Accident Summary
On 2 August 2015, at approximately 1530 UTC, a Flight Design CTSW, registration G-CETF, experienced a sudden engine power loss while on final approach to Boston Aerodrome, Lincolnshire. The pilot was forced to land the aircraft in a field of unharvested corn short of the runway threshold. During the landing, the aircraft nosed over and came to rest inverted. Both occupants—the pilot and one passenger—escaped unaided and were uninjured. The aircraft sustained damage to the landing gear, propeller, engine covers, tailplane, rudder, and extensive airframe damage.
Flight Details
The flight had been an uneventful trip to Cromer, Norfolk, and was returning to Boston Aerodrome. The pilot joined overhead at 2,000 ft, descended on the 'dead side' of the airfield, and joined the circuit at 1,000 ft agl for Runway 09. On the downwind leg, he checked engine temperatures and pressures and noted fuel visible in the sight tubes in the wing roots (there are no conventional fuel gauges on this type). He selected 30° flap on turning onto base leg. After turning onto final approach, the aircraft experienced sink, so the pilot applied more power. As he did so, the engine suddenly stopped. He attempted a restart but was unsuccessful. With insufficient height to glide to the runway, he landed in the field.
Fuel System Description
The CTSW is a high-wing aircraft with integral fuel tanks in the inboard sections of the wings. Each tank has a capacity of 65 litres, and a circular plug in each wing root incorporates a fuel contents sight tube. The wings have a 1.5° dihedral angle, which in balanced flight tends to keep fuel towards the inboard ends. According to the manufacturer's Aircraft Operating Manual, unusable fuel is 3 litres per tank; fuel consumption is 25 litres/hour at maximum continuous power and 18.5 litres/hour at 75% power. The fuel off-take is located close to the sight tube, towards the rear of the tank, and consists of an inlet strainer attached to a short rigid tube that holds the strainer close to the tank floor—there is no sump. The outlets of the left and right tanks are joined in the fuselage, immediately upstream of a simple on/off fuel selector. An 'on' selection draws fuel simultaneously from both tanks. This arrangement, applicable only to UK-registered aircraft, superseded an earlier design with selectable left or right tank. The modification was implemented via Service Bulletin CT125 and mandated by the UK CAA.
Investigation Findings
The pilot reported that, with assistance, the aircraft was returned onto its wheels, though damage to the right main landing gear resulted in a right-wing-low attitude. The aircraft was recovered a couple of days later, by which time virtually all fuel had transferred into the right tank. The pilot drained 22 litres of fuel, all but a couple of litres from the right tank. He was confident no fuel escaped while the aircraft was inverted. Examination by an insurance loss adjustor and a potential repair organisation found no fault that could explain the power loss; no mechanical failure occurred within the engine. The pilot commented that, in his experience, the left tank always emptied before the right, consistent with other pilots' reports of fuel imbalances. His overhead join and circuit entry involved five left turns, and the power loss occurred after he applied a little right bank while aligning with the runway. He considered that one of the fuel off-takes may have become temporarily uncovered, ingesting air. Flap selection tends to pitch the nose down, moving fuel forward away from the off-takes. During downwind checks, fuel was visible in both sight tubes, but the distribution of the approximately 22 litres at the time of the incident could not be determined. Carburettor icing was considered, but other aircraft of the same type operated that day without apparent problems. From the available information, it was not possible to conclude the cause of the engine power loss, though fuel starvation was a possible cause.