There is exfoliation corrosion in several areas of the upper spar caps of both mainplanes. The section shown above is on the outer Right mainplane main spar just outboard of the transport joint rib. The corrosion will need to be removed, the amount of material that has been removed will be assessed and a repair plate fitted. Other areas of the spar caps have previously been repaired as this is a well-known problem with Aluminium Alloy forgings. The repair will need to be referred to Boeing who now holds the historical rights to the Douglas design office. We hold historical reference documents which appear to prohibit repairs to the spar caps inboard of the engine nacelles, further research indicates that it is splice repairs which are not allowed rather than corrosion removal. We have entered into a discussion with Cranfield Aerospace who has agreed in concept there will be no problem with the repairs.
Each repair is estimated to take a minimum of two weeks to accomplish, there are six areas of corrosion. A limited inspection of the spar webs has been carried out in the centre section and inside the inboard engine nacelles. No obvious defects were found and the spar webs and booms appear to be sound. The skins are secured to the structure using Magnesium Alloy rivets. Over long periods the structure of the rivet is weakened and the rivet will fail as has happened in the above picture in the area of the right-wing outer mainplane transport joint. The repair for this will be to remove the failed rivets and replace with a modern alternative. There is also evidence of corroded structure below the skin (as evidenced by the skin swelling up) which will require further inspection and investigation. A conservative estimate would be that the repairs would take between eight and twelve weeks to complete.