Investigators have determined that an Air Vanuatu ATR 72-500 crew misdiagnosed an engine failure as an electrical smoke problem and consequently referred to the wrong checklist, unwittingly disengaging systems which were not malfunctioning.
This left several crucial systems, including brakes and steering, unavailable to the pilots who were then unable to prevent the ATR veering off the runway at Port Vila and crashing into a pair of parked aircraft.
The aircraft, bound for Port Vila from Tanna on 28 July last year, suffered a right-hand engine failure while in cruise.
Papua New Guinea's accident investigation commission says smoke travelled from the engine, through the air-conditioning system, into the cabin, cockpit and avionics bay.
This triggered a smoke detector in the avionics and electrical compartment and generated an electrical smoke warning – misleading the crew as to the nature of the problem, and prompting them to follow the 'electrical smoke' emergency checklist.
The checklist required de-activation of generators and, as a result, the aircraft's main hydraulic pumps were no longer available. This mean the aircraft's main-gear brakes and nose-wheel steering were rendered inoperative.
Investigators state that the crew also shut down the right-hand engine after a low oil-pressure warning.
Shutting down the engine meant the rudder's travel limitation unit remained locked in high-speed mode, restricting rudder deflection. The aircraft was flying at 201kt at the time.
This limitation unit would normally switch automatically to low-speed mode, offering greater rudder authority, when the aircraft decelerated through 180kt.
Engine shutdown inhibited this automatic switching, however, and required the pilots to switch to low-speed mode manually to free the rudder. But the crew did not consult the 'before landing' checklist, which meant the rudder stayed locked and offered significantly limited directional authority when the aircraft landed.
As the ATR touched down and rolled out, both engine power levers were set to maximum reverse-thrust. With only the left-hand engine operating this generated asymmetric thrust, and the aircraft veered to the left of runway 29.
"Use of reverse thrust under the prevailing circumstances was inappropriate," the commission says.
The crew was unable to use either the brakes or the nose-wheel steering – owing to the loss of the hydraulic pumps – or the locked rudder to correct the deviation or slow the aircraft. Although the emergency brake was available, the pilots failed to use it.
After veering off the runway the aircraft rolled across a taxiway and slowed to around 45kt before colliding with two parked Britten-Norman BN-2 Islanders, substantially damaging both.
The inquiry stresses that the engine failure did not cause the landing accident, and pilots are normally trained to land aircraft with an engine inoperative.
"Apart from the engine, none of the aircraft systems, including electrical and hydraulic systems, malfunctioned in-flight," it states. "The loss and unavailability of these systems, was induced by flight crew action."
Although the ATR sustained serious damage, none of the 39 passengers and four crew members was injured.