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266 

14 CFR Ch. I (1–1–24 Edition) 

§ 25.623 

may be reduced when an approved qual-
ity control procedure is established. 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–139, 79 FR 59429, Oct. 2, 
2014] 

§ 25.623

Bearing factors. 

(a) Except as provided in paragraph 

(b) of this section, each part that has 
clearance (free fit), and that is subject 
to pounding or vibration, must have a 
bearing factor large enough to provide 
for the effects of normal relative mo-
tion. 

(b) No bearing factor need be used for 

a part for which any larger special fac-
tor is prescribed. 

§ 25.625

Fitting factors. 

For each fitting (a part or terminal 

used to join one structural member to 
another), the following apply: 

(a) For each fitting whose strength is 

not proven by limit and ultimate load 
tests in which actual stress conditions 
are simulated in the fitting and sur-
rounding structures, a fitting factor of 
at least 1.15 must be applied to each 
part of— 

(1) The fitting; 
(2) The means of attachment; and 
(3) The bearing on the joined mem-

bers. 

(b) No fitting factor need be used— 
(1) For joints made under approved 

practices and based on comprehensive 
test data (such as continuous joints in 
metal plating, welded joints, and scarf 
joints in wood); or 

(2) With respect to any bearing sur-

face for which a larger special factor is 
used. 

(c) For each integral fitting, the part 

must be treated as a fitting up to the 
point at which the section properties 
become typical of the member. 

(d) For each seat, berth, safety belt, 

and harness, the fitting factor specified 
in § 25.785(f)(3) applies. 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–23, 35 FR 5674, Apr. 8, 
1970; Amdt. 25–72, 55 FR 29776, July 20, 1990] 

§ 25.629

Aeroelastic stability require-

ments. 

(a) 

General.  The aeroelastic stability 

evaluations required under this section 
include flutter, divergence, control re-
versal and any undue loss of stability 

and control as a result of structural de-
formation. The aeroelastic evaluation 
must include whirl modes associated 
with any propeller or rotating device 
that contributes significant dynamic 
forces. Compliance with this section 
must be shown by analyses, wind tun-
nel tests, ground vibration tests, flight 
tests, or other means found necessary 
by the Administrator. 

(b) 

Aeroelastic stability envelopes. The 

airplane must be designed to be free 
from aeroelastic instability for all con-
figurations and design conditions with-
in the aeroelastic stability envelopes 
as follows: 

(1) For normal conditions without 

failures, malfunctions, or adverse con-
ditions, all combinations of altitudes 
and speeds encompassed by the V

D

/M

D

 

versus altitude envelope enlarged at all 
points by an increase of 15 percent in 
equivalent airspeed at both constant 
Mach number and constant altitude. In 
addition, a proper margin of stability 
must exist at all speeds up to V

D

/M

D

 

and, there must be no large and rapid 
reduction in stability as V

D

/M

D

is ap-

proached. The enlarged envelope may 
be limited to Mach 1.0 when M

D

is less 

than 1.0 at all design altitudes, and 

(2) For the conditions described in 

§ 25.629(d) below, for all approved alti-
tudes, any airspeed up to the greater 
airspeed defined by; 

(i) The V

D

/M

D

envelope determined by 

§ 25.335(b); or, 

(ii) An altitude-airspeed envelope de-

fined by a 15 percent increase in equiv-
alent airspeed above V

C

at constant al-

titude, from sea level to the altitude of 
the intersection of 1.15 V

C

with the ex-

tension of the constant cruise Mach 
number line, M

C

, then a linear vari-

ation in equivalent airspeed to M

C

+ .05 

at the altitude of the lowest V

C

/M

C

 

intersection; then, at higher altitudes, 
up to the maximum flight altitude, the 
boundary defined by a .05 Mach in-
crease in M

C

at constant altitude. 

(c) 

Balance weights. If concentrated 

balance weights are used, their effec-
tiveness and strength, including sup-
porting structure, must be substan-
tiated. 

(d) 

Failures, malfunctions, and adverse 

conditions.  The failures, malfunctions, 
and adverse conditions which must be 

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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

DF

M

DF

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

DF

/M

DF

, and that 

there is no large and rapid reduction in 
damping as V

DF

/M

DF

, 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

FC

/M

FC

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 

V

C

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] 

C

ONTROL

S

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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