A precise search area was difficult to define, as even small differences in estimates of the aircraft's speed, or the environmental conditions along the flight path (which varied by location and altitude), changed Cooper's projected landing point considerably. An important variable was the length of time he remained in free fall before pulling his ripcord—if indeed he succeeded in opening a parachute at all. Neither of the Air Force fighter pilots saw anything exit the airliner, either visually or on radar, nor did they see a parachute open; but at night, with extremely limited visibility and cloud cover obscuring any ground lighting below, an airborne black-clad human figure could easily have gone undetected. The T-33 pilots never made visual contact with the 727.