What happened
On December 14, 1998, an ATR 42-300 departed Dresden for Poznan, Poland. During the climb, the aircraft entered clouds, and the crew subsequently activated the autopilot at 3,400 ft. As the aircraft climbed, the crew noted moderate icing and activated the aircraft's anti-icing and de-icing systems. While the wing boots appeared to be working correctly, rime ice became visible on the unheated portions of the forward side windows.
As the climb progressed, the crew noticed a significant reduction in the rate of climb. To maintain stability, they reduced the climb speed from 160 kt to 155 kt and transitioned to altitude hold mode at approximately FL 135. Shortly thereafter, the stall warning activated unexpectedly, and the autopilot disconnected. The aircraft immediately entered an uncontrolled flight phase characterized by severe left and right rolling motions, known as "wing rocking," with bank angles reaching as much as 103 degrees to the right. After approximately 50 seconds of instability, the crew regained control, declared an emergency, and performed a safe landing at Berlin-Schönefeld.
The investigation
The BFU examined flight data from the Digital Flight Data Recorder (DFDR), which revealed that while the pilot was unaware, the angle of attack (AOA) had been steadily increasing. The investigation established that the aircraft had been flying in severe icing conditions for at least 13 minutes prior to the autopilot disconnection. The DFDR showed that the AOA reached a critical threshold of 11 degrees, triggering the stall warning. The investigation also noted that the crew's attempts to counter the rolling motion with full aileron input contributed to the sustained wing rocking motion.
Findings
- The primary cause of the loss of control was the entry into severe icing conditions for which the aircraft was not certified.
- The crew did not recognize the rime ice on the cockpit side windows as a visual indicator of severe icing, as specified in the Aircraft Flight Manual (AFM).
- A local loss of lift, likely caused by ice accumulation on the upper surface of the wing beyond the protected de-icing boots, triggered the roll excursion.
- The crew failed to recognize that the significant drop in climb rate and airspeed were indicators of severe icing.
- The pilot's reactive full aileron inputs during the excursion exacerbated the wing rocking phenomenon.