Preliminary analysis indicates that a temperature data-entry error introduced during take-off calculations resulted in a Sunwing Airlines Boeing 737-800's almost failing to become airborne during departure from Belfast.
The aircraft's thrust setting was "grossly low", says the UK Air Accidents Investigation Branch, and it barely managed to lift off from runway 07.
It was low enough to strike an approach light, just 35cm tall, located 29m beyond the end of the runway. Some 1,500m past the runway end the aircraft was still only 220ft above the airport's elevation.
Investigators determined that the thrust setting delivered an engine speed of just 81.5% of N1 which, the inquiry says, was "significantly below" the correct level of 93.3%.
Examination of the crew's electronic flightbag calculations showed they had been performed correctly before entry into the flight-management computer.
But simulation of the entry procedure showed that erroneously entering top-of-climb air temperature – rather than airport temperature – into a specific field on the computer would have generated the low thrust setting. The inquiry says this is the only "credible" path which could have created the situation.
The aircraft involved in the incident (C-FWGH) was equipped with an earlier version of flight-management computer software, which did not feature a cross-check between the temperature entered by the crew and that detected by the aircraft's probes.
Investigators believe the aircraft reached its V1 decision speed of 144kt with around 900m of the runway remaining. The crew had realised the aircraft was not accelerating normally.
Crucially an analysis of the take-off performance found that, had the aircraft suffered an engine failure at V1, it "might not" have had sufficient performance to climb safely away – although it would have had sufficient room to stop if the crew had aborted take-off.
The inquiry found that the crew did not advance the thrust levers above the 81.5% N1 setting until it had reached a height of 800ft.
Investigators have recommended that US regulators mandate implementation of the updated flight-management software, including the cross-check.
None of the occupants – comprising 179 passengers and six crew – was injured during the incident, which occurred on 21 July as the aircraft departed for Corfu.