Mooneycpf

Mechanical/Process/Power and Energy Consulting Engineer currently building large combined cycle gas turbine power plants and part time M20C wing rebuilder

From one in New Zealand who has replaced both lower spar caps, doublers and centre splice in my 1965 M20C with lower spar cap corrosion. From my obseravations of the two photos posted, there is exfoliation between the skin and the lower face of the spar cap (an angle section of approximately 2 1/2" x 2" and in the main fuel tank the section is about 5/16" thick) . Problem here is there is also corrosion on the upper surface of the horizontal leg riveted to the lower skin that has tracked under the sealant, so there is corrosion on both sides of the leg. Unfortuantely, the cap is a 7075T extrusion which is very strong and hard, but prone to intergranular corrosion which is what we are looking at on the exposed surfaces. This form of corrosion is like cancer - it tracks out of sight under the surface along the grain of the extrusion and grinding it out could involve removal of a lot of metal. Some of will be some considerable distance from what is seen on the surface, but will generally be along the grain (the direction of extrusion). The redeeming feature here is that there is a large cross section of metal in the spar cap, which gives an opportunity to grind deep into the section before reaching the 10% reduction in cross-section limit. It is also fortunate that it is easy to access with the fuel tank covers removed. Anywhere else in the wing would be nightmare due to lack of accessability and having to do the job with mirrors BUT with the exfoliation corrosion on the underside of the cap too, it will be necessary to remove that corrosion entirely and clean up the mating surfaces otherwise the problem will recur. This will require deriveting the tank skin along the length of the skin - cap joint plus a few rivets back along the ribs to allow the skin to be peeled back to provide access to the mating surfaces. The surfaces can then be whiz wheeled with a 3M abrasive disk back to clean metal. It is fortunate that the tank has had the sealant already removed, so that major drama is over. I would suggest when re-riveting this skin back, wet rivet it with PRC 1442 B-2 and this will prevent water ever tracking from the wheel well back into the joint and kicking off the corrosion process all over again. Also this has the advantage that if a leak ever occurs in that joint it can't track to some other point between the skins and that makes finding the leak source easy as there is no route for the leak to track along the joint and make its appearance some distance away form the source. Further advantage is that PRC has anticorrosive properties too. The job described above is relatively easy to carry out, particularly when the tank sealant has already been removed. Rivet line would require repainting, but that is not a big deal. Deriveting and cleaning out the joint would take about a day and another half day to re-rivet. If the corrosion is deep enough to cause more than 10% of the section to be removed, then we are talking a totally different ball game and is the route I had to follow due to intergranular (dissimilar metals) corrosion between both (left and right) 4130 steel reinforcing plates where the lower fuselage longerons bolt to the lower spar caps. Cause was attributed to the situation related in one of the earlier contributions where the carpet had been wet some time back in history, combined with a high atmospheric salt content we have in NZ due to the proximity to coastlines and big seas producing a lot of salt air. If the a/c has been operated in coastal loctions, it would also pay to have look directly behind the stall switch too as air is forced through the gap in the switch and salt tends to accumulate on the spar face and if condensation occurs it tracks downward to sit between the lower wing skin and the 1" x 1" x 1/8" angle outer spar cap and produces the same exfoliation seen in the photos. For the record, it has taken me five years of working virtually every weekend to replace the caps along with the making from scratch two outer skins on the left wing that had been damaged and repaired early in the aircraft's life. Material costs haven't been great - guessing at around 15k$, but labour doesn't bear thinking about and if doing it again I would seriosly look at finding a second hand wing that is in good condition. If one is available at around 8 k$ I wouldn't even think about replacing the spar cap. Replacement route is full of traps for the unwary as the wing was never designed to have the caps replaced. Drilling of some holes and driving some rivets and Huck bolts can be nigh on impossible as when the wing was originally built the cap was the first item to be attached to the spar web and there was'nt things like skins and ribs installed to get in the way of rivet and Huck guns. Removal of the wing is reasonably easy and couple of guys should be able to do it in about 3 days or less. Mine has root fairlings and removing them took longer than any other job as the rivets had been faired over with filler and there was a mission involved finding them to drill out without damaging the GRP fairing. I also removed the engine to reduce the weight to be lifted when removing the fuselage.