proven by limit and ultimate load tests in which the actual stress conditions are simulated in the fitting and the surrounding structure. This factor shall apply to all portions of the fitting, the means of attachment, and the bearing on the members joined.

(2) In the case of integral fittings the part shall be treated as a fitting up to the point where the section properties become typical of the member.

(3) The fitting factor need not be employed where a type of joint made in accordance with approved practices is based on comprehensive test data, e. g., continuous joints in metal plating, welded joints, and scarf joints in wood.

(4) A fitting factor need not be employed with respect to the bearing surface of a part if the bearing factor used (see paragraph (b) of this section) is of greater magnitude than the fitting factor.

§ 4b.308 Flutter, deformation, and vibration

Compliance with the following provisions shall be shown by such calculations, resonance tests, or other tests as are found necessary by the Administrator.

(a) Flutter prevention. The airplane shall be designed to be free from flutter of wing and tail units, including all control and trim surfaces, and from divergence (i. e. unstable structural distortion due to aerodynamic loading), at all speeds up to 1.2 VD. A smaller margin above VD shall be acceptable if the characteristics of the airplane (including the effects of compressibility) render a speed of 1.2 VD unlikely to be achieved, and if it is shown that a proper margin of damping exists at speed VD. In the absence of more accurate data, the terminal velocity in a dive of 30 degrees to the horizontal shall be acceptable as the maximum speed likely to be achieved. If concentrated balance weights are used on control surfaces, their effectiveness and strength, including supporting structure, shall be substantiated. If control surface flutter dampers are installed to meet the requirements of this section, it shall be shown that a single failure in the flutter damper system will not preclude continued safe flight of the airplane at any speed up to VD.

(b) Loss of control due to structural deformation. The airplane shall be designed to be free from control reversal and from undue loss of longitudinal,

lateral, and directional stability and control as a result of structural deformation, including that of the control surface covering, at all speeds up to the speed prescribed in paragraph (a) of this! section for flutter prevention.

(c) Vibration and buffeting. The airplane shall be designed to withstand all vibration and buffeting which might occur in any likely operating conditions. [Amdt. 4b-6, 17 F.R. 1093, Feb. 5, 1952 as amended by Amdt. 4b-11, 24 F.R. 7069, Sept. 1, 1959]

The requirements of §§ 4b.311 through 4b.313 shall apply to the design of fixed and movable control surfaces.

§ 4b.311 Proof of strength.

(a) Control surface limit load tests shall be conducted to prove compliance with limit load requirements.

(b) Control surface tests shall include the horn or fitting to which the control system is attached.

(c) Analyses or individual load tests shall be conducted to demonstrate compliance with the special factor requirements for control surface hinges. (See §§ 4b.307 and 4b.313 (a).) § 4b.312 Installation.

(a) Movable tail surfaces shall be so installed that there is no interference between any two surfaces when one is held in its extreme position and all the others are operated through their full angular movement.

(b) When an adjustable stabilizer is used, stops shall be provided which will limit its travel, in the event of failure of the adjusting mechanism, to a range equal to the maximum required to trim the airplane in accordance with § 4b.140. § 4b.313 Hinges.

(a) Control surface hinges, except ball and roller bearings, shall incorporate a special factor of not less than 6.67 with respect to the ultimate bearing strength of the softest material used as a bearing.

(b) For hinges incorporating ball or roller bearings, the approved rating of the bearing shall not be exceeded.

(c) Hinges shall provide sufficient strength and rigidity for loads parallel to the hinge line.

All controls and control systems shall operate with ease, smoothness, and positiveness appropriate to their function. The elements of the flight control system shall incorporate design features or shall be distinctively and permanently marked to minimize the possibility of incorrect assembly which could result in malfunctioning of the control system. Tab control systems shall be such that disconnection or failure of any element at speeds up to Vc cannot jeopardize the safety of flight.

(See also §§ 4b.308,

4b.350, and 4b.353.) An adjustable stabilizer shall incorporate means to permit, after the occurrence of any reasonably probable single failure of the actuating system, such adjustment as would be necessary for continued safety of the flight.

(b) Power boost and power-operated control systems shall be designed in accordance with the provisions of subparagraphs (1) and (2) of this paragraph.

(1) When a power boost or poweroperated control system is used, an alternate system shall be immediately available such that any single failure in the power portion shall not preclude continued safe flight and landing. Such alternate system may be a duplicate power portion or a manually operated mechanical system. The power portion shall include the power source (e.g., hydraulic pumps), and such items as valves, lines, and actuators. The failure of mechanical parts (such as piston rods and links) and the jamming of power cylinders need not be considered if such failure or jamming is considered to be extremely remote.

