240 

14 CFR Ch. I (1–1–19 Edition) 

§ 25.341 

rigid body motions. The limit loads 
must be determined for all critical alti-
tudes, weights, and weight distribu-
tions as specified in § 25.321(b), and all 
critical speeds within the ranges indi-
cated in § 25.341(b)(3). 

(1) Except as provided in paragraphs 

(b)(4) and (5) of this section, the fol-
lowing equation must be used: 

P

L

= P

L

¥

1

g

± 

U

σ

A

¯  

Where— 

P

L

= limit load; 

P

L

¥

1g

= steady 1g load for the condition; 

A

= ratio of root-mean-square incremental 

load for the condition to root-mean- 
square turbulence velocity; and 

U

σ

= limit turbulence intensity in true air-

speed, specified in paragraph (b)(3) of this 
section. 

(2) Values of 

A

must be determined 

according to the following formula: 

Where— 

H(

W

) = the frequency response function, de-
termined by dynamic analysis, that re-

lates the loads in the aircraft structure 
to the atmospheric turbulence; and 

F

(

W

) = normalized power spectral density of 

atmospheric turbulence given by— 

Where— 

W 

= reduced frequency, radians per foot; and 

L = scale of turbulence = 2,500 ft. 

(3) The limit turbulence intensities, 

U

σ

, in feet per second true airspeed re-

quired for compliance with this para-
graph are— 

(i) At airplane speeds between V

B

and 

V

C

: 

U

σ

= U

σ

ref

F

g

 

Where— 

U

σ

ref

is the reference turbulence intensity 

that varies linearly with altitude from 90 
fps (TAS) at sea level to 79 fps (TAS) at 
24,000 feet and is then constant at 79 fps 
(TAS) up to the altitude of 60,000 feet. 

F

g

is the flight profile alleviation factor de-

fined in paragraph (a)(6) of this section; 

(ii) At speed V

D

: U

σ

is equal to 

1

⁄

2

the 

values obtained under paragraph 
(b)(3)(i) of this section. 

(iii) At speeds between V

C

and V

D

: U

σ

 

is equal to a value obtained by linear 
interpolation. 

(iv) At all speeds, both positive and 

negative incremental loads due to con-
tinuous turbulence must be considered. 

(4) When an automatic system affect-

ing the dynamic response of the air-
plane is included in the analysis, the 
effects of system non-linearities on 
loads at the limit load level must be 
taken into account in a realistic or 
conservative manner. 

(5) If necessary for the assessment of 

loads on airplanes with significant non- 
linearities, it must be assumed that 
the turbulence field has a root-mean- 
square velocity equal to 40 percent of 
the U

σ

values specified in paragraph 

(b)(3) of this section. The value of limit 
load is that load with the same prob-
ability of exceedance in the turbulence 
field as 

A

U

σ

of the same load quantity 

in a linear approximated model. 

(c) 

Supplementary gust conditions for 

wing-mounted engines. 

For airplanes 

equipped with wing-mounted engines, 
the engine mounts, pylons, and wing 
supporting structure must be designed 

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