What happened
On July 12, 2013, a Flight Design CTLS-ELA, registration HB-WYS, was performing a routine flight from La Côte (LSGP) to Neuchâtel (LSGN). During the initial circuit, the pilot noted a "FUEL PRESSURE LOW" warning on the engine monitoring system after reducing power. Although the warning disappeared and the pilot successfully completed the circuit and landed, the issue recurred during the subsequent takeoff attempt.
As the aircraft climbed past the trees at the end of runway 04, the engine began to sputter. The pilot observed a warning—possibly indicating oil pressure failure—before the engine abruptly stopped. The pilot managed to perform an emergency landing in a nearby grain field. The aircraft sustained no damage, and the pilot was uninjured, though there was minor damage to the field.
The investigation
SUST investigators examined engine monitoring system data, radar recordings, and the aircraft's fuel system architecture. The investigation focused on why the engine failed despite normal engine parameters being recorded during the preceding flight segments. The inquiry also reviewed the installation manuals for the Rotax 912 ULS engine, noting discrepancies between the German and English versions regarding the necessity of an auxiliary electric fuel pump.
Findings
- The engine failure was caused by insufficient fuel supply resulting from the presence of air bubbles within the fuel lines.
- The specific design of the fuel system on HB-WYS prevented the effective removal of these bubbles. Unlike the engine manufacturer's recommendation, which suggests routing the fuel return line back to the tank to eliminate vapor, the return line on this aircraft was routed directly into the gascolator.
- The aircraft was not equipped with an electric auxiliary fuel pump, a component recommended in the German version of the engine installation manual to assist with fuel delivery and vapor management.
Safety action
SUST issued Safety Recommendation No. 505, calling on EASA and the manufacturer, Flight Design GmbH, to implement measures to minimize vapor formation and ensure adequate bubble elimination in the CTLS model. Following the investigation, the engine manufacturer updated its English installation manual to align with the German version, and the aircraft manufacturer began reviewing design changes, including the implementation of automatic venting valves and modified return line routing.