(2) Both the primary and alternate systems shall be operable in the event of a single engine failure. For airplanes with more than two engines, at least one system shall be operable in the event of failure of any two engines. It shall be shown by analysis that in the event of loss of power on all engines, the airplane is not uncontrollable.

[Amdt. 4b-3, 21 F.R. 992, Feb. 11, 1956, as amended by Amdt. 4b-6, 22 F.R. 5564, July 16, 1957; Amdt. 4b-11, 24 F.R. 7069, Sept. 1, 1959]

§ 4b.321 Two-control airplanes.

Two-control airplanes shall be capable of continuing safely in flight and landing in the event of failure of any one

connecting element in the directionallateral flight control system,

§ 4b.322 Trim controls and systems.

(a) Trim controls shall be designed to safeguard against inadvertent or abrupt operation.

(b) Each trim control shall operate in the plane and with the sense of motion of the airplane. (See fig. 4b-16.)

(c) Means shall be provided adjacent to the trim control to indicate the direction of the control movement relative to the airplane motion.

(d) Means shall be provided to indicate the position of the trim device with respect to the range of adjustment. The indicating means shall be clearly visible.

(e) Trim devices shall be capable of continued normal operation in the event of failure of any one connecting or transmitting element of the primary flight control system.

(f) All trim control systems shall be designed to prevent creeping in flight. Trim tab controls shall be irreversible, unless the tab is appropriately balanced and shown to be free from flutter.

(g) Where an irreversible tab control system is employed, the portion from the tab to the attachment of the irreversible unit to the airplane structure shall consist of a rigid connection.

[15 F.R. 3543, June 8, 1950, as amended by Amdt. 4b-8, 23 F.R. 2591, Apr. 19, 1958]

§ 4b.323 Wing flap controls.

(a) The wing flap controls shall operate in a manner to permit the flight crew to place the flaps in all of the takeoff, en route, approach, and landing positions established under § 4b.111 and to maintain these positions thereafter without further attention on the part of the crew, except for flap movement produced by an automatic flap positioning or load limiting device.

(b) The wing flap control shall be located and designed to render improbable its inadvertent operation.

(c) The rate of motion of the wing flap in response to the operation of the control and the characteristics of the automatic flap positioning or load limiting device shall be such as to obtain satisfactory flight and performance characteristics under steady or changing conditions of air speed, engine power, and airplane attitude.

(d) The wing flap control shall be designed to retract the flaps from the fully extended position during steady flight at maximum continuous engine power at all speeds below Vp+10 (m. p. h.).

(e) Means shall be provided to indicate the take-off, en route, approach, and landing flap positions.

(f) If any extension of the flaps beyond the landing position is possible, the flap control shall be clearly marked to identify such range of extension. § 4b.324 Wing flap interconnection.

(a) The motion of wing flaps on opposite sides of the plane of symmetry shall be synchronized by a mechanical interconnection unless the airplane is demonstrated to have safe flight characteristics while the flaps are retracted on one side and extended on the other. When a mechanical interconnection is employed, means shall be provided to insure against hazardous unsymmetrical operation of the wing flaps after any reasonably possible single failure of the flap actuating system.

(b) Where a wing flap interconnection is used, it shall be designed to account for the applicable unsymmetrical loads, including those resulting from flight with the engines on one side of the plane of symmetry inoperative and the remaining engines at take-off power. For airplanes with flaps which are not subjected to slipstream conditions, the structure shall be designed for the loads imposed when the wing flaps on one side are carrying the most severe load occurring in the prescribed symmetrical conditions and those on the other side are carrying not more than 80 percent of that load.

[15 F. R. 3543, June 8, 1950, as amended by Amdt. 4b-6, 17 F. R. 1093, Feb. 5, 1952; Amdt. 4b-3, 21 F. R. 992, Feb. 11, 1956]

§ 4b.324-1 Procedure for demonstrating wing flaps that are not interconnected (FAA policies which apply to § 4b.324(a)).

If the wing flaps are not mechanically interconnected, tests should be conducted to simulate flap malfunctioning (to the extent of the flaps being retracted on one side and extended on the other) during take-offs, approaches, and landings to demonstrate that the airplane is safe under these conditions. [Supp. 24, 19 F. R. 4461, July 20, 1954]

§ 4b.325 Control system stops.

(a) All control systems shall be provided with stops which positively limit the range of motion of the control surfaces.

(b) Control system stops shall be so located in the system that wear, slackness, or take-up adjustments will not affect adversely the control characteristics of the airplane because of a change in the range of surface travel.

(c) Control system stops shall be capable of withstanding the loads corresponding with the design conditions for the control system.

§ 4b.326 Control system locks.

