Federal Aviation Administration, DOT

Section 25.415

PILOT CONTROL FORCE LIMITS (SECONDARY
CONTROLS)

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 assumed to be static loads, and the hinge
moments H must be computed from the
formula:
H = K (1/2) ro V2 c S

Control

Limit pilot forces

Miscellaneous:
*Crank, wheel, or lever ..

Twist ...............................
Push-pull ........................

((1 + R) / 3)  50 lbs., but not
less than 50 lbs. nor more
than 150 lbs. (R = radius).
(Applicable to any angle within 20Section of plane of control).
133 in.-lbs.
To be chosen by applicant.

*Limited to flap, tab, stabilizer, spoiler, and landing gear operation controls.

Section 25.407

Trim tab effects.

The effects of trim tabs on the control surface design conditions must be
accounted for only where the surface
loads are limited by maximum pilot effort. In these cases, the tabs are considered to be deflected in the direction
that would assist the pilot, and the deflections are - 
(a) For elevator trim tabs, those required to trim the airplane at any
point within the positive portion of the
pertinent flight envelope in Section 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.
Section 25.409

Tabs.

spaschal on DSK3GDR082PROD with CFR

(a) Trim tabs. Trim tabs must be designed 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 effort forces up to those specified in
Section 25.397(b).
(b) Balancing tabs. Balancing tabs
must be designed for deflections consistent with the primary control surface loading conditions.
(c) Servo tabs. Servo tabs must be designed for deflections consistent with
the primary control surface loading
conditions obtainable within the pilot
maneuvering effort, considering possible opposition from the trim tabs.
Section 25.415

Ground gust conditions.

(a) The flight control systems and
surfaces must be designed for the limit

Where - 
K = hinge moment factor for ground gusts
derived in paragraph (c) of this section;
ro = 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
(1) Aileron ....................

(2)
(3)
(4)
(5)
(6)

Aileron ....................
Elevator ..................
Elevator ..................
Rudder ....................
Rudder ....................

K
0.75

* Section0.50
* Section0.75
* Section0.75
0.75
0.75

Position of
controls
Control column locked
or lashed in mid-position.
Ailerons at full throw.
Elevator full down.
Elevator full up.
Rudder in neutral.
Rudder at full throw.

* A positive value of K indicates a moment tending to depress the surface, while a negative value of K indicates a moment 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 control surface. A 1.25 factor on the computed hinge moments must be used in
calculating limit control system loads.
(e) Where control system flexibility
is such that the rate of load application in the ground gust conditions
might produce transient stresses appreciably higher than those corresponding
to static loads, in the absence of a rational analysis substantiating a different dynamic factor, an additional
factor of 1.6 must be applied to the control system loads of paragraph (d) of
this section to obtain limit loads. If a

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