What happened
On May 11, 2008, an ATR 42-500, registration F-GPYB, operated by Airlinair, was performing a scheduled passenger flight from Béziers to Orly. While cruising at FL180 southwest of Clermont-Ferrand, the crew encountered significant convective activity. To avoid thunderstorms, the crew utilized the onboard weather radar to identify a corridor through the cells, requesting a heading change to 015°.
As the aircraft progressed, the crew activated anti-icing systems, followed by de-icing systems due to rapid ice accumulation. Shortly thereafter, the aircraft entered a zone of turbulence. At 16:23 UTC, a series of technical failures occurred: the VOR 1 failed, followed by the failure of the number 1 Attitude and Heading Reference System (AHRS), which subsequently caused the autopilot to disengage. The pilot in command transferred control to the co-pilot.
As the aircraft moved through the convective area, the weather radar image became degraded. The crew then encountered severe turbulence, leading to fluctuations in airspeed and altitude. To manage the turbulence, the crew reduced power, but this resulted in severe icing conditions. Following the "SEVERE ICING" emergency procedure, the crew increased speed, which necessitated a climb to 18,350 ft and a subsequent descent to 17,550 ft to exit the icing conditions.
At 16:31 UTC, the commander issued a PAN PAN message to notify air traffic control of the weather radar failure and requested a westward heading to regain visual meteorological conditions (VMC). During the frequency handover between control sectors, the crew's emergency message was not fully processed by the receiving controller, who was occupied with other traffic. The crew eventually regained VMC and stabilized the aircraft at 18,100 ft and 208 kt.
The investigation
The investigation focused on the sequence of instrument failures and the crew's ability to navigate without radar assistance. Investigators examined the aircraft's systems and found no evidence of lightning strikes. It was noted that the loss of the AHRS 1 likely contributed to the degradation of the weather radar image.
Additionally, the investigation reviewed the meteorological information available to both the crew and air traffic control. While the crew had received a flight folder, it did not contain recent SIGMETs regarding intensifying thunderstorms in the area. Furthermore, investigators found that while a regional control center had access to advanced convective monitoring tools (ASPOC), this information was not directly visible on the controllers' radar screens, limiting their ability to provide proactive guidance to the aircraft.