What happened
On 19 April 2018, a Cessna F172H Skyhawk, registration G-AWUX, was conducting a private local flight departing from Perranporth Airfield. After reaching an altitude of 2,200 ft, the pilot began a descent at idle power to avoid approaching cloud layers. Upon reaching 1,200 ft, the pilot attempted to level off by applying full power, at which point an explosive bang and intense, violent vibrations occurred.
As the pilot attempted to manage the power settings, the vibration persisted and worsened with increased power, making it difficult to maintain control of the aircraft or read instruments. The pilot declared a MAYDAY and prepared for a forced landing. While initially aiming for a long grass field, the pilot identified stone hedges obstructing the path at low altitude and diverted to a nearby harrowed field. Upon landing, the aircraft's wheels dug into the ground, causing the nosewheel to detach and the aircraft to flip onto its back. The 1 occupant sustained minor injuries.
The investigation
Investigators examined the Continental O-300-D engine at the accident site and later via an insurance loss adjustor. The engine showed no obvious external mechanical defects, and the oil level was normal. While the carburettor had broken off, the engine turned over normally during testing. An examination of the spark plugs revealed a layer of dark soot, suggesting the engine had been running in an over-rich condition.
Meteorological data from the region indicated that temperature and dewpoint levels were close enough to create conditions highly conducive to carburettor icing. A maintenance engineer noted that this specific engine series is particularly susceptible to ice formation due to its induction system design.
Findings
- The primary cause of the engine malfunction could not be definitively determined through external examination.
- Carburettor icing is considered a likely factor, as the atmospheric conditions were suitable for ice formation and the engine was found to have been running rich.
- The sudden bang and vibration may have been the result of detonation in the exhaust system caused by incomplete combustion.
- The pilot's decision to divert to a harrowed field led to the aircraft flipping during the landing sequence.