Investigators have determined that an engine failure on an Air Vanuatu ATR 72-500 did not directly lead to the runway excursion and ground collision with a pair of Britten-Norman Islanders at Port Vila.
The aircraft began suffering problems with its right-hand engine while in cruise at 16,000ft, and 60nm from Port Vila, on 28 July last year. The captain took control of the ATR from the first officer.
Smoke entered the cabin and cockpit and the pilots put on oxygen masks as they prepared the aircraft for descent, subsequently shutting down the right-hand engine.
The ATR approached from the south, making a left turn to align with runway 29.
But as it entered the flare the aircraft experienced an "uncommanded" pitch-up, says the Papua New Guinea Accident Investigation Commission, which has not indicated the reason behind it.
This pitch increase prompted the captain to push the ATR's control column forward, to lower the nose, using both hands, while the first officer placed his hand on the throttle control.
Analysis of the flight-data recorder indicates that, almost immediately after touchdown, the throttles were placed in the maximum reverse position.
This reverse thrust induced a "significant" left yaw as the aircraft travelled along the ground, the inquiry states in preliminary findings. It veered off the runway 7s after touchdown before crossing a taxiway and colliding with the Islanders about 5s later.
The ATR came to rest some 3,600ft (1,100m) from the threshold of runway 29 and 380ft to the left of the centreline.
Investigators state that the aircraft "did not have" hydraulically-powered nose-wheel steering and main landing-gear brakes, without elaborating.
"The crew reported that they could neither control, nor stop the aircraft during the landing roll," the inquiry says.
ATR 72-500s have two hydraulic systems – the blue system supplies nose-wheel steering and emergency braking while the green system provides normal braking.
While the rudder could be used for aerodynamic steering, the inquiry says this was "substantially limited" because a switch for manual operation was "not set to the appropriate setting".
The initial engine problem – which involved a rapid increase of turbine temperature, as well as loud banging – was determined to relate to a compressor stall.
Temperatures stabilised when the throttle levers were retarded but rose again when they were advanced. The pilots received a low oil pressure warning for the engine and subsequently shut it down.
Investigators state that an examination of the failed Pratt & Whitney Canada PW127 engine has provided "detailed information" on the root cause of the problem. But the inquiry points out: "The investigation found that the engine failure was not directly causal to the accident."
Source: Cirium Dashboard