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9/8/98 AC 43.13-1B 4-36. FLUTTER AND VIBRATION PRECAUTIONS. To prevent the occurrence of severe vibration or flutter of flight control surfaces during flight, precautions must be taken to stay within the design balance limita tions when performing maintenance or repair. a. Balance Changes. The importance of retaining the proper balance and rigidity of air craft control surfaces cannot be overempha sized. The effect of repair or weight change on the balance and center of gravity is proportion ately greater on lighter surfaces than on the older heavier designs. As a general rule, repair the control surface in such a manner that the weight distribution is not affected in any way, in order to preclude the occurrence of flutter of the control surface in flight. Under certain conditions, counter-balance weight is added forward of the hinge line to maintain balance. Add or remove balance weights only when necessary in accordance with the manufac turer’s instructions. Flight testing must be ac complished to ensure flutter is not a problem. Failure to check and retain control surface bal ance within the original or maximum allow able value could result in a serious flight hazard. b. Painting and Refinishing. Special emphasis is directed to the effect of too many extra coats of paint on balanced control sur faces. Mechanics must avoid adding addi tional coats of paint in excess of what the manufacturer originally applied. If available consult the aircraft manufacturer’s instructions relative to finishing and balance of control surfaces. c. Trapped Water or Ice. Instances of flutter have occurred from unbalanced condi tions caused by the collection of water or ice within the surface. Therefore, ventilation and drainage provisions must be checked and re tained when maintenance is being done. d. Trim Tab Maintenance. Loose or vi brating trim tabs will increase wear of actuat ing mechanisms and hinge points which may develop into serious flutter conditions. When this happens, primary control surfaces are highly susceptible to wear, deformation, and fatigue failures because of the buffeting nature of the airflow over the tab mechanism. Trail ing-edge play of the tab may increase, creating an unsafe flutter condition. Careful inspection of the tab and its mechanism should be con ducted during overhaul and annual inspection periods. Compared to other flight control sys tems on the aircraft, only a minor amount of tab-mechanism wear can be tolerated. (1) Free play and stiffness may best be measured by a simple static test where “up ward” and “downward” (or “leftward” and “rightward”) point forces are applied near the trailing edge of the tab at the span-wise at tachment of the actuator (so as not to twist the tab). The control surface to which the trim tab is attached should be locked in place. Rota tional deflection readings are then taken near the tab trailing edge using an appropriate measuring device, such as a dial gauge. Sev eral deflection readings should be taken using loads first applied in one direction, then in the opposite. If the tab span does not exceed 35 percent of the span of the supporting con trol surface, the total free play at the tab trail ing edge should not exceed 2 percent of the tab chord. If the tab span equals or exceeds 35 percent of the span of the supporting con trol surface, the total free play at the tab trail ing edge should not exceed 1 percent of the distance from the tab hinge line to the trailing edge of the tab perpendicular to the tab hinge line. For example, a tab that has a chord of Par 4-36 Page 4-9 SECTION 3. PRECAUTIONARY MEASURESPDF Image | AFS-640
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