[onewhippedpuppy]

Originally Posted by Jolly Amaranto

Objects like broken wreckage would sink to the bottom relatively unaltered. It is not like it is being run over by a giant steam roller. Anywhere that water can seep into an airspace it will fill the inside and push out at the same pressure as the water pushing in. Things like spent oxygen bottles will be squashed flat if they were well sealed but a section of broken fuselage would remain intact. If the entire cabin of the aircraft remained air tight, it would probably float and not sink anyway. However, if it were somehow (very unlikely) dragged down under and remain completely sealed it would reach a depth where it would collapse catastrophically.

When I was working in deep sea seismic research, we would sink ocean-bottom seismometer units to the ocean floor where they would remain for a week or so recording data before detaching from their anchor and bobbing to the surface to be retrieved. The delicate electronics was enclosed in a crush resistant aluminum sphere. For fun we attached a Styrofoam coffee cup to one of the OBS before it was deployed. When it was retrieved, the cup was a miniature replica. The gas in each of the foam cells was compressed until it dissolved through the foam and into the water leaving behind only a tiny plastic blob for each of the cells, still attached to each other in their original shape. Only thing, the shape was reduced in size.

Great post, it's all about pressure differential. If the airplane filled with water and sank, there would be no pressure differential between the water outside and the water inside, so no crush. But if it somehow landed with the pressure vessel (cockpit/cabin) intact, then sank, eventually the water pressure would exceed the strength of the fuselage and crumple it, filling it with water and equalizing the pressure.

That assumes that the fuselage is airtight, which is incorrect. A number of the fuselage seals on a pressurized airplane are inflatable and bleed air driven, without the engines running they would leak down. Further, pressurized fuselages have an acceptable leak rate, none of them are 100% sealed. Even if you were able to gently land an airplane on the water (Hudson River), it would eventually fill with water and sink.