(b) Each stop must be located so that wear, slackness, or take-up adjustments will not adversely affect the control characteristics of the airplane because of a change in the range of surface travel.
(c) Each stop must be able to withstand any loads corresponding to the design conditions for the control system.
[Amdt. 25-38, 41 FR 55466, Dec. 20, 1976]
b. Intent of Rule. The purpose of this rule is to provide a positive means of limiting the movement of the control surfaces. It also requires that the stops be designed to withstand the design loads on the control system.
c. Background. This rule was carried forward from § 4b.325 of the Civil Air Regulations (CAR).
Amendment 25-38 made editorial changes to paragraph (a).
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-38, 41 FR 55466, December 20, 1976.
e. References. None.
813 - 818. [RESERVED]
819. SECTION 25.677 - TRIM SYSTEMS.
a. Rule Text.
(a) Trim controls must be designed to prevent inadvertent or abrupt operation and to operate in the plane, and with the sense of motion, of the airplane.
(b) There must be means adjacent to the trim control to indicate the direction of the control movement relative to the airplane motion. In addition, there must be clearly visible means to indicate the position of the trim device with respect to the range of adjustment.
(c) Trim control systems must be designed to prevent creeping in flight. Trim tab controls must be irreversible unless the tab is appropriately balanced and shown to be free from flutter.
(d) If an irreversible tab control system is used, the part from the tab to the attachment of the irreversible unit to the airplane structure must consist of a rigid connection.
[Amdt. 25-23, 35 FR 5675, Apr. 8, 1970] 924
b. Intent of Rule. The purpose of this rule is to ensure that the trim system is designed to preclude inadvertent out of trim conditions and flutter problems.
c. Background. This rule was carried forward from § 4b.322 of the Civil Air Regulations (CAR).
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-23, 35 FR 5675, April 8, 1970.
e. References. Advisory Circular (AC) 25.629-1, or latest revision, Flutter Substantiation of Transport Category Airplanes.
820 - 827. [RESERVED]
828. SECTION 25.679 - CONTROL SYSTEM GUST LOCKS.
a. Rule Text.
(a) There must be a device to prevent damage to the control surfaces (including tabs), and to the control system, from gusts striking the airplane while it is on the ground or water. If the device, when engaged, prevents normal operation of the control surfaces by the pilot, it must -
(1) Automatically disengage when the pilot operates the primary flight controls in a normal manner; or
(2) Limit the operation of the airplane so that the pilot receives unmistakable warning at the start of takeoff.
(b) The device must have means to preclude the possibility of it becoming
inadvertently engaged in flight.
b. Intent of Rule. The purpose of this rule is to protect the flight controls from ground gust conditions and to prevent the gust lock system from interfering with the normal operation of the airplane.
c. Background. This rule was carried forward from § 4b.326 of the Civil Air Regulations (CAR).
d. Acceptable Compliance Methods. Compliance with this requirement may be shown by test(s) and/or analysis.
e. References. None.
829 - 835. [RESERVED] 925
836. SECTION 25.681 - LIMIT LOAD STATIC TESTS.
a. Rule Text.
(a) 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.
(b) Compliance must be shown (by analysis or individual load tests) with the special factor requirements for control system joints subject to angular motion.
b. Intent of Rule. The purpose of this rule is to ensure that the control systems are able to withstand any loads that are likely to occur in service, and to validate that deformations will not be hazardous.
c. Background. This rule was carried forward from § 4b.327 of the Civil Air Regulations (CAR).
d. Acceptable Compliance Methods. Compliance with this requirement may be shown by tests(s).
e. References. None.
837 - 844. [RESERVED]
845. SECTION 25.683 - OPERATION TESTS.
a. Rule Text.
It must be shown by operation tests that when portions of the control system subject to pilot effort loads are loaded to 80 percent of the limit load specified for the system and the powered portions of the control system are loaded to the maximum load expected in normal operation, the system is free from -
(a) Jamming;
(b) Excessive friction; and
(c) Excessive deflection.
