23.673 Primary flight controls No policy available as of June 30, 2001.
23.675 Stops No policy available as of June 30, 2001.
23.677 Trim systems Original Issue and Subsequent The trim system should prevent inadvertent, improper or abrupt trim operation. The direction of trim movement and its relation to its range of adjustment should be designed to prevent confusion.
Trim devices should be designed to continue normal operation with one failure of any connecting or transmitting element in the primary flight control system for (1) longitudinal trim in a single-engine airplane, and (2) longitudinal and directional trim in multiengine airplanes.
Amendment 23-7 and Subsequent The amendment requires there be adequate control for safe flight and landing (rather than to “continue normal operation”) using the trim devices following the failure of a connecting/transmitting element in the primary controls. Thus, the control system element failure must not cause a failure of the trim system.
Failures of the trim system must not prevent safe flight and landing.
Amendment 23-34 and Subsequent Probable powered trim runaways should be demonstrated for all part 23 airplanes so equipped. See AC 23-8A, Change 1, Flight Test Guide for Certification of part 23 Airplanes, for the procedure.
Even if trim runaways have been determined to be improbable using the guidance in AC
23.1309-1C, Equipment, Systems, and Installations in Part 23 Airplanes, appropriate trim runaway demonstrations in all axes are required to demonstrate that the airplane has no unsafe features. The FAA has accepted demonstration of control restrained trim runaways during malfunction testing for systems without a monitor/limiter regardless of the reliability and those with a monitor/limiter whose reliability is less than extremely improbable.
However, the FAA has determined this procedure is not acceptable in itself for failure conditions shown to be less than extremely improbable. To allow expansion of the 0 to 2g envelope, as specified in AC 23-8A, the FAA suggests a test procedure that incorporates both control restrained and unrestrained malfunctions. The following test matrix considers the probability of trim runaways, high airframe limit loads, control stick/wheel configuration and absence of an autopilot system. Because rudder trim can be adjusted without the pilot directly in the control loop (i.e., feet on the floor),
6 restrained runaways for rudder trim are not considered acceptable. (See Table 1 in this section.)
TABLE 1. TRIM SYSTEMS REQUIREMENTS Maximum Maximum Force Maximum Axis Time Load(g) Attitude (restrained and Rate of Force (unrestrained) Change unrestrained) Change (unrestrained) (restrained) ______________________________________________________________________________ ____ Pitch recognition structural +/-45 degrees 60 pounds 20 pounds/sec +3 seconds limits NTE
3.5g ______________________________________________________________________________ ____ Roll recognition structural +/-90 degrees 30 pounds 10 pounds/sec +3 seconds limits ______________________________________________________________________________ ____ Yaw recognition structural +/-30 degrees 150 pounds N/A +3 seconds limits (unrestrained only) Note 1: Restrained means the pilot is in the control loop (hands on) and unrestrained means the pilot is not in the control loop (hands off).
Note 2: Trim systems with a monitor/limiter will be tested at a magnitude just below that required for monitor/limiter trip.
Note 3: NTE is Not to Exceed.
23.679 Control system locks Original Issue and Subsequent Section 23.679(a) of part 23 and § 3.341(a) of the CAR require that if there is a device to lock the control system, there should be a means to give unmistakable warning to the pilot when the lock is engaged. Several accidents have occurred because the pilot did not remove the control system lock prior to takeoff. Many such accidents relate to internally applied locks, mostly pins installed at the control wheel column. Misuse and alteration of these installed locking devices, together with neglect by the pilot to perform a control freedom check before takeoff, contributed to such accidents.
When evaluating a control lock system, the following factors should be considered in finding compliance with the applicable regulation:
a. The warning should be easily observable during both day and night operations. Color, location, shape, and accessibility of the device, ease of removal with the pilot seated in the flying position, and legibility of any placards, etc., should be considered.
b. The system operation should be obvious. It should be possible to apply the lock only in such a manner that the required warning is provided.
c. When engaged, the lock should, by design, limit the operation of the airplane so that the pilot receives unmistakable warning in the cockpit before or at the start of takeoff by an effective means, such as one of the following:
(1) Preventing the application of sufficient engine power to attempt a takeoff.
(2) Displacement of primary pilot controls, such as the control wheel full forward.
(3) An aural warning device that cannot be disengaged.
For airplanes with separate locks for throttle and control column, where one lock (e.g., throttle) can be removed independently of the other, each lock should independently meet the criteria of paragraph (c) above.
23.681 Limit load static tests No policy available as of June 30, 2001.
23.683 Operation tests 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 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, that 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. Secondly, 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 Section 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 operation tests by operating the controls from the pilot's 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 for 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 pilot's control until the control 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 repeated in the opposite direction.
(f) Minimum control surface travel from the 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 torque’s specified by the regulations may be met.
ALTERNATE MEANS OF COMPLIANCE Applicants and FAA Aircraft Certification Offices (ACO's) 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 coordinate deviations from these guidelines with the Small Airplane Directorate when the applicant demonstrates a suitable need. To ensure standardization, the ACO should coordinate deviation from this policy with the Small Airplane Directorate. Recently, airplanes have been built with smaller gaps between control surfaces and structure than has been done in the past. In this case, this alternate means of compliance is appropriate. This
11 method also can have the added advantage of demonstrating compliance with several regulations in a single test series.
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 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 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 caseby-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 nonlinear 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
14 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).