Incident Overview
On July 22, 2011, at 01:00 UTC, an Airbus A340-313 registered as F-GLZU, operated by Air France, was en route from Caracas to Paris. The aircraft was cruising at Flight Level 350 (FL350) over the North Atlantic near coordinates 18N 060W when it entered a zone of moderate turbulence.
Sequence of Events
The flight crew reported no significant precipitation on their weather radar, which was set to maximum gain. However, at 01:00:24 UTC, approximately two nautical miles past the 18N 060W point, the aircraft encountered turbulence that triggered an overspeed alarm. The Mach number reached 0.87, exceeding the operational limit of 0.86.
The pilot not flying (PNF) manually disconnected the autopilot and applied a significant pitch-up input, which was followed by roll inputs. This action caused the aircraft to climb rapidly. Despite the pilot flying (PF) deploying speed brakes and selecting lower Mach targets, the aircraft continued to ascend, reaching an altitude of 38,150 ft at 01:01:17 UTC while decelerating to Mach 0.66.
Crew Actions and Recovery
The crew became aware of the significant altitude deviation only after reaching approximately 38,000 ft. When attempting to initiate a descent, the flight directors disappeared due to low speed protections with the autopilot disconnected. The PF realized the autopilot was not engaged and manually reconnected it at 35,400 ft. The aircraft subsequently stabilized back at FL350.
Investigation Findings
The Bureau d’Enquêtes et d’Analyses (BEA) noted that while pre-flight weather information did not show significant phenomena, infrared satellite images later revealed isolated cumulonimbus clouds with tops near FL380 in the area. The investigation highlighted challenges in detecting such convective cells using standard weather radar settings, particularly regarding tilt and gain adjustments required for accurate visualization of high-altitude precipitation structures.
Conclusion
The incident was attributed to the unexpected encounter with moderate turbulence associated with isolated convective activity that was not adequately detected by the onboard weather radar. The subsequent uncontrolled climb resulted from crew actions taken in response to the overspeed alarm while the autopilot was disconnected.