What happened
On the morning of January 20, 2024, a pilot operating an Aquila AT01, registration LN-NRC, departed from Kjeller airport in Norway for a local flight. The ambient temperature was approximately -24 °C. Shortly after takeoff, while climbing over the Nitelva river at an altitude of roughly 600 feet, the pilot observed a drop in engine RPM. This was immediately followed by a loud mechanical impact, after which the propeller ceased rotation.
With the aircraft only about 250 feet above the terrain, the pilot declared a Mayday and initiated an emergency descent. Utilizing familiarity with the local area, the pilot executed a left-hand turn to reach a known emergency landing site in a snow-covered field at Skjetten. The landing was performed as smoothly as possible to avoid residential structures, though the sudden deceleration caused the nose-wheel leg to fracture. The pilot sustained minor injuries, but the engine was destroyed during the incident.
The investigation
The Norwegian Safety Investigation Authority (NSIA) conducted a detailed examination of the engine, which was sent to the manufacturer, Rotax, in Austria. Disassembly of the Rotax 912 S3 engine revealed that connecting rod #3 had punctured through the crankcase. Investigators also found that the big end bearings for both rods #2 and #3 had suffered significant heat damage.
While the oil tank was found to be at full capacity, the investigation focused on the flow of oil through the system. The aircraft was equipped with a cold weather operations kit, including heating pads for the oil tank and crankcase. However, the NSIA noted that these heaters do not heat the oil within the lines or the oil cooler. The investigation also highlighted conflicting temperature and viscosity recommendations between the aircraft and engine manufacturers regarding the use of 10W-40 oil in extreme cold.
Findings
- The primary cause of the engine failure was insufficient oil lubrication to the bearings.
- It is believed that during the transition from idle to high power during takeoff, the oil pump experienced cavitation. This was likely caused by the oil pump attempting to pull a large volume of oil through a cold oil cooler containing very cold, viscous oil.
- The engine preheating system provided a false sense of security by warming the oil in the tank and crankcase, while leaving the oil in the cooler and lines significantly colder.
- Conflicting documentation between the aircraft and engine manufacturers regarding oil viscosity limits and temperature thresholds contributed to the difficulty in determining safe operating limits.
Safety action
Following the investigation, the Nedre Romerike flying club implemented a new operational limit, prohibiting the use of the aircraft in temperatures colder than -15 °C. Additionally, the NSIA expects the aircraft and engine manufacturers to work together to clarify and unify the instructions provided to users regarding oil types and preheating procedures.