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14 CFR Ch. I (1–1–19 Edition) 

§ 25.679 

(c) Trim control systems must be de-

signed to prevent creeping in flight. 
Trim tab controls must be irreversible 
unless the tab is appropriately bal-
anced and shown to be free from flut-

(d) If an irreversible tab control sys-

tem 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] 

§ 25.679

Control system gust locks. 

(a) There must be a device to prevent 

damage to the control surfaces (includ-
ing tabs), and to the control system, 
from gusts striking the airplane while 
it is on the ground or water. If the de-
vice, when engaged, prevents normal 
operation of the control surfaces by the 
pilot, it must— 

(1) Automatically disengage when the 

pilot operates the primary flight con-
trols in a normal manner; or 

(2) Limit the operation of the air-

plane so that the pilot receives unmis-
takable warning at the start of takeoff. 

(b) The device must have means to 

preclude the possibility of it becoming 
inadvertently engaged in flight. 

§ 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 angu-
lar motion. 

§ 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 pow-
ered portions of the control system are 
loaded to the maximum load expected 

in normal operation, the system is free 

(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 applica-
tion of pitch, roll, and yaw limit ma-
neuver 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 vi-

bration 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] 

§ 25.685

Control system details. 

(a) Each detail of each control sys-

tem must be designed and installed to 
prevent jamming, chafing, and inter-
ference from cargo, passengers, loose 
objects, or the freezing of moisture. 

(b) There must be means in the cock-

pit to prevent the entry of foreign ob-
jects 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, 

§ 25.689

Cable systems. 

(a) Each cable, cable fitting, turn-

buckle, splice, and pulley must be ap-
proved. In addition— 

(1) No cable smaller than 



inch in 

diameter may be used in the aileron, 
elevator, or rudder systems; and 

(2) Each cable system must be de-

signed so that there will be no haz-
ardous change in cable tension 
throughout the range of travel under 
operating conditions and temperature 

VerDate Sep<11>2014 

12:50 Apr 30, 2019

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