What happened
On April 24, 2020, a Cessna 172S, registration LN-KAB, attempted a landing on the frozen surface of Tisleifjorden in Norway. The pilot had planned the flight to land on the ice, having previously confirmed suitable conditions for a ski-equipped aircraft earlier that week. On the morning of the accident, the pilot performed a brief ground inspection of the intended landing area, noting a thin layer of frost that appeared to offer moderate braking capability.
During the landing approach, the aircraft touched down further from the shore than originally intended. While the initial 300 meters of the rollout proceeded smoothly, the aircraft began to vibrate. The nose gear suddenly pierced through a top layer of porous ice, penetrating a 3–5 cm layer of slush trapped between the surface ice and the solid ice below. This sudden deceleration caused the aircraft to pivot sharply, leading the right wingtip to strike the ice. The impact resulted in significant structural damage, including a bent propeller, damage to the nose gear strut, and injuries to the wingtip and exhaust system.
The investigation
The Norwegian Safety Investigation Authority (NSIA) examined the ice composition and the pilot's preparations. Glaciological analysis from the Norwegian Water Resources and Energy Directorate (NVE) revealed that the landing site consisted of stratified ice: a top layer of surface ice, a middle layer of water or slush, and a solid base. This type of layering often occurs when snow melts and refreezes or when heavy snowfall creates saturated layers.
The investigation also reviewed the pilot's pre-flight intelligence. While the pilot had consulted an experienced mountain pilot five days prior and had landed on the same lake four days earlier using a different aircraft with skis, the NSIA found these references insufficient for a wheeled aircraft. Furthermore, the pilot's physical inspection on the day of the accident was limited to a small area away from the actual landing site.
Findings
- The primary cause was the unstable, stratified ice structure consisting of a slush layer beneath the surface ice, which could not support the weight of the aircraft's wheels.
- The pilot's preparations were inadequate to ensure the landing site was suitable for a wheeled aircraft, as the previous successful landing was performed with a ski-equipped plane.
- The physical inspection of the ice on the day of the accident was insufficient and did not cover the intended touchdown zone.
- The pilot failed to perform ice thickness testing, such as core drilling, which would have revealed the dangerous presence of the slush layer.
- The NVE ice warning for the region at the time indicated "large variation" in ice thickness, signaling the potential for unexpected dangerous patches.