What happened
On May 2, 2009, a Boeing 737-300, registration F-GFUF, was performing a charter flight from Marseille to Antalya. While on approach at approximately 30 NM from the airport, the aircraft encountered significant turbulence. The crew was utilizing the autopilot and autothrottle, navigating through a layer of scattered cumulus clouds.
As the aircraft performed a right-hand avoidance maneuver to bypass a large cumulus cell, it entered a zone of intense turbulence. During this period, a reduction in thrust was recorded, followed by the crew manually advancing the thrust levers. This manual intervention, combined with a vertical acceleration of 1.45 g, led to a rapid increase in the roll rate. The aircraft experienced a significant bank angle, reaching approximately 102 degrees to the right, accompanied by the activation of the bank angle warning and the stick shaker.
The aircraft then entered a stall, with the pitch attitude dropping sharply to -24.8 degrees and a descent rate reaching 12,000 ft/min. The loss of control in roll lasted approximately 18 seconds. The crew eventually recovered the aircraft, returning to the initial altitude of 11,000 ft and stabilizing the flight parameters.
The investigation
The investigation focused on the aerodynamic behavior of the aircraft during the encounter with orographic waves. Investigators analyzed flight data recorder (FDR) parameters, including vertical accelerations, engine N1 levels, and flight control movements. The investigation also examined the meteorological conditions, specifically the presence of orographic waves caused by the Taurus Mountains, which were not identified in the pre-flight meteorological briefing.
Findings
- The primary cause of the loss of control was the combination of manual thrust increases and low airspeed during a period of intense turbulence.
- The manual advancement of the thrust levers likely created a pitch-up moment and a slight thrust asymmetry, which increased the right roll rate.
- The aircraft's margin to stall was significantly reduced due to the low airspeed (187 kt) and the high bank angle.
- The crew was not aware of the potential for orographic wave phenomena in the area, as this was not included in the provided meteorological documentation.
Safety action
Following the incident, the operator implemented several safety measures, including:
- Providing specialized meteorological training via Météo France to assist pilots in recognizing complex weather phenomena.
- Conducting an additional four-hour simulator training session for pilots.
- Increasing pilot awareness regarding the sudden and violent nature of environmental phenomena that can exceed the response capabilities of automated flight systems.