History of the Flight
The flight was a circuit training exercise with a student pilot and an instructor. Pre-flight checks and engine run-up were normal. After takeoff, the carburettor heat was left on following downwind checks, which the instructor noted but did not correct as heat would soon be needed for descent. The student reduced power, possibly more than usual, to extend flaps and begin the approach. When the throttle was advanced, the engine did not respond. The instructor took control, declared a MAYDAY, and changed fuel tanks. The aircraft descended rapidly, and power cables became visible on the approach path. To avoid stalling while attempting to fly over them, the instructor descended to go under the cables. The aircraft touched down short of the intended field, struck a hedge, and came to rest on its right side.
Evacuation
The fuselage came to rest on its right side, preventing the cabin door from being opened. The occupants smelled fuel and saw it leaking from the detached wings. Unable to dislodge the windscreens, they escaped through the left side window. They used the fire extinguisher to break the window, using the direct vision window cut-out as a weak point. No fire occurred. The aerodrome dispatched emergency services, and the instructor called operations via mobile phone. Assistance arrived minutes later.
Engineering Examination
The aircraft was examined at the operator's maintenance facility. Throttle and mixture controls were connected and operated normally. The carburettor heat control was connected but restricted due to distortion of the air box from the accident. The primer was in and locked. The fuel system had no defects; the gascolator filter was clean. The engine was removed and tested, running normally.
Weather Conditions
The METAR for London Heathrow Airport, 10 nm east, reported wind from 190° at 6 kt, scattered cloud at 1,200 ft with temporary broken cloud, temperature 15°C, and dew point 14°C.
Carburettor Icing
The CAA Safety Sense Leaflet No14 describes carburettor icing as a combination of temperature drop from fuel vaporisation and pressure reduction. Plotting the reported temperature and dew point on the leaflet's chart indicates serious carburettor icing was likely at any power setting. Risk factors present included high humidity, a low power setting, and the engine likely not at full operating temperature.
Discussion
In the absence of mechanical defects, carburettor icing is considered the most likely cause of the power loss, given the risk factors. The student noted he had not been shown how to remove the fire extinguisher or that the left side window offered an alternative escape route.