What happened
During a routine high-speed descent, the Boeing 737-800 aircraft, registration VH-VZZ, encountered an increase in headwind. This change in wind conditions caused the airspeed to rise toward the aircraft's maximum operating speed. In an attempt to prevent an overspeed condition, the pilot flying applied input to the control column. Because of a specific modification to the aircraft's autopilot system, this input caused the autopilot to disengage entirely rather than reverting to a steering mode. The sudden disengagement triggered an abrupt elevator deflection and a significant change in pitch and g-loading, which resulted in two cabin crew members sustaining injuries.
The investigation
An internal investigation by Qantas revealed that the aircraft's autopilot had been modified such that applying sufficient back pressure on the control column would lead to disengagement instead of reverting to Control Wheel Steering (CWS-P) mode. The investigation also identified a common, though undocumented, practice among crews operating the Boeing 73 37-800 fleet to descend at 320 kt—which is 20 kt below the VMO—when cleared for high-speed descents. This practice was intended to prevent overspeed events but was not a formal Qantas or Boeing procedure. Because this practice was not documented, the potential risks associated with it were not evaluated when the autopilot modification was implemented. Additionally, the flight crew had no prior indication of turbulence risks, as the flight plan's wind information did not suggest significant changes in wind direction during the descent.
Findings
- The increase in headwind during the descent caused the airspeed to approach the aircraft's maximum allowable limit.
- The autopilot disengagement was the primary driver of the abrupt pitch change.
- A modified autopilot system prevented the crew from overriding the system into CWS-P mode via manual intervention.
- There was a documented history of 47 overspeed occurrences involving the airline's Boeing 737-800 fleet over the previous six years.
- The practice of descending at 320 kt to avoid overspeed was a common but undocumented crew habit.