245 

Federal Aviation Administration, DOT 

§ 25.395 

be conservatively estimated, consid-
ering the characteristics of the par-
ticular engine-propeller-airplane com-
bination. 

(b) Pilot corrective action may be as-

sumed to be initiated at the time max-
imum yawing velocity is reached, but 
not earlier than two seconds after the 
engine failure. The magnitude of the 
corrective action may be based on the 
control forces specified in § 25.397(b) ex-
cept that lower forces may be assumed 
where it is shown by anaylsis or test 
that these forces can control the yaw 
and roll resulting from the prescribed 
engine failure conditions. 

§ 25.371

Gyroscopic loads. 

The structure supporting any engine 

or auxiliary power unit must be de-
signed for the loads, including gyro-
scopic loads, arising from the condi-
tions specified in §§ 25.331, 25.341, 25.349, 
25.351, 25.473, 25.479, and 25.481, with the 
engine or auxiliary power unit at the 
maximum rotating speed appropriate 
to the condition. For the purposes of 
compliance with this paragraph, the 
pitch maneuver in § 25.331(c)(1) must be 
carried out until the positive limit ma-
neuvering load factor (point A

2

in 

§ 25.333(b)) is reached. 

[Amdt. 25–141, 79 FR 73468, Dec. 11, 2014] 

§ 25.373

Speed control devices. 

If speed control devices (such as 

spoilers and drag flaps) are installed 
for use in en route conditions— 

(a) The airplane must be designed for 

the symmetrical maneuvers prescribed 
in §§ 25.333 and 25.337, the yawing ma-
neuvers in § 25.351, and the vertical and 
lateral gust and turbulence conditions 
prescribed in § 25.341(a) and (b) at each 
setting and the maximum speed associ-
ated with that setting; and 

(b) If the device has automatic oper-

ating or load limiting features, the air-
plane must be designed for the maneu-
ver and gust conditions prescribed in 
paragraph (a) of this section, at the 
speeds and corresponding device posi-
tions that the mechanism allows. 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–72, 55 FR 29776, July 20, 
1990; Amdt. 25–86, 61 FR 5222, Feb. 9, 1996; 
Amdt. 25–141, 79 FR 73468, Dec. 11, 2014] 

C

ONTROL

S

URFACE AND

S

YSTEM

L

OADS

 

§ 25.391

Control surface loads: Gen-

eral. 

The control surfaces must be de-

signed for the limit loads resulting 
from the flight conditions in §§ 25.331, 
25.341(a) and (b), 25.349, and 25.351, con-
sidering the requirements for— 

(a) Loads parallel to hinge line, in 

§ 25.393; 

(b) Pilot effort effects, in § 25.397; 
(c) Trim tab effects, in § 25.407; 
(d) Unsymmetrical loads, in § 25.427; 

and 

(e) Auxiliary aerodynamic surfaces, 

in § 25.445. 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–86, 61 FR 5222, Feb. 9, 
1996; Amdt. 25–141, 79 FR 73468, Dec. 11, 2014] 

§ 25.393

Loads parallel to hinge line. 

(a) Control surfaces and supporting 

hinge brackets must be designed for in-
ertia loads acting parallel to the hinge 
line. 

(b) In the absence of more rational 

data, the inertia loads may be assumed 
to be equal to 

KW, where— 

(1) 

K = 24 for vertical surfaces; 

(2) 

K = 12 for horizontal surfaces; and 

(3) 

W  = weight of the movable sur-

faces. 

§ 25.395

Control system. 

(a) Longitudinal, lateral, directional, 

and drag control system and their sup-
porting structures must be designed for 
loads corresponding to 125 percent of 
the computed hinge moments of the 
movable control surface in the condi-
tions prescribed in § 25.391. 

(b) The system limit loads of para-

graph (a) of this section need not ex-
ceed the loads that can be produced by 
the pilot (or pilots) and by automatic 
or power devices operating the con-
trols. 

(c) The loads must not be less than 

those resulting from application of the 
minimum forces prescribed in 
§ 25.397(c). 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–23, 35 FR 5672, Apr. 8, 
1970; Amdt. 25–72, 55 FR 29776, July 20, 1990; 
Amdt. 25–141, 79 FR 73468, Dec. 11, 2014] 

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