248 

14 CFR Ch. I (1–1–19 Edition) 

§ 25.427 

rational analysis is used, the addi-
tional factor must not be less than 1.2. 

(f) For the condition of the control 

locks engaged, the control surfaces, the 
control system locks, and the parts of 
any control systems between the sur-
faces and the locks must be designed to 
the resultant limit loads. Where con-
trol locks are not provided, then the 
control surfaces, the control system 
stops nearest the surfaces, and the 
parts of any control systems between 
the surfaces and the stops must be de-
signed to the resultant limit loads. If 
the control system design is such as to 
allow any part of the control system to 
impact with the stops due to flexi-
bility, then the resultant impact loads 
must be taken into account in deriving 
the limit loads due to ground gust. 

(g) For the condition of taxiing with 

the control locks disengaged, or where 
control locks are not provided, the fol-
lowing apply: 

(1) The control surfaces, the control 

system stops nearest the surfaces, and 
the parts of any control systems be-
tween the surfaces and the stops must 
be designed to the resultant limit 
loads. 

(2) The parts of the control systems 

between the stops nearest the surfaces 
and the flight deck controls must be 
designed to the resultant limit loads, 
except that the parts of the control 
system where loads are eventually re-
acted by the pilot need not exceed: 

(i) The loads corresponding to the 

maximum pilot loads in § 25.397(c) for 
each pilot alone; or 

(ii) 0.75 times these maximum loads 

for each pilot when the pilot forces are 
applied in the same direction. 

[Amdt. 25–141, 79 FR 73468, Dec. 11, 2014] 

§ 25.427

Unsymmetrical loads. 

(a) In designing the airplane for lat-

eral gust, yaw maneuver and roll ma-
neuver conditions, account must be 
taken of unsymmetrical loads on the 
empennage arising from effects such as 
slipstream and aerodynamic inter-
ference with the wing, vertical fin and 
other aerodynamic surfaces. 

(b) The horizontal tail must be as-

sumed to be subjected to unsymmet-
rical loading conditions determined as 
follows: 

(1) 100 percent of the maximum load-

ing from the symmetrical maneuver 
conditions of § 25.331 and the vertical 
gust conditions of § 25.341(a) acting sep-
arately on the surface on one side of 
the plane of symmetry; and 

(2) 80 percent of these loadings acting 

on the other side. 

(c) For empennage arrangements 

where the horizontal tail surfaces have 
dihedral angles greater than plus or 
minus 10 degrees, or are supported by 
the vertical tail surfaces, the surfaces 
and the supporting structure must be 
designed for gust velocities specified in 
§ 25.341(a) acting in any orientation at 
right angles to the flight path. 

(d) Unsymmetrical loading on the 

empennage arising from buffet condi-
tions of § 25.305(e) must be taken into 
account. 

[Doc. No. 27902, 61 FR 5222, Feb. 9, 1996] 

§ 25.445

Auxiliary aerodynamic sur-

faces. 

(a) When significant, the aero-

dynamic influence between auxiliary 
aerodynamic surfaces, such as out-
board fins and winglets, and their sup-
porting aerodynamic surfaces, must be 
taken into account for all loading con-
ditions including pitch, roll, and yaw 
maneuvers, and gusts as specified in 
§ 25.341(a) acting at any orientation at 
right angles to the flight path. 

(b) To provide for unsymmetrical 

loading when outboard fins extend 
above and below the horizontal surface, 
the critical vertical surface loading 
(load per unit area) determined under 
§ 25.391 must also be applied as follows: 

(1) 100 percent to the area of the 

vertical surfaces above (or below) the 
horizontal surface. 

(2) 80 percent to the area below (or 

above) the horizontal surface. 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–86, 61 FR 5222, Feb. 9, 
1996] 

§ 25.457

Wing flaps. 

Wing flaps, their operating mecha-

nisms, and their supporting structures 
must be designed for critical loads oc-
curring in the conditions prescribed in 
§ 25.345, accounting for the loads occur-
ring during transition from one flap po-
sition and airspeed to another. 

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