Swedish-based operator West Atlantic has modified the ‘before take-off’ checklist for its Boeing 737s after an incident in which an aircraft departed with an incorrect configuration.

The 737-300 freighter had been flying from Aberdeen to East Midlands on 6 March last year.

Its first officer, who was flying, had recently converted to 737s having previously flown ATR 72 turboprops.

On the ATR, selecting 15° take-off flap involves moving the flap lever a single detent from the flaps-up position.

For the 737 departure the captain called for ‘flap 5’ and the first officer confirmed this had been completed. The captain recalled seeing a cockpit indicator light which showed the flap lever was no longer in the zero position.

But the flaps had actually been extended to the ‘flap 1’ position, a single detent from flaps-up.

“It is possible the [first officer] reverted to the motor memory of selecting one flap detent, which was correct on the ATR 72 he had recently flown,” says the UK Air Accidents Investigation Branch.

West Atlantic 737-300 freighter-dxme Creative Commons

Source: dxme/Creative Commons

Pilots of the jet, similar to this one, did not discover the flap error until after take-off

Snowfall during taxiing reduced visibility and the inquiry says the conditions introduced “several distractions” to the cockpit. These included an adverse-weather supplementary procedure which changed the point at which the flaps were set during departure preparations.

The incorrect flap setting remained undetected before the aircraft commenced its take-off roll from runway 34.

Although the performance calculation for a ‘flap 1’ take-off would have generated speeds similar to those for the intended ‘flap 5’ departure, the inquiry points out that under different circumstances – such as increased weight or reduced runway length – the aircraft could have been exposed to greater risk.

Its crew realised the error only after the 737 (G-JMCU) became airborne, as the flaps were retracted during climb. The pilots maintained the configuration until the jet accelerated to flap clean-up speed.

West Atlantic subsequently amended the ‘before take-off’ checklist to ensure crews not only acknowledge the flap indicator light but also the planned and indicated flap settings.

It has also drawn attention to the potential hazard of incorrect configuration, particularly for those pilots who risk “reverting to type” after converting from the ATR to the 737.