The flaps in any position wouldn't have any affect on the stairs. the slower the plane, the lower the stairs could go. they would have less stress (wind load). once they started slowing down on approach the stairs began to drop until finally in taxi they were dragging.
The stuntman in the movie appears to be crouched at the bottom of the stairs at 150 knots. in the down and locked position, the stairs measure just shy of 7 feet for clearance. odds are that all of those who jumped were probably in a slight crouched position'
The railing stops before the last step. Cooper was probably on the second or third step up from the bottom..
Also, if the code was the same in 1971, the railing would be at 42" in height. when you go below that, it's hard to grab onto while go up or down. that's 3 1/2 feet.
McNally went down on his ass, feet first rolling over and allowing his body to merge off the stairs.
A total of 11 steps (treads) from the hinge point to the ground. the steps above that are fixed to the aircraft.
But, the stairs during the Cooper jump and the sled test were UNLOCKED - free to move. As the attached shows.
Unlocked stairs apparently ride in the slipstream where the sled test photo shows them riding.
When stairs are unlocked and free to move they are free to mechanically vibrate. As they move acoustic effects in the cabin changes - like a musical instrument! It is important to distinguish the two effects: mechanical vibrations of the stairs vs. Bernoulli effects in cabin pressure due to the fuselage hole changing as the stairs move. Its like a wind instrument, say a trumpet or trombone.
FJ is saying all vibrations and bumps can only happen when the stairs are at their fullest extent, due to weight being on them. That is the same as saying: a trombone can produce all of its notes ONLY when the slider bar is at its fullest extent! FJ does not understand the principle of how and why simple resonating systems work! In fact I am rather sure FJ will contradict everything I am saying because he does not know what he's saying, or the implications of what he is saying.
When the airstairs are in the unlocked position they are like a large tuning fork that vibrates in the air stream. Since the stair assembly opens and closes a hole there are also acoustic Bernoulli effects in cabin air, like pressure notes/frequencies! Mechanical Oscillations vs pressure bumps/notes! Both types of effects were noted by the crew during the sled test and during the hijacking once the airstairs were unlocked.