What happened
On September 18, 2020, a privately owned Kitfox 6, registration HB-YLH, departed from Grenchen Airport (LSZG) for a flight to Bad Ragaz. During the initial climb phase, shortly after takeoff, the pilot experienced a sudden loss of engine power. While the engine maintained enough performance to sustain level flight at approximately 20 meters altitude for about 15 seconds, the aircraft was unable to gain further altitude or airspeed.
In an attempt to return to the airport, the pilot initiated a right-hand turn. During this maneuver, the aircraft's airspeed dropped below the minimum required speed, leading to a stall. The aircraft subsequently struck a field at low altitude, resulting in heavy damage to the airframe. The pilot sustained light injuries and was able to exit the wreckage independently.
The investigation
Investigators examined the aircraft's fuel system, engine, and flight data from the Flarm device and airport webcams. The technical examination of the fuel system revealed significant installation errors. Specifically, at the firewall, the fuel supply and return hoses were swapped. Furthermore, two fuel hoses near the fuel distributor were installed with a radius that was too small, creating kinks that restricted flow.
Testing demonstrated that even a slight movement of these restricted lines could reduce fuel flow from 50 l/h to below 20 l/h. The investigation also noted that the spark plugs for cylinders 1 and 3 were heavily fouled, suggesting an asymmetric fuel-air mixture, potentially caused by the cockpit's choke control mechanism failing to return both carburetors to a fully closed position.
Findings
- The primary cause of the accident was the pilot's decision to execute a high-risk turning maneuver, often referred to as "the impossible turn," at a low altitude following engine power loss.
- The engine power loss was likely caused by vapor lock (the formation of vapor bubbles) within the fuel system.
- Vapor lock was facilitated by the incorrect installation of fuel hoses at the firewall, which swapped the supply and return lines.
- The formation of vapor bubbles was further exacerbated by fuel line kinks and the use of MOGAS, which is more prone to such issues in this configuration.
- The pilot's decision to attempt a 180-degree turn at low altitude significantly increased the risk of a stall and subsequent loss of control.