What happened
On May 9, 2009, at approximately 20:05 local time, an Airbus A321 operating a flight from Munich to Lisbon encountered severe turbulence at approximately 30,000 ft. While climbing, the crew identified a thunderstorm cell roughly 80 nautical miles ahead, north of the waypoint RAVED. To maintain safe separation, the captain executed a course change of approximately 5 degrees to the south.
Approximately 20 nautical miles south of the thunderstorm cell, near the RAVED waypoint, the aircraft entered a period of turbulence lasting between 8 and 10 seconds. During this event, the aircraft experienced a sudden sink rate of 5,000 ft/min. The turbulence was accompanied by precipitation, though the onboard weather radar did not show any precipitation echoes at the time of the encounter. The turbulence was severe enough to cause the autopilot to disconnect, requiring the captain to briefly assume manual control before re-engaging the system.
The incident resulted in 15 persons lightly injured, specifically two flight attendants and 13 passengers who were not wearing seatbelts. One passenger required supplemental oxygen. Following the event, the captain elected to perform an unscheduled diversion to Geneva, where the aircraft landed safely at approximately 20:30 local time.
The investigation
The BFU examined flight data from the Flight Data Recorder (FDR) and the Cockpit Voice Recorder (CVR). The FDR analysis confirmed significant vertical accelerations during the event, reaching a peak of +1.85 g and a single negative gust of -0.5 g. The CVR analysis was limited as the recordings from the time of the turbulence had been overwritten, as the device had remained active from the time of the event until landing.
Meteorological analysis by the German Meteorological Service (DWD) revealed that a low-pressure system moving across Germany had created an unstable, warm, and moist air mass. This led to the development of convective clouds and clusters of thunderstorms. While the weather radar showed active cells, the specific area encountered by the aircraft did not show precipitation echoes on the radar screen during the encounter.
Findings
- The primary cause of the turbulence was the encounter with convective activity and thunderstorm cells developing in the region.
- At the time of the encounter, the aircraft was flying near the edge of a thunderstorm cell.
- While SIGMET warnings had been issued for the Munich and Langen FIR between 17:31 and 17:40 UTC, these warnings were issued after the aircraft had already departed.
- The flight crew's pre-flight weather briefing did not indicate that significant cumulonimbus clouds would be present on the initial segment of the route, as the weather at Munich showed no thunderstorm activity and Zurich predicted only a 30% probability of storms.