23.681 Limit load static tests
No policy available as of December 31, 2007.
The corresponding rule in CAR 3 is CAR 3.342.
The corresponding airworthiness criteria is section 4.18, Airship Design Criteria, FAA-P-8110-2, Change 2.
23.683 Operation tests
The corresponding rule in CAR 3 is CAR 3.343.
The corresponding airworthiness criteria is section 4.19, Airship Design Criteria, FAA-P-8110-2, Change 2.
Original Issue and Subsequent
The 1.25 factor of part 23, § 23.395(a)(1) does not apply to the control system operational test of this section.
Compliance with this section is required whether or not the airplane has a significant flight test history. Proof of structure is accomplished by ground tests because required flight tests may not subject the airplane to limit loads for all possible flight conditions.
Amendment 23-7 and Subsequent
This amendment adopted a proposed revision in NPRM 67-14 that stated:
“This proposal would preserve consistency with Sec. 23.397 which provides that the airloads on the surfaces need not exceed those that would result from application of the forces in Sec.
23.397 (b).”
Part 23, § 23.683, and CAR Section 3.343 require showing by operation tests, when the controls are operated from the pilot compartment with the system loaded, the system is free from jamming, excessive friction, and excessive deflection. This section has not been uniformly applied. Some airplanes were certified using 50 percent of the control surface travel with no load as criteria for meeting the excessive deflection requirements for the operation tests. Other airplanes were not required to meet any specific travel as long as the airplane had adequate flight characteristics.
Requiring a specific large travel while under limit load could result in control system authority that is greater than desired or needed. However, some travel of the control surface should exist when the system is loaded to limit load. No travel could indicate there was a possible fault, such as a jammed system. Second, with little or no travel, operation of the controls would have such a limited effect on the maneuverability of the airplane that it could have questionable flight characteristics.
Acceptable Means of Compliance
One method, but not the only method, for showing compliance with the control system operation test requirements of § 23.683 and CAR § 3.343 is as follows:
a. This method may be used when clearances around control surfaces are no less than 3/16 inch.
b. Conduct the control system ope ration tests by operating the controls from the pilots’ compartment with the entire system loaded so as to correspond to the limit control forces established by the regulations for the control system being tested. The following conditions should be met:
(1) Under limit load, check each control surface fo r travel and detail parts for deflection.
This may be accomplished as follows:
(a) Support the control surface being tested while positioned at the neutral position.
(b) Load the surface using loads corresponding to the limit control forces established in the regulations.
(c) Load the pilots control until the co ntrol surface is just off the support.
(d) Determine the available travel, which is the amount of movement of the surface from neutral when the control is moved to the system stop.
(e) The above procedure should be repe ated in the opposite direction.
(f) Minimum control surface travel from th e neutral position in each direction being measured should be 10 percent of the control surface travel with no load on the surface.
Regardless of the amount of travel of the surface when under limit load, the airplane should have adequate flight characteristics, as specified in § 23.141. Any derivative airplane of a previous type certificated airplane need not exceed the control surface travel of the original airplane; however, the flight characteristics should be flight tested to ensure compliance.
(2) Under limit load, no signs of jamming or of any permanent set of any connection, bracket, attachment, etc., may be present.
(3) Friction should be minimized so that the limit control forces and torques specified by the regulations may be met.
Alternate Means of Compliance
NOTE: Recently, airplanes have been built with smaller gaps between control surfaces and structure than has been done in the past. If the gap is less than 3/16 inches, this alternate means of compliance should be considered.
Applicants and FAA aircraft certification offices (ACOs) involved with certification of small airplanes should follow this policy. Applicants should expect the ACO would consider this information when making findings of compliance. However, in determining compliance with certification standards, each ACO has the discretion to coordinate deviations from these guidelines with the Small Airplane Directorate when the applicant demonstrates a suitable
23 need. To assure standardization, the ACO should coordinate deviation from this policy with the Small Airplane Directorate. In this case, this alternate means of compliance is appropriate. This method also can have the added advantage of showing compliance with several regulations in a single test series.
Regardless of the amount of travel of a control surface when tested as, 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.
