233 

Federal Aviation Administration, DOT 

§ 25.321 

(2) The deformations involved are 

fully accounted for in the analysis; or 

(3) The methods and assumptions 

used are sufficient to cover the effects 
of these deformations. 

(c) Where structural flexibility is 

such that any rate of load application 
likely to occur in the operating condi-
tions might produce transient stresses 
appreciably higher than those cor-
responding to static loads, the effects 
of this rate of application must be con-
sidered. 

(d) [Reserved] 
(e) The airplane must be designed to 

withstand any vibration and buffeting 
that might occur in any likely oper-
ating condition up to V

D

/M

D

, including 

stall and probable inadvertent excur-
sions beyond the boundaries of the buf-
fet onset envelope. This must be shown 
by analysis, flight tests, or other tests 
found necessary by the Administrator. 

(f) Unless shown to be extremely im-

probable, the airplane must be designed 
to withstand any forced structural vi-
bration resulting from any failure, 
malfunction or adverse condition in 
the flight control system. These must 
be considered limit loads and must be 
investigated at airspeeds up to V

C

/M

C

. 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–23, 35 FR 5672, Apr. 8, 
1970; Amdt. 25–54, 45 FR 60172, Sept. 11, 1980; 
Amdt. 25–77, 57 FR 28949, June 29, 1992; Amdt. 
25–86, 61 FR 5220, Feb. 9, 1996] 

§ 25.307

Proof of structure. 

(a) Compliance with the strength and 

deformation requirements of this sub-
part must be shown for each critical 
loading condition. Structural analysis 
may be used only if the structure con-
forms to that for which experience has 
shown this method to be reliable. In 
other cases, substantiating tests must 
be made to load levels that are suffi-
cient to verify structural behavior up 
to loads specified in § 25.305. 

(b)–(c) [Reserved] 
(d) When static or dynamic tests are 

used to show compliance with the re-
quirements of § 25.305(b) for flight 
structures, appropriate material cor-
rection factors must be applied to the 
test results, unless the structure, or 
part thereof, being tested has features 
such that a number of elements con-
tribute to the total strength of the 

structure and the failure of one ele-
ment results in the redistribution of 
the load through alternate load paths. 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–23, 35 FR 5672, Apr. 8, 
1970; Amdt. 25–54, 45 FR 60172, Sept. 11, 1980; 
Amdt. 25–72, 55 FR 29775, July 20, 1990; Amdt. 
25–139, 79 FR 59429, Oct. 2, 2014] 

F

LIGHT

L

OADS

 

§ 25.321

General. 

(a) Flight load factors represent the 

ratio of the aerodynamic force compo-
nent (acting normal to the assumed 
longitudinal axis of the airplane) to the 
weight of the airplane. A positive load 
factor is one in which the aerodynamic 
force acts upward with respect to the 
airplane. 

(b) Considering compressibility ef-

fects at each speed, compliance with 
the flight load requirements of this 
subpart must be shown— 

(1) At each critical altitude within 

the range of altitudes selected by the 
applicant; 

(2) At each weight from the design 

minimum weight to the design max-
imum weight appropriate to each par-
ticular flight load condition; and 

(3) For each required altitude and 

weight, for any practicable distribution 
of disposable load within the operating 
limitations recorded in the Airplane 
Flight Manual. 

(c) Enough points on and within the 

boundaries of the design envelope must 
be investigated to ensure that the max-
imum load for each part of the airplane 
structure is obtained. 

(d) The significant forces acting on 

the airplane must be placed in equi-
librium in a rational or conservative 
manner. The linear inertia forces must 
be considered in equilibrium with the 
thrust and all aerodynamic loads, 
while the angular (pitching) inertia 
forces must be considered in equi-
librium with thrust and all aero-
dynamic moments, including moments 
due to loads on components such as 
tail surfaces and nacelles. Critical 
thrust values in the range from zero to 
maximum continuous thrust must be 
considered. 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–23, 35 FR 5672, Apr. 8, 
1970; Amdt. 25–86, 61 FR 5220, Feb. 9, 1996] 

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