Hola NSEU -
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A severe pressure differencial inbalance between Upper Deck and Main Deck...
Would lead to a complete (structural) collapse of the floor.
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I have flown 747F, 747C and 747SF. Does not matter if not 400s.
A collapse of the upper deck floor would would impede/destroy cables.
Cables linking flight deck to all tail flight controls hydraulic units.
While I agree that generally pressure would remain the same through out the aircraft, I disagree that the floor would collapse. The aft "cosmetic" bulkhead of the upper deck is the weak point. It would also suck that wimpy upper deck smoke door right out.
Been years since I have been on a 747, I remember some classics with a smoke valve that I thought connected to a tube to connect to the main deck to dump overboard, I don't know if boeing thought it would reduce the amount of smoke below the cockpit.
Those packs put out more air than any other aircraft I have been on, I remember closing the out flow valve with the doors closed on the grund and the overhead hatch open and burning a cigarette at the F/E panel, I could flick my ashes and they would go straight up and out.