On 2 May 2009, a Boeing 737-300 being operated by French airline Europe Airpost on a passenger charter flight from Marseille to Antalya, Turkey was descending in day Visual Meteorological Conditions (VMC) towards destination when a sudden and severe turbulence encounter led to a temporary loss of control and stall. Recovery was achieved and none of the 115 occupants was injured and the aircraft was undamaged.
An Investigation was carried out by the French Bureau d'Enquêtes et d'Analyses (BEA). It was established that a south westerly airflow had prevailed which had created conditions conducive to mountain wave conditions for which evidence was subsequently found on a contemporaneous satellite picture. However, no mountain wave activity had been included on forecasts or reports made available to the flight crew either prior to or during the flight.
At the time of the encounter, at approximately 30 nm north of destination, the cabin had been prepared for landing and all occupants were secured by their seat belts / harnesses. The First Officer had been acting as PF.
It was established that the aircraft had been level at FL110 with the Autopilot (AP) engaged and maintaining a speed of 210 KIAS and beginning a deviation to the right of track around a discrete Cumulus top about 1000 feet higher than the aircraft in VMC with ATC approval. As this turn was commenced, sudden turbulence was encountered and the Auto Throttle (A/T) reduced thrust as a result. The A/T was overridden and high, but not fully symmetric thrust was applied. Then, with a 25° bank angle set on the MCP, a very high roll rate occurred which triggered a “bank angle“ warning and the stick shaker activated. Flight Data Recorder (FDR) data recorded a maximum bank angle of 102° to the right at a minimum speed of 181 knots. Pitch increased to 25° with a nose-up elevator position and the descent rate reached about 12000 fpm in Instrument Meteorological Conditions (IMC). Eventually, the pitch up was relaxed and the wings rolled level and the aircraft was unstalled. FDR data showed that the roll upset lasted eighteen seconds with a minimum altitude reached during it being just above 7,500 ft. Once control had been regained, the aircraft was returned to FL110 and ATC advised of the occurrence.
Aircraft track reproduced from the official report
The Investigation concluded that conditions conducive to mountain wave turbulence had been encountered and note that for the prevailing aircraft weight of 48 tonnes at 10,000 ft with flight idle thrust, the stall speed at 60° bank would have been 198 KIAS compared to the wings level stall speed of 138 KIAS.
The Investigation considered that it was “not possible to determine or to precisely quantify the causes of the strong increase in the roll rate that led to the loss of roll control and (aircraft) stall” but it was thought reasonable to attribute it to the prevailing mountain wave situation.
It was considered that “making the crew aware of potential mountain wave meteorological conditions over high ground would have made them more vigilant” especially in relation to automatic systems and flight instrument indications and corresponding safety action was implemented by the Operator.
The Final Report was published in an English language translation in April 2010: BEA Incident Report "F-GFUF - 2 May 2009". No Safety Recommendations were made.