Provision shall be made to prevent damage to the control surfaces (including tabs) and the control system which might result from gusts striking the airplane while it is on the ground or water (see also § 4b.226). If a device provided for this purpose, when engaged, prevents normal operation of the control surfaces by the pilot, it shall comply with the following provisions.

(a) The device shall either automatically disengage when the pilot operates the primary flight controls in a normal manner, or it shall limit the operation of the airplane in such a manner that the pilot receives unmistakable warning at the start of take-off.

(b) Means shall be provided to preclude the possibility of the device becoming inadvertently engaged in flight. [Amdt. 4b-6, 17 F. R. 1093, Feb. 5, 1952] § 4b.327 Static tests.

Tests shall be conducted on control systems to show compliance with limit load requirements in accordance with the following provisions.

(a) The direction of the test loads shall be such as to produce the most severe loading in the control system.

(b) The tests shall include all fittings, pulleys, and brackets used in attaching the control system to the main structure.

(c) Analyses or individual load tests shall be conducted to demonstrate compliance with the special factor requirements for control system joints subjected to angular motion. (See §§ 4b.307 and 4b.329 (b).)

All details of control systems shall be designed and installed to prevent jamming, chafing, and interference from cargo, passengers, and loose objects. Precautionary means shall be provided in the cockpit to prevent the entry of foreign objects into places where they would jam the control systems. Provisions shall be made to prevent the slapping of cables or tubes against other parts of the airplane. The following detail requirements shall be applicable with respect to cable systems and joints.

(a) Cable systems. (1) Cables, cable fittings, turnbuckles, splices, and pulleys shall be of an approved type.

(2) Cables smaller than 8-inch diameter shall not be used in the aileron, elevator, or rudder systems.

(3) The design of cable systems shall be such that there will be no hazardous change in cable tension throughout the range of travel under operating conditions and temperature variations.

(4) Pulley types and sizes shall correspond with the cables used.

(5) All pulleys and sprockets shall be provided with closely fitted guards to prevent the cables and chains being displaced or fouled.

(6) Pulleys shall lie in the plane passing through the cable within such limits that the cable does not rub against the pulley flange.

(7) Fairleads shall be so installed that they do not cause a change in cable direction of more than 3°.

(8) Clevis pins (excluding those not subject to load or motion) retained only by cotter pins shall not be used in the control system.

(9) Turnbuckles attached to parts having angular motion shall be installed to prevent positively any binding throughout the range of travel.

(10) Provision for visual inspection shall be made at all fairleads, pulleys, terminals, and turnbuckles.

(b) Joints. (1) Control system Joints subjected to angular motion in

push-pull systems, excepting ball and roller bearing systems, shall incorporate a special factor of not less than 3.33 with respect to the ultimate bearing strength of the softest material used as a bearing.

(2) It shall be acceptable to reduce the factor specified in subparagraph (1) of this paragraph to a value of 2.0 for joints in cable control systems.

(3) The approved rating of ball and roller bearings shall not be exceeded. [15 F. R. 3543, June 8, 1950, as amended by Amdt. 4b-6, 17 F. R. 1093, Feb. 5, 1952] § 4b.329-1 Installation of turnbuckles (FAA policies which apply to § 4b.329 (a)).

Fork ends of turnbuckles should not be attached directly to control surface horns or to bellcrank arms unless a positive means (such as the use of shackles, links, universal joints, spacer bushings, ball bearings, etc.) is used to prevent binding of turnbuckles relative to the horns or bellcrank arms or unless it can be shown that turnbuckles have adequate strength assuming one end fixed to the horn or arm and the design cable loads pulling off the other end at 5° to the turnbuckle axis. There should be no interference between the horns or bellcrank arms and the fork ends of turnbuckles throughout the range of motion of the control surfaces.

[Supp. 25, 20 F. R. 2278, Apr. 8, 1955] § 4b.329-2 Safetying of turnbuckles (FAA policies which apply to § 4b.329).

Section 4b.300 requires in part that there be no design features or details which experience has shown to be hazardous or unreliable. Experience has shown that the reliability of turnbuckles should be insured by safetying with wire as shown in figure 5. After safetying the turnbuckle, no more than three threads should be exposed on either side of the turnbuckle barrel and the ends of each safety wire should be securely fastened by at least four wraps. A turnbuckle safetying guide is given in table 1. [Supp. 25, 20 F. R. 2278, Apr. 8, 1955]

§ 4b.329-3 Approval of control system components (FAA policies which apply to § 4b.329 (a)).

The Administrator does not issue specific approvals as such for cables, cable fittings, turnbuckles, splices, pulleys, etc., for general use on aircraft. Approval is