Repair of Debonding

From Bondline

Aside from problems with the infamous purple glue, used in late '74 through early '76 models, that resulted in AD 76-17-03, there can be random debonding on the rest of the fleet. Often this is the result of corrosion.

The staggered holes in the bonded area were drilled to allow the skin and ribs to be clecoed together in the same alignment that they were bonded, before the extent of the corrosion was determined. This created a "matched hole tooling" effect, much like Van's RV series uses on their airframes to reduce the amount of jigging that must be done to assemble the planes.

The factory provided data to be used in doing repairs of all types of debonding with the publication of Service Kit 125A. The short version is that nearly any debonding can be repaired with this data. That is good news for owners and mechanics as we attempt to preserve the aging fleet and repair various damage that is unrelated to the spontaneous debonding. The factory data doesn't speak to HOW or WHY the debonding occurred, so that means that if we intentionally debond the parts to replace a corroded or dented part, we are still allowed to use the SK-125A methods to install the replacement parts.

SK-125A calls out an "inhibited poly-sulfide sealant", which is the same Mil-Spec as high adhesion fuel tank sealant, and NAS1097 reduced head rivets to replace the original factory bond. Cleaning of the surfaces to be re-bonded is also explained, and care must be taken not to introduce contaminants that might initiate future corrosion.

Grumman American Service Kit No. 125A

Thanks to the support and generosity of FletchAir Parts, and Garner Rice in particular, a copy of SK-125A is available. This data was released by Grumman American before the copyright law changed in 1987, which had required an explicit "©" copyright symbol to denote that there was copyright on a document. The data is hosted here as a service to the community, and is presumed to be in the Public Domain.

SK125A.