b. Backing Plate Repair for Webs

TM-1-1520-240-BD Army CH-47D Heilcopter (NSN 1520-01-088-3869) (EIC:RCD) Manual
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TM 1-1520-240-BD (7) Insert coin patch in cutout from inside skin. Drill rivet holes through backing plate and skin. NOTE Use rivets MS20470AD4, for 0.025- inch thick skin, and rivets MS20470AD5, for skin 0.03 inch or thicker. Rivet pitch should be approximately 3/4 inch. (8) Remove coin patch from cutout after establishing a rivet pattern and seal faying surfaces of cutout and coin patch. (9) If damage is less than 1/2 inch and larger than 3/16 inch and located over an undamaged frame or stiffener, repair damage by omitting backing plate. Rivet coin patch to frame or stiffener. (10) Install and set rivets. NOTE Repair magnesium skin as directed in preceding steps, providing dissimilar metals are insulated. Use rivets MS20470B. b. Backing Plate Repair for Webs. This repair is intended for damage occurring in webs, such as holes, tears or sharp dents that can be cut out. It must be repaired within an area not to exceed 1-1/2-inch diameter. When repaired, it must clear adjacent structure by 1/8 inch. Not more than two repairs are permitted in any web. (1) Remove damage by cutting or drilling. The diameter of cutout should not be less than 3/16 inch. (2) Smooth edges of cutout with a file. (3) If damage can be removed by drilling, drill a 3/16-inch-diameter hole or smaller. Insert a rivet MS20470-AD and install a washer AN960D of a size to fit shank of rivet. Set rivet. (4) If diameter of cutout is larger than 3/16 inch, make a circular backing plate from same material as web, in next heavier thickness. NOTE The backing plate must be large enough to cover cutout and overlap web a minimum of four times diameter of rivet. If backing plate cannot be circular, all comers must have a minimum radius of 1/4 inch. (5) Smooth edges of backing plate with a file. (6) Center backing plate over cutout. Drill a rivet pattern having a pitch of 1/2 to 3/4 inch. The number of rivet holes will be determined by size of backing plate. Maintain a pattern that will allow a minimum edge distance of two-times diameter of rivet. Use rivets MS20470AD4, for webs not thicker than 0.025 inch. Use rivets MS20470AD5, for webs 0.03 inch or thicker. Use monel rivets for corrosion-resistant steel webs. (7) Apply a thin even coat of primer (item 113, Appendix C) to faying surfaces of backing plate and web. Allow primer to dry. (8) Assemble backing plate to web by installing and setting rivets. 2-60

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