title-14-part-25-sec25-672

268

14 CFR Ch. I (1–1–24 Edition)

§ 25.655

(b) Compliance with the special fac-

tors requirements of §§ 25.619 through
25.625 and 25.657 for control surface
hinges must be shown by analysis or
individual load tests.

§ 25.655

Installation.

(a) Movable tail surfaces must be in-

stalled so that there is no interference
between any surfaces when one is held
in its extreme position and the others
are operated through their full angular
movement.

(b) If an adjustable stabilizer is used,

it must have stops that will limit its
range of travel to the maximum for
which the airplane is shown to meet
the trim requirements of § 25.161.

§ 25.657

Hinges.

(a) For control surface hinges, in-

cluding ball, roller, and self-lubricated
bearing hinges, the approved rating of
the bearing may not be exceeded. For
nonstandard bearing hinge configura-
tions, the rating must be established
on the basis of experience or tests and,
in the absence of a rational investiga-
tion, a factor of safety of not less than
6.67 must be used with respect to the
ultimate bearing strength of the soft-
est material used as a bearing.

(b) Hinges must have enough

strength and rigidity for loads parallel
to the hinge line.

[Amdt. 25–23, 35 FR 5674, Apr. 8, 1970]

ONTROL

YSTEMS

§ 25.671

General.

(a) Each control and control system

must operate with the ease, smooth-
ness, and positiveness appropriate to
its function.

(b) Each element of each flight con-

trol system must be designed, or dis-
tinctively and permanently marked, to
minimize the probability of incorrect
assembly that could result in the mal-
functioning of the system.

analysis, tests, or both, to be capable
of continued safe flight and landing
after any of the following failures or
jamming in the flight control system
and surfaces (including trim, lift, drag,
and feel systems), within the normal
flight envelope, without requiring ex-
ceptional piloting skill or strength.

Probable malfunctions must have only
minor effects on control system oper-
ation and must be capable of being
readily counteracted by the pilot.

(1) Any single failure, excluding jam-

ming (for example, disconnection or
failure of mechanical elements, or
structural failure of hydraulic compo-
nents, such as actuators, control spool
housing, and valves).

(2) Any combination of failures not

shown to be extremely improbable, ex-
cluding jamming (for example, dual
electrical or hydraulic system failures,
or any single failure in combination
with any probable hydraulic or elec-
trical failure).

(3) Any jam in a control position nor-

mally encountered during takeoff,
climb, cruise, normal turns, descent,
and landing unless the jam is shown to
be extremely improbable, or can be al-
leviated. A runaway of a flight control
to an adverse position and jam must be
accounted for if such runaway and sub-
sequent jamming is not extremely im-
probable.

(d) The airplane must be designed so

that it is controllable if all engines
fail. Compliance with this requirement
may be shown by analysis where that
method has been shown to be reliable.

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as
amended by Amdt. 25–23, 35 FR 5674, Apr. 8,
1970]

§ 25.672

Stability augmentation and

automatic and power-operated sys-
tems.

If the functioning of stability aug-

mentation or other automatic or
power-operated systems is necessary to
show compliance with the flight char-
acteristics requirements of this part,
such systems must comply with § 25.671
and the following:

(a) A warning which is clearly distin-

guishable to the pilot under expected
flight conditions without requiring his
attention must be provided for any
failure in the stability augmentation
system or in any other automatic or
power-operated system which could re-
sult in an unsafe condition if the pilot
were not aware of the failure. Warning
systems must not activate the control
systems.

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§ 25.681

(b) The design of the stability aug-

mentation system or of any other auto-
matic or power-operated system must
permit initial counteraction of failures
of the type specified in § 25.671(c) with-
out requiring exceptional pilot skill or
strength, by either the deactivation of
the system, or a failed portion thereof,
or by overriding the failure by move-
ment of the flight controls in the nor-
mal sense.

single failure of the stability aug-
mentation system or any other auto-
matic or power-operated system—

(1) The airplane is safely controllable

when the failure or malfunction occurs
at any speed or altitude within the ap-
proved operating limitations that is
critical for the type of failure being
considered;

(2) The controllability and maneuver-

ability requirements of this part are
met within a practical operational
flight envelope (for example, speed, al-
titude, normal acceleration, and air-
plane configurations) which is de-
scribed in the Airplane Flight Manual;
and

(3) The trim, stability, and stall char-

acteristics are not impaired below a
level needed to permit continued safe
flight and landing.

[Amdt. 25–23, 35 FR 5675 Apr. 8, 1970]

§ 25.675

Stops.

(a) Each control system must have

stops that positively limit the range of
motion of each movable aerodynamic
surface controlled by the system.

(b) Each stop must be located so that

wear, slackness, or take-up adjust-
ments will not adversely affect the
control characteristics of the airplane
because of a change in the range of sur-
face travel.

stand any loads corresponding to the
design conditions for the control sys-
tem.

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as
amended by Amdt. 25–38, 41 FR 55466, Dec. 20,
1976]

§ 25.677

Trim systems.

(a) Trim controls must be designed to

prevent inadvertent or abrupt oper-
ation and to operate in the plane, and

with the sense of motion, of the air-
plane.

(b) There must be means adjacent to

the trim control to indicate the direc-
tion of the control movement relative
to the airplane motion. In addition,
there must be clearly visible means to
indicate the position of the trim device
with respect to the range of adjust-
ment. The indicator must be clearly
marked with the range within which it
has been demonstrated that takeoff is
safe for all center of gravity positions
approved for takeoff.

signed to prevent creeping in flight.
Trim tab controls must be irreversible
unless the tab is appropriately bal-
anced and shown to be free from flut-
ter.

(d) If an irreversible tab control sys-

tem is used, the part from the tab to
the attachment of the irreversible unit
to the airplane structure must consist
of a rigid connection.

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as
amended by Amdt. 25–23, 35 FR 5675, Apr. 8,
1970; Amdt. 25–115, 69 FR 40527, July 2, 2004]

§ 25.679

Control system gust locks.

(a) There must be a device to prevent

damage to the control surfaces (includ-
ing tabs), and to the control system,
from gusts striking the airplane while
it is on the ground or water. If the de-
vice, when engaged, prevents normal
operation of the control surfaces by the
pilot, it must—

(1) Automatically disengage when the

pilot operates the primary flight con-
trols in a normal manner; or

(2) Limit the operation of the air-

plane so that the pilot receives unmis-
takable warning at the start of takeoff.

(b) The device must have means to

preclude the possibility of it becoming
inadvertently engaged in flight.

§ 25.681

Limit load static tests.

(a) Compliance with the limit load

requirements of this Part must be
shown by tests in which—

(1) The direction of the test loads

produces the most severe loading in the
control system; and

(2) Each fitting, pulley, and bracket

used in attaching the system to the
main structure is included.