Aircraft Sheet Metal Repair

by Jan Zumwalt - 2014.09.27

References

Introduction

There is nothing as frustrating as discovering a small dent or damaged area to an aircraft skin then bewildered by what safe method should be used to patch it.

There are many ways to calculate the number of rivets needed and/or the rivet spacing. The exact number of rivets will vary over a surprising number of variations and all methods are approved by the FAA. So don't be alarmed by the differences. However, the method used for the calculation should be included in the permanent aircraft maintenance record

This discussion will lead us through the repair process for a patch on .040 sheet metal using 1/8" rivets. But, we provide all the information needed to repair any other thickness of aluminium sheet metal.

Steps

  1. Draw a circle around the damaged area.
  2. Drill a pilot hole in the circle so a larger uni-bit, fly-cutter, or metal saw can cut out the damaged area.
  3. De-bur the newly cut hole.
  4. Cut and de-bur a patch from a sheet of aluminium that is the same thickness as the original aircraft skin. The diameter will depend on the number of rows of rivets you're planning on installing.
  5. Center the patch over the hole in the aircraft skin and mark its outline.
  6. We need to calculate the edge distance for the rivets. Since the aluminium being used here is 0.040" thick, and the rivets are 1/8" dia. The edge distance must be between 1/4" (2x) and 1/2" (4x). So, we will use 3/8".
  7. Draw lines for the edge distance.
  8. Now we calculate the rivet pitch (distance between rivets). We can either do a single row of tightly spaced rivets or a double row. The double row gives more room to work. From the table we see a .040 sheet with 1/8" rivets needs 7.7 rivets per inch. If we stager the rivets in two rows, then each row needs 3.3 per inch (1/3 of an inch). So, we will use 5/16 rivet spacing in each row.
  9. A rule of thumb for rivet spacing on a new part is a minimum of 4x the rivet diameter and a maximum of 12x the rivet diameter. Therefor, 10x the rivet diameter is often used. For patches, double the number of rivets needed for a new part - so 5x rivet dia would be good.
  10. Draw a mark for each rivet's location. Keep the spacing between each rivet the same.
  11. Next, drill pilot holes where the rivet makings are located with a small drill bit so it is possible to drill the holes to full size, without deformation of the aluminium sheet.
  12. Secure the patch on the aircraft skin and drill pilot holes in the patch. Use the holes previously drilled in the skin as a guide.
  13. With the patch still secured to the aircraft skin, expand the holes to the proper diameter. Use clecos to make sure the patch does not move while drilling.

  14. Rivet
    size
    Best
    drill
    Common
    drill
    Hole
    size
    3/32 #40 7/64 0.096
    1/8 #30 9/64 0.128
    5/32 #21 11/64 0.159
    3/16 #11 13/64 0.191
    7/32 #10 15/64 0.225
    1/4 #F 17/64 0.257
    5/16 #P 21/64 0.323
    3/8 #W 25/64 0.386
    7/16 29/64 29/64 0.453
    1/2 33/64 33/64 0.516
    Rivet Hole Drill Size

  15. The hole does not need neccessarly need to be de-burred if the parts were held firmly together during the hole drilling process and the hole has clean edges. if the hole edges are not clean, remove the patch from the aircraft skin and de-bur the holes with rough edges.
  16. Re-attach the patch on the skin using clecos.
  17. Using the proper rivets, secure the patch to the aircraft skin.
  18. Make an appropriate entry in the permanent aircraft maintenance log.

Edge distance

The edge distance for all rivets, except those with a flush head, should be a minimum of 2x rivet shank dia, but no more than 4x rivet shank dia. Flush-head rivets require an edge distance of at least 2 1/2 times the diameter.

If rivets are placed to close to the edge of the sheet, the sheet is apt to warp, crack or pull away from the rivets. If they are placed too far away from the edge, the sheet is apt to turn up at the edge.

NOTE: On most repairs, the general practice is to use the same rivet spacing and edge distance that the manufacturer used in the surrounding area, or refer to the structural repair manual for the aircraft

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Rivet pitch (distance)

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