What happened
On the evening of 20 January 2020, a De Havilland DHC-8-311, registration LN-WFO, operated by Widerøe, was performing a scheduled flight from Kristiansund Airport Kvernberget to Bergen Airport Flesland. During the climbout phase, the aircraft inadvertently entered an area of severe icing. Although the crew attempted to mitigate the situation by adjusting their altitude and flight path, the aircraft remained exposed to heavy icing for an extended period.
As the aircraft approached Flesland, ice that had accumulated within the engine air inlets began to detach. This falling ice entered the engines, causing the left engine to lose power, followed by the right engine, and subsequently the left engine once more. While the aircraft's automatic ignition system successfully restarted the engines, the delay in the start-up sequence left the aircraft momentarily without engine power. Despite the critical nature of the engine failures, the crew managed to land the aircraft safely at Bergen.
The investigation
The Norwegian Safety Investigation Authority (NSIA) examined flight data, cockpit voice recordings, and interviews with the crew. The investigation focused on why the aircraft entered the severe icing zone and why the engine flameouts occurred. Investigators reviewed the airline's operational manuals, meteorological reports from the Norwegian Meteorological Institute, and technical documentation from the manufacturer, De Havilland.
Findings
- The aircraft entered a zone of severe icing due to a direct clearance from air traffic control that placed the flight on a course through the icing area.
- Ice accumulation inside the engine nacelles and air inlets was a primary factor; as this ice broke away, it disrupted airflow or entered the combustion chambers as slush, leading to the flameouts.
- There was a lack of clear, explicit definitions for "moderate" versus "severe" icing within joint European regulations, which can complicate decision-making for pilots and meteorologists.
- The investigation noted that the aircraft's engine air inlets were not specifically checked for ice during the pre-flight inspection, as the ambient temperature was 9°C and it was raining.