Significant torque split between the throttle levers of a Fairchild Metro contributed to the pilots’ losing control of the aircraft during a low-visibility approach to Cork, a fatal accident inquiry has disclosed.
Although the first officer was the flying pilot, the captain took control of the throttle levers on the final approach to runway 17 – the crew’s third landing attempt.
He then retarded the levers below flight-idle, against normal procedures, and the Irish Air Accident Investigation Unit believes this would have been “unexpected” by the first officer.
Such was the power asymmetry that the left-hand engine entered a negative torque regime, normally only used during ground manoeuvring, and the propeller’s automatic feathering mechanism began to activate.
The aircraft suddenly rolled 40° to the left and the crew began to execute a go-around at about 100ft.
As the throttle levers were advanced, the aircraft rolled quickly to the right. The investigation could not determine whether the first officer had turned the control wheel, to counter the initial left roll, but states that the rapid increase in power probably contributed to the subsequent roll to the right.
The Metro’s right wing-tip struck the runway and the aircraft inverted, coming to rest off the right side of the runway.
Examination of flight-data recordings revealed that the torque from the right-hand engine had consistently been exceeding that delivered by the left-hand engine, by about 5%, for more than 100h. This was traced to a defective sensor, but the fault had not been recorded in the technical log.
Investigators found that crews on the aircraft had been adjusting the throttle levers in flight to compensate for the difference in torque from the engines. Under normal conditions the levers should not be misaligned by more than 0.05in (1.27mm).
Although the split condition did not “materially affect” the normal operation of the aircraft, says the inquiry, it “became significant” after the Metro entered a prohibited flight regime, caused when the throttles were brought below flight-idle.
The inquiry says that moving the throttles below flight-idle during flight can generate “uncontrollable” roll rates as a result of sudden airspeed loss, drag and power asymmetry.
It adds that the fatal approach, on 10 February 2011, had been continued despite not having the required minima, and that the pilots proceeded below decision height without adequate visual references. The low height left no margin to recover from the loss of control.
Investigators say the stall warning activated for the last few seconds of flight but believe this was the result of aerodynamic disturbances and loading from sharp manoeuvring during the loss of control.
Six of the 12 occupants survived the accident but both pilots were among the fatalities. The aircraft had been operated by Spanish carrier Flightline on behalf of Manx2.
Source: Cirium Dashboard
