Accident Sequence
The instrument-rated commercial pilot was conducting a cross-country flight and approaching the destination airport in night instrument meteorological conditions. According to radar track data and air traffic control communications, while receiving radar vectors to the final approach course, the pilot did not always immediately comply with assigned headings and allowed the airplane to descend below assigned altitudes on several occasions.
Based on airplane performance calculations from radar track and GPS data, the pilot made an engine power reduction about 2.5 minutes before the accident while maneuvering toward the final approach fix. Following the power reduction, the airplane's airspeed decreased from 162 to 75 knots calibrated airspeed, and the angle of attack increased from 2.7° to 14°. Approximately 4 miles from the final approach fix, the airplane descended below the specified minimum altitude for that segment of the instrument approach.
The tower controller alerted the pilot of the low altitude, and the pilot replied that he would climb. At the time of the alert, the airplane was 500 ft below the specified minimum altitude of 2,000 ft mean sea level. According to performance calculations, 5 seconds after the controller told the pilot to check his altitude, the pilot made an abrupt elevator-up input that further decreased airspeed, and the airplane entered an aerodynamic stall.
A witness observed the airplane abruptly transition from a straight-and-level flight attitude to a nose-down, steep left bank, vertical descent toward the ground, consistent with a stall. Security camera footage confirmed the transition from a wings-level descent to a near-vertical spiraling descent.
Pilot Information
The pilot held a commercial pilot certificate with an instrument rating and had monocular vision due to a childhood injury that resulted in very limited vision in his left eye. He had passed a medical flight test and received a Statement of Demonstrated Ability. Having flown for several decades with monocular vision, his lack of binocular depth perception likely did not impede his ability to monitor cockpit instrumentation during the accident flight. The pilot had recently purchased the airplane and obtained make-and-model specific training about one month before the accident, accumulating approximately 10 hours on the airplane before the accident flight. The pilot's instrument proficiency and night currency could not be determined from available records.
Aircraft Examination
A post-accident examination of the airplane revealed no anomalies that would have precluded normal operation during the accident flight.
Findings
The official probable cause was the pilot's failure to maintain adequate airspeed during the instrument approach in night instrument meteorological conditions, which resulted in the airplane exceeding its critical angle of attack and an aerodynamic stall/spin at low altitude.