CS 25.629 Aeroelastic stability requirements

(See AMC 25.629)

(a)     General. The aeroelastic stability evaluations required under this paragraph 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 paragraph must be shown by analyses, tests, or some combination thereof as found necessary by the Agency.

(b)     Aeroelastic stability envelopes. The aeroplane 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 V_D/M_D versus altitude envelope enlarged at all points by an increase of 15 percent in equivalent airspeed at 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 approached. 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 CS 25.629(d) below, for all approved altitudes, any airspeed up to the greater airspeed defined by:

(i)      The V_D/M_D envelope determined by CS 25.335(b); or,

(ii)     An altitude-airspeed envelope defined by a 15 percent increase in equivalent airspeed above V_C at constant altitude, from sea level to the altitude of the intersection of 1.15 V_C with the extension of the constant cruise Mach number line, M_C, then a linear variation 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 increase in M_C at constant altitude; and

(iii)     Failure conditions of certain systems must be treated in accordance with CS 25.302.

(3)     For failure conditions in those systems covered by CS 25.302, the margins defined in Appendix K of CS-25 apply.

(c)      Balance weights. If 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 paragraph 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 damper or flutter control system.

(3)     For aeroplanes 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 aeroplanes 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 sub-paragraphs (d)(4) and (d)(5) of this paragraph.

(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 CS 25.571. The single structural failures described in sub-paragraphs (d)(4) and(d)(5) of this paragraph need not be considered in showing compliance with this paragraph if;

(i)      The structural element could not fail due to discrete source damage resulting from the conditions described in CS 25.571(e) and CS 25.903(d); and

(ii)     A damage tolerance investigation in accordance with CS 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)     The following flight control system failure combinations where aeroelastic stability relies on flight control system stiffness and/or damping:

(i)      any dual hydraulic system failure;

(ii)     any dual electrical system failure; and

(iii)     any single failure in combination with any probable hydraulic system or electrical system failure.

(10)    Any damage, failure or malfunction, considered under CS 25.631, CS 25.671, CS 25.672, and CS 25.1309.

(11)    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 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 effect on the aeroelastic stability. These tests must demonstrate that the aeroplane 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 condition is simulated during flight test in showing compliance with sub-paragraph (d) of' this paragraph, the maximum speed investigated need not exceed V_FC/M_FC if it is shown, by correlation of the flight test data with other test data or analyses, that the aeroplane is free from any aeroelastic instability at all speeds within the altitude-airspeed envelope described in sub-paragraph (b)(2) of this paragraph.

[Amdt No: 25/1]

[Amdt No: 25/18]

[Amdt No: 25/24]