ATR 42-300 Icing Incident Near Cottbus in 1998

Incident Overview and Flight Details On December 14, 1998, an Aerospatiale-Alenia ATR 42-300 aircraft departed Dresden at 11:09 UTC, bound for Poznań, Poland. During the climb, the aircraft entered clouds at approximately 2,000 feet, and the autopilot was engaged at 3,400 feet. The crew observed light icing at Flight Level (FL) 80, which intensified to moderate icing by FL 120. Both anti-icing and de-icing systems were activated, and visual checks confirmed the de-icing boots were functioning without ice accumulation. However, rime ice was visible on the unheated portions of the forward side windows. ## Uncontrolled Flight and Emergency Landing As the climb rate decreased, the crew reduced speed to 155 knots and interrupted the climb at FL 135 to regain speed. Shortly after activating altitude hold mode, a stall warning activated unexpectedly at 155 knots, causing the autopilot to disengage. The aircraft entered an uncontrolled state with alternating left and right roll movements, reaching extreme bank angles of 103° right and 48° left. The pilot flying countered the roll with full aileron deflections. After approximately 50 seconds, the crew regained control, declared an emergency, and landed safely at Berlin-Schönefeld Airport at 11:49 UTC. No injuries or third-party damage occurred, and the aircraft was not damaged. ## Investigation Findings and Aircraft Systems The German Federal Aviation Accident Investigation Office (BFU) analyzed flight data recorder (DFDR) data, which showed the angle of attack (AOA) reached approximately 12° before the stall warning triggered at 11:28:54 UTC. The aircraft’s flight manual warned against autopilot use when visual cues of severe icing were present, specifically rime ice on the unheated forward side windows. The crew had not classified the icing as severe. The aircraft was equipped with pneumatic de-icing boots and electrical anti-icing systems, and it had previously undergone an extension of the de-icing boots on the outer wing. The investigation noted that the transition to an uncontrolled flight state was completely unexpected for the crew, with no instrument indications of an impending stall and no evidence of mistrim.