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Engine Pylon.

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  • Engine Pylon.

    Something in this picture caught my attention.

    From just about where the 2nd upper access door is(near the vertical firewall i think) there seems to be a doubler in the structure. I would assume this is due to the high loading and stress of the area? If this is the case how are they integrated into the structure?


  • #2
    This happens to be one of the 10 B-747-249 / -245 aircraft that I helped maintain until 1990 when the aircraft were sold to Polar.

    The doublers are added to reinforce the pylon skin which tends to crack in this area. The doubler is one gage thicker than the pylon skin so it takes 75 to 80% of the load in the corner. The doubler is attached using existing fastener holes in primary structure (bulkheads) that are over sized to accept the appropriate Hi-Loc (trade mark of Hi-Shear Corp) fasteners. Hi-Loc's are close tolerance threaded fasteners that fit into precision holes to transfer stress loads from one member to another. The remainder of the doubler is used to bleed out the load remaining in the doubler to the surrounding structure with out over stressing any one point.

    Most repair doublers are installed using Hi-Loc's. Engineers love them because we can transfer the loads using fewer fasteners than would be required using conventional "AD" aluminum rivets. In general, the fewer holes, the better, assuming the loads are transferred properly.
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    • #3
      Thanks. Very informative post.