[Federal Register Volume 66, Number 197 (Thursday, October 11, 2001)]
[Notices]
[Pages 51991-51994]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 01-25085]


-----------------------------------------------------------------------

DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration


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 316-946-4111; fax 316-946-4407; email [email protected].

SUPPLEMENTARY INFORMATION:

[[Page 51992]]

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 Sec. 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 Sec. 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 Sec. 23.683 of the regulations. Testing, not analysis, must be used 
to show compliance with Sec. 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 Sec. 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 
Secs. 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 
Sec. 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 
Sec. 23.683, in some cases analysis may be acceptable for showing 
compliance with Sec. 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 
Sec. 23.305, paragraph (a), in accordance with Sec. 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 
Sec. 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 
Sec. 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 Sec. 23.305, paragraph (a), to be shown by tests, the 
following test procedure is one means to simultaneously demonstrate 
compliance with both Sec. 23.305, paragraph (a), and Sec. 23.683. It 
also demonstrates compliance with Sec. 23.681,

[[Page 51993]]

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 (Sec. 23.683, 
paragraph (b)(1)). Per Sec. 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 Sec. 23.683, paragraphs (a)(1), 
(2), and (3). They also support the demonstration that structural 
deformations not interfere with safe operation as required by 
Sec. 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 Sec. 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 Sec. 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 
Sec. 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 Sec. 23.683, paragraphs (a)(1) and (2). They 
also demonstrate that structural deformations do not interfere with 
safe operation, as required by Sec. 23.305, paragraph (a). These tests 
meet the limit load static test requirements of Sec. 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

[[Page 51994]]

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 Sec. 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.
[FR Doc. 01-25085 Filed 10-10-01; 8:45 am]
BILLING CODE 4910-13-P