ATR 42-500 Incident: Loss of Weather Radar in Thunderstorms

Flight Details and Initial Conditions On Sunday, May 11, 2008, an Airlinair ATR 42-500, registration F-GPYB, departed Béziers for Orly at 15:51 UTC. The flight carried three crew members and 44 passengers. The aircraft was cleared to Flight Level 180 and proceeded toward the MOU VOR. As the aircraft approached the Massif Central, the crew visually observed cumulonimbus clouds that had been forecast in their flight plan. Using the weather radar, they identified a corridor to cross the convective sector while remaining close to their route. At 16:13 UTC, the crew requested and received authorization to turn to a heading of 015 degrees to avoid the weather. The captain maintained Flight Level 180 and a cruise speed of 220 knots. ## System Failures and Emergency Procedures At 16:16:12 UTC, the crew activated the anti-icing systems. They observed rapid ice formation and activated the de-icing systems at 16:20:31 UTC. Around 16:22:53 UTC, the aircraft entered a zone of turbulence. At 16:23:35 UTC, a CAUTION warning light illuminated for eight seconds. The crew identified a failure of VOR 1, followed by a failure of the number 1 attitude and heading reference system (AHRS). The autopilot disengaged at 16:24:06 UTC. The captain transferred control to the co-pilot, whose instruments were functioning. At 16:27:24 UTC, the crew informed the controller that they were on a heading of 310 degrees to navigate between cumulonimbus clouds. Shortly after, with no external visual references, the crew noted that the weather radar was providing a degraded image. Approximately thirty seconds later, the turbulence intensified, causing variations in altitude and speed. The crew reduced torque to decrease speed and observed severe icing. They executed the SEVERE ICING emergency procedure at 16:29:04 UTC, which required increasing the red line speed by 10 knots, setting torque to maximum continuous thrust, and exiting the icing conditions. After climbing to 18,350 feet at 16:29:16 UTC and descending to 17,550 feet at 16:29:34 UTC, the variations in altitude and speed subsided. The speed was 228 knots at 16:29:37 UTC. ## Emergency Communication and Recovery At 16:31:27 UTC, the captain issued an emergency message stating the loss of the weather radar and requested a course alteration to the west to regain visual meteorological conditions (VMC). This message was issued while the aircraft was crossing a sector boundary and a sector handover was in progress. The controller acknowledged and transferred the crew to the next sector frequency (LO). The captain announced on the LO frequency using the prefix PAN PAN, requested to resume the route to the MOU VOR, and stated that conditions were clear again. A new controller, who had just opened the LO sector, authorized direct navigation to MOU without using the PAN PAN prefix. Immediately after, this controller was continuously occupied on the radio by two other crews. At 16:33:14 UTC, the pilot canceled the emergency situation, stating they were en route to MOU in clear skies. The controller responded that they had not received the emergency message. At 16:35:03 UTC, the autopilot was reactivated, and the aircraft stabilized at 18,100 feet and 208 knots. The remainder of the flight proceeded without further incident. Upon arrival, an inspection of the aircraft and verification of the systems were performed, and no anomalies were detected. ## Meteorological Information and Investigation Findings During the stop at Béziers, where the aircraft arrived at 14:50 UTC from Montpellier, the crew received a printed weather briefing at 14:33 UTC. The briefing indicated isolated cumulonimbus clouds over the Massif Central between Flight Level 50 and Flight Level 340, with the 0°C isotherm at Flight Level 100. A SIGMET from the Marseille Flight Information Region, valid from 14:00 to 18:00 UTC, indicated embedded cumulonimbus clouds observed and forecast north of 43°50'N, with tops at Flight Level 360, moving west and intensifying. A SIGMET from the Bordeaux Flight Information Region, valid from 14:00 to 18:00 UTC, forecast embedded thunderstorms east of 001°30'E. These two SIGMETs were not included in the briefing provided to the crew. The pilot stated that they expected radar guidance while in instrument meteorological conditions (IMC) among cumulonimbus clouds without a weather radar. The pilot believed the controller had radar information on the position of the storm cells. The crew was surprised to be transferred to another control frequency following their initial emergency message and to receive no assistance on that frequency. The crew navigated on a west/northwest heading toward a zone they remembered as clear on the weather radar before it ceased functioning. The turbulence intensified, and the crew decided to reduce speed, which led to severe icing. They executed the SEVERE ICING procedure, which required increasing speed despite the turbulence. The pilot noted that, in the rush, they did not inform the controller of the severe icing conditions and did not upgrade the emergency to a distress call. The chosen heading allowed the aircraft to exit the storm area and regain VMC quickly. The AHRS failure procedure was then applied, and the systems were recovered a few minutes later. The head of the south-east Regional Air Traffic Control Centre had access to SIGMET messages to adapt sector workload to meteorological constraints. They also had a terminal (ASPOC) that displayed a synthetic image of convective clouds and their predicted trajectories, updated every five minutes. This information was not directly available on the controllers' radar screens. The airline did not find any lightning strike on the airframe. The loss of the weather radar was not reported to the equipment manufacturer. The AHRS contributes to the stabilization of the radar in pitch and roll. It is possible that the loss of AHRS 1 caused the degradation of the weather radar image. The cause of the AHRS 1 failure was not determined. ## Conclusion The incident was due to the difficulty for the crew to adopt a course of action when confronted with severe icing associated with severe turbulence while the weather radar was no longer operational. The controller did not have the means to visualize convective zones on their radar screen, which prevented them from assisting the crew.