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A Cessna Caravan that overturned on Lizard Island in Far North Queensland last year suffered an engine malfunction that caused it to increase speed on its own, the ATSB has said.
VH-NWJ, a Cessna 208B operated by East Air, was returning to Lizard Island on 8 January 2024 with nine passengers on board after the pilot noticed a significant uncommanded increase in engine power, but overshot the runway, flipped over and came to rest 127 metres away.
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“During the climb [from Lizard Island], the pilot heard a change in the engine and noted that the aircraft was accelerating,” the ATSB said.
“The gauges showed engine torque and temperature above the redline, and no indications for gas generator speed or fuel flow. Propeller revolutions were high but within the normal operating range.
“The pilot elected to return to land at Lizard Island. However, the uncontrollable excessive engine power led to a high‑speed approach, and the aircraft touching down long on the short runway.
“The pilot applied maximum braking, but the aircraft overran the end of the runway at 92 kt and continued across undulating sandy soil, and low vegetation, until its left wingtip struck the ground.”
All 10 occupants of the plane, including a 14-year-old girl, were able to escape with minor injuries and were airlifted to Cairns Hospital.
According to Kerri Hughes, director of transport safety for the ATSB, data provided by the plane’s engine manufacturer, Pratt & Whitney Canada, indicated that uncommanded acceleration happens more often than “roll back to idle” malfunctions, but despite this, there is little training for the scenario.
“The ATSB investigation concluded the uncommanded engine acceleration was likely due to a malfunction of the engine’s fuel control unit,” she said.
“Pilots of PT6A turbine single engine aircraft are trained for ‘roll back to idle’ malfunctions, but not trained for uncommanded engine acceleration malfunctions.
“Consequently, as demonstrated in this accident, there is limited awareness by pilots and operators of how to safely respond to an uncommanded engine acceleration event in aircraft powered by PT6A engines.”
As part of its investigation, the ATSB contacted Cessna Caravan training operators and found “limited awareness of this type of event”.
“While not as immediately dire as an engine failure or roll back to idle, an uncommanded engine acceleration is alarming and requires prompt action to control the aircraft and prevent overstress of the engine and airframe,” Hughes said.
“When faced with non‑normal or emergency scenarios for which there are no procedures, pilots are required to exercise judgement based on their experience, skills and knowledge.
“Operators should consider updating their procedures and providing appropriate training to ensure that their pilots are adequately prepared for such events.”
The ATSB has issued a safety advisory notice to single-engine PT6A-powered aircraft operators to consider, document and train potential responses to this type of malfunction.
East Air has since published a temporary revision to its Quick Reference Handbook for the Cessna 208B to include an emergency item for unscheduled power increase during flight.
