Summary
On 19 April 2018 at 1338 UTC, a Cessna F172H Skyhawk (registration G-AWUX) was involved in an accident 3 miles southwest of Perranporth airfield, Cornwall. The private flight carried only the pilot, who suffered minor injuries. The aircraft sustained damage to the propeller, nosewheel, cowling, left wing strut, and tailplane.
History of the Flight
The pilot departed from Perranporth Airfield after completing power checks. He climbed to 2,200 ft and headed west in clear air and visual meteorological conditions. After reaching the coast north of St Agnes, he tracked southbound along the coastline before deciding to return to Perranporth to avoid an incoming cloud layer. He selected idle power and began a descending left turn onto a south-easterly heading.
At 1,200 ft (QFE), the pilot applied full power. He reported a sudden explosive bang accompanied by heavy, violent vibration and shuddering. Reducing power to idle eased the vibration, but it continued at a reduced yet severe level. Reapplying power intensified the vibration, making instruments difficult to read and the yoke hard to hold. The pilot selected idle power again, made a MAYDAY call, and prepared for a forced landing. He initially aimed for a long narrow grass field, but at approximately 50 ft he noticed stone hedges across its width and veered left toward a harrowed field. Upon touchdown, the wheels dug in, the nosewheel was torn off, and the aircraft nosed over onto its back. The pilot exited via the passenger door.
Engine Examination
The aircraft was powered by a Continental Motors Corp O-300-D six-cylinder piston engine with a carburettor. A maintenance engineer inspected the engine at the accident site. Apart from the carburettor having broken off, there was no external damage. The oil level was within the normal range, and there were no leaks. The propeller was turned by hand, revealing six compressions, and the starter motor engaged with normal rotation. The engineer noted that the O-300 series engines were prone to carburettor ice due to the carburettor’s narrow throat and high gas-speed induction system. An insurance loss adjustor later examined the engine after the aircraft was moved. He confirmed the oil level and normal starter rotation without abnormal sounds or mechanical interference. The exhaust and manifold were firmly attached. All six upper spark plugs exhibited a layer of dark soot, indicating an over-rich running condition; the adjustor believed this was short-term as the electrode insulators were not fully covered.
Meteorological Conditions
Approximately 10 minutes after the accident, the temperature and dewpoint at Newquay (8 nm northeast of Perranporth) were 14°C and 11°C, respectively. An upper air sounding for Camborne (11 nm southwest) showed at 2,000 ft a temperature of 9.8°C and dewpoint of 9.2°C, and at 1,000 ft, 11°C and 10.5°C. These conditions placed the risk of carburettor icing as 'serious icing – any power setting' according to CAA Safety Sense Leaflet No 14.
Analysis
The engine examinations did not reveal any mechanical defects that would explain the reported heavy vibration. The meteorological conditions were conducive to carburettor icing, and the engine type was noted to be prone to such ice. The pilot could not recall whether he selected carburettor heat before the descent. If carburettor heat was not applied, carburettor ice was more likely to form, leading to airflow restriction and a rich mixture – evidence of which was seen on the spark plugs. Although severe vibration is not typical of carburettor ice, the six-cylinder engine might exhibit different symptoms. The sudden bang could have been caused by detonation in the exhaust system from incomplete combustion due to an over-rich mixture. Alternatively, a fault possibly related to the valves could not be ruled out without a full engine teardown. The cause of the engine problem could not be determined.