On August 16, 2021, about 1515 mountain daylight time, a Europa Trigear, N133AC, was substantially damaged when it was involved in an accident near Hanksville, Utah. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that during the preflight, he noticed that the coolant in the cooling system’s expansion tank was low. He removed the cowling and did not notice any pooling of coolant in the engine compartment, so he replaced the coolant with about 16 ounces of water. He completed the preflight and ground run without further incident and continued with the departure, which he characterized as a “stair-stepped” climb out to assist with the engine cooling on a hot day over rough terrain. After 30 minutes, the pilot reported that the engine’s CHT was pegged in the red arc [maximum admissible head cylinder temperature of 180°C (356?)] and he immediately reduced the throttle setting to idle and elected to conduct a precautionary landing. While the airplane was descending, he cycled the engine off and on to reduce the CHT, but the temperature remained in the red arc. During the landing roll on an unimproved dirt road, the airplane impacted rough terrain. Postaccident examination of the cooling system revealed that the expansion tank, coolant hoses, and water pump were unremarkable, and that the system was void of any coolant. The manufacturer states that the coolant capacity for the engine is about 0.4 US gallons (about 1.5 liters). TEST AND RESEARCH The pilot stated that about 2 weeks before the accident flight, he removed the original radiator (pressure screw) cap and replaced it with a non-original equipment manufactured radiator cap from a third-party vendor. The original cap and the installed cap were both labeled to maintain system pressure up to 1.2 bar (18 psi) and both caps contained an upper pressure seal, lower pressure seal, pressure relief valve, and return(vacuum) valve. A pressure test of the original/replaced cap revealed that the pressure relief valve opened about 0.8 bar (12 psi). The accident radiator cap was also pressure tested and revealed that the valve opened about 0.2 bar (3 psi). According to the manufacturer, the engine boiling point is influenced by the system pressure and that if temperature exceeds the boiling point, the engine can quickly overheat due to loss of coolant. According to the manufacturer, the radiator cap sits on top of the expansion tank and provides a tight seal to maintain pressurization of the cooling system, which increases the system’s boiling point. As the system pressure reaches the cap’s rated pressure, the cap’s pressure relief valve opens, and the pressurized coolant flows into the overflow bottle; this allows for the expansion of the heated coolant. Conversely, as the temperature drops and the coolant contracts, a vacuum is created within the cooling system. The vacuum seal within the cap opens and allows the coolant to flow from the overflow bottle back into the radiator.

Figure 1. Rotax 914 Cooling System Flow Diagram According to Rotax Service Letter SL-914-012R6 Revision 13 May 2022: The use of any third-party replacement kits, modifications and non-genuine ROTAX® components or accessories for ROTAX® engines is NOT approved by ROTAX® and is very strongly discouraged. Use of any improper third-party replacement kits, improper modifications and non-genuine ROTAX® components or accessories may cause engine damage resulting in catastrophic engine failure. As stated in the manufacturer’s maintenance manual, a rise in the cylinder head temperature above normal operations is a clear signal for a failure in the cooling system, to include the pressure relief valve.

Figure 2. Cylinder Head Table from Rotax 914 Maintenance Manual 05-50-00 Edition 2, Revision 2