What happened
On 7 May 2019, a Cessna FA152 Aerobat, registration G-WACH, was conducting a navigation progress check flight near Quainton, Buckinghamshire. The flight, involving a student pilot and an instructor, proceeded normally at 2,500 ft until the return leg to Wycombe Air Park. During this period, the instructor noticed the engine speed fluctuating between 2,200 and 1,900 rpm.
Upon taking control, the instructor applied additional carburettor heat and increased the throttle, but the engine speed continued to drop, eventually settling between 1,500 and 1,800 rpm. Realising there was insufficient power to reach Westcott airfield, the instructor opted for a forced landing in a nearby field. After avoiding telegraph wires, the aircraft touched down successfully. However, due to damp grass reducing braking effectiveness, the aircraft struck a hedge and a hidden ditch, resulting in damage to the nose and wings. There were no injuries to the two occupants.
The investigation
The investigation involved an examination of the engine and a review of the flying school's maintenance records. The engine's fuel, oil, and ignition systems were inspected, and cylinder compression ratios were found to be good. The carburettor was also tested at a specialist facility, which revealed no operational anomalies.
Meteorological data showed a surface temperature of 12°C and a dewpoint of 5°C. Analysis of these figures indicated that atmospheric conditions were highly conducive to the formation of serious carburettor icing at any power setting.
Findings
- The primary cause of the engine power loss could not be definitively identified, though insufficient or inconsistent fuel flow was considered a likely factor by the flying school.
- The possibility of carburettor icing could not be ruled out, given the weather conditions and the fact that the student pilot had not been applying carburettor heat at the recommended intervals.
- The successful forced landing was attributed to the instructor's training and ability to identify a suitable landing site.
- The subsequent collision with the hedge and ditch was caused by limited braking effectiveness on the damp grass.