Circumstances:
At 0646LT, the single engine airplane departed Lizard Island Airport on a non-scheduled passenger service to Cairns. On board were the pilot and 9 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. At 0652LT, passing 3,400 feet at an indicated airspeed of 102 kt, the pilot noted a change to the engine sound and that the aircraft was accelerating. The pilot checked the engine gauges on the Garmin G1000 multifunction flight display. They noted the values indicated on both the engine torque and interstage turbine temperature gauges were above the redline and the gas generator speed and fuel flow were not indicating, being marked with a diagonal red cross through the gauge locations. The propeller revolutions per minute was high but within the green operating range. Within 10 seconds, the pilot initiated a left turn with the purpose of returning to Lizard Island and broadcast a PAN PAN call on the area frequency advising their intentions. Despite trying to arrest it, the aircraft continued to climb and accelerate over the next 2 minutes reaching 4,000 feet and 166 kt. The pilot started troubleshooting the issue by moving the power, propeller control and emergency power levers with the only response coming from the propeller control lever. The pilot also partially moved the fuel condition lever through the gate from low idle to cut-off and noted the engine power cutting in and out and returned the condition lever to low idle. At the same time, the pilot conducted a large orbit around the island attempting a shallow descent while the engine continued to produce excessive power. Further manipulation of the propeller control lever somewhat reduced the engine thrust. The pilot updated ATC on their situation and progressively deployed the flaps to create drag in an attempt to slow the aircraft. After one orbit of the island, the pilot felt they were low enough to attempt a landing with a 2 NM (4 km) final approach to runway 12 and notified ATC of their intentions. The pilot reported they were concerned with the populated resort accommodation and maintenance buildings on the approach end of the runway and the possibility of injury to those on board and on the ground if the aircraft did not have the energy to make the runway. To ensure they cleared potential obstacles, the pilot elected to perform a powered-on approach. The pilot continued to adjust the propeller lever in an attempt to reduce the engine thrust. The final approach was started at 147 kt (the pilot operating handbook stated the normal approach speed with full flaps was 75–85 kt). The pilot was able to reduce the airspeed to 123 kt by the runway threshold and reported attempting to shut down the engine. The aircraft floated along the down-sloping runway, bouncing and touching down at around 100 kt, an estimated two-thirds of the way along the runway. The pilot applied maximum braking, but the aircraft exited the end of the runway at 92 kt. The aircraft continued across undulating sandy soil and low vegetation before the left wingtip struck the ground, which spun and flipped the aircraft, coming to rest inverted, 127 metres from the end of the runway. All 10 occupants escaped with minor injuries and the airplane was damaged beyond repair.
Probable cause:
The accident was the consequence of the following:
- Shortly after departure, the fuel control unit very likely malfunctioned resulting in an uncommanded engine acceleration event beyond limits, necessitating a return to the airport.
- The engine power was unable to be reduced and the engine was not successfully shut down on final approach. As a result, the aircraft could not be slowed sufficiently to prevent a runway overrun.
The following contributing factor was identified:
- While uncommanded engine acceleration or inability to reduce power events occur at a higher rate than any other type of fuel control unit malfunction in Pratt & Whitney Canada PT6A single-engine aircraft, there were no flight manual procedures addressing this type of occurrence. Consequently, there was limited awareness by pilots and operators on how to identify and safely respond to an uncommanded engine acceleration event.
