What happened
On 21 November 2010, a Boeing 737-76N, registration 5N-MJI, was preparing for a non-revenue repositioning flight from Southend Airport to Lagos, Nigeria. The flight was already delayed due to the commander's late arrival. While the aircraft was taxiing for a departure from Runway 24, Air Traffic Control changed the active runway to Runway 06.
To accommodate the runway change, the co-pilot reprogrammed the Flight Management Computer (FMC). During this process, an incorrect assumed temperature of approximately 50°C was entered, rather than the setting required for maximum thrust. The crew proceeded to line up at the displaced threshold of Runway 0 6. During the takeoff roll, the commander noticed a lack of acceleration and requested maximum thrust; however, the engine parameters did not reflect this request.
Despite the reduced thrust, the aircraft became airborne at 140 kt, passing through the 35 ft screen height after 4,317 ft of travel. The aircraft climbed away on its programmed route without further incident.
The investigation
The AAIB examined the flight data and the crew's performance calculations. The investigation established that the crew had used the Quick Reference Handbook (QRH) to calculate takeoff speeds for the new runway. Because the crew could not locate a specific Airport Analysis Table for Southend, they relied on the QRH and prevailing weather conditions.
Investigators found that the use of a high assumed temperature significantly reduced the available thrust. Analysis by the aircraft manufacturer revealed that had the takeoff been rejected just before V1, the aircraft would have run out of runway. Furthermore, had an engine failure occurred near V1, the aircraft would not have been able to maintain the required 35 ft screen height by the end of the runway.
Findings
- The crew was operating under time pressure due to previous delays.
- The co-pilot entered an incorrect assumed temperature of 50°C into the FMC, which was much higher than the actual ambient temperature of 6°C.
- The crew failed to notice the error during the reprogramming of the FMC.
- There was a breakdown in adherence to Standard Operating Procedures (SOPs) during the runway change.
- The takeoff roll began at the displaced threshold, which effectively reduced the available runway length.
Safety action
Following the incident, the operator implemented several safety measures, including:
- A structured training package for crew members returning to line flying, specifically involving takeoff and landing exercises at Southend Airport.
- A limitation of operations at similar airfields to daylight hours only to ensure visibility is not impaired.
- The introduction of 'Special Briefings' via the Operations Control Centre for unusual or limiting airfields.