Norway’s Accident Investigation Board (AIBN) has concluded that the dual flameout of a 2015 Airbus H145 helicopter last year was caused by ice-clogged inlet barrier filters (IBFs).
The Norsk Luftambulanse flight was conducting a search-and-rescue mission last November 20 in the Kongsvikdalen valley in Lofoten during localized snowy conditions when the left engine abruptly shut down in flight. The pilots successfully executed an immediate emergency landing.
Once on the ground, the right engine also shut down. None of the six aircraft occupants were injured and the helicopter was later airlifted to a maintenance facility. The post-incident investigation found damage to compressor blades in both engines’ axial compressors and Norsk subsequently removed the IBFs from its fleet.
Before losing the left engine, the pilot observed an “IBF Clog Trend” alert that is triggered when clogging reaches 40 percent. Following the procedure, he opened a bypass door that feeds air to the engine from the main gearbox compartment; however, that door was already open due to an actuator issue and the action actually opened the right IBF bypass door.
The AIBN noted that the incident helicopter spent substantial time in hover and on the ground with engines running while awaiting the arrival of the party being rescued and concluded that ice probably built up in the IBF system during this time. It noted that post-incident flight tests conducted by Airbus concluded that “under certain weather conditions, significant amounts of ice can build up in the IBF system and this ice can enter the engine unhindered. Especially on the lower side of the bypass grid when the bypass door is open. If the temperature varies and goes above 0 °C, the ice can quickly melt on the contact surfaces leading to dislodging of ice or slush.”
The H145 is not certified for flight into known icing, and shortly after the incident Airbus published Safety Information Notice 3515-71-Rev1 that prohibits flights in snowy weather or in temperatures below 5 degrees C when moisture is visible for helicopters with IBFs installed.