What happened
On the evening of June 21, 1998, a CUBY II microlight, identified by the number W 54-OE, was conducting a private flight near Bures, France. The aircraft was operating at maximum weight capacity as part of a flight intended to obtain the aircraft's identification card.
At approximately 18:30 local time, while performing a low-altitude pass over the lake at the Bures leisure base, witnesses observed the engine beginning to misfire during a steep left-hand turn. Shortly after, the engine ceased operation as the aircraft leveled out on a horizontal flight path. The pilot attempted to restart the engine, but the effort was unsuccessful. The aircraft subsequently pitched up, entered a stall, and collided with a tree, resulting in the death of both the pilot and the passenger and the destruction of the aircraft.
The investigation
Investigators examined the wreckage of the CUBOT II and its HIRTH F30 A1 80 hp engine. The technical inspection focused on the engine's ignition and exhaust components to determine the cause of the sudden loss of power.
Findings
Analysis of the engine components revealed that the four spark plugs were not properly secured; they had become unscrewed by between one and two threads. Additionally, investigators found that a retaining screw for an exhaust pipe had partially unscrewed, with half of its thread no longer engaged. The investigation established that the unsecured spark plugs were the primary cause of the engine failure.