What happened
On 6 March 2023, a Boeing 737-301, registration G-JMCU, departed Aberdeen Airport on a cargo flight bound for East Midlands Airport. During the departure, the aircraft took off with the flaps set to position 1, despite the takeoff performance calculations being based on a flap 5 configuration.
The crew only became aware of the discrepancy during the initial climb when attempting to retract the flaps according to the standard sequence. Due to heavy snow and reduced visibility, the crew had implemented supplementary procedures to keep the flaps retracted during taxi to prevent contamination. This altered the standard pre-departure sequence for setting the flaps.
The investigation
The AAIB examined the flight data and cockpit voice recordings. The investigation found that while the commander visually confirmed that the green flap extension light was illuminated, this only indicated that the flaps were set to a position greater than zero, not the specific required setting.
Analysis of the flight data recorder showed that the takeoff performance was adequate for the flap 1 setting on this specific occasion, as the acceleration and climb rates were consistent with previous departures using flap 5. However, the investigation noted that the co-pilot had recently transitioned to the 737 from an ATR 72, an aircraft where the first flap detent is the standard takeoff setting. The investigation also identified interference from the aircraft's electrical system on the cockpit voice recorder channels.
Findings
- The crew departed with an incorrect flap setting because the takeoff configuration was not verified against the planned setting.
- Adverse weather conditions, including heavy snow and low visibility, increased the workload and mental capacity required for taxiing.
- The use of supplementary procedures for de-icing disrupted the normal sequence for flap selection.
- The co-pilot may have reverted to motor memory from a previous aircraft type where only one flap detent is used for takeoff.
- The flap position indicator light only confirms that the flaps are extended, not the specific degree of extension.
Safety action
Following the incident, the operator updated the 'Before Take-off' checklist. The amended procedure now requires the crew to verbalise both the planned and the indicated flap settings to prevent future mis-selections.