A malfunctioning fuel control unit caused the PT6A turbine engine in a Cessna Grand Caravan to accelerate to full power, leading to a runway over-run and crash, according to an ATSB investigation report released late last week.
VH-NWJ had departed Lizard Island near Cairns with 10 people on board in January 2024 when the pilot detected a change in engine noise and increase in airspeed six minutes into the flight. The pilot noted the engine torque and temperature gauges were above the red line.
The engine did not respond to power lever movement, but did respond to propeller pitch commands. The pilot elected to return to Lizard Island rather than continue to Cairns.
The excessive power cause the approach to be high and fast, leading to the aircraft touching down too far into the runway and over-running the end. The aircraft turned over in the sandy soil and low vegetation and was substantially damaged. Passengers were able to evacuate with only minor injuries.
Investigators determined the fuel control unit was faulty, which resulted in the uncommanded increase in power, but also noted that pilots were generally not trained to respond to that condition.
“The ATSB investigation concluded the uncommanded engine acceleration was likely due to a malfunction of the engine’s fuel control unit,” ATSB Director Transport Safety Kerri Hughes 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.”
Engine manufacturer Pratt & Whitney Canada provided data to the ATSB that showed an uncommanded acceleration was actually more common than a power roll-back.
However, ATSB investigators found there was little awareness of the potential for uncommanded acceleration among Caravan operators, and issued a Safety Notice encouraging them to consider responses to an event at different phases of flight.
“While not as immediately dire as an engine failure or roll back to idle," Hughes said, "an uncommanded engine acceleration is alarming and requires prompt action to control the aircraft and prevent overstress of the engine and airframe.”
The aircraft operator 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.
The full report is on the ATSB website, and a video outlining the accident sequence is on You Tube.