History of the Flight
The pilot was conducting a private cross-country flight from Cumbernauld Airport to Perth. While cruising at 1,700 feet just below cloud base approaching the Ochil Hills, he initiated a climb as the clouds over the hills were higher. During the climb through 2,400 feet, the engine began to run rough and lost power. The pilot applied carburettor heat, confirmed the fuel cock was on, the mixture was set to rich, and the master switch was on. Shortly after, the engine lost all power. He attempted a restart without success and prepared for a forced landing on a hill ridge. During the approach, the engine restarted but ran rough and produced insufficient power to climb. The pilot extended full flap, shut off the fuel, turned off the master switch, and reduced speed towards stall, successfully stalling the aircraft onto the upward slope of a valley. The left wheel touched down first, followed by the nosewheel, and the aircraft flipped upside down within 5 to 8 metres of touchdown. The pilot exited through the window unassisted.
Weather
An aftercast from the Meteorological Office estimated the temperature and dewpoint at ground level as 14°C and 12°C respectively (relative humidity 87%). At 1,700 to 2,400 feet, temperatures were slightly lower and humidity slightly higher. The carburettor induction system icing probability chart in Safety Sense Leaflet 14 of LASORS indicated a serious risk of icing at any power setting for a typical light aircraft piston engine without carburettor hot air selected.
Aircraft Examination
Before recovery, the aircraft was heavily vandalised. The propeller and wheels were taken, and all flight instruments were stripped. No examination of the engine, induction system, or fuel system was carried out by the operator.
Pilot's Assessment
The pilot reported applying carburettor heat as part of cruise checks approximately 5 minutes before the engine ran rough. He believed carburettor ice could still be the cause. He reported that his application of carburettor heat after the engine started to run rough might have dislodged a build-up of ice, causing the engine to ingest water, resulting in sudden engine failure.
Analysis
The AAIB noted that a problem with the engine, induction system, fuel system, or fuel could not be ruled out as potential factors, as no examination was carried out. However, the atmospheric conditions at the time and the symptoms experienced by the pilot indicated that carburettor ice could have been a factor. The AAIB referenced its Safety Recommendation 2004-01 regarding research on carburettor ice and noted that the CAA had conducted research, with a report pending.