What happened
On July 28, 2011, a single-engine Rutan Aircraft Factory COZY III departed from Arnbruck airfield for a local flight. Approximately ten minutes into the flight, the aircraft entered a descent that led it into a wooded area south of the airfield. The aircraft struck the treetops of a 30-meter-high coniferous forest approximately 80 meters before the impact site, resulting in the total destruction of the airframe. The pilot sustained serious injuries and was rescued from the wreckage.
The investigation
The BFU investigation focused on the mechanical state of the engine and the fuel used during the flight. Although the aircraft was a French-registered experimental build, its permit to fly in Germany had expired at the end of 2010. Investigators examined the wreckage and found that the nose gear had been retracted during the flight. While no immediate technical defects were visible at the crash site, laboratory analysis of fuel samples revealed that the aircraft was not using the manufacturer-recommended JET A-1. Instead, the tanks contained heating oil (EL).
Furthermore, a detailed examination of the WAM-120 engine revealed a mechanical failure in one of the three cylinders, specifically a sticking exhaust valve.
Findings
- The primary cause of the accident was a significant loss of engine power caused by a malfunctioning exhaust valve.
- Because the aircraft was already on its final approach, the pilot lacked sufficient altitude to maneuver to a safe landing area once the power loss occurred.
- The engine was operated using heating oil instead of the required JET A-1 fuel.
- The aircraft's flight permit for Germany had expired prior to the accident.