What happened
On March 18, 2007, an ATR 42-300, registration F-GVZY, operated by Airlinair, was performing a scheduled passenger flight from Lyon to Montpellier. During the climb, the aircraft entered icing conditions, prompting the crew to activate anti-icing and de-icing systems.
Upon reaching a cruise altitude of FL 150, the crew noted a significant drop in indicated airspeed, falling well below the selected speed bug. Due to the presence of snow and an external temperature of approximately -13°C, the crew attempted to climb to FL 170 to find warmer air. However, as the aircraft reached FL 170, the crew observed rapid and uniform ice accumulation on the wings, engine inlets, and cockpit windows. The airspeed continued to degrade, dropping to between 160 and 170 knots.
Recognizing the severity of the icing, the commander decided to descend to FL 130. During this descent, as the aircraft passed FL 135, the left engine stopped. The crew disconnected the autopilot and continued the descent to FL 130. The left engine underwent an automatic relight 12 seconds after the shutdown. The flight proceeded to Montpellier without further incident, though ice remained visible on parts of the airframe upon landing.
The investigation
The BEA examined the meteorological conditions, the aircraft's performance, and the crew's decision-making process. Investigators analyzed the weather charts available to the crew, noting that while a TEMSI chart indicated moderate icing, the crew had misinterpreted the symbols, associating the icing area with a different region of the map.
Technical examinations included a borescope inspection of the high and low-pressure turbines and the engine air intake of the left engine. No mechanical defects were found in the engine components. The investigation also evaluated the potential benefits of the Aircraft Performance Monitoring (APM) system, which was not installed on this specific aircraft at the time of the event.