Commercial airplanes have only recently used composite wings, under large regulatory suspicion and are used in a far more benign environment than 4000 m of wate

Commercial airplanes have only recently used composite wings, under large regulatory suspicion and are used in a far more benign environment than 4000 m of water head. I really dont think ppl appreciate what 4000 m of water means! 1 atmosphere is only a 10 meter water column. 4000 m is 400 atm. Actually more because water can no longer be assumed incompressible. EDIT: to add another point - submarines are subject to implosion, which is a buckling failure. That sets up a vicious cycle where a small deformation gets worse, whereas a vessel at positive pressure will "straighten itself out"