Accident Overview
The airplane was destroyed by impact forces with no evidence of fire. Wreckage examinations revealed crushing and bending consistent with a moderate angle of descent and a moderate right-wing-down attitude at impact. The engine was producing power at the time of impact, and no preexisting powerplant, system, or structural failures were found. All recovered parts were at the crash site, scattered over an area of about 600 feet.
Flight Path and Traffic Advisory
Radar and performance data indicated that the accident airplane initiated a descent from 3,000 feet immediately after the pilot received a second traffic advisory from air traffic control. The pilot reported that the traffic was above him and stated, "I needed to deviate, I needed to deviate." Data showed that the airplane was in or entering an uncontrolled descent at that time. Performance simulation indicated that beginning about 15 seconds before the pilot's last transmission, his view of the traffic (a DC-10) moved diagonally across the windscreen from left to straight ahead while tripling in size.
Wake Turbulence Analysis
The closest identified airplane to the accident airplane was a DC-10 at an altitude of about 4,000 feet, with a horizontal distance of about 1.1 nautical miles and a vertical distance of about 1,600 feet. The accident airplane was never in a location where wake turbulence from the DC-10 would have intersected its flightpath; wake turbulence was determined not to be a factor.
Pilot's Actions and Spatial Disorientation
Shortly after the pilot stated a need to deviate, the airplane experienced high bank and pitch angles. Simulation showed the airplane rolling through 90° about 13 seconds after the transmission, continuing to a peak of about 150° three seconds later, with nearly recovering at impact. Performance data indicated the airplane must have remained structurally and aerodynamically intact to reach the ground from the point of upset. The meaning of the pilot's statement could not be determined. A sound spectrum study found increased background noise and activation of the overspeed warning, consistent with extreme fragmentation. Radar transponder data was lost below 2,400 feet, likely due to unusual attitudes masking transmissions.
Weather Conditions
The accident occurred at night with the moon obscured by low clouds. Instrument meteorological conditions prevailed, with visual conditions reported between cloud layers. The terminal aerodrome forecast included a possible cloud layer at 3,000 feet. Weather data and observations by the DC-10 pilot indicated that after flying about 100 to 500 feet above a cloud layer and soon after sighting the DC-10, the accident airplane would have entered clouds. Several conditions conducive to spatial disorientation were present: no visible horizon references due to night conditions, varying eye and head movements during visual search, and the DC-10 appearing as a moving light source against a featureless background.
Probable Cause
The pilot's spatial disorientation, which resulted in loss of airplane control. Contributing to the accident was the night instrument meteorological conditions with variable cloud layers.