There was a 5% disparity in the torque delivered by the two turboprop engines powering the Swearingen Metro III that stalled on short final approach to Cork, Ireland and crashed causing six fatalities in February last year.
In its interim statement on the 10 February crash, the Irish Air Accident Investigation Unit said when the crew lost control of the aircraft on short final approach to runway 17 at Cork, the stall warning was sounding and the pilots had just advanced the throttle levers to a high power setting with the intention of going around.
Almost simultaneously the aircraft rolled left, then rapidly right, causing the starboard wingtip to hit the ground. The right roll continued and the aircraft impacted the runway fully inverted, then it slid to a halt on soft ground to the right hand side of the runway. The pilots and four passengers were killed, four passengers were seriously injured, and the remaining two passengers received minor injuries, according to the statement.
The AAIU says that the flight data recorder (FDR) revealed that, for more than 100h of flying before the accident, the two engines had been delivering different power levels for the same throttle lever settings, with a disparity of up to 5%. The No 2 (starboard) engine was responding more quickly and providing more power. Working with engine manufacturer Garrett and the US National Transportation Safety Board, the AAIU determined that the cause of the power differential was a faulty pressure/temperature sensor on the No 2 engine that affected the performance of its fuel control unit.
On short final approach to Cork's runway 17 in poor visibility, with eight seconds to go before impact, the throttle levers were below the idle setting, and the left engine was developing -9% (negative) torque, the right engine 0% torque. At 7s before impact the stall warning began to sound and the throttle levers were simultaneously advanced to a high power setting.
The AAIU is not able to say yet whether the power disparity had a bearing on the loss of control. It said: "The investigation is currently carrying out a flight path analysis with the assistance of the NTSB, which will analyse the complex aerodynamic factors involved during the final phase of flight. This analysis, together with the limited FDR data available, may provide additional information regarding the loss of control of the aircraft."
The AAIU confirmed information reported in its original interim statement, including the fact that the crew had elected to continue descent well below their decision height with no indication that they had sight of the runway. In this report the investigators reveal that, apart from the sensor fault leading to the engine power disparity, the aircraft, its engines, propellers and avionics all appear to have been serviceable.
The AAIU is taking a great interest in the supervisory relationship between the ticket seller - Manx2 - the aircraft operator and the aircraft owner, explaining: "The Investigation is also examining the operational control, the regulatory oversight of the operation and the effectiveness of that oversight. The examination of the complex relationships between the AOC [aircraft operator's certificate] holder, the undertaking selling the service, and the undertaking which supplied the aircraft and flight crew, is continuing."
The aircraft owner is Airlada of Seville, Spain; the operator was Flightline BCN of Barcelona; and Manx2 in whose name the flights were sold is based in the Isle of Man. The aircraft, EC-ITP, is registered in Spain, so regulatory oversight was provided by the Agencia Estatal de Seguridad Aérea (AESA) which, according to the AAIU, has since renewed Flightline BCN's AOC "following corrective actions by the company," but has removed its right to operate Metro IIIs.
The full accident investigation into the Manx2 flight is not yet complete, and the just-issued AAIU information is provided in an interim statement released against the legal requirement to provide information about the investigation's progress a year after the accident.