01-25085. Policy for Control System Operation Tests

AGENCY:

Federal Aviation Administration, DOT.

ACTION:

Notice of policy statement.

SUMMARY:

This document announces the issuance of a policy statement pertaining to operation tests of normal, utility, acrobatic, and commuter category airplane control systems. This material is neither mandatory nor regulatory in nature and does not constitute a regulation.

DATES:

On February 22, 2001, the Small Airplane Directorate issued a proposed policy statement. On March 9, 2001, (66 FR 14243) we published the proposed policy statement for public comments. The final policy statement becomes effective on the issue date, which is shown at the end of this policy statement.

FOR FURTHER INFORMATION CONTACT:

Lester Cheng, Federal Aviation Administration, Small Airplane Directorate, Regulations and Policy Branch, ACE-111, 901 Locust, Room 301, Kansas City, Missouri 64106; telephone [phone]; fax [phone]; email [e-mail].

SUPPLEMENTARY INFORMATION:

Background

On March 9, 2001, (66 FR 14243) we published the proposed policy statement for public comments. Several comments were received, and those comments have been resolved. A copy of the final policy statement will be posted on the internet and directions to the location will be found at the “Latest News” page, which has the following address: http://www.faa.gov/​programs_​rsvp2/​smart/​faa_​home_​page/​certification/​ aircraft/small_ airplane_directorate_ news_latest.html

After reviewing the compliance methods in Advisory Circular (AC) 23-17, the Directorate determined there was additional information related to the compliance methods in AC 23-17, paragraph 23.683, that might be beneficial.

Policy

What Is the General Effect of This Proposed Policy?

Applicants and FAA Aircraft Certification Offices (ACO) involved with certification of small airplanes should generally follow this policy. Applicants should expect that the ACO would consider this information when making findings of compliance. However, in determining compliance with certification standards, each ACO has the discretion to deviate from these guidelines when the applicant demonstrates a suitable need. To ensure standardization, the ACO should coordinate deviation from this policy with the Small Airplane Directorate.

As with all advisory material, this statement of policy identifies one method, but not the only method, of compliance.

Regardless of the amount of travel of a control surface when tested as described above, 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 ensure compliance.

The method of showing compliance with § 23.683 presented in AC 23-17, paragraph 23.683, Operation Tests, discusses only the control system. It does not explicitly specify the consideration of loading on adjacent structures and elements. This 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 the following:

1. The first one, which is noted in AC 23-17, is section 23.305, paragraph (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.”

2. Section 23.307, (Subpart C—Structure, General) Proof of Structure, states that “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.”

3. Section 23.655, paragraph (a), (Subpart D—Design and Construction, Control Surfaces) Installation, requires that “Moveable surfaces must be installed so that there is no interference between any 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.”

4. Section 23.681, paragraph (a), [Subpart D—Design and Construction, Control Surfaces] Limit Load Static Tests, requires that “Compliance with the limit load requirements of this part must be shown by tests in which—

(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.”

5. Section 23.141, (Subpart B—Flight, Flight Characteristics) 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, not exceeding the maximum operating altitude established under § 23.1527, and without requiring exceptional piloting skill, alertness, or strength.”

To ensure 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 set-up may be required.

While testing is required for demonstration of compliance to § 23.683, in some cases analysis may be acceptable for showing compliance with § 23.305, paragraph (a). Section 23.307, paragraph (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, paragraph (a), in accordance with § 23.307, paragraph (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 demonstrate that clearance between controls and adjacent structure (under load) meets § 23.305, paragraph (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 demonstrate compliance with § 23.305, paragraph (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) The amount of 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 is possible that 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, paragraph (a), to be shown by tests, the following test procedure is one means to simultaneously demonstrate compliance with both § 23.305, paragraph (a), and § 23.683. It also demonstrates compliance with § 23.681, paragraph (a). These tests may be conducted as follows:

Except where otherwise specified, the tests described below in sections (1), (2), and (3) should be conducted within the following parameters (a. through h.)

