Incorrect data entry during take-off calculations has been identified as the reason that a Thomson Airways Boeing 767-300 laboured to become airborne from Manchester last year, and suffered a tail-strike in the process.
After receiving the loadsheet the crew had inadvertently entered the zero-fuel weight, about 118t, into the computer-based system for calculating take-off speeds, instead of the proper figure of 172t.
This data-entry error, says the UK Air Accidents Investigation Branch, would have generated "significantly slower" take-off speeds than required. The calculated velocity for rotation was 21kt lower than the true figure.
During the take-off roll for the flight to Montego Bay, however, the captain felt the aircraft might be heavier than calculated, and delayed the 'V1' call by around 10-15kt after sensing "sluggish" acceleration.
Nevertheless, as the aircraft rotated, its tailskid struck the runway. The captain - while not the flying pilot - applied full power. In response to a brief stick-shaker activation, the co-pilot reduced pitch and the 767 climbed away safely, although the crew opted to dump fuel and return to Manchester.
The AAIB indicates that the crew may have been distracted by taxiway works in progress at Manchester as well as time pressures from a 15min pushback delay.
Since the 13 December incident, it adds, Thomson Airways has instructed pilots to extract take-off weight data from loadsheets independently, and reminded crews that checking loadsheets for "gross errors" remains "good practice".