What happened
On Saturday, September 19, 1998, at approximately 18:00, a Mike-type ultralight (identified as 31-JZ) departed from the Gardouch airfield for a local flight. Shortly after takeoff, while maintaining an altitude of approximately 250 meters, the pilot noticed that the engine water temperature had exceeded 100°C.
In an attempt to return to the airfield, the pilot began a maneuver to rejoin the circuit. However, at an altitude of roughly 250 meters above the ground, the engine ceased operation. Realizing the runway was unreachable, the pilot executed a left turn to avoid a road and a plowed field, ultimately selecting a wheat field for an emergency landing. During the descent, the aircraft struck a hedge, causing the landing gear to break and the firewall to be torn away.
The investigation
The investigation focused on the engine failure and the mechanical state of the aircraft. The engine, a Rotax 582 water-cooled unit, was examined to determine the cause of the sudden stoppage. Investigators also reviewed the aircraft's recent maintenance history and technical modifications.
Findings
Technical analysis revealed that the engine had seized as a direct result of overheating. The investigation established that the pilot, who was also the owner and builder of the aircraft, had experienced a similar engine failure due to overheating on August 30 of the same year.
To address this recurring issue, the pilot had installed an additional radiator in series with the existing one to improve cooling. This modification was not approved by the engine manufacturer and was not documented in the aircraft's official technical file updated in September 1998. This unauthorized cooling modification failed to prevent the overheating that led to the engine seizure.