Parameters

a. Conduct the control system operation tests by operating the controls from the pilot's compartment.

b. All the control surfaces must be installed to their adjacent fixed surface on the airframe (according to the type design).

c. The entire control system and adjacent fixed structure should be loaded.

d. The adjacent fixed surfaces (wings, horizontal stabilizers, vertical stabilizers, and so forth) should be loaded to provide deflections equivalent to critical limit load flight conditions.

e. The structural deflections should correspond 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.

f. The entire control system must be loaded to either the limit airloads or the limit pilot forces, whichever is less (§ 23.683, paragraph (b)(1)). Per § 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.

g. Minimum clearances around control surfaces and minimum tensions in cable systems should be defined and incorporated in the airplane's instructions for continued airworthiness. The test article should incorporate these minimum clearances and tensions, unless you otherwise account for them.

h. If reductions in the minimum clearances described in paragraph g above are possible due to 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.

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.

(1) 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, paragraphs (a)(1), (2), and (3). They also support the demonstration that structural deformations not interfere with safe operation as required by § 23.305, paragraph (a). Accomplish the following:

(i) Load the adjacent fixed aerodynamic surface (wing, horizontal tail, or vertical tail) in accordance with one of the conditions of paragraphs d, e, and f above.

(ii) Support the control surface being tested while it is located in the neutral position.

(iii) Load the control surfaces to the critical limit loads, as described in paragraph f above, and evaluate their proximity to the fixed adjacent structure for interference (contact).

(iv) Load the pilot's control until the control surface is just off the support.

(v) 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.

(vi) The control surface under loads described in paragraph f above must have adequate flight characteristics as specified in § 23.141.

(vii) 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.

(viii) The above procedure should be repeated in the opposite direction.

(ix) 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.

(x) 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.

(xi) 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.

(xii) Repeat this process for each of the critical loading conditions as defined by paragraphs d and f above.

Section (2)

(2) The tests described in this section support the demonstration that structural deformations not interfere with safe operation as required by § 23.305, paragraph (a). Accomplish the following:

(i) Load the adjacent fixed aerodynamic surface (wing, horizontal tail, or vertical tail) in accordance with one of the conditions of paragraph d and e above.

(ii) Operate the unloaded control system from stop to stop.

(iii) No signs of interference (contact) may be present.

(iv) The control system should operate freely without excessive friction.

(v) Repeat this process for each of the critical adjacent fixed surface loading conditions as defined by paragraphs d and e above.

Note 1:

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 per type design. A sufficiently representative mockup of remaining control system components must be used to ensure that the full length 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)

(3) The tests described in this section will demonstrate that the control system is free from excessive deflection as required by § 23.683, paragraph (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, paragraphs (a)(1) and (2). They also demonstrate that structural deformations do not interfere with safe operation, as required by § 23.305, paragraph (a). These tests meet the limit load static test requirements of § 23.681, paragraph (a). Accomplish the following:

(i) With the adjacent fixed surface (wing, horizontal tail, or vertical tail) unloaded, support the control surface being tested while it is located in the neutral position.

(ii) Load the control surfaces to the critical limit loads, as described in paragraph f above, and evaluate their proximity to the fixed adjacent structure for jamming or contact.

(iii) Load the pilot's control until the control surface is just off the support.

(iv) Operate the cockpit control in the direction opposite the load to the extent of its travel.

(v) The above procedure should be repeated in the opposite direction.

(vi) The minimum loaded control surface travel must have adequate flight characteristics as specified in Sec. 23.141.

(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 2:

The tests described in section (3) above are normally accomplished using a complete airplane. As a minimum, they must be completed using an airframe/control system that completely represents the final product from the cockpit controls to the control surface.

Regardless of the amount of travel of a control surface when tested as described above, the airplane must have adequate flight characteristics as specified in § 23.141. Any airplane that is a close derivative of a previous type 1 certificated airplane need not exceed the control surface travel of the original airplane; however, the flight characteristics should be tested to ensure compliance.

Issued in Kansas City, Missouri, on September 12, 2001.

Michael Gallagher,

Manager, Small Airplane Directorate, Aircraft Certification Service.