247 

Federal Aviation Administration, DOT 

§ 25.415 

P

ILOT

C

ONTROL

F

ORCE

L

IMITS

(S

ECONDARY

 

C

ONTROLS

) 

Control 

Limit pilot forces 

Miscellaneous: 

*Crank, wheel, or lever ..

((1 + R) / 3) 

× 

50 lbs., but not 

less than 50 lbs. nor more 
than 150 lbs. (R = radius). 
(Applicable to any angle with-
in 20

° 

of plane of control). 

Twist ...............................

133 in.–lbs. 

Push-pull ........................

To be chosen by applicant. 

*Limited to flap, tab, stabilizer, spoiler, and landing gear op-

eration controls. 

§ 25.407

Trim tab effects. 

The effects of trim tabs on the con-

trol surface design conditions must be 
accounted for only where the surface 
loads are limited by maximum pilot ef-
fort. In these cases, the tabs are con-
sidered to be deflected in the direction 
that would assist the pilot, and the de-
flections are— 

(a) For elevator trim tabs, those re-

quired to trim the airplane at any 
point within the positive portion of the 
pertinent flight envelope in § 25.333(b), 
except as limited by the stops; and 

(b) For aileron and rudder trim tabs, 

those required to trim the airplane in 
the critical unsymmetrical power and 
loading conditions, with appropriate 
allowance for rigging tolerances. 

§ 25.409

Tabs. 

(a) 

Trim tabs. 

Trim tabs must be de-

signed to withstand loads arising from 
all likely combinations of tab setting, 
primary control position, and airplane 
speed (obtainable without exceeding 
the flight load conditions prescribed 
for the airplane as a whole), when the 
effect of the tab is opposed by pilot ef-
fort forces up to those specified in 
§ 25.397(b). 

(b) 

Balancing tabs. 

Balancing tabs 

must be designed for deflections con-
sistent with the primary control sur-
face loading conditions. 

(c) 

Servo tabs. 

Servo tabs must be de-

signed for deflections consistent with 
the primary control surface loading 
conditions obtainable within the pilot 
maneuvering effort, considering pos-
sible opposition from the trim tabs. 

§ 25.415

Ground gust conditions. 

(a) The flight control systems and 

surfaces must be designed for the limit 

loads generated when the airplane is 
subjected to a horizontal 65-knot 
ground gust from any direction while 
taxiing and while parked. For airplanes 
equipped with control system gust 
locks, the taxiing condition must be 
evaluated with the controls locked and 
unlocked, and the parked condition 
must be evaluated with the controls 
locked. 

(b) The control system and surface 

loads due to ground gust may be as-
sumed to be static loads, and the hinge 
moments H must be computed from the 
formula: 
H = K (1/2) 

r

o

V

2

c S 

Where— 

K = hinge moment factor for ground gusts 

derived in paragraph (c) of this section; 

r

o

= density of air at sea level; 

V = 65 knots relative to the aircraft; 
S = area of the control surface aft of the 

hinge line; 

c = mean aerodynamic chord of the control 

surface aft of the hinge line. 

(c) The hinge moment factor K for 

ground gusts must be taken from the 
following table: 

Surface K 

Position of 

controls 

(1) Aileron ....................

0 .75  Control column locked 

or lashed in mid-po-
sition. 

(2) Aileron ....................

*

±

0 .50  Ailerons at full throw. 

(3) Elevator ..................

*

±

0 .75  Elevator full down. 

(4) Elevator ..................

*

±

0 .75  Elevator full up. 

(5) Rudder ....................

0 .75  Rudder in neutral. 

(6) Rudder ....................

0 .75  Rudder at full throw. 

* A positive value of K indicates a moment tending to de-

press the surface, while a negative value of K indicates a mo-
ment tending to raise the surface. 

(d) The computed hinge moment of 

paragraph (b) of this section must be 
used to determine the limit loads due 
to ground gust conditions for the con-
trol surface. A 1.25 factor on the com-
puted hinge moments must be used in 
calculating limit control system loads. 

(e) Where control system flexibility 

is such that the rate of load applica-
tion in the ground gust conditions 
might produce transient stresses appre-
ciably higher than those corresponding 
to static loads, in the absence of a ra-
tional analysis substantiating a dif-
ferent dynamic factor, an additional 
factor of 1.6 must be applied to the con-
trol system loads of paragraph (d) of 
this section to obtain limit loads. If a 

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