What do the Boeing 727 airstair tests tell us about D B Cooper?

Question

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.

Answer 1

Yes. The result suggests that the hydraulic actuators for the door stall out at that extension, that is, they can't push any harder against air loads, probably by design by limiting extension pressure to the hydraulic cylinders in some way if that was necessary, and the test was to demonstrate that the airplane is manageable in that condition (uncommanded door extension), which is only 5 degrees beyond the free-floating position.

The question becomes, could Cooper crawl out through a 13-14 degree opening while wearing a parachute? Not a very big gap, especially with a military style backpack emergency rig on your back, but apparently, yes.

Answer 2

The tests say nothing about DB Cooper the man, his motives, or whether he survived the jump, but they do provide some insight into the experience of exiting a 727.

However you might envision it happening, he wouldn’t have just walked to the bottom of the stairs and done a swan dive since they cannot be fully extended in flight. The illustration below from the included link depict the stairs mostly extended, but in reality they would have been closer to an aerodynamic trail position, or roughly horizontal. (the accompanying article also has some insight into his instructions for the pilots to slow down and deploy flaps that I was previously unaware of…)

Article picture was taken from

It is difficult to know exactly how much room is available at that extension, but it is likely that he had to crawl along the stairs to squeeze through the exit at the far rear. The still photos below are taken from the linked YouTube video. It is difficult to estimate the exact angle, but I attempted to pause at roughly 14 degrees, which as you can see is roughly parallel to the fuselage and would offer much less drag than further extension.

YouTube video of stairs operating

In the second photo taken from the interior you can get at least some perspective of the greatly reduced amount of space there would be in a partial extension. Snug, and not enough room to standup, but enough to get out.

Finally, 3000 PSI is pretty standard system pressure for aircraft hydraulic system. It is highly unlikely that an aircraft in normal passenger service would have a system with boosted pressure or a different actuator arrangement intended to allow greater extension than normal. Unless you can make a positive connection between a surplus airframe modified for jumping that the airline leased and subsequently never modified, and was in service on this particular route on this day, it is "reasonable to conclude that via the control lever, with or without hydraulic power, he would have been unable to lower the stairs by more than 13.5 degrees."