Glueball & 411A - I know that sometimes there are airlines that don't do the job right and there are occasions when stress calculations go awry. When cracks first occur, possible propagation paths are sometimes miscalculated. Lu is a real professional and I have the utmost respect for him, but I couldn't let his suggestion that the manufacturer always knows best, go unchallenged. I'm sure that Lu will be the first to agree that there are times when he reflects upon his work and worries if he got it right. Manufacturers don't always get it right and those in the Tech Services side of the business [like me for instance] earn our living from picking up the pieces and sweeping up after them.
The second crack in the external patch descibed by 411A is the result of poor understanding of the stresses involved in the original damage - the outflow valve area is notoriously difficult to get right. As it was, the inspection process caught the new crack at an early stage [don't skimp on your walk-rounds, they are the first step in the process], corrective action resulted regardless of the commercial costs and that is as it should be. As to aircraft patched with incorrect materials - there goes a rogue engineering outfit - draw your own conclusions and be careful who you fly with; it isn't as if they don't have a history.
In our company we aren't an approved design organization so all our repairs are passed through Boeing for review and given 8110-3 certification. But I don't sleep any better at night just because "Uncle Bill" Boeing says our repair is OK, its knowing the men with the rivetting guns personally that does that. There are airlines that don't do things properly but manufacturers need to be watched carefully too. Its discussions like this one that get things out in the open and let us keep everyone on the straight and narrow so, thanks for the chance to open out the discussion Lu. Lets keep professionalism alive...
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Through difficulties to the cinema