History of the Flight
The aircraft, a Boeing 757-258 registered G-STRZ, began its takeoff roll from Accra, Ghana, at 2334 UTC on 28 January 2009 with the commander as pilot flying. Before reaching 80 knots, the commander noticed his airspeed indicator was not functioning. He elected to continue the takeoff using the co-pilot’s and standby instruments, believing the weather conditions would allow resolution in the air.
During the initial climb below 400 feet, Engine Indication and Crew Alerting System (EICAS) messages illuminated: airspeed unreliable, mach/speed trim, and rudder ratio. The commander handed control to the co-pilot. At 1,000 feet, the co-pilot selected Vertical Navigation (VNAV) mode and engaged the right autopilot. During flap retraction, the crew noted abnormal acceleration and disconnected the autopilot to fly manually. They requested assistance from a company engineer on board.
The right autopilot was re-engaged, and the Quick Reference Handbook (QRH) consulted. The engineer advised that the EICAS messages indicated the left Air Data Computer (ADC) was unserviceable; he had previously experienced a similar defect when a bug blocked the left pitot tube on another aircraft. Following the QRH, the commander selected the air data switch to ALTN, believing he isolated the left ADC. He resumed control and continued the climb using the right autopilot with Lateral Navigation (LNAV) and Flight Level Change (FLCH) modes.
Passing FL250, the commander selected VNAV. At approximately FL320, the co-pilot noticed an increasing climb rate and decreasing airspeed. The commander attempted to select vertical speed mode but the Mode Control Panel alternated between VS mode with a 4,000 fpm climb and altitude hold. As indicated airspeed reduced to about 250 knots, the commander felt the control column move forward and believed a stick pusher activated (the Boeing 757 has no stick pusher; the commander had flown aircraft with one). He disconnected the autopilot and auto-throttles, advanced the thrust levers, and pitched nose-down to 3–5 degrees. He became aware the co-pilot was also on the controls and handed control to him while transmitting a mayday. A nearby aircraft observed from its TCAS that G-STRZ was at FL310. The flight directors were disengaged, and the aircraft returned to Accra with the co-pilot flying. As the aircraft neared Accra and appeared to operate normally, the mayday was downgraded to a pan. The commander flew an uneventful approach and overweight landing.
Engineering Investigation
Operator engineers discovered the remains of a “beetle-like creature” in the left-hand pitot system. No faults were found with the ADC, autopilots, or other relevant systems.
Flight Data Recording
The aircraft was equipped with a Cockpit Voice Recorder (CVR), Flight Data Recorder (FDR), and Quick Access Recorder (QAR). The CVR recordings had been overwritten by the time the event was reported. The FDR and QAR data, analysed with manufacturer assistance, showed that the captain’s computed airspeed lagged behind ground speed initially, reaching 70 knots at rotation while ground speed was 155 knots. As altitude increased, trapped pitot pressure caused the captain’s indicated airspeed to rise. When the right autopilot channel was selected at 8,000 feet, the Flight Control Computers (FCCs) used right ADC data. At 18,150 feet, selection of alternate air data caused the captain’s computed airspeed to drop from 350 to 280 knots. Passing 31,600 feet, VNAV mode became active; the Flight Management Computers (FMCs) used left ADC data, sensed an overspeed, and commanded a pitch-up. At 32,500 feet, vertical speed mode activated with a 4,000 fpm climb, then transitioned to altitude capture. Shortly after, the autopilot was disconnected and the aircraft manually pitched nose-down, reaching a maximum descent rate of 6,919 fpm.
Previous Occurrences
The source references a 1996 accident involving a Boeing 757 off the Dominican Republic where blocked pitot tubes caused erroneous airspeed indications leading to confusion and fatal consequences. The present incident’s operator amended engineering procedures to include fitting pitot covers and blanks during ground turnaround.