What happened
On January 3, 2020, a Cessna 172S, registration LN-AZA, was conducting an instrument flight rules (IFR) training mission. The flight, operated by OSM Aviation Academy, originally planned a route through several Norwegian aerodromes before returning to Arendal. After a stop at Sola airport, the crew—consisting of an instructor and a student—decided to fly back toward Arend to traverse mountainous regions.
While navigating between flight levels FL100 and FL110, the aircraft encountered rapidly developing clouds. The crew attempted to climb to FL110 to clear what they believed were cloud tops, but the clouds grew around the aircraft, trapping them in instrument meteorological conditions (IMC). The aircraft entered significant icing conditions, leading to visible ice accumulation on the windshield and wing leading edges. The crew reported that flight controls became heavy and the aircraft began to vibrate. For approximately 17 minutes, the crew struggled within these conditions before requesting a descent to lower altitudes. With the assistance of air traffic services, they successfully reached warmer air, which allowed the ice to melt, and eventually landed safely at Sola airport.
The investigation
The investigation focused on the flight planning and the decision-making processes used by the crew. Investigators examined weather reports, including METAR, TAF, and icing forecasts, which indicated that while convective activity (CB cells) was present, specific icing warnings outside those cells were not explicitly noted. The investigation also reviewed the flight school's "Threat and Error Management" (TEM) procedures, noting that the crew had not established a specific contingency plan for encountering unexpected icing.
Findings
- The Cessna 172S was certified for IMC but was not certified for flight in icing conditions.
- The crew's decision to fly over the mountains was based on an optimistic assessment of cloud tops, failing to account for the rapid vertical growth of convective clouds.
- The flight path over the mountains presented a significant risk because the terrain height made it difficult to descend to warmer, non-icing altitudes without violating minimum safe altitudes.
- The crew's decision-making was influenced by a desire to complete the training objectives, which may have led to "pushing the weather."
- The 0-degree isotherm (freezing level) was significantly lower than the Minimum Safe Altitude (MSA) in the area, meaning any encounter with clouds at the planned altitude would likely result in icing.
Safety action
Following the incident, the flight school implemented new procedures for planning long-duration IFR flights. A key new directive prohibits flying in IMC if the forecasted 0-degree isotherm is lower than the Minimum Safe Altitude (MSA). This measure is intended to ensure that if a pilot unexpectedly enters icing conditions, they have the altitude available to descend into warmer air without risking terrain impact.