Sec. 25.629 - Aeroelastic stability requirements.
(a) General. The aeroelastic
stability evaluations required under this section include flutter,
divergence, control reversal and any undue loss of stability and control
as a result of structural deformation. 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 tunnel 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 configurations and design conditions within the aeroelastic stability
envelopes as follows:
(1) For normal conditions without
failures, malfunctions, or adverse conditions, all combinations of
altitudes and speeds encompassed by the VD/MD
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 VD/MD and, there must be no large and
rapid reduction in stability as VD/MD is
approached. The enlarged envelope may be limited to Mach 1.0 when MD
is less than 1.0 at all design altitudes, and
(2) For the conditions described in
§25.629(d) below, for all approved altitudes, any airspeed up to the
greater airspeed defined by;
(i) The VD/MD
envelope determined by §25.335(b); or,
(ii) An altitude-airspeed envelope
defined by a 15 percent increase in equivalent airspeed above VC
at constant altitude, from sea level to the altitude of the intersection
of 1.15 VC with the extension of the constant cruise Mach
number line, MC, then a linear variation in equivalent
airspeed to MC+.05 at the altitude of the lowest VC/MC
intersection; then, at higher altitudes, up to the maximum flight
altitude, the boundary defined by a .05 Mach increase in MC at
(c) Balance weights. If
concentrated balance weights are used, their effectiveness and strength,
including supporting structure, must be substantiated.
(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
conditions, not shown to be extremely improbable, which may result from
mismanagement of fuel.
(2) Any single failure in any flutter
(3) For airplanes not approved for
operation in icing conditions, the maximum 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 externally mounted aerodynamic body
(such as an external fuel tank).
(5) For airplanes with engines that have
propellers or large rotating devices capable of significant dynamic
forces, any single failure of the engine structure that would reduce the
rigidity of the rotational axis.
(6) The absence of aerodynamic or
gyroscopic forces resulting from the most adverse combination of feathered
propellers or other rotating devices capable of significant dynamic
forces. In addition, the effect of a single feathered propeller or
rotating device must be coupled with the failures of paragraphs (d)(4) and
(d)(5) of this section.
(7) Any single propeller or rotating
device capable of significant dynamic forces rotating at the highest
(8) Any damage or failure condition,
required or selected for investigation by §25.571. The single structural
failures described in paragraphs (d)(4) and (d)(5) of this section need
not be considered in showing compliance with this section if;
(i) The structural element could not
fail due to discrete source damage resulting 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 element.
(9) Any damage, failure, or malfunction
considered under §§25.631, 25.671, 25.672, and 25.1309.
(10) Any other combination of failures,
malfunctions, or adverse conditions not shown to be extremely improbable.
(e) Flight flutter testing. Full
scale flight flutter tests at speeds up to VDF/MDF
must be conducted for new type designs and for modifications to a type
design unless the modifications have been shown to have an insignificant
effect on the aeroelastic stability. These tests must demonstrate that the
airplane has a proper margin of damping at all speeds up to VDF/MDF,
and that there is no large and rapid reduction in damping as VDF/MDF,
is approached. If a failure, malfunction, or adverse condition is
simulated during flight test in showing compliance with paragraph (d) of
this section, the maximum speed investigated need not exceed VFC/MFC
if it is shown, by correlation of the flight test data with other test
data or analyses, 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]