title-14-part-25-sec25-651

267

Federal Aviation Administration, DOT

§ 25.651

considered in showing compliance with
this section are:

(1) Any critical fuel loading condi-

tions, not shown to be extremely im-
probable, which may result from mis-
management of fuel.

(2) Any single failure in any flutter

damper system.

(3) For airplanes not approved for op-

eration in icing conditions, the max-
imum likely ice accumulation expected
as a result of an inadvertent encounter.

(4) Failure of any single element of

the structure supporting any engine,
independently mounted propeller shaft,
large auxiliary power unit, or large ex-
ternally mounted aerodynamic body
(such as an external fuel tank).

(5) For airplanes with engines that

have propellers or large rotating de-
vices capable of significant dynamic
forces, any single failure of the engine
structure that would reduce the rigid-
ity of the rotational axis.

(6) The absence of aerodynamic or gy-

roscopic forces resulting from the most
adverse combination of feathered pro-
pellers or other rotating devices capa-
ble of significant dynamic forces. In
addition, the effect of a single feath-
ered propeller or rotating device must
be coupled with the failures of para-
graphs (d)(4) and (d)(5) of this section.

(7) Any single propeller or rotating

device capable of significant dynamic
forces rotating at the highest likely
overspeed.

(8) Any damage or failure condition,

required or selected for investigation
by § 25.571. The single structural fail-
ures described in paragraphs (d)(4) and
(d)(5) of this section need not be consid-
ered in showing compliance with this
section if;

(i) The structural element could not

fail due to discrete source damage re-
sulting from the conditions described
in § 25.571(e), and

(ii) A damage tolerance investigation

in accordance with § 25.571(b) shows
that the maximum extent of damage
assumed for the purpose of residual
strength evaluation does not involve
complete failure of the structural ele-
ment.

(9) Any damage, failure, or malfunc-

tion considered under §§ 25.631, 25.671,
25.672, and 25.1309.

(10) Any other combination of fail-

ures, malfunctions, or adverse condi-
tions not shown to be extremely im-
probable.

(e)

Flight flutter testing. Full scale

flight flutter tests at speeds up to V

must be conducted for new type

designs and for modifications to a type
design unless the modifications have
been shown to have an insignificant ef-
fect on the aeroelastic stability. These
tests must demonstrate that the air-
plane has a proper margin of damping
at all speeds up to V

, and that

there is no large and rapid reduction in
damping as V

, is approached. If a

failure, malfunction, or adverse condi-
tion is simulated during flight test in
showing compliance with paragraph (d)
of this section, the maximum speed in-
vestigated need not exceed V

if it

is shown, by correlation of the flight
test data with other test data or anal-
yses, that the airplane is free from any
aeroelastic instability at all speeds
within the altitude-airspeed envelope
described in paragraph (b)(2) of this
section.

[Doc. No. 26007, 57 FR 28949, June 29, 1992]

§ 25.631

Bird strike damage.

The empennage structure must be de-

signed to assure capability of contin-
ued safe flight and landing of the air-
plane after impact with an 8-pound bird
when the velocity of the airplane (rel-
ative to the bird along the airplane’s
flight path) is equal to

at sea level,

selected under § 25.335(a). Compliance
with this section by provision of redun-
dant structure and protected location
of control system elements or protec-
tive devices such as splitter plates or
energy absorbing material is accept-
able. Where compliance is shown by
analysis, tests, or both, use of data on
airplanes having similar structural de-
sign is acceptable.

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

ONTROL

URFACES

§ 25.651

Proof of strength.

(a) Limit load tests of control sur-

faces are required. These tests must in-
clude the horn or fitting to which the
control system is attached.

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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.