Section 25.631 14 CFR Ch. I (1-1-19 Edition) 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] Section 25.631 Bird strike damage. The empennage structure must be designed to assure capability of continued safe flight and landing of the airplane after impact with an 8-pound bird when the velocity of the airplane (relative to the bird along the airplane-s flight path) is equal to VC at sea level, selected under Section 25.335(a). Compliance with this section by provision of redundant structure and protected location of control system elements or protective devices such as splitter plates or energy absorbing material is acceptable. Where compliance is shown by analysis, tests, or both, use of data on airplanes having similar structural design is acceptable. [Amdt. 25-23, 35 FR 5674, Apr. 8, 1970] CONTROL SURFACES Section 25.651 Proof of strength. (a) Limit load tests of control surfaces are required. These tests must include the horn or fitting to which the control system is attached. (b) Compliance with the special factors requirements of SectionSection 25.619 through 25.625 and 25.657 for control surface hinges must be shown by analysis or individual load tests. spaschal on DSK3GDR082PROD with CFR Section 25.655 Installation. (a) Movable tail surfaces must be installed so that there is no interference between any surfaces when one is held in its extreme position and the others are operated through their full angular movement. (b) If an adjustable stabilizer is used, it must have stops that will limit its range of travel to the maximum for which the airplane is shown to meet the trim requirements of Section 25.161. Section 25.657 Hinges. (a) For control surface hinges, including ball, roller, and self-lubricated bearing hinges, the approved rating of the bearing may not be exceeded. For nonstandard bearing hinge configurations, the rating must be established on the basis of experience or tests and, in the absence of a rational investigation, a factor of safety of not less than 6.67 must be used with respect to the ultimate bearing strength of the softest material used as a bearing. (b) Hinges must have enough strength and rigidity for loads parallel to the hinge line. [Amdt. 25-23, 35 FR 5674, Apr. 8, 1970] CONTROL SYSTEMS Section 25.671 General. (a) Each control and control system must operate with the ease, smoothness, and positiveness appropriate to its function. (b) Each element of each flight control system must be designed, or distinctively and permanently marked, to minimize the probability of incorrect assembly that could result in the malfunctioning of the system. (c) The airplane must be shown by analysis, tests, or both, to be capable of continued safe flight and landing after any of the following failures or jamming in the flight control system and surfaces (including trim, lift, drag, and feel systems), within the normal flight envelope, without requiring exceptional piloting skill or strength. Probable malfunctions must have only minor effects on control system operation and must be capable of being readily counteracted by the pilot. (1) Any single failure, excluding jamming (for example, disconnection or failure of mechanical elements, or structural failure of hydraulic components, such as actuators, control spool housing, and valves). (2) Any combination of failures not shown to be extremely improbable, excluding jamming (for example, dual 268 VerDate Sep<11>2014 12:50 Apr 30, 2019 Jkt 247046 PO 00000 Frm 00278 Fmt 8010 Sfmt 8010 Y:\SGML\247046.XXX 247046