233 

Federal Aviation Administration, DOT 

§ 25.305 

need not be exceeded. In addition, the 
entry speeds for flight test demonstra-
tions at normal acceleration values 
less than 1 g must be limited to the ex-
tent necessary to accomplish a recov-
ery without exceeding V

DF

/M

DF

. 

(f) In the out-of-trim condition speci-

fied in paragraph (a) of this section, it 
must be possible from an overspeed 
condition at V

DF

/M

DF

to produce at 

least 1.5 g for recovery by applying not 
more than 125 pounds of longitudinal 
control force using either the primary 
longitudinal control alone or the pri-
mary longitudinal control and the lon-
gitudinal trim system. If the longitu-
dinal trim is used to assist in pro-
ducing the required load factor, it must 
be shown at V

DF

/M

DF

that the longitu-

dinal trim can be actuated in the air-
plane nose-up direction with the pri-
mary surface loaded to correspond to 
the least of the following airplane 
nose-up control forces: 

(1) The maximum control forces ex-

pected in service as specified in §§ 25.301 
and 25.397. 

(2) The control force required to 

produce 1.5 g. 

(3) The control force corresponding to 

buffeting or other phenomena of such 
intensity that it is a strong deterrent 
to further application of primary longi-
tudinal control force. 

[Amdt. 25–42, 43 FR 2322, Jan. 16, 1978] 

Subpart C—Structure 

G

ENERAL

 

§ 25.301

Loads. 

(a) Strength requirements are speci-

fied in terms of limit loads (the max-
imum loads to be expected in service) 
and ultimate loads (limit loads multi-
plied by prescribed factors of safety). 
Unless otherwise provided, prescribed 
loads are limit loads. 

(b) Unless otherwise provided, the 

specified air, ground, and water loads 
must be placed in equilibrium with in-
ertia forces, considering each item of 
mass in the airplane. These loads must 
be distributed to conservatively ap-
proximate or closely represent actual 
conditions. Methods used to determine 
load intensities and distribution must 
be validated by flight load measure-
ment unless the methods used for de-

termining those loading conditions are 
shown to be reliable. 

(c) If deflections under load would 

significantly change the distribution of 
external or internal loads, this redis-
tribution must be taken into account. 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–23, 35 FR 5672, Apr. 8, 
1970] 

§ 25.303

Factor of safety. 

Unless otherwise specified, a factor of 

safety of 1.5 must be applied to the pre-
scribed limit load which are considered 
external loads on the structure. When a 
loading condition is prescribed in 
terms of ultimate loads, a factor of 
safety need not be applied unless other-
wise specified. 

[Amdt. 25–23, 35 FR 5672, Apr. 8, 1970] 

§ 25.305

Strength and deformation. 

(a) The structure must be able to 

support limit loads without detri-
mental permanent deformation. At any 
load up to limit loads, the deformation 
may not interfere with safe operation. 

(b) The structure must be able to 

support ultimate loads without failure 
for at least 3 seconds. However, when 
proof of strength is shown by dynamic 
tests simulating actual load condi-
tions, the 3-second limit does not 
apply. Static tests conducted to ulti-
mate load must include the ultimate 
deflections and ultimate deformation 
induced by the loading. When analyt-
ical methods are used to show compli-
ance with the ultimate load strength 
requirements, it must be shown that— 

(1) The effects of deformation are not 

significant; 

(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 

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