So for the 8:11pm bump not to be Coop's jump, there would have to be a way to exit the plane without creating a bump right ?  Or am I missing something ?  Unless it's possible for some other factor that could cause the crew to not have noticed ?

I think you're mistaken in assuming the crew detected all possible pressure bumps and reported them.

Imagine this experiment:

Crew on a plane. They feel a pressure bump and say Cooper has exited the plane. Shortly after that, they fly around an airport, requiring increased attention. A second pressure bump (maybe less) occurs. (Cooper jumped then).

Does the crew always detect and report the second pressure bump? They already thought Cooper had jumped before that. And they were maybe more mentally focused on other things at this time?

Also, the stairs have to be substantially lowered for Cooper to have enough room to jump and he has to be near the bottom of the stairs for this to happen.  Cooper jumping will result in the stairs slamming back into the fuselage.

The FBI did one experiment that achieved what you say.

Doesn't mean that every time someone jumps the aft stairs, there is *always* a pressure bump like that.

Sure it means it's possible to cause a pressure bump. Does it mean *always* ? or is the pressure bump of the same magnitude? What factors affect it?

Are you saying no factors affect it and it is always the same? How would you know that?

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Also, the stairs have to be substantially lowered for Cooper to have enough room to jump and he has to be near the bottom of the stairs for this to happen.  Cooper jumping will result in the stairs slamming back into the fuselage.

The FBI did one experiment that achieved what you say.

Doesn't mean that every time someone jumps the aft stairs, there is *always* a pressure bump like that.

Sure it means it's possible to cause a pressure bump. Does it mean *always* ? or is the pressure bump of the same magnitude? What factors affect it?

Are you saying no factors affect it and it is always the same? How would you know that?

First, due to the side panels on the stairs, Cooper has to go off the end of the stairs.  Second, he has to be close to the end of the stairs when he jumps or he is going to get smacked when his weight is removed from the stairs and before he clears them.  Third, remember that the pivot point for the stairs is at the bottom of the fuselage structure and the stairs have to be down quite a bit for Cooper to clear the fuselage cutout even if he is crawling down them on his hands and knees.  The FBI tests perfectly illustrate this.

Other things being equal, the position of the unlocked stairs is going to be determined solely by the dynamic pressure from the airstream.  And it will be repeatable.   

A plane flying straight will have the airstream in line with the aftstairs.

a plane turning will have some amount of airstream at an angle across the aftstairs.


How can it be that one knows that a turning airplane has dynamic pressure on the aftstairs the same way a plane flying straight does?

This is just an example of how there is variablity.

Now you can argue you *know* that the variability doesn't affect things. But since no experiment was done, you're just speculating.

thinking about the stability/instability of a loose aft stair on a plane flying

the pressure on the bottom of the airstair is immense, due to the speed.

But: apparently the additional weight of only around 200-220 lbs on the end of the airstair, is enough to open it, and when removed, cause the air stair to "pop up" causing a pressure bump.

this is kind of amazing, as it shows the relative instability of the airstair position. 220 lbs is not much compared to all the wind forces in place

So if the plane was turning and air was running over the top of the airstair, and maybe a twisting force was applied to the hinge (due to air force on the fabric on the handrails, and also the vertical risers of the stairs)...it's very unpredictable what would happen..
Small changes apparently cause big effects (i.e. the 220 lb change causes a big effect). Nonlinearity is always the tricky thing to model.

at one end of the thought experiment, I think we could agree that if the air was flowing at 90 degrees to the airstairs, the upward and downward pressures would probably be equal (unless there was some airfoil effect due to the different shapes on top and bottom of the airstair) and the airstair wouldn't move (and the friction at the hinges would be high due to high twisting forces)


So if the plane was turning, the overall effects would be somewhere between that extreme, and the one known data point (the FBI drop experiment)

It's not a simple linear analysis. Lots of random effects

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Other things being equal, the position of the unlocked stairs is going to be determined solely by the dynamic pressure from the airstream.  And it will be repeatable.

So you're saying the behavior of whatever hinges the aft stairs pivots on, is always the same?  (and I suppose any hydraulic arm connections)

I think your statement that the dynamic pressure from the airstream is the *only* factor that affects the things leading up to the pressure bump, is not supported by anything other than speculation.

Snowmman, let's go over this again.  Did you notice that I said "other things being equal" along with the repeatable remark?  I have never seen any comments on the Internet about the stairs behavior during turning flight and my comments applied to straight and level flight as have all the previous Internet discussions that I am aware of.  Admittedly, if the airliner is flying upside down and backwards then the stairs will behave differently than they did during the Cooper hijacking and the FBI tests.

The pivot point for the stairs is fixed permanently in the airframe structure.  There are several steps above that pivot point to get to the cabin floor level that are also permanently fixed in the airframe and have nothing to do with the movement of the stairs below the pivot point.

Further, the stairs below the pivot point have side "modesty" panels that are made of fabric or fiberglass and are not load-bearing panels.  In the hijacking, pictures of the aircraft in Reno show that these modesty panels had been heavily damaged and partially blown away.  In the FBI tests, these panels appear to have been removed to facilitate the photography.  But they are not an important factor in the aerodynamics related to the stairs.  It is going to be extremely windy and cold on those lowered stairs.

Again, given the same aircraft configuration, altitude, airspeed, weight on the stairs, etc., the results will be repeatable from one test to another. 

was just noticing someone else commenting on the randomness of airstair behavior (likely).

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Although the hydraulic actuators would have provided damping, the swirling wakes from the landing gear and flaps—not to mention the aircraft turbulence inherent when flying low and slow over mountains in a storm—undoubtedly wrenched the stairs up and down considerably. Cooper's exit has been romanticized as a high-dive into the unknown, but it was more like trying to exit a springboard during an earthquake, with his own weight just exacerbating the problem.

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I'm going to go as far as to suggest there were multiple pressure bumps up until the plane landed in Reno.

To get a "pressure bump" the stairs are going to have to slam back into the fuselage.  And that is only going to happen once.  That once is when Cooper jumped off the end of the stairs.

okay, you can believe that.
I'm just not agreeing with you.

nice video of a 727 air stair being tested...just to see the up/down motion in normal operation

Snowmman, I suggest that you get a few minutes of flying time in a vertical wind tunnel.  If you live in the San Francisco area, I am sure that 377 can tell you where to find just such a tunnel.

The wind speed that you will be exposed to in a vertical wind tunnel is only about 100 MPH.  At the time Cooper jumped, the airliner was doing about 225 MPH.  So you will be exposed to a much lower wind speed than Cooper experienced.

After the vertical wind tunnel experience, you will probably have somewhat more respect for aerodynamic forces.