Would a 90 kg object jettisoned from an aircraft create a detectable movement? 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.
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.