What happened
On 24 May 2004, a Thruster TST Mk1 microlight, registration G-MTKD, was conducting a leisure flight after departing from Kilrush Airfield. While climbing to an altitude between 1,500 and 2,000 feet, the engine RPM suddenly decreased before the engine ceased operation entirely.
In response to the failure, the pilot transitioned the aircraft into a glide, shut off the fuel and ignition, and notified local air traffic control. The pilot identified a suitable landing site in a field south of Kilrush, Co. Kild to execute the approach. The landing resulted in no injuries to the pilot, though the aircraft sustained damage to its composite nose and pitot head after the nose wheel struck a sharp depression in the ground.
The investigation
The investigation focused on the mechanical cause of the engine failure and the integrity of the fuel system. Examination of the fuel supply system, which utilizes a Rotax 503 engine with a primer bulb, revealed that the engine had suffered from fuel starvation.
Investigators analyzed the internal components of the primer bulb, a device used to prime the engine during cold starts. The investigation established that the non-return valve within the bulb, which relies on a small ball to regulate fuel flow, had malfunctioned. Specifically, the inlet ball valve had become detached from its position due to wear, allowing it to migrate through the bulb and obstruct the outlet.
Findings
- The primary cause of the engine stoppage was fuel starvation caused by a blockage in the fuel supply line.
- The internal inlet ball valve of the primer bulb became loose due to mechanical wear.
- This loose component moved into the outlet of the primer bulb, physically blocking the flow of fuel to the engine.