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| '''Structural Repairs'''
| | == Introduction == |
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| 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. | | 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. |
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| 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 [http://www.airweb.faa.gov/Regulatory_and_Guidance_Library%5CrgAdvisoryCircular.nsf/0/99C827DB9BAAC81B86256B4500596C4E?OpenDocument AC 43.13-1B Change 1], that deals with bonded and honeycomb repairs. | | 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 [http://www.airweb.faa.gov/Regulatory_and_Guidance_Library%5CrgAdvisoryCircular.nsf/0/99C827DB9BAAC81B86256B4500596C4E?OpenDocument AC 43.13-1B Change 1], that deals with bonded and honeycomb repairs. |
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| '''Repair of Debonding'''
| | == Common Structural Repairs == |
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| Aside from problems with the infamous purple glue, used in late '74 through early '76 models, that resulted in [http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/0/15ABBB4154F287DA86256A38005E36C8?OpenDocument AD 76-17-03], there can be random debonding on the rest of the fleet. Often this is the result of corrosion.
| | * [[Repair of Debonding]] |
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| | * [[Lower Cowling Split]] |
| [[Image:Corrosion_under_bonds.jpg]] [[Image:corrosion2.jpg]]
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| 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 [http://vansaircraft.com/public/kit-std.htm Van's RV series] uses on their airframes to reduce the amount of jigging that must be done to assemble the planes.
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| 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.
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| SK-125A calls out an "inhibited poly-sulfide sealant", which is the same Mil-Spec as high adhesion fuel tank sealant, and [http://www.pegasusautoracing.com/bigpicture.asp?RecId=3466 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.
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| '''Lower Cowling split'''
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| 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.
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| 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. [[Image:Bottom_sm.jpg]]
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| 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 [http://www.tinmantech.com/html/powerhouse_wheeling_machines.php with an English Wheel] to smooth the ripples that had been filled with bondo during previous work by persons unknown (no log entry!).
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| [[Image:LH_Cowl.jpg]]
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| 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.
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| [[Image:Left_inside.jpg]]
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| '''Another Approach'''
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| 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:
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| * Removed the nut plate at the front on the nose gear boot mount, and clamped the strip in place.
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| * 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.
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| * 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.
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| * Removed the strip.
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| * Drilled all holes in the cowling and new strip #19 for #8 screws and nutplates riveted to the new strip.
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| Only now, after the cowling is completey drilled, is the original joining strip cut down the middle with a [http://www.hobbypeople.net/gallery/516443.asp 2 inch cutting wheel on a Dremel tool]. Working from the inside, this avoids harming the (new) exterior paint.
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| 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.
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| This technique ensures that the original alignment of the two halves of the cowling is precisely maintained, allowing accurate #19 holes (a slip fit for #8 screws) and fixed, not floating, nutplates to be used.
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| This photo shows the left side cowl and the center strip. Note that the center strip end that is shown at the bottom of the picture is actually the end that is attached to the nose bowl. The slight joggle in the strip can just be seen. This joggle keeps the same thicknesses of metal at the surfaces that are screwed to the nose bowl.
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| [[Image:Cowl-small-1.JPG]]
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Introduction
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.
Common Structural Repairs
