Incident Overview
On November 19, 2024, at approximately 10:50 UTC, a Textron B200GT King Air with registration TF-NLE experienced instrument anomalies while in cruise flight at Flight Level 180 between Skagafjörður and Blönduós. The aircraft was operating as a medical transport from Akureyri to Ísafjörður, with a subsequent leg to Reykjavík planned. On board were two pilots, a medical attendant, and one patient.
Operational Context and De-icing
Prior to departure, the aircraft was de-iced at Akureyrarflugvöllur due to snow accumulation on the wings and fuselage. The de-icing process involved spraying a 50/50 mixture of Type II fluid and hot water for cleaning, followed by a 75/25 mixture for anti-icing. The pilot noted that the fluid appeared thicker than usual during the taxi phase, requiring wipers to clear the windshield.
In-Flight Anomalies
After reaching cruise altitude, the aircraft pitched down on autopilot and lost 200 feet of altitude. Upon disconnecting the autopilot, the pilots observed the altimeter fluctuating by ±500 feet and the attitude indicator rocking in level flight. Suspecting unreliable instruments, the crew diverted to Reykjavík, where visual meteorological conditions prevailed. They requested a restricted airspace corridor due to the unreliable altitude readings and flew using pitch/power procedures.
Investigation Findings
Post-landing inspection revealed frozen fluid over the static air port. The Icelandic Transport Authority (RNSA) noted that no sample of the frozen fluid was collected for analysis because the operator did not report the incident until December 13, 2024. Consequently, it could not be definitively determined whether the freeze resulted from the de-icing fluid or residual slush. However, RNSA found it likely that the 50/50 de-icing mixture, which has a freezing point of -10.6°C, began to freeze as the aircraft climbed into colder temperatures (-2°C per 1,000 feet). The static port lacks an in-flight heater, and while an alternate static source exists, it did not resolve the issue during the flight.