JAR 25.629 Flutter, deformation, and
failsafe criteria
(a) General. Compliance with this paragraph
must be shown by calculations, resonance tests, or
other tests found necessary by the Authority. Full
scale flight flutter tests at speeds up to VDF/MDF
for the critical aeroplane flutter modes must be
conducted when –
(1) MD is equal to or greater than
0·8 M;
(2) The adequacy of flutter analysis and
wind tunnel tests have not been established by
previous experience with aircraft having
similar design features; or
(3) The conditions specified in sub-paragraph
(a)(1) or (2) of this paragraph exist,
and modifications to the type design have a
significant effect on the critical flutter modes.
(b) Flutter and divergence prevention. The
dynamic evaluation of the aeroplane must include
an investigation of the significant elastic, inertia,
and aerodynamic forces associated with the
rotations and displacements of the plane of the
propeller. In addition, the following apply:
(1) The aeroplane must be designed to
be free from flutter and divergence (unstable
structural distortion due to aerodynamic
loading) for all combinations of altitude and
speed encompassed by the VD/MD versus
altitude envelope enlarged at all points by an
increase of 20% in equivalent air-speed at both
constant Mach number and constant altitude,
except that the envelope may be limited to a
maximum Mach number of 1·0 when MD is less
than 1·0 at all design altitudes and the
following is established:
(i) A proper margin of damping
exists at all speeds up to MD; and
(ii) There is no large and rapid
reduction in damping as MD is
approached.
(2) If concentrated balance weights are
used on control surfaces, their effectiveness
and strength, including supporting structure,
must be substantiated.
(c) Loss of control due to structural
deformation. The aeroplane must be designed to
be free from control reversal and from undue loss
of longitudinal, lateral, and directional stability
and control, as a result of structural deformation
(including that of the control surface covering) at
speeds up to the speed prescribed in sub-paragraph
(b) of this paragraph for flutter
prevention.
(d) Fail-safe criteria. The following fail-safe
criteria must be met:
(1) It must be shown, by analysis or
tests, that the aeroplane is free from such
flutter or divergence that would preclude safe
flight, at any speed up to VD, after each of the
following:
(i) Each of the failures,
malfunctions, or adverse conditions listed
in sub-paragraph (d)(4) of this paragraph.
(ii) Any other combination of
failures, malfunctions, or adverse
conditions not shown to be extremely
improbable.
(2) If a failure, malfunction, or adverse
condition described in sub-paragraph (d)(4) of
this paragraph is simulated during a flight test
in showing compliance with this paragraph, the
maximum speed investigated need not exceed
VFC if it is shown, by correlation of the flight
test data with other test data or analyses, that
hazardous flutter or divergence will not occur
at any speed up to VD.
(3) The structural failures described in
sub-paragraphs (d)(4)(i) and (ii) of this
paragraph need not be considered in showing
compliance with this paragraph if engineering
data substantiate that the probability of their
occurrence is negligible by showing that the
structural element is designed with –
(i) Conservative static strength
margins for each ground and flight
loading conditions specified in this
JAR–25; or
(ii) Sufficient fatigue strength for
the loading spectrum expected in
operation.
(4) The failures, malfunctions, or
adverse conditions used to show compliance
with this paragraph are as follows:
(i) 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).
(ii) Any single failure of the
engine structure, on turbo-propeller
aeroplanes, that would reduce the yaw or
pitch rigidity of the propeller rotational
axis.
(iii) Absence of propeller
aerodynamic forces resulting from the
feathering of any single propeller, and,
for aeroplanes with four or more engines,
the feathering of the critical combination
of two propellers. In addition, any single
feathered propeller must be paired with
the failures, specified in (d)(4)(i) of this
sub-paragraph, involving failure of any
single element of the structure supporting
any engine or independently mounted
propeller shaft, and the failures specified
in (d)(4)(ii) of this sub-paragraph.
(iv) Any single propeller rotating
at the highest likely overspeed.
(v) Failure of each principal
structural element selected for
compliance with JAR 25.571 (b). Safety
following a failure may be substantiated
by showing that losses in rigidity or
changes in frequency, mode shape, or
damping are within the parameter
variations shown to be satisfactory in the
flutter and divergence investigations.
(vi) Any single failure or mal-function,
or combinations thereof, in the
flight control system considered under
JAR 25.671, 25.672 and 25.1309, and
any single failure in any flutter damper
system. Investigation of forced structural
vibration other than flutter, resulting from
failures, malfunctions, or adverse
conditions in the automatic flight control
system may be limited to airspeed up to
WC.
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