Accident Sequence
On September 1, 2017, at approximately 1711 eastern daylight time, a Cessna U206G airplane, registration N4615X, was substantially damaged during a forced landing near Harpswell, Maine. The pilot, the sole occupant, was not injured. The airplane was operated by Waters Aero-Marine, doing business as Penobscot Island Air, as a positioning flight under Title 14 Code of Federal Regulations Part 91.
The flight had departed Knox County Regional Airport (RKD) in Rockland, Maine, at about 1645, en route to Portland International Jetport (PWM), Portland, Maine, under a company visual flight rules (VFR) flight plan. According to the pilot, while cruising at 2,500 feet approximately 6 nautical miles from the departure end of runway 19L at Brunswick Executive Airport (BXM), Brunswick, Maine, a loud bang was heard, followed by a complete loss of engine power. The pilot immediately broadcast a distress call to air traffic control and declared an intention to land at BXM. He performed emergency procedures to secure the engine and continued with the emergency landing.
Upon realizing the airplane was not within gliding distance of BXM, the pilot transmitted his intention to make an off-airport landing. The airplane bounced, traveled about 150 feet, and touched down again. The nose landing gear then dropped into a small drainage ditch, causing the airplane to nose over and come to rest inverted. Substantial damage resulted to both wings, the aft and forward fuselage, and the nose gear assembly.
Engine Examination
Post-accident examination of the engine revealed that the crankshaft would not rotate. Three cracks were visible on the crankcase: two above the No. 1 cylinder and one between the Nos. 1 and 3 cylinders, extending from the No. 1 cylinder saddle toward the spine on the top right half of the crankcase. Metallic debris was found in the oil filler neck and port in the crankcase. The externally visible portion of the crankshaft was intact and appeared undamaged. All rear accessories remained attached.
Both magnetos produced spark at all terminal leads when tested. The fuel manifold and fuel injection lines were undamaged. The exhaust system showed normal operating signatures with no signs of induction system leaks.
The oil sump was intact and contained multiple metallic pieces and particles consistent with bearing material. The oil pick-up tube and screen were undamaged but contained metallic particles. The oil pump was intact and undamaged; when disassembled, its housing displayed scoring consistent with hard particle passage. The oil filter was secure and undamaged; its filter pleats contained a significant amount of metallic material. The oil cooler had minor impact damage and contained small flakes of metal internally.
All valves and rocker arms were intact and undamaged with normal operating signatures. All six cylinders remained attached. Examination of the cylinders and pistons revealed that the bottom of the No. 1 cylinder, the skirt of the No. 1 piston, and the skirt of the No. 2 piston had damage consistent with contacting portions of the engine’s crankshaft assembly.
The No. 1 main bearings had shifted slightly out of the support and displayed deformation. The No. 2 main bearings showed bearing shift signatures: one was missing, and the other had extruded out of the bearing support. Multiple pieces of the missing bearing were found in the oil sump. The No. 3 main bearings were intact but had moved in a clockwise direction when viewed from the back of the engine, had shifted slightly out of the support, and the oil passage holes were partially blocked. The Nos. 4 and 5 main bearings were intact and undamaged.
The crankshaft was fractured between the No. 2 main journal and the No. 2 connecting rod journal. The No. 2 connecting rod journal displayed incipient signatures of lubrication distress. The remaining journals showed normal lubrication signatures. The counterweights and connecting rods were secured. The connecting rod bearings displayed normal signatures except for the No. 2 connecting rod bearings, which had incipient lubrication distress.
Maintenance History
According to Continental Aerospace Technologies, cylinder removal should not be performed unless a borescope inspection reveals a definite problem. The engine incorporates through-bolts that hold the crankcase halves together; proper installation requires a multiple-step torquing process. Continental warned that failure to torque through-bolt nuts on both sides of the engine can result in a loss of main bearing crush, leading to main bearing shift and subsequent engine failure.
Review of engine maintenance records revealed that all cylinders had been replaced with overhauled cylinders, new pistons, and reconditioned lifters and seals at 1,250 total hours of operation since major overhaul, which was about 500 hours before the accident.