Evidence suggested that Cooper was knowledgeable about flying technique, aircraft, and the terrain. He demanded four parachutes to force the assumption that he might compel one or more hostages to jump with him, thus ensuring he would not be deliberately supplied with sabotaged equipment. He chose a 727-100 aircraft because it was ideal for a bail-out escape, owing not only to its aft airstair but also to the high, aftward placement of all three engines, which allowed a reasonably safe jump despite the proximity of the engine exhaust. It had "single-point fueling" capability, a then-recent innovation that allowed all tanks to be refueled rapidly through a single fuel port. It also had the ability (unusual for a commercial jet airliner) to remain in slow, low-altitude flight without stalling, and Cooper knew how to control its air speed and altitude without entering the cockpit, where he could have been overpowered by the three pilots. In addition, Cooper was familiar with important details, such as the appropriate flap setting of 15 degrees (which was unique to that aircraft), and the typical refueling time. He knew that the aft airstair could be lowered during flight—a fact never disclosed to civilian flight crews, since there was no situation on a passenger flight that would make it necessary—and that its operation, by a single switch in the rear of the cabin, could not be overridden from the cockpit. He also may have known that the Central Intelligence Agency was, at the time, using 727s to drop agents and supplies behind enemy lines during the Vietnam War.