West Air Sweden Flight 294

Investigation

The Swedish Accident Investigation Authority (Statens Haverikommission, or SHK) opened an investigation into the accident. On 9 January 2016, the flight data recorder (FDR) was found severely damaged as well as parts of the Cockpit Voice Recorder (CVR). The CVR was, however, not intact, and the part containing the memory functions was missing. The following day, the missing parts of the CVR were found, alongside human remains. On 12 January, SHK reported that the distress call from the pilots contained the word "Mayday" repeated, with no further information. On 26 January, Statens Haverikommission reported that they had managed to read both CVR and FDR, and were analysing and validating the recordings.

On 19 March, in their interim report, SHK revealed:

After 17 seconds from the start of the event, the maximum speed (VMO) of 315 knots was exceeded. The overspeed warning was activated and the vertical acceleration turned to positive values.

Another 16 seconds later, the first officer transmitted a "MAYDAY" message that was confirmed by air traffic control. The indicated airspeed then exceeded 400 knots and the stabiliser trim was reactivated and reduced to 0.3 degrees nose down. The Pilot in Command called "Mach trim" after which engine power was reduced to idle.

During the further event, the last valid FDR value shows that the speed continued to increase up to 508 knots while the vertical acceleration values were positive, with maximum values of approximately +3G.

FDR data shows that the aircraft's ailerons and spoilerons mainly were deflected to the left during the event.

Investigators discovered that the inertial reference unit (IRU; which provides information to the flight crew's instruments) no. 1 (connected to the captain's instruments and the flight data recorder) had malfunctioned in-flight (the SHK was unable to determine the cause of the malfunction), causing the captain's artificial horizon to display a nose-up pitch, when in fact the aircraft was perfectly level.

The captain responded by pushing the yoke down in an attempt to regain level flight. Instead, the aircraft's nose lowered and it departed level flight. The aircraft then entered a steep dive traveling over 510 knots, nearly inverted, and banked over 40 degrees, causing the "bank angle" warning sounded in the cockpit.

However, this alarm is connected to the inertial unit number 2, which provides information to the first officer's instruments. This indicated that only the captain's instruments had failed and the first officer's were functioning properly, but neither pilots cross-checked their instruments. The first officer, despite having the correct attitude indication displayed, reacted insufficiently.

Unable to find a visual reference as it was nighttime, the pilots became spatially disoriented and were further unable to manage the situation due to high G-loads.

The final report was published by SHK on 12 December 2016. The inquiry reached the following conclusion:

The accident was caused by insufficient operational prerequisites for the management of a failure in a redundant system.

Contributing factors were: The absence of an effective system for communication in abnormal and emergency situations.

The flight instrument system provided insufficient guidance about malfunctions that occurred.

The initial manoeuver that resulted in negative G-load probably affected the pilots' ability to manage the situation in a rational manner.

The SHK issued 14 safety recommendations.