Incident Overview
At 0646 local time, a single-engine airplane departed Lizard Island Airport on a non-scheduled passenger service to Cairns. On board were the pilot and nine passengers. After takeoff from runway 12, the pilot turned right, tracked south-southwest, and made a departure call to Brisbane Centre air traffic control (ATC). At 1,750 feet above mean sea level, the pilot turned left onto the departure track in a cruise climb and engaged the autopilot.
Engine Anomaly and Return
At 0652, while passing 3,400 feet at an indicated airspeed of 102 knots, the pilot noted a change in engine sound and that the aircraft was accelerating. Checking the engine gauges on the Garmin G1000 multifunction flight display, the pilot observed that the engine torque and interstage turbine temperature indications exceeded the redline, while the gas generator speed and fuel flow gauges were unserviceable, marked with a diagonal red cross. The propeller revolutions per minute were high but within the green operating range. Within ten seconds, the pilot initiated a left turn to return to Lizard Island and broadcast a PAN PAN call on the area frequency, advising their intentions.
Despite attempts to arrest the acceleration, the aircraft continued to climb and accelerate over the next two minutes, reaching 4,000 feet and 166 knots. The pilot moved the power, propeller control, and emergency power levers; only the propeller control lever elicited a response. The pilot partially moved the fuel condition lever through the gate from low idle to cut-off, noting the engine power cutting in and out, and then returned the condition lever to low idle. The aircraft conducted a large orbit around the island while the pilot attempted a shallow descent, but the engine continued to produce excessive power. Further manipulation of the propeller control lever somewhat reduced engine thrust.
Approach and Overrun
After one orbit, the pilot felt low enough to attempt a landing on runway 12 with a 2-nautical-mile (4 km) final approach, and notified ATC. Concerned about populated resort accommodation and maintenance buildings near the runway threshold, the pilot elected a powered-on approach to ensure clearance of potential obstacles. The final approach began at 147 knots, significantly above the normal approach speed of 75–85 knots specified in the pilot operating handbook for full flaps. By the runway threshold, the pilot reduced airspeed to 123 knots and reported attempting to shut down the engine.
The aircraft floated along the down-sloping runway, bouncing and touching down at approximately 100 knots, about two-thirds of the way along the runway. Maximum braking was applied, but the aircraft exited the end of the runway at 92 knots. It continued across undulating sandy soil and low vegetation before the left wingtip struck the ground, causing the aircraft to spin and flip. It came to rest inverted 127 meters from the runway end. All ten occupants escaped with minor injuries; the aircraft was damaged beyond repair.
Probable Cause
The official investigation determined that the accident was the consequence of a very likely malfunction of the fuel control unit shortly after departure, resulting in an uncommanded engine acceleration event beyond limits, necessitating a return to the airport. The engine power could not be reduced and the engine was not successfully shut down on final approach, preventing the aircraft from slowing sufficiently to avoid a runway overrun. A contributing factor was that, while uncommanded engine acceleration or inability to reduce power events occur at a higher rate than any other fuel control unit malfunction in Pratt & Whitney Canada PT6A single-engine aircraft, no flight manual procedures addressed this type of occurrence, leading to limited awareness among pilots and operators on how to identify and respond to such an event.