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Federal Aviation Administration, DOT 

§ 25.345 

for the maximum response at the na-
celle center of gravity derived from the 
following dynamic gust conditions ap-
plied to the airplane: 

(1) A discrete gust determined in ac-

cordance with § 25.341(a) at each angle 
normal to the flight path, and sepa-

(2) A pair of discrete gusts, one 

vertical and one lateral. The length of 
each of these gusts must be independ-

ently tuned to the maximum response 
in accordance with § 25.341(a). The pene-
tration of the airplane in the combined 
gust field and the phasing of the 
vertical and lateral component gusts 
must be established to develop the 
maximum response to the gust pair. In 
the absence of a more rational anal-
ysis, the following formula must be 
used for each of the maximum engine 
loads in all six degrees of freedom: 




= limit load; 



= steady 1g load for the condition; 



= peak incremental response load due to 

a vertical gust according to § 25.341(a); 



= peak incremental response load due to 

a lateral gust according to § 25.341(a). 

[Doc. No. 27902, 61 FR 5221, Feb. 9, 1996; 61 FR 
9533, Mar. 8, 1996; Doc. No. FAA–2013–0142; 79 
FR 73467, Dec. 11, 2014; Amdt. 25–141, 80 FR 
4762, Jan. 29, 2015; 80 FR 6435, Feb. 5, 2015] 

§ 25.343

Design fuel and oil loads. 

(a) The disposable load combinations 

must include each fuel and oil load in 
the range from zero fuel and oil to the 
selected maximum fuel and oil load. A 
structural reserve fuel condition, not 
exceeding 45 minutes of fuel under the 
operating conditions in § 25.1001(e) and 
(f), as applicable, may be selected. 

(b) If a structural reserve fuel condi-

tion is selected, it must be used as the 
minimum fuel weight condition for 
showing compliance with the flight 
load requirements as prescribed in this 
subpart. In addition— 

(1) The structure must be designed 

for a condition of zero fuel and oil in 
the wing at limit loads corresponding 

(i) A maneuvering load factor of + 

2.25; and 

(ii) The gust and turbulence condi-

tions of § 25.341(a) and (b), but assuming 
85% of the gust velocities prescribed in 
§ 25.341(a)(4) and 85% of the turbulence 
intensities prescribed in § 25.341(b)(3). 

(2) Fatigue evaluation of the struc-

ture must account for any increase in 
operating stresses resulting from the 

design condition of paragraph (b)(1) of 
this section; and 

(3) The flutter, deformation, and vi-

bration requirements must also be met 
with zero fuel. 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–18, 33 FR 12226, Aug. 30, 
1968; Amdt. 25–72, 55 FR 37607, Sept. 12, 1990; 
Amdt. 25–86, 61 FR 5221, Feb. 9, 1996; Amdt. 
25–141, 79 FR 73468, Dec. 11, 2014] 

§ 25.345

High lift devices. 

(a) If wing flaps are to be used during 

takeoff, approach, or landing, at the 
design flap speeds established for these 
stages of flight under § 25.335(e) and 
with the wing flaps in the cor-
responding positions, the airplane is 
assumed to be subjected to symmet-
rical maneuvers and gusts. The result-
ing limit loads must correspond to the 
conditions determined as follows: 

(1) Maneuvering to a positive limit 

load factor of 2.0; and 

(2) Positive and negative gusts of 25 

ft/sec EAS acting normal to the flight 
path in level flight. Gust loads result-
ing on each part of the structure must 
be determined by rational analysis. 
The analysis must take into account 
the unsteady aerodynamic characteris-
tics and rigid body motions of the air-
craft. The shape of the gust must be as 
described in § 25.341(a)(2) except that— 



= 25 ft/sec EAS; 

H = 12.5 c; and 
c = mean geometric chord of the wing (feet). 

(b) The airplane must be designed for 

the conditions prescribed in paragraph 
(a) of this section, except that the air-
plane load factor need not exceed 1.0, 

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