What happened
On 28 June 2003, a Zenair CH 601UL Zodiac, registration G-RUVI, was performing its first flight following recent construction. Prior to the flight, the aircraft had undergone several ground tests and had been parked in the open for over an hour. During the pre-flight sequence, the engine was run for approximately 12 minutes using premium unleaded motor fuel.
As the aircraft departed from the grass airstrip at Droppingwells Farm, the engine began to lose power. At an altitude of roughly 100 feet, the engine failed to produce sufficient power to maintain flight. The pilot attempted to land back on the grass strip but determined there was insufficient room, necessitating a forced landing in a nearby field of standing corn. During the landing, the aircraft's left wing contacted the crop, causing the aircraft to slew and the right main landing gear to collapse. The pilot escaped the aircraft without injury, though the aircraft sustained damage to the right wing, the centre section, and the right main gear.
The investigation
Investigators examined the engine and fuel system, finding no mechanical faults or defects in the fuel delivery components. Testing showed that both the electric and mechanical fuel pumps were operating at capacities well above the requirements for the Rotax 912 engine.
The investigation focused on the fuel system configuration. The aircraft was using unleaded motor fuel, which has a higher vapour pressure than Avgas, making it more susceptible to vapour formation. The fuel lines for the mechanical pump were positioned over the top of the engine, making them vulnerable to heat soak after the engine had been run and then left sitting in the open.
Findings
- The engine power loss was likely caused by vapour lock within the fuel lines leading to the mechanical engine-driven pump.
- This vapour lock was facilitated by heat soak in the fuel lines following ground operations and the use of highly volatile unleaded motor fuel.
- The pilot had failed to activate the electric fuel pump during the pre-flight checks and throughout the flight.
- Had the electric pump been operational, it likely would have provided enough secondary fuel flow to prevent the power loss, as its position on the firewall was less exposed to engine heat.
- The aircraft lacked a vapour return line, a feature intended to mitigate such issues by routing fuel/vapour back to the tank.
Safety action
- Safety Recommendation 2003-124: The PFA should notify all owners of Zenair CH601 aircraft equipped with Rotax 912 engines and operating under permits to fly regarding the necessity of installing a vapour return fuel line to ensure compliance with CAA AWN 98B.