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Photo © Christos Psarras
Sorry I didn't use this one earlier, but I didn't even think to look for it...duh! OK, now I know this happens with 747s and L-1011s--do any other aircraft do this?
Regarding FRA_to_usas questions, of course it doesn't threaten the structural integrity--it's designed to do this! It does, however, add drag so there would be a performance penalty (then again, any cargo would affect performance). Also, why would an airline want to pay someone else to ship it? That would take time and great expense--it would be much quicker and cost-effective to transport it along with the rest of the cargo on their own revenue flight. I've seen aircraft sit for days at a remote location after some mishap waiting for a new engine to arrive by truck--what a waste! Also, I know on the 747, they mount devices on the spare engine to reduce drag, since it is inoperative.