Investigators believe a Boeing 757-200 crew's struggle to establish stable flight after a pitot tube blockage could have been avoided if the pilots had aborted take-off at low speed rather than opt to continue.
The Ghana International Airlines service from Accra was being operated by UK wet-lease carrier Astraeus.
Investigators discovered that the left pitot system had become blocked by an insect, causing discrepancies in the captain's airspeed indicator. Although the problem was detected during the take-off roll, below 80kt (150km/h), the captain chose to resolve it once airborne.
Pitot tube blockage was held responsible for two fatal 757-200 accidents in 1996, and the UK Air Accidents Investigation Branch warns that flying a large aircraft with incorrect instrument readings is "demanding" and that crews can become "task-saturated".
"There were times during this flight were the flightcrew were confused as to what was happening," it adds.
Although the right-hand autopilot - which used the right-hand air data computer - was selected as the aircraft climbed through 8,000ft (2,440m), the captain selected the alternate air data source at about flight level 180. In doing so, says the inquiry, the captain believed he had isolated the problem with the left-hand air data computer.
But as the 757 reached FL316, and the vertical navigation mode became active, the flight control computers adjusted the flight profile according to left-hand air data. While a fault would normally result in a switch to the other air data computer, the AAIB says that the computer might not interpret a pitot blockage as a system failure.
The false air data information resulted in the aircraft automatically pitching up to counter a possible overspeed. The unusual behaviour greatly concerned the co-pilot, who countered by pitching the aircraft's nose down.
Having declared an emergency and decided to return to Accra, the crew managed to bring the aircraft under control.
The inquiry says that rejecting initial the take-off at low speed would have been "more appropriate".
Astraeus has since advised crews to reject take-off if a problems is recognised at speeds below 80kt. It has also changed procedures to ensure fitting of pitot covers during long turnarounds, and the carrier includes blocked-pitot simulation in its recurrent training.
Source: Flight International
