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Hide details for Sec. 25.629Sec. 25.629

Part 25 AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY AIRPLANES
Subpart D--Design and Construction
General

Sec. 25.629

Flutter, deformation, and fail-safe criteria.

[(a) General. Compliance with this section must be shown by calculations, resonance tests, or other tests found necessary by the Administrator. Full scale flight flutter tests at speeds up to VDF/MDF for the critical airplane flutter modes must be conducted when--
(1) MD is equal to or greater than 0.8M;
(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 subparagraph (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 airplane 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 airplane 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 percent in equivalent airspeed at both constant Mach number and constant altitude, except that Mach effects for Mach numbers greater than 1.0 need not be included 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 airplane 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 paragraph (b) of this section 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 airplane is free from such flutter or divergence that would preclude safe flight, at any speed up to VD, after--
(i) Each of the failures, malfunctions, or adverse conditions listed in subparagraph (4) of this paragraph; and
(ii) Any other reasonably probable single failure, malfunction, or adverse condition affecting flutter or divergence.
(2) If a failure, malfunction, or adverse condition described in subparagraph (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 subparagraph (4)(i) and (4)(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 condition specified in this part; 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 feature of the engine structure, on turbopropeller airplanes, 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 airplanes 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 subdivision (i) of this subparagraph, involving failure of any single element of the structure supporting any engine or independently mounted propeller shaft, and the failures specified in subdivision (ii) of this subparagraph.
(iv) Any single propeller rotating at the highest likely overspeed.
(v) Failure of each principal structural element for which compliance with the alternative provisions of Sec. 25.571 (c) is selected. Safety following a failure may be substantiated by showing that possible losses in rigidity or changes in frequency, modal form, or damping, resulting from the failure, are within the general parameter variations covered in the flutter and divergence investigations.
(vi) Failure, malfunction, or disconnection of any single element in the main flight control system (including automatic flight control systems, if installed), in any tab control system, or in any flutter damper connected to a control surface or tab. Investigation of the forced structural vibrations, other than flutter resulting from failures, malfunctions, or adverse conditions in the automatic flight control system, may be limited to airspeeds up to VC.

Amdt. 25-23, Eff. 5/8/70


Hide details for Document HistoryDocument History
Notice of Proposed Rulemaking Actions:
Notice of Proposed Rulemaking. Notice No. 68-18; Issued on 08/16/68.

Final Rule Actions:
Final Rule. Docket No. 9079; Issued on 04/01/70.