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AC 43.13-1B 9/8/98 4 inches and less than or equal to 35 percent of the control surface span would have a maxi mum permissible free play of 4 inches x 0.020 or 0.080 inches (total motion up and down) measured at the trailing edge. Correct any free play in excess of this amount. (2) Care must also be exercised during repair or rework to prevent stress concentration points or areas that could increase the fatigue susceptibility of the trim tab system. Advisory Circular (AC) 23.629-1A, Means of Compli ance with Section 23.629, “Flutter,” contains additional information on this subject. NOTE: If the pilot has experienced flutter, or thinks he/she has, then a complete inspection of the aircraft flight control system and all related components including rod ends, bearings, hinges, and bellcranks must be accomplished. Suspected parts should be replaced. 4-37. LOAD FACTORS FOR REPAIRS. In order to design an effective repair to a sheet metal aircraft, the stresses that act on the structure must be understood. a. Six types of major stresses are known and should be considered when making re pairs. These are tension, compression, bend ing, torsion, shear, and bearing b. The design of an aircraft repair is complicated by the requirement that it be as light as possible. If weight were not critical, repairs could be made with a large margin of safety. But in actual practice, repairs must be strong enough to carry all of the loads with the required factor of safety, but they must not have too much extra strength. A joint that is too weak cannot be tolerated, but neither can one that is too strong because it can create stress risers that may cause cracks in other lo cations. 4-38. TRANSFER OF STRESSES WITH- IN A STRUCTURE. An aircraft structure must be designed in such a way that it will ac cept all of the stresses imposed upon it by the flight and ground loads without any permanent deformation. Any repair made must accept the stresses, carry them across the repair, and then transfer them back into the original structure. These stresses are considered as flowing through the structure, so there must be a con tinuous path for them, with no abrupt changes in cross-sectional areas along the way. Abrupt changes in cross-sectional areas of aircraft structure that are subject to cycle load ing/stresses will result in stress concentration that may induce fatigue cracking and eventual failure. A scratch or gouge in the surface of a highly-stressed piece of metal will cause a stress concentration at the point of damage. a. Multirow Fastener Load Transfer. When multiple rows of rivets are used to se cure a lap joint, the transfer of stresses is not equal in each row. The transfer of stress at each row of rivets may be thought of as trans ferring the maximum amount capable of being transferred without experiencing rivet shear failure. b. Use Of Stacked Doublers. A stacked doubler is composed of two or more sheets of material that are used in lieu of a single, thicker sheet of material. Because the stress transferred at each row of rivets is dependent upon the maximum stress that can be trans ferred by the rivets in that row, the thickness of the sheet material at that row need only be thick enough to transfer the stress applied. Employing this principle can reduce the weight of a repair joint. 4-39.4-49. [RESERVED.] Page 4-10 Par 4-36PDF Image | AFS-640
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