A comment about the stairs: I feel like the group that thinks Cooper was very knowledgeable about the 727 references that he knew the stairs could be lowered in flight. Does this same group say that he was knowledgeable about the 727 because he thought the plane could take off with the stairs lowered? I'm guessing no, because I can't think of a situation where the 727 would have taken off with stairs down, even in Vietnam.
Daly's World Airways 727-100 that took off from Da Nang while the North was taking over, had the stairs down during takeoff and people hanging on/falling off.
NYT 1975 article
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Loginvideo of the plane taxiing with stairs down starts here
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Loginthe narrator of the video says people are clinging to the stairs as it takes off
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LoginYou can see the plane is airborne with the stairs still down, starting here
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LoginPeople stilling clinging to stairs, 7 fell off
Daly pulled one onboard from the stairs.
So yes, you can take off with people on the stairs.
Here he says that in flight, they couldn't close the stairs because they were damaged
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LoginSo you can take off, but likely you will damage the stairs?