Friends have been asking me “How did a metal chunk of an airplane float to Réunion? Why didn’t it sink?” Or, as Monty Python might have asked, “What also floats in water?”
I talked to a former Boeing engineer. It seems that the 777 flaperon is hollow, comprising ribs and an aluminum skin. The same riveting techniques used on the pressure vessel are used on on the flaperon and therefore everything is airtight by default and therefore water-tight. He didn’t remember if there were any drain holes in the flaperon, as there are for some control surfaces, but they would have been very small and easily clogged, thus trapping air.
Trivia: The 777 was originally designed with folding wingtips so that it could fit into gates designed for the B767. This was abandoned at some point but the idea is back with the 777X (gizmodo).
It would appear from this photo:
http://i4.mirror.co.uk/incoming/article6160168.ece/ALTERNATES/s615/plane.jpg
that the flaperon is not only hollow but also separated into compartments by the ribs which would in effect form bulkheads. So that even if one or more of the compartments was pierced that the flaperon would remain buoyant as long as some of the others were intact.
Seems curious given its lifetime of transitioning from 1 atm to 1/4 atm (at 35K feet) they would intentionally make an airtight compartment in a flaperon. Wouldn’t that add additional stress to the various fittings (if not outright swell the structure.)
I would have thought they would intentionally make the flaperon with air holes in it allowing pressure to equalize.