Investigations into a sluggish take-off involving an Airbus A321 at Manchester have again uncovered erroneous performance calculations undetected by crew cross-checks.
Thomas Cook Airlines had been operating the flight, bound for Heraklion earlier this year, and the incident once more highlights the difficulty of avoiding inadvertent use of incorrect aircraft data.
The flight was 17min behind schedule by the time the loadsheet was generated. As the captain read the aircraft weights to the co-pilot, to begin the performance calculations, he mistakenly gave the zero fuel weight, 69.6t, as the actual take-off weight.
Since the actual take-off weight of the A321 was 17t higher, the discrepancy had a "significant effect" on the thrust and speed computations, said the UK Air Accidents Investigation Branch.
It notes that the crew more often flew the smaller A320 whose maximum take-off weight is comparable to the maximum zero-fuel weight of the A321.
Although the oversight was made early in the calculation process, the AAIB said there were "a number of missed opportunities" to detect the error through standard operating procedures.
The captain, having written the incorrect take-off weight on the navigation log, carried out an initial numerical check, but compared the written figure with the estimated zero-fuel weight - to which it was similar - instead of the estimated take-off weight.
Another standard procedure required the crew to cross-check the 'green dot' lift-to-drag speed, generated by laptop computer calculations, against that generated by the flight-management system.
But while the pilots cross-checked the laptop figures with one another, they "missed" the flight-management system check, said the AAIB.
During acceleration along Manchester's Runway 05L the captain felt that the A321's side-stick control "felt heavier than expected" during rotation. The aircraft climbed at a slower rate than normal, and as a precaution the captain lowered the pitch attitude and covered the thrust levers in case more power was needed.
None of the 223 passengers and eight crew members was injured in the 29 April event.
While the AAIB said Thomas Cook had since changed the layout of the navigation log as well as procedures regarding the check of 'green dot' speeds, and stressed the importance of accurate performance calculations to crews, it pointed out that similar data miscalculations had occurred among operators in recent years.
"Many errors which occur are successfully detected but there is no single solution to ensure that such errors are always prevented or captured," it added. "Industry awareness of the frequency of these errors has been raised but a solution has yet to be found."