An Example Repair

From Bondline

This page details an example of a Major Repair.

Degree of difficulty:

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.

This damage is to the lower side of a Cheetah horizontal stabilizer that had an unfortunate encounter with a runway light. The 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 Tuf-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, and no further enlargement of the hole was needed.

The repair plan 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.

The spar cap was damaged over the center of one of the spar lightening holes, so an 0.040 metal piece was fabricated.

Shown clamped in place here.

After the spar cap was straightened with steel bars and and a C-clamp forming a press, this piece was bonded into place and held with 4 CherryMax rivets.

Doublers for the patch were cut from 0.040 2024-T3, with two being required, for and aft of the spar. The marks on the skin are for the 5/16 set back for the rivets - 2D being the minimum, which is 1/4 inch for our 1/8 rivets.

Pilot holes (#40) were drilled in the skin, the doublers cleoced in place, the doublers were then match drilled to the skin holes and clecoed.

A patch was cut and trimmed out of 0.020 2024-T3 aluminum, the same as the original skin, put in place, and drilled and clecoed.

With everything fitted, the holes were drilled out to #30, the patch and doubler were removed, the patch and skin holes dimpled, and the doubler holes countersunk. The doublers were primed, and then riveted in place. Solid rivets were used were a squeezer would fit, and CherryMax rivets for all other holes. Squeezing the rivets is important, since the repetitive hitting of a rivet gun could damage adjacent bond joints.

The patch was now installed, with CherryMax rivets and bonded to the spar cap. After a exterior prime and a little filler, this repair should be essentially invisible, and structurally sound.

NOTE: This is a major structural repair, and due to the bonded nature of our Grummans, it is hard to find references in the Maintenance Manual and AC 43.13 that cover all cases of damage. A DER was hired... contact Fletchair to find names of DERs that are familiar with our Grumman bonded assemblies.

The DER used for this repair was:-

Rocky Howard

AERODESIGN Aircraft Engineering, Inc.

16006 Canard Circle,

Austin, TX 78734

512-266-0610