What happened
On 28 January 2009, a Boeing 757-258, registration G-STRZ, departed Accra, Ghana, for a commercial passenger flight. During the initial takeoff roll, the commander noticed that his airspeed indicator was not functioning correctly. Despite the discrepancy, the commander elected to continue the takeoff, utilizing the co-pilot's and standby instruments to proceed.
As the aircraft climbed, the crew encountered several EICAS messages regarding unreliable airspeed and rudder ratio. During the climb, the commander noticed the control column moving forward; believing a stick-pusher had activated, he manually pitched the aircraft nose-down and handed control to the co-pilot. The crew declared a Mayday and eventually returned to Accra. During the descent, the crew selected the alternate air data source, believing this would isolate the faulty left Air Data Computer (ADC).
The investigation
An investigation involving AAIB inquiries, company reports, and Flight Data Recorder (FDR) analysis established that the aircraft's airspeed indicated a significant lag behind ground speed during takeoff. As the aircraft climbed, the trapped pressure in the blocked pitot tube caused the airspeed to eventually over-read.
At approximately 31,600 ft, the Flight Management Computers (FMCs), which were still utilizing data from the left ADC, sensed an overspeed condition. This triggered a pitch-up command from the autopilot. The investigation also noted that the crew experienced periods of task saturation and confusion regarding the aircraft's automated flight modes.
Findings
- The primary cause of the airspeed errors was a blocked left pitot tube.
- An engineering inspection revealed the remains of a beetle-like creature inside the left pitot system.
- The commander's decision to continue the takeoff after 80 kt, despite the instrument failure, contributed to the subsequent complex emergency.
- The aircraft's automated systems reacted to erroneous speed data, leading to unexpected pitch commands.
Safety action
Following the incident, the operator amended its engineering procedures to require the use of pitot covers and blanks on the aircraft during long periods on the ground.