What happened
On 20 November 2021, an Airbus Helicopters MBB-BK117 D-2, registration LN-OOS, was conducting a search and rescue mission in the Kongsvikdalen valley, Norway. Operated by Norsk Luftambulanelle AS, the crew was attempting to locate a group of hikers lost in the mountains. The mission was conducted at night using night vision goggles amidst snow showers.
While the helicopter was hovering to wait for the hikers to reach the landing site, the crew received a warning regarding the clogging of the Inlet Barrier Filter (IBF) on the right engine. During the subsequent flight, the left engine suddenly suffered a flameout. The pilot managed to perform an emergency landing without incident, and no injuries were reported among the crew or the passengers. However, once the aircraft was on the ground and the right engine was set to idle, it also shut down.
The investigation
The Norwegian Safety Investigation Authority (NSIA) examined the aircraft, engine components, and flight data. The investigation focused on why both engines failed despite no apparent mechanical defects being present in the engines themselves. Testing conducted by the manufacturer, Airbus Helicopters Deutschland, was also reviewed to understand the interaction between the weather and the aircraft's intake system.
Findings
- The primary cause of the dual engine shutdown was flameout caused by the ingestion of ice.
- The ice likely accumulated within the Inlet Barrier Filter (IBF) system while the helicopter was hovering in the snow.
- Under specific coastal winter conditions—characterized by temperatures near 0°C and high humidity—significant ice buildup can occur in the IBF, which can then enter the engines unhindered.
- The investigation found no technical faults or mechanical failures within the engines themselves.
- The IBF system did not meet the necessary certification requirements for adequate icing protection under these specific environmental conditions.