What happened
On 17 January 2012, a Thruster T600N 450, registration G-CBIO, was performing a private flight from Sandown Airfield to Compton Abbas. While cruising at 1,700 ft, the pilot experienced a reduction in engine power. The atmospheric conditions, with an air temperature of +2°C and a dewpoint of +1°C, were highly conducive to the formation of carburettor ice.
In an attempt to clear the issue, the pilot applied carburettor heat and reduced power to 2,000 rpm to position the aircraft for a landing on the downwind leg of Runway 08. However, when the pilot attempted to increase the throttle, the engine power failed to respond. Realising the aircraft was too low to reach the runway, the pilot declared a MAYDAY and selected a nearby field for an emergency landing. Upon touchdown, the aircraft hit soft ground, causing it to decelerate rapidly and overturn. There were no injuries to the two occupants.
The investigation
The AAIB investigation focused on the effectiveness of the aircraft's carburettor heat system. While the aircraft was originally manufactured with a production-standard oil-heated jacket, it had been modified with an electrically heated jacket. This unapproved modification was not documented in the aircraft maintenance manual or the Type Approval Data Sheet (TADS).
Testing conducted by the AAIB revealed that the electrical system was significantly less effective than the original design. In a controlled test, the electric heater raised the butterfly valve temperature to only 6°C after six minutes, whereas the original oil-heated system would have provided much greater warmth. Furthermore, the investigation found that the throttle control cable showed signs of resistance, though water freezing within the cable was considered unlikely given the ambient temperatures.
Findings
- The reduction in engine power was likely caused by carburettor ice formation restricting the movement of the throttle butterfly valve.
- The installation of an unapproved electrical carburettor heat system significantly increased the aircraft's vulnerability to icing.
- The modification had not been approved by the manufacturer or the BMAA, and its presence went undetected during previous maintenance inspections.
- The pilot's inability to maintain altitude or reach the runway was a direct result of the power loss caused by the ineffective heating system.