History of the flight
The aircraft, a Piper PA-31-350 Chieftain registered N66886, had been purchased in Seattle, USA, in August 2013 and was being delivered to a customer in Thailand. The intended route was eastward across the continental USA, then across the North Atlantic via Canada, Greenland, Iceland, Scotland, and France. In late December 2013, the flight was abandoned at Sondrstrom, Greenland, after low oil pressure indications on both engines, possibly due to the use of incorrect engine oil for cold weather. The aircraft remained there until 28 February 2014, when a replacement ferry pilot was engaged. No oil change or maintenance was performed in Greenland before departure. The flight then continued uneventfully to Wick, Scotland, arriving on 3 March 2014. At Wick, the right engine was noted to be running roughly; investigation revealed low compression on No 4 cylinder. A replacement cylinder set was fitted and certified on 27 March 2014.
On 9 April 2014, the ferry pilot arrived and departed Wick at 14:50 local time for Le Touquet, France. About 25 minutes after takeoff, while at Flight Level 090 routing from Aberdeen to the St Abbs Head VOR, the pilot noted falling manifold pressure and fuel flow on the right engine. He attempted restart procedures without success. He increased power on the left engine and informed Aberdeen Air Traffic Control of the problem, requesting a diversion. While altering course towards Aberdeen, the left engine also lost power. Although landing clearance was received, it became apparent the aircraft could not reach Aberdeen. The pilot opted to land in a ploughed field. Shortly before touchdown, both engines failed completely. The pilot reduced airspeed, landed heavily, and the aircraft slid briefly before stopping. There was no fire, and the pilot was uninjured.
Examination of the engines
Both engines were removed and disassembled under AAIB supervision. The left engine had very little oil remaining (approximately a quarter of a litre); oil had likely escaped when the engine was placed upside down without the dipstick. The oil filler flap was open and the dipstick missing. The propeller was not feathered. Disassembly revealed severe heat damage to the No 6 piston (rear left), with burning and melting around the circumference, extending to the skirt and rings. The piston ring gaps were aligned, contrary to normal practice, and the ring gaps had widened to 0.5–1.0 inches due to erosion. This damage indicated blow-by, where combustion gases bypass the piston into the crankcase. The No 5 connecting rod had extruded bearing shell material, and the No 5 big end bearing was partially seized. The No 1 cylinder had a loose oil jet nozzle that had jammed against the cylinder skirt, damaging the big end bearing cap. No significant defects were found in other cylinders, and combustion deposits were normal. The No 1 and No 6 cylinders were PMA (Parts Manufacturer Approval) components, not original Lycoming.
The right engine was difficult to turn, suggesting partial seizure or crankshaft distortion from impact. Oil level was low, with 3.5 litres drained from the sump. The propeller was not feathered. The No 3 piston had similar blow-by damage, with the piston crown less severely damaged but the skirt in worse condition. The rings had broken into segments, likely due to piston damage. The remaining cylinders, including the replaced No 4 cylinder at Wick, showed no additional defects. The No 3 cylinder was a PMA component; the rest were Lycoming. Combustion deposits in unaffected cylinders were normal.
Oil analysis
Oil samples from both engines were sent for detailed analysis.