title-14-part-25-sec25-427

247

Federal Aviation Administration, DOT

§ 25.427

(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)

c S

Where—

K = hinge moment factor for ground gusts

derived in paragraph (c) of this section;

= 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.

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
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

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248

14 CFR Ch. I (1–1–24 Edition)

§ 25.445

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.

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.

§ 25.459

Special devices.

The loading for special devices using

aerodynamic surfaces (such as slots,
slats and spoilers) must be determined
from test data.

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as
amended by Amdt. 25–72, 55 FR 29776, July 20,
1990]

ROUND

OADS

§ 25.471

General.

(a)

Loads and equilibrium. For limit

ground loads—

(1) Limit ground loads obtained

under this subpart are considered to be
external forces applied to the airplane
structure; and

(2) In each specified ground load con-

dition, the external loads must be
placed in equilibrium with the linear
and angular inertia loads in a rational
or conservative manner.

(b)

Critical centers of gravity. The crit-

ical centers of gravity within the range
for which certification is requested
must be selected so that the maximum
design loads are obtained in each land-
ing gear element. Fore and aft,
vertical, and lateral airplane centers of
gravity must be considered. Lateral
displacements of the c.g. from the air-
plane centerline which would result in
main gear loads not greater than 103
percent of the critical design load for
symmetrical loading conditions may be
selected without considering the ef-
fects of these lateral c.g. displacements
on the loading of the main gear ele-
ments, or on the airplane structure
provided—

(1) The lateral displacement of the

c.g. results from random passenger or
cargo disposition within the fuselage or
from random unsymmetrical fuel load-
ing or fuel usage; and

(2) Appropriate loading instructions

for random disposable loads are in-
cluded under the provisions of
§ 25.1583(c)(2) to ensure that the lateral