[Amdt. 25-23, 35 FR 5675, Apr. 8, 1970] 926
b. Intent of Rule. The purpose of this rule is to ensure that the control systems are free from jamming, excessive friction, and excessive deflections under likely operating loads.
c. Background. This rule was carried forward from § 4b.328 of the Civil Air Regulations (CAR).
d. Acceptable Compliance Methods. For guidance with this requirement, refer to the preamble of Amendment 25-23, 35 FR 5675, April 8, 1970.
e. References. None.
846 - 851. [RESERVED]
852. SECTION 25.685 - CONTROL SYSTEM DETAILS.
a. Rule Text.
(a) Each detail of each control system must be designed and installed to prevent jamming, chafing, and interference from cargo, passengers, loose objects, or the freezing of moisture.
(b) There must be means in the cockpit to prevent the entry of foreign objects into places where they would jam the system.
(c) There must be means to prevent the slapping of cables or tubes against other parts.
(d) Sections 25.689 and 25.693 apply to cable systems and joints.
[Amdt. 25-38, 41 FR 55466, Dec. 20, 1976]
b. Intent of Rule. The purpose of this rule is to ensure that the control systems will not become jammed due to foreign objects or ice formation.
c. Background. This rule was carried forward from § 4b.329 of the Civil Air Regulations (CAR).
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-38, 41 FR 55466, December 20, 1976.
e. References. None.
853 - 860. [RESERVED] 927
861. SECTION 25.689 - CABLE SYSTEMS.
a. Rule Text.
(a) Each cable, cable fitting, turnbuckle, splice, and pulley must be approved. In addition
(1) No cable smaller than 1/8 inch in diameter may be used in the aileron,
elevator, or rudder systems; and
(2) Each cable system must be designed so that there will be no hazardous change in cable tension throughout the range of travel under operating conditions and temperature variations.
(b) Each kind and size of pulley must correspond to the cable with which it is used. Pulleys and sprockets must have closely fitted guards to prevent the cables and chains from being displaced or fouled. Each pulley must lie in the plane passing through the cable so that the cable does not rub against the pulley flange.
(c) Fairleads must be installed so that they do not cause a change in cable
direction of more than three degrees.
(d) Clevis pins subject to load or motion and retained only by cotter pins may not be used in the control system.
(e) Turnbuckles must be attached to parts having angular motion in a manner that will positively prevent binding throughout the range of travel.
(f) There must be provisions for visual inspection of fairleads, pulleys, terminals, and turnbuckles.
b. Intent of Rule. The purpose of this rule is to prevent damage to control cable runs due to wear, fatigue, fire, lightning, and accidental damage.
c. Background. This rule was carried forward from § 4b.329 of the Civil Air Regulations (CAR).
d. Acceptable Compliance Methods. Compliance with this requirement may be shown by design review.
e. References. None.
862 - 867. [RESERVED] 928
868. SECTION 25.693 - JOINTS.
a. Rule Text.
Control system joints (in push-pull systems) that are subject to angular motion, except those in ball and roller bearing systems, must have a special factor of safety of not less than 3.33 with respect to the ultimate bearing strength of the softest material used as a bearing. This factor may be reduced to 2.0 for joints in cable control systems. For ball or roller bearings, the approved ratings may not be exceeded.
[Amdt. 25-72, 55 FR 29777, Jul. 20, 1990]
b. Intent of Rule. This rule is intended to account for wear, abuse, and shock loading in the bearing.
c. Background. This rule was carried forward from § 4b.329 of the Civil Air Regulations (CAR).
Amendment 25-72 removed the reference to MIL-HDBK-5.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-72, 55 FR 29777, July 20, 1990.
e. References. None.
869 - 875. [RESERVED]
876. SECTION 25.695 - POWER-BOOST AND POWER-OPERATED CONTROL SYSTEM. [Removed] NOTE: This section was removed since it became extraneous and superseded by the proposed general requirements under 25.671(c) and (d). Certification basis earlier than Amendment 25-23 should include § 25.695.
a. Rule Text.
(a) If a power-boost or power-operated control system is used, an alternate system must be immediately available to allow continued safe flight and landing in the event of a single failure in the power portion.
(b) Each alternate system may be a duplicate power portion or a manually operated mechanical system. The power portion includes the power source, such as hydraulic pumps, and such items as valves, lines, and actuators.
(c) The failure of mechanical parts (such as piston rods and links) and the jamming of power cylinders must be considered unless they are extremely remote.
