What happened
On November 19, 2024, a Textron B200GT (registration TF-NLE) was operating a medical transport flight from Akureyri to Ísafjörður, with a final destination of Reykjavík. The aircraft was carrying two pilots, a medical professional, and one patient. Prior to departure, the aircraft had been stored outside a hangar during snowy conditions, resulting in significant snow and ice accumulation on the wings and fuselage.
Following recommendations from a ground handler, the crew performed de-icing using a 50/50 mixture of Type II de-icing fluid and hot water, followed by anti-icing with a 75/25 mixture. During the takeoff roll, the pilot noted that the fluid appeared thicker than usual, adhering to the windscreen and requiring the use of wipers to maintain visibility.
While cruising at FL180 between Skagafjörður and Blönduós, the aircraft suddenly pitched down and lost 200 feet of altitude while on autopilot. Upon disconnecting the autopilot, the pilot observed the altimeter and vertical speed indicator oscillating by approximately +/- 500 feet. Due to the unreliable instruments, the crew diverted to Reykjavík, flying via visual flight rules (VFR) using a pitch-and-power technique to ensure safety. The aircraft landed safely in Reykjavík, where a post-flight inspection revealed that frozen fluid had blocked the static air port.
The investigation
The RNSA investigated the cause of the instrument failure and the de-icing procedures used at Akureyri Airport. The investigation examined the freezing points of the specific de-icing fluid mixtures used and the ambient temperature conditions at the time of application. METAR data indicated temperatures around -10°C at the time of de-icing.
Investigators also reviewed the aircraft's systems, noting that the static port system lacks an internal heater. While the crew attempted to use the alternate static air source, the issue persisted. The investigation also looked into the suitability of Type II de-icing fluid for aircraft with low takeoff speeds, noting that Type II fluid is more viscous and does not run off the airframe as easily as Type I fluid.
Findings
- The primary cause of the instrument failure was frozen fluid blocking the static air port.
- The 50/50 de-icing fluid mixture used during the de-icing phase had a freezing point of approximately -10.6°C. Given that the ambient temperature was near -10°C, the fluid was at risk of freezing.
- As the aircraft climbed, the ambient temperature dropped (typically -2°C per 1,000 feet), likely causing the fluid or accumulated slush to freeze over the sensor.
- The pilot's observation of thicker-than-normal fluid on the windscreen during takeoff suggested the fluid was already near its physical limits.
Safety action
- The RNSA directed the de-icing service provider at Akureyri Airport to review procedures for preparing de-icing fluids to prevent using concentrations that are at risk of freezing.
- The RNSA directed Norlandair to review the operational limits of various de-icing fluid concentrations, specifically regarding both de-icing and anti-icing applications, with their pilots and ground personnel.