267 

Federal Aviation Administration, DOT 

§ 25.629 

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 
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 

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