(d) Both the primary and alternate systems must be operable if any engine fails.
For airplanes with more than two engines, at least one system must be operable if any two engines fail. It must be shown by analysis that the airplane is controllable if all engines fail.
[Amdt. 25-23, 35 FR 5675, Apr. 8, 1970]
877. SECTION 25.697 - LIFT AND DRAG DEVICES, CONTROLS.
a. Rule Text.
(a) Each lift device control must be designed so that the pilots can place the device in any takeoff, en route, approach, or landing position established under § 25.101(d). Lift and drag devices must maintain the selected positions, except for movement produced by an automatic positioning or load limiting device, without further attention by the pilots.
(b) Each lift and drag device control must be designed and located to make inadvertent operation improbable. Lift and drag devices intended for ground operation only must have means to prevent the inadvertent operation of their controls in flight if that operation could be hazardous.
(c) The rate of motion of the surfaces in response to the operation of the control and the characteristics of the automatic positioning or load limiting device must give satisfactory flight and performance characteristics under steady or changing conditions of airspeed, engine power, and airplane altitude.
(d) The lift device control must be designed to retract the surfaces from the fully extended position, during steady flight at maximum continuous engine power at any speed below VF+9.0 (knots).
[Amdt. 25-23, 35 FR 5675, Apr. 8, 1970; Amdt. 25-46, 43 FR 50595, Oct. 30, 1978; Amdt. 25-57, 49 FR 6848, Feb. 23, 1984]
b. Intent of Rule. This rule is intended to ensure safe operation of the lift and drag devices under all operating conditions expected in service.
c. Background. This rule was carried forward from § 4b.323 of the Civil Air Regulations (CAR).
Amendment 25-23 made a major editorial change to the rule. Amendments 25-46 and 25-57 made additional editorial changes.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-23, 35 FR 5675, April 8, 1970; Amendment 25-46, 43 FR 50595, October 30, 1978; and Amendment 25-57, 49 FR 6848, February 23, 1984.
e. References. None.
878 - 886. [RESERVED]
887. SECTION 25.699 - LIFT AND DRAG DEVICE INDICATOR.
a. Rule Text.
(a) There must be means to indicate to the pilots the position of each lift or drag device having a separate control in the cockpit to adjust its position. In addition, an indication of unsymmetrical operation or other malfunction in the lift or drag device systems must be provided when such indication is necessary to enable the pilots to prevent or counteract an unsafe flight or ground condition, considering the effects on flight characteristics and performance.
(b) There must be means to indicate to the pilots the takeoff, en route, approach, and landing lift device positions.
(c) If any extension of the lift and drag devices beyond the landing position is possible, the controls must be clearly marked to identify this range of extension.
[Amdt. 25-23, 35 FR 5675, Apr. 8, 1970] NOTE: Refer to Advisory Circular (AC) 25-XX, Mechanical Systems Handbook, for guidance.
888. SECTION 25.701 - FLAP AND SLAT INTERCONNECTION.
a. Rule Text.
(a) Unless the airplane has safe flight characteristics with the flaps or slats retracted on one side and extended on the other, the motion of flaps or slats on opposite sides of the plane of symmetry must be synchronized by a mechanical interconnection or approved equivalent means.
(b) If a wing flap or slat interconnection or equivalent means is used, it must 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 takeoff power.
(c) For airplanes with flaps or slats that are not subjected to slipstream conditions, the structure must be designed for the loads imposed when the wing flaps or slats 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.
(d) The interconnection must be designed for the loads resulting when interconnected flap or slat surfaces on one side of the plane of symmetry are jammed and immovable while the surfaces on the other side are free to move and the full power of the surface actuating system is applied.
[Amdt. 25-23, 35 FR 5675, Apr. 8, 1970; Amdt. 25-46, 43 FR 50595, Oct. 30, 1978; Amdt. 25-72, 55 FR 29777, Jul. 20, 1990]
b. Intent of Rule. The purpose of this rule is to require the installation of interconnection systems unless the airplane has safe flight capability with a full asymmetric deployment of the flaps or slats, and to establish design load conditions for the interconnection system.
c. Background. This rule was carried forward from § 4b.324 of the Civil Air Regulations (CAR).
