What happened
On 10 October 2007, a Vans RV-9, registration HB-YML, departed from Kägiswil aerodrome for a VFR private flight. Shortly after liftoff, at an altitude of approximately 50 to 100 meters, the pilot experienced a significant power drop accompanied by high-frequency vibrations. While the pilot attempted to maintain airspeed by transitioning to horizontal flight, the engine power eventually dropped to zero during a left turn as the pilot attempted to return to the runway.
To avoid a large building in the flight path, the pilot performed an emergency landing. The aircraft struck several parked cars and a garage building, causing the landing gear, wingtips, and tail to be torn from the airframe. The pilot sustained minor injuries, but the aircraft was destroyed.
The investigation
Investigators examined the fuel system and the engine's injection components. Testing of the fuel quality showed that the AVGAS 100LL met specifications. However, the investigation focused on the fuel lines and the booster pump return line.
Upon inspecting the fuel selector casing, investigators discovered that the return line from the booster pump to the fuel selector had separated. Detailed examination of the aluminum fuel lines revealed that the flares did not comply with the SAEJ5SB3 standard. Specifically, the wall thickness at the flare of the torn line was greatly reduced, indicating improper fabrication. Furthermore, the investigation found that the injection nozzle for cylinder No. 1 was partially blocked, which likely contributed to irregular engine performance.
Findings
- The engine failure was caused by air entering the injection system through a fissure in the improperly fabricated fuel return line.
- The improperly fabricated fuel line was the primary cause of the engine failure.
- The fuel line's wall thickness was significantly reduced at the flare, making it prone to failure under pressure or vibration.
- The engine's performance was also affected by a partially blocked injection nozzle in the first cylinder.
- The aircraft's mass and center of gravity were within permitted limits at the time of the accident.