245 

Federal Aviation Administration, DOT 

§ 25.391 

the conditions specified in paragraph 
(e) of this section. In addition, reason-
able design precautions must be taken 
to minimize the probability of parts 
becoming detached and injuring occu-
pants while in their seats. 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–54, 45 FR 60172, Sept. 
11, 1980; Amdt. 25–71, 55 FR 13477, Apr. 10, 
1990; Amdt. 25–72, 55 FR 29776, July 20, 1990; 
Amdt. 25–87, 61 FR 28695, June 5, 1996] 

§ 25.367

Unsymmetrical loads due to 

engine failure. 

(a) The airplane must be designed for 

the unsymmetrical loads resulting 
from the failure of the critical engine. 
Turbopropeller airplanes must be de-
signed for the following conditions in 
combination with a single malfunction 
of the propeller drag limiting system, 
considering the probable pilot correc-
tive action on the flight controls: 

(1) At speeds between 

V

MC

and 

V

D,

the 

loads resulting from power failure be-
cause of fuel flow interruption are con-
sidered to be limit loads. 

(2) At speeds between 

V

MC

and 

V

C,

the 

loads resulting from the disconnection 
of the engine compressor from the tur-
bine or from loss of the turbine blades 
are considered to be ultimate loads. 

(3) The time history of the thrust 

decay and drag build-up occurring as a 
result of the prescribed engine failures 
must be substantiated by test or other 
data applicable to the particular en-
gine-propeller combination. 

(4) The timing and magnitude of the 

probable pilot corrective action must 
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 

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