With the autopilot calculating the takeoff speed based on a temperature of -52C, instead of 16C, the plane lifted off at the extreme end of the runway, striking a 36cm-high light on the ground almost 30 metres after takeoff.
Pilots eventually increased the thrust after the 737 had travelled two and a half miles and climbed to just 800ft, far below the usual steep climb on takeoff.
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Investigators said only the benign nature of the clearway after the runway, and the surrounding areas lack of obstacles, saved the plane from further collision. They said any engine failure that might have occurred as the plane struggled to get off the ground would have been catastrophic.
The incident happened on a Sunwing holiday flight from Belfast to Corfu on 21 July last year. Crew on the Canadian carrier could not account for why they had put in the wrong temperature, but investigators found that one of the pilots may have been suffering from jetlag.
The crew did not report the incident themselves. They only became aware something was wrong with the acceleration as the aircraft was rapidly approaching the end of the runway. After applying the full thrust, the plane climbed and continued its flight to Greece safely.
Airport staff who had seen the low takeoff and checked the broken light found it marked with rubber from the 737s tyre.
The Air Accidents Investigation Branch recommended that all Boeing 737s have a software upgrade, which had been available but not installed on the Sunwing flight, to ensure the plane crosschecks the information typed into the flight management system with its own temperature sensors.
The jet, operated by Canada’s Sunwing Airlines, avoided crashing only because of the “benign nature” of the area beyond the runway at Belfast International airport in Northern Ireland and the low elevation of surrounding terrain, the U.K. Air Accident Investigation Branch said in the report on Wednesday.
It has also recommended that safety systems be introduced to warn pilots of unusually low acceleration before take-off, and low altitude afterwards.
A “catastrophic” incident was narrowly avoided at Belfast International Airport last year after pilots entered the wrong outside temperature on a Boeing 747 plane carrying 185 passengers.
Setting the temperature to -52C instead of 16C, the plane struggled to take off before it reached the end of the runway and flew low for two and a half miles after that.
The Boeing 737 tried to take off without enough power to meet regulations and hit an approach light as it left the runway.
Its engines were only able to deliver 60% thrust which the crew did not realise until it reached the end of the strip.
The plane, which was on a Sunwing holiday flight from Belfast to Corfu at 3.39pm on July 21 last year, began to rotate and climbed at a very slow rate.
Full thrust was applied around 4km away from the airport – when they were just 800ft off the ground – but continued its flight to Greece.
The Canadian carrier was undamaged but it was only the benign nature of the runway, the terrain elevation and the lack of obstacles that allowed it to climb safely.
An investigation was launched three days later to prevent any repeat incident but was not commissioned to point blame.
It found the outside air temperature put into the flight management computer was significantly below what was needed for the aircraft weight and environmental conditions.
The report said the incorrect data and the abnormal acceleration along the runway was not realised by the crew until the end of the runway and no action was taken to either abort the takeoff or increase engine thrust.
Further factors that contributed to the incident included the computer not having the capability to alert the flight crew to the fact they entered the incorrect temperature.
The investigation concluded that the crew were unlikely to detect the abnormally low acceleration because of normal limitations in human performance.
It suggested safety systems should be introduced to tell pilots when a plane has an unusually low acceleration before take-off.