Flight and Maintenance Background
The accident flight was a repositioning operation conducted by two airline transport pilots. It was the first flight after an aviation maintenance technician (AMT) had replaced the left engine propeller with an overhauled unit. The AMT performed an engine run, verifying correct power settings and blade angles.
In-Flight Event
Review of flight data recorder (FDR) data showed that about 2 seconds after rotation, the left engine propeller rpm decreased to 60 percent, and the left engine torque increased off-scale (beyond 5,000 ft lbs). This is consistent with the left propeller traveling to the feathered position and the engine torque increasing in an attempt to maintain rpm. Approximately 30 seconds later, the flight crew shut down the left engine and attempted to return to the departure airport.
Postaccident Examination Findings
Postaccident examination of the rudder trim actuator revealed it was at its full-right limit. This position would have been set to counteract left engine drag before its shutdown. Based on evidence, it is likely that the flight crew did not readjust the trim when the drag was alleviated. As a result, the airplane operated in a crosscontrolled attitude for about 50 seconds, with a left bank and full-right rudder trim. Although the airplane should have been able to climb at about 500 ft per minute on one engine, it slowed and descended from 300 ft in the crosscontrolled attitude until it stalled. A stall warning was recorded by the cockpit voice recorder, and the airplane subsequently impacted terrain.
Mechanical Examination
Examination of the wreckage, including teardown of the left engine and propeller, revealed no preimpact mechanical anomalies. Review of the airplane maintenance manual indicated instructions to check the propeller reversing linkage (controlling the beta valve) for proper rigging during propeller installation. A warning in the manual stated that misadjustment of the beta valve can cause unplanned feathering and result in possible hazard and over torque damage. However, beta valve rigging could not be verified postaccident due to impact damage.
Additionally, the ground/flight idle solenoid energizes when weight is off wheels, further opening the beta valve. This could exacerbate an existing misrigged condition as soon as the airplane becomes airborne, which is when the uncommanded feathering occurred.
Checklist Noncompliance
The FDR data were consistent with the flight crew not performing the Before Takeoff (Runup) checklist. One item on that checklist is a low-pitch solenoid test, which would have energized the solenoid and possibly driven the left propeller uncommanded to feather during ground operations rather than in flight. A similar test during the post-maintenance engine run would have had the same results.