An Example Repair

This page details an example of a Major Repair. Degree of difficulty xxxx. As with all Major Repairs, a Form 337 is required, and in this case an FAA Designated Engineering Representative (DER) was involved in the structural analysis.

The damage to the lower side of a Cheetah horizontal stabilizer that had an unfortunate encounter with a runway light. This picture was taken after stripping two layers of paint off an area of the stabilizer, ensuring no stripper came anywhere near an bond lines. PTI stripper proved to be very effective, but read all warning labels, it will damage skin!

The repair is major since the required patch is over 6 inches in one direction, and the spar cap is damaged, which is defined as a primary structure in the Maintenance Manual.

The damaged area was cut away to the marked lines in the picture. First cuts were made with a Dremel tool with a 2 inch Tuff-grind disk just short of the line. The hole was then trimmed to the line with a 2 inch sanding disk in an angle die grinder. Care was taken over the spar, and the metal was not cut through at this location. A strip of aluminum was left bonded to the spar cap. After the rest of the hole was exposed, this strip was removed with pliers and a peeling motion. Epoxy bonds do not work well in peel, and the strip came off easily. Interestingly, after the hole was cut to the marks, the skin distortions visible in the picture forward of the spar just popped out.

The repair plane worked out with the DER involved a spar reinforcement and a skin patch similar to that defined for wing leading edges in the Maintenance Manual.