EUROPA - TRIGEAR N133AC accident near Hanksville, Utah

Probable cause

The pilot’s installation of a non-original equipment manufactured radiator cap, which resulted in the loss of coolant and the engine’s exceedance of the maximum admissible cylinder head temperature. Contributing to the accident was the lack of suitable terrain for the forced landing.

NTSB analysis

The pilot replaced lost fluid in the airplane engine’s cooling system with water and after a successful ground run, the pilot departed. During a “stair-stepped” climbout, the engine’s cylinder head temperature (CHT) increased into the red arc of the CHT gauge (the maximum admissible cylinder head temperature of 180°C [356°F]). The pilot immediately reduced the throttle setting to ide and elected to conduct a forced landing. The airplane sustained substantial damage to the wings and fuselage after impacting rough terrain during the landing.

Postaccident examination of the cooling system revealed that system components were secured and void of all fluid. Testing of the accident radiator cap revealed that it was a non-original equipment manufactured (OEM) part by a third-party vendor and that the pressure relief valve opened and released air pressure about 0.2 bar (3 psi), less than the manufacturer’s recommended coolant system maximum pressure value of 1.2 bar (18 psi).

It is likely that the lack of pressure created by the non-OEM radiator cap reduced the coolant boiling point and system pressure. The pressure reduction and lowering of the boiling point would have allowed the coolant to evaporate, leading to a loss of coolant in the system and the engine’s exceedance of the maximum admissible cylinder head temperature of 180°C (356°F).

Text from the NTSB. Open the NTSB docket for WPR21LA325 for the full record.