Swedish investigators believe a Bombardier CRJ200 freighter entered a fatal descent after its crew reacted to a sudden, but inaccurate, indication of a change in attitude.
The West Atlantic aircraft had been cruising at 33,000ft with an airspeed of 275kt, at night on 8 January, with its pilots briefing for a descent to Tromso.
Swedish investigation authority SHK says the pilots were probably paying attention to approach charts, with the maps illuminated and the cockpit lights on.
Their external visual references would have been “virtually non-existent”, the inquiry adds, and the pilots would have been “completely dependent” on the jet’s attitude indicators.
But a malfunction in one of the inertial reference units generated an erroneous attitude indication on the left-hand pilot’s display. The display appeared to show the aircraft’s pitch rapidly increasing at 6° per second over the course of 6s.
Although the flight-data recorder registered no change of altitude, speed or angle-of-attack, the attitude indication prompted an exclamation from the captain.
Recorder information showed a shift in the elevator towards the nose-down position, and the autopilot disengaged automatically. The recorder also revealed that the horizontal stabiliser was moved manually, using the left-hand control column, for 19s in the nose-down direction.
SHK says the apparent increasing pitch on the captain’s display – which also showed urgent red chevrons as the pitch angle became excessive – probably led to an “instinctive” reaction to counter the unusual attitude.
Investigators state that a comparator function on the aircraft constantly compares the information on the left-hand display with that on the right, and highlights any discrepancy in such parameters as pitch.
But as the indicated pitch on the left-hand display exceeded 30°, the comparator ceased highlighting the discrepancies, because of a “decluttering” feature designed to remove excess information in the event of a serious attitude upset.
Within 13s of the initiating event, the pilots faced two “contradictory” attitude indicators, with red chevrons on the left-hand display urging ‘nose down’ and those on the correctly-functioning right-hand display urging ‘nose up’. By this point, as a result of the decluttering function, neither display showed any indication pointing to an inconsistency in the two pilots’ data.
The pilots were probably subjected to spatial disorientation as the aircraft descended, the result of the dynamic forces, the exterior darkness, and the jet’s movements not corresponding to those on the captain’s display.
SHK says the suddenness of the event and a lack of routinely-practised procedures for abnormal attitudes meant there was no clear behaviour to which the crew could revert, and the pilots resorted to “improvisation”. They failed to communicate with one another effectively – saying almost nothing for several seconds after the initial event – and became focused on their own flight display.
The crew had communicated normally up until the upset, but the inquiry believes incomprehension over the rapidly-developing situation, and the effect of varying dynamic forces, hampered the pilots' ability to discuss and troubleshoot the problem.
Although the aircraft (SE-DUX) exceeded 500kt in the descent, analysis of the dynamics as well as the impact site – from which the rudder, wing-tips and other structures were recovered – indicated that the CRJ did not break up in the air.
Investigators believe an internal malfunction is the only “plausible” explanation for the incorrect pitch value of the inertial reference unit, but neither the inquiry nor the manufacturer could find evidence of a similar error by the equipment involved.