Accident Overview
An airline transport pilot was conducting a cross-country repositioning flight. While en route, the pilot informed air traffic control that he was beginning a descent. No further communications were received. Radar and GPS data indicated that the airplane began descending in a westerly direction around the time of the last transmission. The final GPS recorded position placed the aircraft approximately 200 feet southwest of the initial impact point, at an altitude of 90 feet above ground level and a groundspeed of 66 knots. The wreckage was located in an open field, with impact patterns consistent with a stall/spin leading to a near-vertical collision at low speed.
Engine and Systems Examination
Examination revealed that the right propeller blades were in the feathered position. The right engine's oil gauge housing extension was improperly secured to the oil gauge housing, resulting in a loss of engine oil. Further inspection found a hole in the right engine's crankcase, metal debris in the oil sump, and evidence consistent with insufficient lubrication. The cockpit switch positions corresponded to an in-flight shutdown of the right engine. No anomalies were found with the left engine or the airframe that would have prevented normal operation.
Pilot Actions and Autopilot Use
A pilot who had previously flown with the accident pilot reported that the pilot typically used the autopilot. Examination of the autopilot system showed that the roll, heading, and pitch modes were active. During the descent, no significant heading changes were recorded, and the direction of travel before the stall was not optimal for landing before a fence line. It is likely that the autopilot was controlling the airplane's flightpath before the stall.
Stall/Spin Analysis
Despite one operating engine, the pilot did not maintain adequate airspeed and exceeded the airplane's critical angle-of-attack, resulting in an aerodynamic stall/spin. Correcting the last GPS-recorded groundspeed for prevailing wind, the airplane's indicated airspeed would have been approximately 72 knots, which is above the aircraft's zero-bank stall speed. However, an undetermined amount of bank would have been applied to maintain heading, which would have increased the stall speed. The investigation could not determine why the pilot failed to maintain proper airspeed or notify air traffic control of the engine problem.
Medical and Distraction Considerations
A review of the pilot's medical history revealed several historical conditions. Toxicology tests detected multiple sedating allergy medications in his system. It was inconclusive whether these conditions or medications impaired the pilot's ability to fly or if the pilot was incapacitated. The possibility that the pilot was distracted by the loss of oil from the right engine was considered, but based on available evidence, the investigation could not determine the reason for the pilot's lack of corrective actions.
Probable Cause
The official probable cause is the pilot's failure to maintain adequate airspeed and his exceedance of the airplane's critical angle-of-attack for reasons that could not be determined based on the available evidence, which resulted in an aerodynamic stall/spin. Contributing to the accident was the improperly installed oil gauge housing extension, which resulted in a loss of oil quantity and right engine power.