Amendment 25-23 added the requirement to consider flap jam loads on the interconnection.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-23, 35 FR 5675, April 8, 1970; Amendment 25-46, 43 FR 50595, October 30, 1978; and Amendment 25-72, 55 FR 29777, July 20, 1990. Advisory Circular (AC) 25 14, or latest revision, provides guidance information for showing compliance with the requirements for structural and functional safety standards for high lift and drag devices and their operating systems.
e. References. Advisory Circular 25-14, or latest revision, High Lift and Drag Devices; and Advisory Circular 25.1309-1A, or latest revision, System Design Analysis.
889 - 896. [RESERVED] 932
Section 4. LANDING GEAR
897. SECTION 25.721 - GENERAL.
a. Rule Text.
(a) The main landing gear system must be designed so that if it fails due to overloads during takeoff and landing (assuming the overloads to act in the upward and aft directions), the failure mode is not likely to cause
(1) For airplanes that have a passenger seating configuration, excluding pilots seats, of nine seats or less, the spillage of enough fuel from any fuel system in the fuselage to constitute a fire hazard; and
(2) For airplanes that have a passenger seating configuration, excluding pilots seats, of 10 seats or more, the spillage of enough fuel from any part of the fuel system to constitute a fire hazard.
(b) Each airplane that has a passenger seating configuration excluding pilots seats, of 10 seats or more must be designed so that with the airplane under control it can be landed on a paved runway with any one or more landing gear legs not extended without sustaining a structural component failure that is likely to cause the spillage of enough fuel to constitute a fire hazard.
(c) Compliance with the provisions of this section may be shown by analysis or tests, or both.
[Amdt. 25-15, 32 FR 13262, Sep. 20, 1967; Amdt. 25-23, 35 FR 5676, Apr. 8, 1970; Amdt. 25-32, 37 FR 3969, Feb. 24, 1972]
b. Intent of Rule. The purpose of this rule is to ensure that if the landing gear fails due to overloads during takeoff and landing, the failure itself will not cause the spillage of enough fuel from any part of the fuel system to constitute a fire hazard.
c. Background. This rule was added by Amendment 25-15 (Sept. 20, 1967). Amendments 25 23 and 25-32 extended the application of the rule to include all fuel systems.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-15, 32 FR 13262, September 20, 1967; Amendment 25-23, 35 FR 5676, April 8, 1970; and Amendment 25-32, 37 FR 3969, February 24, 1972.
e. References. None.
898 - 907. [RESERVED] 1007
1008
908. SECTION 25.723 - SHOCK ABSORPTION TESTS.
a. Rule Text.
(a) It must he shown that the limit load factors selected for design in accordance with § 25.473 for takeoff and landing weights, respectively, will not be exceeded.
This must be shown by energy absorption tests except that analyses based on earlier tests conducted on the same basic landing gear system which has similar energy absorption characteristics may be used for increases in previously approved takeoff and landing weights.
(b) The landing gear may not fail in a test, demonstrating its reserve energy absorption capacity, simulating a descent velocity of 12 f.p.s. at design landing weight, assuming airplane lift not greater than the airplane weight acting during the landing impact.
[Amdt. 25-23, 35 FR 5675, Apr. 8, 1970; Amdt. 25-46, 43 FR 50595, Oct. 30, 1978; Amdt. 25-72, 55 FR 29777, July 20, 1990]
b. Intent of Rule. The purpose of this rule is to ensure that the landing gear has sufficient strength and energy absorption capability to withstand any landing conditions likely to be encountered in service.
The rule also ensures that the airplane limit load factors will not be exceeded.
c. Background. This rule was carried forward from §§ 4b.330, 4b.331, and 4b.332 of the Civil Air Regulations (CAR). Amendment 25-72 changed the rule to allow analysis for gross weight increases even when some modifications to the basic landing gear were required. In addition to the changes in air and oil charges, these modifications might include changes in the metering pins and orifice plates.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-23, 35 FR 5675, April 8, 1970; Amendment 25-46, 43 FR 50595, October 30, 1978; and Amendment 25-72, 55 FR 29777, July 20, 1990.
e. References. None.
909 - 913. [RESERVED] 1009