What happened
On July 21, 2007, a Spring-Bok 2000 ultralight, identified as 11-GR, was performing a ferry flight from Lézignan-Corbières to Névian. During the initial climb following takeoff, the Rotax 582 engine ceased operation. Unable to find a suitable landing area directly ahead, the pilot attempted a 180-degree turn to execute a forced landing in a nearby field. During the final approach, the aircraft's main landing gear struck a fence, causing the nose of the aircraft to impact the ground heavily. The aircraft sustained heavy damage.
The investigation
The investigation focused on the engine failure and the recent maintenance history of the aircraft. Two days prior to the accident, the same aircraft had experienced an engine failure during a training flight, which resulted in a field landing. Following that event, a mechanic replaced a slightly cracked fuel hose, and subsequent ground tests appeared normal.
Investigators examined the fuel system, noting that the previous owner had implemented several modifications, including lengthening the fuel lines, adding a filter and a flow meter, and altering the position of the fuel tank vent. The current owner had also added an additional translucent filter to monitor fuel flow. While the engine itself showed no mechanical anomalies and the fuel tank was not empty, the investigation scrutinized the impact of these modifications on fuel delivery.
Findings
- The primary cause of the engine failure was inadequate fuel pressure caused by improper modifications to the fuel system.
- The modified fuel circuit, which included extra filters and extended lines, created significant pressure drops.
- While gravity and tank pressure were sufficient to maintain fuel flow during ground operations, the modified position of the tank vent caused a vacuum (depression) to form inside the tank during flight. This prevented the mechanical fuel pump from adequately supplying the engine.