Introduction
On 20 August 2006 at 1140 UTC, a Reims Cessna F172N Skyhawk, registration G-DENR, was involved in an accident at Lower Widdon Farm near Ashburton, Devon. The aircraft was on a private Visual Flight Rules (VFR) flight from Bournemouth, Dorset, to Truro, Cornwall. On board were one pilot and two passengers; none were injured. The aircraft sustained severe damage and was deemed beyond economic repair.
Flight History
The aircraft departed Bournemouth at approximately 1000 hrs. Before departure, the owner (not the pilot) filled the right fuel tank to FULL and the left tank to two gallons short of FULL, checking quantities visually. The owner advised the pilot to take off with the fuel selector on LEFT and to select BOTH after takeoff. The pilot complied and selected BOTH during a routine in-flight check over Dorchester, 23 track miles from Bournemouth. He also applied precautionary carburettor heat.
As the aircraft approached Lyme Regis, the pilot contacted Exeter radar and requested a Flight Information Service. He reported overhead Exmouth at 2,000 ft amsl, having flown a further 38 track miles. A second routine in-flight check was performed, including carburettor heat, before commencing a cruise climb to 3,300 ft amsl at 95 kt and 2,300 rpm.
While climbing through 2,800 ft amsl, the engine began to run roughly and the rpm reduced slightly. The pilot selected carburettor heat ON with no initial change. He reduced indicated airspeed to the best glide speed of 65 kt. The engine rpm then fluctuated and stopped. The aircraft was 15 nm south-west of Exmouth. The pilot carried out the Engine Failure checklist from memory and declared an emergency to Exeter ATC.
With the engine not restarting, the pilot focused on a forced landing. The aircraft was on the eastern edge of Dartmoor, where ground elevation ranges from approximately 500 to 1,000 ft amsl. The pilot selected a field, positioned the aircraft downwind, and turned onto a base leg. Realising he had turned too early, he crossed the first field and a tree line to land in the next field. The elevator clipped the top of the trees, and the left wing tip contacted the ground before the aircraft landed heavily on a down slope. The pilot made the aircraft safe, and the occupants vacated uninjured.
Fuel System
The Cessna 172 fuel system includes a fuel selector on the floor below the instrument panel, accessible to both pilots. The selector can feed the engine from the left tank, right tank, or both tanks. To turn fuel off, the pointer is turned directly aft. There is no safeguard on the OFF position. For normal operations, the selector should be in BOTH. A placard required BOTH for takeoff, climb, and landing.
The fuel strainer bowl has a volume of 0.196 litre. If fuel supply is shut off, the bowl's design prevents its contents from being drawn into the carburettor, and the engine will run only as long as fuel in the carburettor float bowl lasts, predicted to be no longer than 20 seconds at cruise power.
The pilot was fully conversant with the fuel selector operation and had recently demonstrated it during a skills test.
Weather
The Met Office provided an aftercast for the accident time. A weak decaying frontal system over the British Isles and a relatively moist westerly flow covered south-west England. The METAR for Exeter, issued 10 minutes before the accident, indicated surface wind from 260º at 12 kt, visibility more than 10 km, scattered cloud at 1,400 ft and broken cloud at 4,000 ft above airfield level. Surface temperature was 18ºC and dew point was 13ºC.
Carburettor icing conditions were assessed using the reported temperature and dew point. The chart indicated Moderate Icing in the cruise power area, close to the 80% humidity line. Given the aircraft was near cloud base, relative humidity likely approached 100%, increasing icing risk. However, the pilot's routine in-flight checks should have cleared any light icing, making carburettor icing an unlikely cause of the engine failure.
Discussion
The pilot stated his recollections of actions before and during the engine failure were imprecise. He considered the possibility that he misidentified the fuel selector position and turned it from BOTH to OFF, though he did not recall doing so. Given the fuel selector design and the pilot's experience (107 hours total, 31 on type, with only 0.5 hours in the last 28 days), such an error could be conceivable. The engine failure might be attributable to a selection error, carburettor icing, or some other unidentified problem. The engine was not inspected due to the severity of the damage, so no mechanical failure could be established.