What happened
During a repositioning flight, a multiengine turboprop experienced a significant mechanical failure shortly after takeoff. The flight, operated by two pilots, was the initial flight following maintenance work where an aviation maintenance technician had installed an overhauled propeller on the left engine. Approximately two seconds after rotation, the left engine propeller RPM dropped to 60 percent, while engine torque increased beyond 5,000 ft lbs. This data indicates the left propeller moved into a feathered position, causing the engine to increase torque in an attempt to maintain RPM.
In response to the engine issue, the crew shut down the left engine roughly 30 seconds later and attempted to return to the departure airport. However, the aircraft was being operated in a cross-controlled state. Investigation of the rudder trim actuator showed it was set to the full-right limit, a position used to counteract the drag from the malfunctioning engine. The crew did not readjust the trim once the engine was shut down, leaving the aircraft in a left bank with full-right rudder trim for approximately 50 seconds. While the aircraft was capable of climbing on a single engine, the cross-controlled attitude caused the plane to lose airspeed and descend from 300 feet. The cockpit voice recorder captured a stall warning before the aircraft impacted the terrain, resulting in fatalities.
Findings
Post-accident analysis of the wreckage and engine components showed no mechanical anomalies prior to the impact. However, the investigation highlighted potential issues regarding the propeller reversing linkage and the beta valve rigging. The aircraft maintenance manual warns that improper adjustment of the beta valve can lead to unplanned propeller feathering. While the impact prevented a full verification of the beta valve rigging, the flight data suggested the propeller feathered as the aircraft became airborne.
Evidence from the flight data recorder indicated that the crew did not complete the Before Takeoff (Runup) checklist. This checklist includes a low-pitch solenoid test, which, if performed, might have triggered the uncommanded feathering during ground operations rather than during the flight phase. Additionally, the ground/flight idle solenoid, which opens the beta valve when the aircraft leaves the ground, may have exacerbated a misrigged condition.