David Learmount/LONDON

A quirk in the flight management system (FMS) of the Crossair Saab 340 which crashed near Zurich, Switzerland, on 10 January led the captain to reverse a cleared left turn and turn right instead just before the crash.

After the manoeuvre, the right bank quickly became extreme, the nose dropped and within 30s the aircraft hit the ground, says an interim report from the Swiss air accident investigation bureau (BEAA) (Flight International, 1-7 February). All seven passengers and three crew, bound for Dresden, Germany, were killed.

After leaving Zurich's runway 28, with the aircraft configured for a standard zero-flap take-off, the captain flew the aircraft manually throughout. It was dark with 6km visibility, wind 290°/3kt (5km/h), temperature 2°C/dew point 1°C in light drizzle, with broken cloud at 500ft (150m).

The aircraft had been cleared for a departure entailing a 234° left turn toward the Zurich East (ZUE) VOR beacon.

The captain started the left turn just before the co-pilot entered ZUE into the FMS; but because a right turn to ZUE is shorter, the captain's flight director then demanded a right turn. Right bank increased to 42° with 0° pitch.

The report says: "For a few seconds, roll and pitch remained constant, [then] the roll rate increased significantly due to a right aileron input.". When right aileron was reapplied the co-pilot advised the captain that he should be turning left. "At this point the aircraft had already entered a high speed, high rate spiral descent," says the report.

The interim report does not draw conclusions, but earlier in the investigation the BEAA had questioned what was showing on the electronic flight instrument system display panel and what caused the crew to veer to the right and bank so sharply.

Source: Flight International