Accident Summary
On April 11, 2014, at about 1800 eastern daylight time, a Cessna U206G, registration N156SA, was forced to land following a total loss of engine power near Louisa, Virginia. The commercial pilot sustained serious injuries, and one passenger received minor injuries. The airplane was substantially damaged. The flight was operated by Laser Mapping Specialists, Inc. under 14 Code of Federal Regulations Part 91 as an aerial laser mapping flight. Day visual meteorological conditions prevailed, and no flight plan was filed. The flight originated from Louisa County Airport (LKU) about 1730.
Pilot Actions
The pilot reported that during a prior flight, engine oil pressure fluctuated within the normal range. He changed the oil and found "a great deal of chrome" in the oil filter. Using a magnet, he determined the particles were non-ferrous. Four cylinders had been replaced in December 2013, which he attributed the particles to break-in. Consulting the pilot's operating handbook, he believed the oil pressure fluctuations were due to a gauge problem or clogged pressure relief valve and elected to continue. On the accident flight, about 30-40 minutes after departure, the engine made a strange sound and quit about 20 seconds later. The pilot force-landed in an open field; the airplane landed hard and sustained structural damage.
Engine Examination and Findings
Post-accident, the engine was removed and shipped to the manufacturer for examination. Before disassembly, rotating the propeller flange revealed that internal continuity to the rear accessory pad was not established, and the crankshaft could only rotate through 90 degrees. Disassembly showed that breakaway torque values of through bolts at cylinder flanges varied; only those associated with the number 6 cylinder met the manufacturer's recommended torque. For cylinders 1-5, top through bolt torque values in the loosen direction ranged from 534 to 705 inch-pounds (recommended 790-810), and bottom through bolts from 458 to 675 inch-pounds (recommended 790-810). The oil pan and oil cooler contained metal particulates. Substantial internal damage was found around the number 2 main bearing, with the crankshaft fractured in that area. According to Continental Motors personnel, a crankshaft failure associated with a bearing shift typically fails in fatigue, and this failure exhibited fatigue signatures. Adjacent areas showed scoring on the engine case, and main bearings 1 and 3 also showed evidence of bearing movement before failure.