The method of showing compliance with § 23.683 presented in AC 23-17, § 23.683, Operation Tests, discusses only the control system. It does not specify loading on adjacent structures and elements. This revision is consistent with the wording in § 23.683 of the regulations. Testing, not analysis must be used to show compliance with § 23.683. There are five other regulations, the control system, the control surfaces, and the adjacent fixed aerodynamic surfaces related to both the control system and the control surfaces, which must also be met. These include:
a. Section 23.305(a), [Subpart C - Structure, General] Strength and Deformation. It requires that "At any load up to limit loads, the deformation may not interfere with safe operation."
b. Section 23.307, [Subpart C - Structure, General] Proof of Structure, states, "Compliance with the strength and deformation requirements of § 23.305 must be shown for each critical load condition. Structural analysis may be used only if the structure conforms to those for which experience has shown this method to be reliable. In other cases, substantiating load tests must be made."
c. Section 23.655(a), [Subpart D - Design and C onstruction, Control Surfaces] Installation, requires that "Moveable surfaces must be installed so that there is no interference between surfaces, their bracing, or adjacent fixed structure, when one surface is held in its most critical clearance positions and the others are operated through their full movement."
d. Section 23.681(a), [Subpart D - Design and C onstruction, Control Surfaces] Limit Load Static Tests, requires that "Compliance with the limit load requirements of this part must be shown by tests as follows:
(1) The direction of the test loads produces the most severe loading in the control system; and
(2) Each fitting, pulley, and bracket used in attaching the system to the main structure is included."
e. Section 23.141, [Subpart B--Flight, Flight Char acteristics] General, states that "The airplane must meet the requirements of §§ 23.143 through 23.253 at all practical loading conditions and operating altitudes for which certification has been requested,
24 not exceeding the maximum operating altitude established under § 23.1527, and without requiring exceptional piloting skill, alertness, or strength."
To assure that these requirements will be satisfied in the conduct of the control system operation test, inclusion of loads on the adjacent structures or elements in the testing setup may be required.
While testing is required for demonstration of compliance to § 23.683, sometimes analysis may be acceptable for showing compliance with § 23.305(a). Section 23.307(a), provides the criterion for when analysis is not acceptable and testing must be performed.
It is not appropriate to define specific quantitative criterion to determine when testing is required to demonstrate compliance with § 23.305(a), in accordance with § 23.307(a).
One specific criterion will not work for all possible airplane designs. It is better that such determinations are made on a case-by-case basis, in which the appropriate details of a particular design can be considered.
However, this policy describes some of the factors that should be considered when determining if tests are required to show that clearance between controls and adjacent structure (under load) meets § 23.305(a). These factors include, but are not limited to, the following:
(1) The clearance between control surfaces and adjacent structure, when at rest.
Suppose an applicant has experience with other airplanes that have a half-inch of clearance between controls and adjacent structure at rest. However, a new design is similar except it now has only a tenth of an inch clearance when at rest. Tests to show compliance with § 23.305(a), may be required because the new structure may not conform to those for which experience has shown this method to be reliable in the past. The accuracy of past methods may not be suitable for the smaller clearances. Critical conditions assessed in past analysis may not have included a condition that is critical for the new smaller clearance.
(2) Deformation (under limit loads) in the control surface or adjacent structure.
If analysis had been shown to be reliable in the past for a wing that had much smaller deflections than a current design, the current structure may not conform to those for which experience has shown this method to be reliable, and testing may be required. Previous analytical methods may no longer be reliable because the new design behaves in a more non-linear manner. It may types of deflection that were neglected in past analysis may now become critical.
(3) New control surface attachment configurations or other local design changes could create new types of deformation that are critical for the new design but were not considered in past analysis.
If the FAA requires (or if an applicant voluntarily chooses) compliance with § 23.305(a), to be shown by tests, the following test procedure is one means to simultaneously show compliance with both § 23.305(a), and § 23.683. It also shows compliance with § 23.681(a). These tests may be conducted as follows:
Except where otherwise mentioned, the tests described below in sections (1), (2), and (3) should be conducted within the following parameters (1 through 8).
PARAMETERS:
1. Conduct the control system operation tests by operating the controls from the pilots' compartment.
2. All the control surfaces must be installe d to their adjacent fixed surface on the airframe (according to the type design).
3. The entire control system and adjace nt fixed structure should be loaded.
4. The adjacent fixed surfaces (wings, horizon tal stabilizers, vertical stabilizers, and so forth) should be loaded to provide deflections equivalent to critical limit load flight conditions.
5. The structural deflections should corr espond to the limit flight conditions that represent the worst case conditions for increased cable tension, decreased cable tension, and control/fixed surface proximity for each control system as appropriate.
6. The entire control system must be loaded to either the limit airloads or the limit pilot forces; whichever is less (§ 23.683(b)(1)). For § 23.397, the automatic pilot effort must be used instead of limit pilot forces if it alone can produce higher control surface loads than the human pilot.
