Polish investigators have determined that a Wizz Air aircraft's sluggish take-off from Warsaw was due to a miscount over the number of passengers on board.
The Airbus A320 (HA-LPO) had been departing Warsaw Modlin, the new secondary airport for the Polish capital, on 31 July this year.
But during the take-off roll the pilots "experienced a slower-than-expected rotation" before the twinjet lifted off, says the state commission for accident investigation PKBWL.
After the jet achieved level flight the crew requested that the number of passengers on board be checked.
PKBWL says that there were 148 passengers compared with just 95 indicated on the load sheet.
The aircraft's zero-fuel weight had also been "poorly defined", it adds. While an aircraft weight of 60t had been used to calculate performance, a review of the data showed the initial weight was 64.7t.
"Improved data was loaded into the on-board computers and used to calculate performance for landing," the inquiry states.
PKBWL has attributed the incident to errors by ground-handling personnel during preparation of the load sheets, adding that there was a "lack of effective weight-control procedures" in place.
It has proposed that Wizz Air considers amending its procedures to take into account the number of tickets sold for flights in order to provide additional comparison data to the flight crew.