Summary
On an unspecified date, an aircraft crashed about 2 miles north of an airport after experiencing a loss of control during a post-maintenance flight. The flight was intended to verify the proper inflight operation of the autofeather system, which had recently undergone maintenance on its pressure sensing switches due to reported issues with the left engine not autofeathering properly in flight. A ground check of the system had been successful before the flight.
Facts
Air traffic control communications indicated that the pilot called ready for takeoff from runway 2 at approximately 1207 and requested to orbit above the airport at 8,000 feet. The airport elevation is 4,740 feet. The pilot reported established in a hold at 8,000 feet around 1213 and was instructed to report leaving the hold. At about 1215, an abbreviated radio transmission, "zero five four," was recorded. The tower controller responded but received no further communication from the aircraft despite repeated attempts.
Witnesses reported that the aircraft banked to the left or west, then entered a spiral from that bank and crashed. One witness noted the aircraft was flying at 200 to 300 feet above ground level when it entered the bank and performed a "skidding" or "sliding" motion about one second before entering the spiral.
Investigation
On-site examination revealed wreckage and impact signatures consistent with an uncontrolled, relatively low-speed, moderate to steep angle (greater than 22 degrees), left-wing-low impact on an easterly flight path. No evidence of flight control system malfunction was found. A large quantity of jet fuel was aboard.
Post-accident engine examination indicated the left engine was most likely operating in a low power range and the right engine in a mid to high power range at impact. No anomalies or distress that would have precluded normal operation were found. Propeller examination showed both propellers were rotating at impact, neither was at or near the feather position, both were being operated with power (exact amount unknown), both were at approximately 14 to 20 degrees blade angle, and there were no propeller failures prior to impact.
The autofeather pressure sensing switches were examined and revealed evidence of alterations, tampering, or modifications made in the field on all but one replacement switch (installed just before the accident flight). All switches except the replacement operated outside their design pressure specifications; the replacement operated within specs. All switches were installed in correct positions. Engineering analyses of expected autofeather system performance with the switches at their as-found pressure settings did not indicate a likelihood of any anomalous or abnormal operation. Cockpit light and switch evidence indicated the autofeather system was not activated at the time of impact.
The combination of probable engine power and propeller pitch on the left engine was noted to be generally consistent with the "zero-thrust" engine torque and propeller RPM settings specified for simulated single-engine practice in the aircraft Information Manual.
Probable Cause
The pilot-in-command's failure to maintain adequate airspeed with an asymmetric thrust condition, resulting in a loss of aircraft control. A factor was an asymmetric engine thrust condition, which was present for undetermined reasons.