Israeli investigators have determined that a 40t weight error in take-off performance calculations caused an El Al Boeing 787-9 to struggle to become airborne from Tel Aviv.
Analysis of the preparations for the 29 March flight to Newark found that the captain had entered a zero-fuel weight of 128t, some 40t lower than the actual figure of 168t.
The twinjet rotated at 154kt, less than its minimum-unstick speed, when it would otherwise have been travelling at around 175kt.
Israel’s air accident investigation authority states that the rotation speed was roughly that of the 787’s stall speed.
As a result, it says, the aircraft responded “lazily” to the rotation command and failed to lift off until its speed increased. The geometry of the situation caused a tail-strike protection system to activate.
“The delay in lifting the nose contributed to the aircraft’s gaining speed before it began to climb and thereby avoiding stalling or loss of control,” says the inquiry.
The aircraft, assisted by ground effect, reached only 35ft some 13s after rotation was initiated.
While the climb was executed at reduced thrust, the speed was sufficient to avoid any significant impact on the manoeuvring margins.
Flap retraction also took place at speeds lower than those required by the aircraft’s actual weight, but there was “no hazardous increase” in the angle of attack, says the inquiry.
Once the aircraft had reached 20,000ft the pilots realised the error, because the flight-management system recommended an optimum cruise altitude of 38,000ft rather than the typical 34,000ft.
This was not only unexpected, for an aircraft transporting 282 passengers and 18 crew members on a transatlantic service, but was also higher than the altitude it could have achieved in the early phase of flight.
“It was impossible to climb to this altitude,” says the inquiry. The crew immediately understood that the take-off performance had been incorrectly calculated, informed the airline’s control centre, and rectified the error in the flight-management computer.
Investigators state that the aircraft had arrived at Tel Aviv late, and this put time pressure on the crew during preparation for the Newark service. Weight and balance documents were transferred to the crew for review immediately upon their arrival at the aircraft.
While the captain entered an incorrect zero-fuel weight during take-off calculations, he noticed the error. But the inquiry says he “probably” entered the wrong data again while trying to correct it.
Investigators say the first officer did not conduct effective cross-checking of the data, as required by operating procedures, and opportunities to identify the error were missed.
El Al has been undertaking a fleet modernisation with 787s. The aircraft involved (4X-EDB) had only been delivered to the carrier five months earlier.
The inquiry suggests the lack of experience on the type contributed to their failure to notice the deviation from performance figures which would have been more reasonable.
Acceleration during the take-off run was substantially lower than normal, but the departure took place in darkness and the inquiry states that abnormal acceleration is difficult to discern under such conditions.