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I was reading the description of an old hijacking which has this snippet of interesting information:

After takeoff Cooper told Mucklow to join the rest of the crew in the cockpit and remain there with the door closed..... At approximately 8:00 pm a warning light flashed in the cockpit, indicating that the aft airstair apparatus had been activated.... The crew soon noticed a subjective change of air pressure, indicating that the aft door was open. At approximately 8:13 pm the aircraft's tail section sustained a sudden upward movement, significant enough to require trimming to bring the plane back to level flight. At approximately 10:15 pm Scott and Rataczak landed the 727, with the aft airstair still deployed, at Reno Airport....an armed search quickly confirmed that he was gone.

Would a 90 kg man jumping off the aft stairs of a B-727 in flight create a detectable "sudden upward moment". Not just a slight one but "significant enough to require re-trimming"?

Doesn't sound like a fluke or the product of a vivid imagination because we further know that:

An experimental re-creation was conducted using the same aircraft hijacked by Cooper in the same flight configuration, piloted by Scott. FBI agents, pushing a 200-pound (91 kg) sled out of the open airstair, were able to reproduce the upward motion of the tail section described by the flight crew at 8:13 pm.

The full story is here:

https://en.wikipedia.org/wiki/D._B._Cooper

I'm estimating the mass of a fully-fueled B-727 as about 60,000 kg. A detectable perturbation by a 90 kg ejected body sounded a tad unlikely.

But I'd love to see a more accurate calculation. I suppose the center of gravity and the long lever arm for the aft stairs location will matter?

PS. It seems far more plausible to me that the movement detected was actually due to the aerodynamic forces on aft stairs and not the effect of the ejected 90 kg mass. The only reason I was rejecting this explanation was that the timing seems off if we are to believe the flight crew report:

They seem to have detected two events: A pressure change and later this reported sudden movement. So the movement does not seem correlated in time with the aft stair deployment.

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In the EMB-145 you had to continually re-trim just from the flight attendant and galley cart moving the length of the cabin. (normally transparently done by the autopilot but if you hand flew it at cruise you definitely noticed it). – casey Jan 19 at 17:59
    
@casey Interesting. Then again, the B-727 would be roughly 5x heavier than the EMB-145? – curious_cat Jan 19 at 18:17
    
depending on model, max weight is in the 20,000 to 25,000 kg range. You'd also see a larger change in the 727 because the moment arm at the far aft airstairs is large. – casey Jan 19 at 18:20
    
Related question, but not duplicate: aviation.stackexchange.com/questions/24447/… – Quora Feans Jan 19 at 22:03

Yes

It's not just weight, it's also aerodynamics. First let's assume that the aircraft was trimmed slightly nose-heavy (no passengers), therefore it's trimmed slightly "tail down" anyway

When the stair was first opened, it would have been held partially closed by the airflow under the fuselage - it's not heavy enough alone to extend fully against a 100 knot wind.

There was then a two-part change from Cooper leaving the aircraft - one of him walking down the stair, pushing it into the airflow and acting much like the elevator.

When he then jumped, the loss of his weight at the very back of the aircraft, along with a presumed "push" against the door, pushing it further into the airstream, would have simultaneously jolted the tail up, then reduced the weight at the very back of the tail "lever".

90kg doesn't sound like very much, but when it's at the very, very back of quite a long lever, and when combined with the airstair, the results seem perfectly reasonable and, indeed, were proven by the FBI.

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A tail at the end creates a pitch up moment. Therefore dropping that weight creates a pitch down moment. The door create a pitch down moment while deployed, so letting them partially close again would create a pitch up moment as described. But the weight itself does not contribute, it works against it. – Jan Hudec Jan 19 at 17:25
    
I'm talking about the combination of walking down the stairs (pushing the door into the airstream) and then pushing off the door to jump out. Pushing the tail up momentarily, combined with the overall loss of his weight (over the course of a few seconds) from the tail. The weight change the trim, the door merely helped by pushing the tail up momentarily for a couple of seconds between the weight being there and then vanishing. – Jon Story Jan 19 at 17:31
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The weight in & of itself is utterly inconsequential. Having paratroopers walk off the ramp of a C-130 is unnoticeable on the controls. (Drop a Humvee -- you notice that!) Only the aerodynamic effects would give a pitching input the pilot would notice. – Ralph J Jan 19 at 17:35

I would suggest that the aft stairs acted as a control surface lifting the tail. Perhaps they will not extend fully against the slipstream until a weight of 90 kg is applied to the end...

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I had thought the same thing. And maybe you are right. I was only discarding that explanation because the description makes it seem that there was a delay between the flight crew detecting the stairway deployment (pressure change) and the actual jump (the movement). – curious_cat Jan 19 at 16:31
    
I'm pretty sure that the 727 airstair acts as a tall stand to keep the plane from tipping from uneven loads. It doesn't just drop down from weight, it opens hydraulically – TomMcW Jan 19 at 18:59
    
@TomMcW Thanks. In light of your comment, then the relevant question would be: How strong are the actuators in the context of what drag forces you would expect on the slide when opened during flight. – curious_cat Jan 19 at 19:26

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