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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