Section 25.679

14 CFR Ch. I (1-1-19 Edition)

(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.
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as
amended by Amdt. 25-23, 35 FR 5675, Apr. 8,
1970; Amdt. 25-115, 69 FR 40527, July 2, 2004]

Section 25.679

Control system gust locks.

(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.
Section 25.681

Limit load static tests.

(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
analyses or individual load tests) with
the special factor requirements for
control system joints subject to angular motion.

spaschal on DSK3GDR082PROD with CFR

Section 25.683

Operation tests.

(a) 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 - 
(1) Jamming;
(2) Excessive friction; and
(3) Excessive deflection.
(b) It must be shown by analysis and,
where necessary, by tests, that in the
presence of deflections of the airplane
structure due to the separate application of pitch, roll, and yaw limit maneuver loads, the control system, when
loaded to obtain these limit loads and
operated within its operational range
of deflections, can be exercised about
all control axes and remain free from - 
(1) Jamming;
(2) Excessive friction;
(3) Disconnection; and
(4) Any form of permanent damage.
(c) It must be shown that under vibration loads in the normal flight and
ground operating conditions, no hazard
can result from interference or contact
with adjacent elements.
[Amdt. 25-139, 79 FR 59430, Oct. 2, 2014]

Section 25.685 Control system details.
(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.
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as
amended by Amdt. 25-38, 41 FR 55466, Dec. 20,
1976]

Section 25.689 Cable systems.
(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.

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