For the first time in the public domain, the FBI has released one of the technical reports on the Boeing Company’s tests, in March 1964, of the Boeing 727 with the air stairs down in flight. The report in question is numbered and titled D6-7771: “Flight Characteristics with Aft Airstair Down – B”. The airplane used was the second prototype, tail number N72700.
The FBI document is at https://vault.fbi.gov/D-B-Cooper%20/d.-b.-cooper-part-71-of-71/view. The Boeing report is on pages 333-356.
I extracted the following key passages:
Purpose of Conditions Performed: Simulate a failure of the aft airstair up-latch in flight, evaluating the characteristics of the extended airstair and the effect on airplane performance and handling.
The airstair was extended both by allowing it to freefall and by utilizing normal hydraulic power. These extensions were made with the airplane trimmed for level flight at 125 knots and flaps at 25 degrees.
The airstair extended a nominal 8.5 degrees when allowed to free-fall and it required only approximately one-tenth unit of nose-down trim to compensate for the stair.
The stair extended a nominal 13.5 degrees with hydraulic power and this caused approximately a three-tenths unit nose-down trim change.
On November 24, 1971, the hijacker of Flight 305 departed the airplane (a production 727-51, tail number N467US) via the aft airstair, at an indicated air speed of between 160 and 170 knots.
On January 6, 1972, the FBI and the US Air Force flew the same airplane (N467US) in an attempt to replicate the hijacker's jump. On releasing the airstair without hydraulic power at IAS of 150 knots, they found that it rotated through 20 degrees. On placing a person's weight on the bottom step of the airstair, they found that the stair rotated through 35 degrees (vs. fully retracted position). I have not seen any document stating whether they ever used hydraulic power.
Can we conclude that the hijacker of Flight 305, using the control lever without hydraulic power, would have been able to lower the stairs by at least 20 degrees? and if engaging hydraulic power, by significantly more than 20 degrees?
Alternatively are there any known differences between the prototype N72700 and the production 727-51, such that the airstair might be easier to extend on the 727-51? Documents in the public domain indicate that the 727 hydraulic system generally had an operating pressure range of 2800-3100 psi, but I have not found documents specific to the prototypes or the 727-51 model.