7. Minimum clearances around control surf aces and minimum tensions in cable systems should be defined and incorporated in the airplanes instructions for continued airworthiness. The test article should incorporate these minimum clearances and tensions, unless you otherwise account for them.
8. If reductions in the minimum clearances described in parameter 7 above are possible because of environmental conditions expected in service, you must account for this. This can be accomplished through analysis or during testing by adjusting the test article clearances to encompass these effects.
26 SECTION (1):
Consider all airplane maneuver and gust loads, and inertial loads, represented by the airplane flight envelope (V-n diagram); consider unsymmetrical load cases.
a. The tests described in this section support the demonstration that the control system is free from jamming, excessive friction, and excessive deflection as required by § 23.683(a)(1), (2), and (3). They also support the demonstration that structural deformations not interfere with safe operation as required by § 23.305(a). Accomplish the following:
(1) Load the adjacent fixed aerodynamic surface (wing, horizontal tail, or vertical tail) in accordance with one of the conditions of parameters d, e, and f above.
(2) Support the control surface being tested while it is found in the neutral position.
(3) Load the control surfaces to the critical limit loads, as described in parameter 6 above, and evaluate their proximity to the fixed adjacent structure for interference (contact).
(4) Load the pilots control until the control surface is just off the support.
(5) Determine the available control surface travel, which is the amount of movement of the surface from neutral when the cockpit control is moved through the limits of its travel.
(6) The control surface under loads described in parameter 6 above must have adequate flight characteristics as specified in § 23.141.
(7) To address the possibility of a critical intermediate control surface loading, gradually remove load from the control surface (while maintaining the load on the adjacent fixed surface) until maximum control surface travel is achieved.
(8) The procedure should be repeated in the opposite direction.
(9) With limit load applied to the adjacent fixed surface and limit or intermediate load applied to the control surface, no signs of jamming, or of any permanent set of any connection, bracket, attachment, and so forth, may be present.
(10) The control system should operate freely without excessive friction. Excessive friction is any increase under limit loads that results in exceeding the limit control forces and torques specified by the regulations.
(11) Cable systems should be checked with the loads applied to ensure that excessive slack does not develop in the system. Excessive slack is any change in cables or cable hardware that results in reduced airplane control surface movement.
(12) Repeat this process for each of the critical loading conditions as defined by paragraphs (4) and (6) above.
SECTION (2):
b. The tests described in this section support the demonstration that structural deformations not interfere with safe operation as required by § 23.305(a). Accomplish the following:
(1) Load the adjacent fixed aerodynamic surface (wing, horizontal tail, or vertical tail) in accordance with one of the conditions of paragraphs (4) and (5) above.
(2) Operate the unloaded control system from stop to stop.
(3) No signs of interference (contact) may be present.
(4) The control system should operate freely without excessive friction.
(5) Repeat this process for each of the critical adjacent fixed surface loading conditions as defined by paragraphs (4) and (5) above.
NOTE: An alternate procedure may be used to accommodate the testing described in sections (1) and (2) above during structural tests of a partial airplane. This method requires that all control system components that are attached to or enclosed by the loaded test structure be installed for type design. A sufficiently representative mockup of remaining control system components must be used to assure the full lengths of any cables, which extend from the loaded test structure, are included. This is necessary to make a reasonable assessment that slack that could develop in control cables is not excessive enough to cause an entanglement or jam. The control surface activation may be input at any convenient location between the mockup terminus and the cockpit.
SECTION (3):
c. The tests described in this section will show the control system is free from excessive deflection as required by § 23.683(a)(3). These tests complete this means of compliance that the control system is free from jamming and excessive friction as required by § 23.683(a)(1) and (2). They also show that structural deformations do not interfere with safe operation as required by § 23.305(a). These tests meet the limit load static test requirements of § 23.681(a). Accomplish the following:
(1) With the adjacent fixed surface (wing, horizontal tail, or vertical tail) unloaded, support the control surface being tested while it is found in the neutral position.
(2) Load the control surfaces to the critical limit loads, as described in parameter 6 above, and evaluate their proximity to the fixed adjacent structure for jamming or contact.
(3) Load the pilots control until the control surface is just off the support.
(4) Operate the cockpit control in the direction opposite the load to the extent of its travel.
(5) The above procedure should be repeated in the opposite direction.
(6) The minimum loaded control surface travel must have adequate flight characteristics as specified in § 23.141.
(7) Under limit load, no signs of jamming, or of any permanent set of any connection, bracket, attachment, and so forth, may be present.
(8) The control system should operate freely without excessive friction.
NOTE: The tests described in section c. 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 effective February 12, 2009, AMC 23.683 is acceptable for FAA certification.
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.”