What happened
On 7 December 2006, a Boeing 747-4Q8, registration G-VHOT, was preparing for a commercial passenger flight from London Heathrow to New York. During the initial takeoff roll, approximately 5 knots before reaching V1, both stick shakers began to operate continuously. The commander, assessing the situation as a spurious warning, elected to continue the takeoff to address the issue in flight.
During the climb, the crew encountered further technical difficulties, including an altitude disagreement and airspeed discrepancies between the pilot displays. After reaching 17,000 feet, the crew decided to return to Heathrow. This required dumping fuel to reach a safe landing weight. Following a period of troubleshooting and the replacement of a right-hand Air Data Computer (ADC) by maintenance engineers, the aircraft attempted a second departure. However, the stick shaker activation recurred, prompting the commander to reject the takeoff, successfully stopping the aircraft two-thirds of the way along the runway.
The investigation
The AAIB investigation focused on the cause of the continuous stick shaker activation and the subsequent instrument discrepancies. Examination of the aircraft's flight data recorders revealed that while the first takeoff did not show actual stall conditions, the second takeoff was interrupted by the recurring warning.
Engineers performed a ground test on the right-hand ADC, which triggered a fault message. Further investigation into the Angle of Attack (AOA) sensors revealed that a previously replaced sensor was also defective. The investigation found that the internal gear train of the AOA sensor had become unsecured, causing the sensor to provide inaccurate data to the ADC. This erroneous data caused the stall warning system to activate because it registered a high angle of an attack that was not actually occurring.
Findings
- The primary cause of the incident was a defective AOA sensor featuring an unsecured internal gear train, which generated erroneous high-angle-of-attack signals.
- The maintenance troubleshooting process was primarily guided by the ADC's Built-In Test Equipment (BITE) and fault codes, which pointed toward the ADC rather than the underlying AOA sensor issue.
- The continuous stick shaker activation was processed by the aircraft systems as a valid warning because the inaccurate sensor data was within normal operating parameters, rather than being identified as a system failure.
- The crew's decision to reject the second takeoff was influenced by the cumulative technical difficulties experienced during the first flight segment.