347.
The corresponding rule in the Airship Design Criteria, FAA-P-8110-2, Change 2, is section 4.21.
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 inservice 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 rule in the Airship Design Criteria, FAA-P-8110-2, Change 2, is section 4.22.
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.
A proposed revision to NPRM 67-14 revised this section by Amendment 23-7 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 is no corresponding rule in CAR 3 or the Airship Design Criteria.
Amendment 23-49 and Subsequent
A proposed revision to NPRM 94-21 states as follows: “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.”
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 Airplane 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 23-8B) -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 September 30, 2003.
This rule was adopted on February 1, 1965 as a recodification of CAR 3.346.
The corresponding rule in the Airship Design Criteria, FAA-P-8110-2, Change 2, is section 4.23.
23.697 Wing flap controls
The corresponding rule in CAR 3 is CAR 3.338.
There is no corresponding rule in the Airship Design Criteria.
Amendment 23-49 and Subsequent
A revision to NPRM 94-21 proposed § 23.867(c) as follows: “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 September 30, 2003.
This rule was adopted on February 1, 1965 as a recodification of CAR 3.338.
There is no corresponding rule in the Airship Design Criteria.
23.701 Flap interconnection
The corresponding rule in CAR 3 is CAR 3.339.
There is no corresponding rule 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 re-trim, 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
A proposed revision to NPRM 89-5 explains this amendment as follows: “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 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 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 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 (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.