What happened
On January 3, 2020, a Cessna 172S, registration LN-AZA, was conducting an instrument flight rules (IFR) training mission. After an initial leg from Kristiansand to Stavanger, the crew—consisting of an instructor and a student—decided to return to Arendal by flying over mountainous regions.
While navigating between flight levels FL100 and FL110, the aircraft entered instrument meteorological conditions (IMC) and encountered heavy icing. The crew reported that the flight controls became sluggish and the aircraft began to vibrate significantly. The aircraft remained in these icing conditions for approximately 17 minutes. To mitigate the danger, the crew requested a change in route and descended to a lower altitude where warmer air allowed the ice to melt. The aircraft subsequently landed safely at Stav and Sola airport.
The investigation
The Norwegian Safety Investigation Authority (NSIA) examined the flight planning and the crew's decision-making processes. The investigation focused on the weather assessments made prior to the flight and the specific route chosen. Investigators reviewed radar data, GPS logs, and weather reports, including the 0-degree isotherm levels and the Minimum Safe Altitude (MSA) for the area. The inquiry also looked into the flight school's internal procedures and the effectiveness of their Threat and Error Management (TEM) protocols regarding unexpected icing encounters.
Findings
- The crew's weather assessment was overly optimistic; while they identified convective activity, they did not account for the potential for rapid cloud growth or icing outside of known cells.
- The flight plan lacked a specific contingency for encountering icing, as the crew's TEM training did not sufficiently cover procedures for unexpected ice accumulation.
- The decision to fly over high terrain was risky because the 0-degree isotherm was lower than the Minimum Safe Altitude (MSA), which restricted the crew's ability to descend into warmer air to clear the ice.
- The aircraft was certified for IMC but not certified for flight in icing conditions.
- The crew's attempt to maintain the training objective led to a situation where they were "pushing the weather," prioritizing the flight program over the safety margins provided by the terrain.
Safety action
Following the incident, the flight school implemented a new policy for long-duration IFR flights. The primary directive now prohibits flying into IMC if the forecast 0-degree isotherm is lower than the MSA. This ensures that if the aircraft encounters icing, the crew has the altitude available to descend into warmer air without risking terrain impact.