What happened
Following recent maintenance on the autofeather system pressure sensing switches, an aircraft took off from runway 2 to perform a flight test. The maintenance had been conducted to address previous issues where the left engine failed to autofeather correctly during flight. At approximately 12:07, the pilot notified air traffic control of readiness for takeoff and requested to enter an orbit at 8,0-00 feet. By 12:13, the pilot confirmed the aircraft was established in a holding pattern.
At roughly 12:15, a brief radio transmission was recorded, after which the aircraft ceased all communication. Despite repeated attempts by the Idaho Falls tower controller to re-establish contact, the aircraft did not respond. Eyewitnesses observed the aircraft bank toward the west and enter a spiral. One witness noted the aircraft was flying at an altitude of 200 to 300 feet above ground level when the bank began, describing a skidding motion just before the spiral developed. The aircraft ultimately crashed about two miles north of the airport.
Findings
Post-accident investigation of the wreckage showed an impact signature consistent with an uncontrolled, low-speed, and steep left-wing-low descent on an easterly path. While the aircraft contained a large amount of jet fuel, no malfunctions in the flight control systems were identified. Analysis of the engines suggested the left engine was at low power while the right engine was at a mid to high power setting, though no engine anomalies were found that would have prevented normal operation. Both propellers were rotating and operating with power at the time of impact, and neither was in the feathered position.
Investigators discovered that the autofeather pressure sensing switches had been modified or tampered with in the field. Specifically, all but one switch operated outside of their intended design pressure specifications. Although the replacement switch installed during recent maintenance functioned correctly, the other switches did not. However, engineering analysis determined that the pressure settings of the existing switches were not likely to cause abnormal autofeather system behavior. Evidence from the cockpit indicated the autofeather system was not active during the accident.