What happened
On 7 November 2002, an Airbus A321-231, registration G-TTIA, was operating a public transport flight enroute to Almeria. While cruising at FL350 across the Pyrenees mountain range, the aircraft encountered turbulence. The disturbance began as light turbulence, prompting the crew to activate the seat belt sign as a precautionary measure.
Shortly after the sign was illuminated, the turbulence intensified into a severe encounter. This event caused the autopilot to disconnect and led to significant aircraft instability. The crew reported difficulty maintaining speed control, unreadable flight instruments, and developing oscillations in both pitch and roll. To manage the situation, the commander transitioned to manual flight, maintaining a constant pitch attitude, while the first officer worked to re-engage the autopilot. The crew subsequently initiated a descent to FL310. During this descent, the atmospheric conditions improved, allowing the flight to continue to its destination. While there were no fatalities, 15 passengers required medical attention for minor injuries.
The investigation
The investigation examined the meteorological conditions and the aircraft's flight parameters during the encounter. It was noted that the aircraft was positioned directly between two north-westerly jetstreams, both of which featured maximum wind speeds exceeding 100 kt. While moderate to severe Clear Air Turbulence (CAT) had been forecast for the easterly side of these jetstreams, no such turbulence was predicted for the specific location where the encounter occurred.
Investigators also reviewed the timing of the crew's actions and passenger positioning. It was determined that there was a roughly 30-second window between the activation of the seat belt sign and the onset of the severe turbulence.
Findings
- The primary cause of the incident was likely mountain wave activity generated by the strong north-westerly flow interacting with the Pyrenees, rather than forecasted CAT.
- Most passengers who sustained minor injuries were not seated with their seat belts fastened at the moment the severe turbulence hit.
- Although the crew had requested passengers return to their seats following the seat belt sign activation, many passengers were slow to comply as they were queuing for the lavatory.