Pilots of a Sunwing Airlines Boeing 737-800 twice entered an incorrect temperature figure during take-off calculations, leading to its departing Belfast with insufficient thrust and virtually overrunning the runway.

As the crew programmed the flight-management computer prior to departure, a figure of minus 47°C was erroneously entered as the outside air temperature.

The flight-management computer uses outside air temperature to calculate the N1 level for engine thrust.

Such an abnormally low temperature figure would cause the N1 calculation to be “significantly below” that required for take-off thrust, says the Air Accidents Investigation Branch.

It points out that minus 47°C was the outside air temperature for the first waypoint after top-of-climb, recorded on the pilot’s log.

“Neither pilot noticed the error,” it adds. The crew then exacerbated the situation by entering a correctly-calculated assumed temperature of 47°C, which reduced the thrust requirement further.

Following a delay to departure, caused by the need for a tyre change, the crew recalculated the take-off performance.

The outside air temperature had increased, and the crew entered a new figure of 48°C for the assumed temperature, but again entered an incorrect figure for outside air temperature – changing it to minus 52°C.

Investigators state that minus 52°C was the top-of-climb outside air temperature on the front page of the log.

The aircraft had been bound for Corfu on 21 July last year.

As the 737 reached 120-130kt during its take-off run along runway 07, having departed from taxiway D, the crew realised it was “not accelerating normally”, says the inquiry. The take-off run available was 2,654m.

No action was taken, either to increase thrust or abort the take-off. Some 900m before the end of the runway the aircraft reached the V1 threshold and rotated shortly afterwards.

“The aircraft, which was seen by multiple witnesses, took a significant time to lift off before climbing at a very low rate,” the inquiry adds.

Its undercarriage struck a runway light just 36cm tall which was situated 29m beyond the take-off run.

Once airborne the crew checked the take-off performance figures, finding that the N1 level had been just 81.5%, far below the 92.7% required. Thrust was not increased until the aircraft had reached 800ft, some 4km after becoming airborne.

Investigators state that the 737 escaped damage and continued to its destination. But the inquiry points out that only the absence of obstacles and a “benign” nature of the runway location and surrounding terrain prevented a potentially “catastrophic” accident, should the jet have suffered engine failure.

The flight-management computer on the twinjet did not have the capability to alert the crew to the abnormal air temperature entry – although, the inquiry says, later versions of the software, available at the time, did.

It adds that the particular electronic flightbag application in use did not display N1 figures, which meant the crew was unable to verify the N1 level calculated by the computer.

Source: Cirium Dashboard