History of the Flight
The instructor and student planned to conduct circuits at Biggin Hill. During the engine power check, the carburettor heat lever was exercised four times; although stiff, it produced a positive RPM drop. After taxiing approximately 1000 metres from the run-up area to the threshold of Runway 21, which took about two to three minutes, carburettor heat was not reapplied. The student initiated a short field takeoff with wheel brakes applied. At full power, RPM indicated approximately 2,250 RPM, and all temperature and pressure gauges were within limits. The takeoff and climb proceeded normally until reaching about 300 feet, when the engine coughed and spluttered, with a small RPM decrease. The instructor took control, leveled off at 450 feet, and suspecting carburettor icing, applied full carburettor heat. Instantly, RPM dropped and the engine lost all power. The instructor exercised both throttle and carburettor heat levers but with no effect. He switched fuel tanks, then realised he was committed to landing in a field, so he switched the fuel selector to SHUTOFF and turned off the fuel pump. A field to the right of the extended runway centreline was selected. Just before touchdown, the nose wheel struck a low wire fence, shearing off the nosewheel strut and causing the aircraft to slew left. The propeller blades bent upon ground impact, and the wing and underside panels suffered minor dents. Both occupants vacated the aircraft immediately.
Observations
The engine was shock loaded by the propeller impact and could not be run for testing. An inspection by the operator's engineer revealed no anomalies that could have caused the power failure. The carburettor heat lever was confirmed to operate the intake flap properly. Engine inspection showed no damage or anomalies; magnetos passed functional checks. The METAR at 1150 hrs reported a temperature of 5°C and a dewpoint of 3°C in light rain, with relative humidity about 80%. The CAA carburettor icing probability chart indicated a serious risk of icing at any power for a typical light aircraft piston engine without carburettor hot air. It is possible that ice formed in the carburettor during the two to three minute taxi after the last carburettor heat selection. However, a noticeable RPM drop at full power would have been expected, but the instructor believed the engine performed normally. Applying carburettor heat after noticing the drop caused a complete power loss, which is unusual for icing alone. Fuel starvation was considered but deemed unlikely: both tanks had been filled to the tabs, used successfully, and fuel was checked for water before and after the flight, with no water found. The electric fuel pump was on. Although no definite conclusion can be drawn, the atmospheric conditions and lack of hardware fault suggest carburettor icing was the likely cause.