Accident Overview
The accident occurred during a skydiving flight where the right-seated pilot was training the left-seated pilot. Before the accident flight, six flights had been completed without incident, and the skydivers had been dropped normally. The right-seated pilot could not completely recollect the minutes leading up to the accident due to his injuries, but he recalled that the airplane was descending as expected with the power at idle.
Flight Path and Engine Performance
Recorded ADS-B data showed that after turning onto final approach, the airplane completed a right 360° turn, presumably because the altitude was too high. The right-seated pilot attempted to increase power by slightly nudging the throttle forward and thought the engine power did not increase as expected. However, a performance study of the last 70 seconds of data revealed a series of speed and thrust oscillations consistent with a pilot increasing and then decreasing the power lever.
Impact and Examination
The right-seated pilot recalled aiming for an open dirt field and observing a berm in the immediate flight path. In an effort to avoid the berm, he maneuvered the airplane into a right turn. The airplane landed short of the runway, resulting in a collision with the berm. The engine was producing power at the time of impact. Postaccident examination revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation.
Pilot Actions and Training
The right-seated pilot was training the left-seated pilot and stated that he took over the controls during the final approach. It is unknown when he took over, so it is unknown which pilot was at the controls during the speed oscillations. The right-seated pilot likely took over the controls too late. The left-seated pilot’s ability to hear changes in engine power might have been hindered because she was listening to music through her headset at an elevated decibel level.
Aircraft Modification
The airplane was modified by a Supplemental Type Certificate, replacing the original Pratt & Whitney PT-6 turbine engine with a Honeywell TPE331 turbine engine. The TPE331 engine’s characteristics are such that if the airplane is on final approach with the power near idle, the throttle sensitivity increases around the transition between the propeller-governing and underspeed-governing modes, which corresponds to a zero-thrust condition. Near this transition point, small movements of the power lever (about ¼ to ½ inch of deflection) can result in relatively large thrust changes that can surprise pilots inexperienced with this behavior and result in pilot-induced oscillations (PIO). Given the thrust oscillations observed shortly before the end of the ADS-B data, it is likely that the left-seated pilot was at the controls and experienced such a PIO on a short final approach to land.