What happened
On January 28, 2000, an ATR 42-300 operated a scheduled IFR flight from Berlin-Tegel to Bremen with 3 crew members and 42 passengers on board. During the initial climb, the aircraft entered instrument meteorological conditions (IMC) at approximately 3000 ft. Shortly thereafter, the crew identified icing on the aircraft's ice evidence probe.
As the aircraft continued its climb, the crew observed freezing rain and significant ice accumulation on the propeller spinners and wing leading edges. By FL 80, the icing had become so severe that the aircraft could no longer maintain its climb rate, despite both engines operating at maximum climb power. The crew noted visual cues of severe icing, specifically ice forming on the cockpit side windows, which, according to the aircraft's flight manual, indicates conditions beyond the aircraft's certification.
To escape the icing zone, the crew requested an immediate descent from air traffic control (ATC). The aircraft descended through FL 60 and FL 40, eventually reaching 2000 ft (QNH). At this altitude, the aircraft entered visual meteorological conditions (VMC) and temperatures rose above freezing, causing the ice to shed. After approximately 10 minutes of flight at 2000 ft, the crew decided to continue to Bremen, climbing to FL 100 and landing without further incident.
The investigation
The BFU examined the flight data recorder (DFDR) and consulted the aircraft manufacturer, Aerospatiale-Alenia (ATR). Analysis of the DFDR confirmed that the aircraft's drag increased significantly during the encounter. At 7900 ft, the drag had increased by approximately 80% compared to the initial climb, although no loss of lift was detected at that stage. The manufacturer attributed this drag increase to ice accumulation on the leading edges and on surfaces well behind the protected areas.
The investigation also reviewed meteorological data from the German Meteorological Service (DWD). While the crew had been briefed on potential light to moderate icing, the specific conditions encountered—severe icing—were not predicted for the Berlin area at the time of departure. The investigation also noted that during the descent to 2000 ft, the aircraft briefly flew below the minimum radar altitude (MRVA) of 3500 ft, but the crew was able to maintain situational awareness as they were in VMC.
Findings
- The primary cause of the incident was the aircraft entering severe icing conditions for which it was not certified.
- The accumulation of ice on the leading edges and unprotected surfaces caused an 80% increase in aerodynamic drag, which severely degraded the aircraft's climb performance.
- The crew correctly identified the visual cues of severe icing and followed the procedures outlined in the Airplane Flight Manual (AFM) by manually flying the aircraft and initiating a descent to exit the icing zone.
- The aircraft's de-icing and anti-icing systems were functioning normally, but they were insufficient to counteract the rate of accumulation in the freezing rain.