269 

Federal Aviation Administration, DOT 

§ 25.681 

(b) The design of the stability aug-

mentation system or of any other auto-
matic or power-operated system must 
permit initial counteraction of failures 
of the type specified in § 25.671(c) with-
out requiring exceptional pilot skill or 
strength, by either the deactivation of 
the system, or a failed portion thereof, 
or by overriding the failure by move-
ment of the flight controls in the nor-
mal sense. 

(c) It must be shown that after any 

single failure of the stability aug-
mentation system or any other auto-
matic or power-operated system— 

(1) The airplane is safely controllable 

when the failure or malfunction occurs 
at any speed or altitude within the ap-
proved operating limitations that is 
critical for the type of failure being 
considered; 

(2) The controllability and maneuver-

ability requirements of this part are 
met within a practical operational 
flight envelope (for example, speed, al-
titude, normal acceleration, and air-
plane configurations) which is de-
scribed in the Airplane Flight Manual; 
and 

(3) The trim, stability, and stall char-

acteristics are not impaired below a 
level needed to permit continued safe 
flight and landing. 

[Amdt. 25–23, 35 FR 5675 Apr. 8, 1970] 

§ 25.675

Stops. 

(a) Each control system must have 

stops that positively limit the range of 
motion of each movable aerodynamic 
surface controlled by the system. 

(b) Each stop must be located so that 

wear, slackness, or take-up adjust-
ments will not adversely affect the 
control characteristics of the airplane 
because of a change in the range of sur-
face travel. 

(c) Each stop must be able to with-

stand any loads corresponding to the 
design conditions for the control sys-
tem. 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–38, 41 FR 55466, Dec. 20, 
1976] 

§ 25.677

Trim systems. 

(a) Trim controls must be designed to 

prevent inadvertent or abrupt oper-
ation and to operate in the plane, and 

with the sense of motion, of the air-
plane. 

(b) There must be means adjacent to 

the trim control to indicate the direc-
tion 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 adjust-
ment. The indicator must be clearly 
marked with the range within which it 
has been demonstrated that takeoff is 
safe for all center of gravity positions 
approved for takeoff. 

(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-
ter. 

(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. 

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