Structural Repair

Structural Repairs

The maintenance manual has an extensive collection of structural repair details. Many cover "Grumman only" procedures like splicing the firewall and cabin floor with a factory repair section, or replacing the entire tailcone from the honeycomb aft.

There are also numerous factory repairs for mistakes made on the factory floor. These can be very helpful in the field to repair worn and damaged components.

The factory also published Service Kit 125A instructions which are the basis for repair of any debonded joint on the plane. The original need was because of the problems with debonding early on from the "purple glue", but there are other debonding and repair issues that can be addressed with the factory supplied data.

Because of the relatively unique construction of the Grummans, most shops are not familiar with repairs to honeycomb and bonded structures. A considerable amount of material has been added to AC 43.13-1B Change 1, that deals with bonded and honeycomb repairs.

Lower Cowling split

Requests for information on splitting the lower cowl on Tigers and Cheetahs was made on the GG and I'll post some pictures and an explanation of the means I used to repair lower cowls that had failed the spot welds and had cracking doublers.

Here is the Tiger lower cowl with failed spot welds and cracked doublers after removing the cowl from the plane and taking the doublers out.

This is a Cheetah LH lower cowl from the outside with larger ramp being drilled and clecoed in place. Paint is off because of the amount of metal work that was needed with an English Wheel to smooth the ripples that had been filled with bondo during previous work by persons unknown (no log entry!).

Here is the inside of that same Cheetah lower cowl half with the ramp in place and the new doubler mounted, ready to attach the other side with screws.

Another Approach

Gil Alexander took a slightly different approach for a Tiger repair, since he didn't want to harm a new paint job. Gil left the doubler in place and added a 0.032 strip of aluminum 1 7/8 wide on the inside. Here's what he did:

• Removed the nut plate at the front on the nose gear boot mount, and clamped the strip in place.

• Drilled #30 holes from the outside for the mounting screws, roughly located where rivets from a previous repair were removed. Clecos were inserted as the holes were drilled.

• Drilled matched #19 holes for #8 screws for the aft screw at the nose gear boot mount and for the central retaining screw into the nose bowl.

• Removed the strip.

• Drilled all holes in the cowling and new strip #19 for #8 screws and nutplates riveted to the new strip.

Only now, after the cowling is completey drilled, is the original joining strip cut down the middle with a 2 inch cutting wheel on a Dremel tool. Working from the inside, this avoids harming the (new) exterior paint.

Only the forward 2 inches of the original joining strip need be removed. This ensures that the extra layer of the new joining strip does not create an extra layer at the center where the cowling is screwed to the nose bowl.