What happened
On January 6, 2016, a Pilatus PC-12/47E, registration N978AF, was performing a positioning flight from Savannah/Hilton Head International Airport (SAV) to Blue Grass Airport (LEX). During the initial climb phase, shortly after takeoff, the crew received a red crew alerting system (CAS) torque warning.
While the pilots reported observing a low torque indication, the aircraft's systems were only designed to trigger warnings for high torque exceedances. Following the warning, the pilot flying initiated a 90° left descending turn to land in an open area between the runways and the terminal. The aircraft landed in the grass with approximately 2,700 feet of runway remaining. During the landing roll, the airplane impacted a drainage ditch, resulting in substantial damage and a postimpact fire. The two crew members sustained 2 minor injuries, and there were 0 fatalities.
The investigation
Investigators examined the engine and the torque indicating system. The engine, a Pratt & Whitney Canada PT6A-67P, showed no evidence of pre-impact anomalies in the compressor section, gearboxes, governor, torque limiter, or fuel control unit (FCU) bellows. Testing of the FCU bellows at a separate facility showed no anomalies.
Examination of the modular avionics unit (MAU) and the custom I/O module revealed a small amount of debris on some internal pins, but all tests related to engine torque signals resulted in values within required specifications. The torque transducer also showed no signs of mechanical damage or electrical faults.
Data from the aircraft condition monitoring system (ACMS) showed that during the initial climb, the torque indication rapidly increased from approximately 45.0 psi to 71.0 psi, which triggered the CAS warning. Four seconds later, the torque decreased to 47.3 psi, coinciding with a reduction in engine power by the crew.