Deadhorse, AK, USA
N206J
CESSNA TU206
The commercial pilot was conducting an on-demand air taxi commuter flight. The pilot reported that, during cruise flight, the engine began vibrating violently and lost power and that the throttle and propeller controls were unresponsive. The cockpit subsequently filled with smoke. The pilot conducted a forced landing to the nearest and driest tundra patch, during which the airplane nosed over. Examination of the engine revealed a large hole on the top left side of the crankcase near the No. 2 cylinder. The No. 2 connecting rod had separated from the crankshaft connecting rod journal. The No. 2 main bearing support mating surfaces exhibited fretting damage. The No. 2 main bearing had shed material from its forward side and had shifted. Measurements of the breakaway torque on the cylinder through bolts indicated that the torque on the Nos. 2 and 3 cylinder through bolts was below the engine manufacturer's specification. It is likely that the insufficient torque on the Nos. 2 and 3 cylinder through bolts resulted in the fretting damage and the No. 2 main bearing shifting and interrupted the oil flow to the No. 2 connecting rod bearing. The interruption of oil flow resulted in the bearing becoming starved of oil and overheating and the subsequent separation of the connecting rod from the crankshaft. A review of maintenance records revealed that the No. 1 cylinder was replaced about 403 flight hours before the accident, which would have required the removal and installation of hardware on the Nos. 1, 2, and 3 cylinder through bolts. It is likely that insufficient torque was applied to the through bolts during the replacement of the No. 1 cylinder.
***This report was modified on April 12, 2016. Please see the docket for this accident to view the original report.*** On September 22, 2014 at 1018 Alaska daylight time, a Cessna TU206, N206J, sustained substantial damage following a loss of engine power and subsequent emergency landing, near Deadhorse, Alaska. The airplane was registered to and operated by 70 North LLC, under the provisions of Title 14 Code of Federal Regulations Part 135 as an on-demand air taxi flight. The commercial pilot and the three passengers were not injured. Visual meteorological conditions prevailed and a company flight plan was filed. The cross-country flight originated from Badami Airport, Alaska at 1006 with a destination of Deadhorse, Alaska. According to the pilot, about 12 minutes after departure, the engine began vibrating violently during a loss of power, and the throttle and propeller controls were unresponsive. Smoke soon filled the cockpit. The pilot executed a forced landing onto the nearest and driest area of tundra. During touchdown onto an area of tundra-covered terrain, the airplane nosed over, sustaining substantial damage to the fuselage, wings, and empennage. Passengers reported that during cruise flight about 800 feet above ground level they heard a loud bang followed by smoke in the cabin area. The engine began to run rough and then suddenly quit. The pilot called out a mayday and initiated a forced landing. One passenger stated that during the landing sequence the airplane's tail touched down first and shortly after the airplane nosed over. Examination of the airplane by a local mechanic revealed that the wings and fuselage were buckled and the vertical stabilizer and rudder were crushed near the top. Oil was observed on the lower side of the fuselage. The wreckage was recovered to a secure location for further examination. The engine was removed from the airframe and sent to the facilities of Continental Motors, Inc. (CMI), Mobile, Alabama, for further examination. The engine was examined on January 14, 2015, by representatives from CMI under the supervision of the NTSB investigator-in-charge (IIC). The examination revealed a large hole on the top left side of the crankcase near the number 2 cylinder. Visual inspection through the hole revealed that the number 2 connecting rod separated from the crankshaft connecting rod journal. The engine was subsequently disassembled and examined. The cylinders were removed from the engine during the examination. The break-away through bolt torque during disassembly indicated that cylinder number 2 and 3 through bolt torque was below CMI specification. The crankcase halves remained secured. The main bearing support mating surfaces for the number 2 bearing revealed fretting signatures. The number 1, 2, 3, and 4 cylinder bays had mechanical damage. The oil galleys on both crankcase halves were free of debris. The number 2 main bearing displayed contamination imbedded in the surface layer and also lubrication distress and displaced babbit material. The number 2 bearing had shed material from its forward side and had shifted on the bearing support such that it covered the oil port that fed the number 2 connecting rod bearing. The bearing lock tab showed lateral movement on the bearing support. The remaining main bearings displayed similar contamination; however there was no sign of lock tab movement or lock slot elongation. The crankshaft remained intact but displayed thermal distress at the number 2 connecting rod journal. The rear set of counterweights were mechanically damaged and were free to move on their hanger blades. The connecting rods were intact and revealed normal operation, with exception to the number 2 connecting rod which sustained thermal and mechanical damage and was separated through the base of the I-beam, separating both yoke sections of the bearing support. The connecting rod cap was bent and had thermal and mechanical damage. The number 1 connecting rod bearing revealed normal operating signatures. The number 3, 4, 5 and 6 connecting rod bearings revealed contamination and hard particle damage. Only fragments of the number 2 connecting rod bearing were found and displayed evidence of thermal discoloration and heat distress associated with a lack of lubrication. According to CMI, the engine was remanufactured on August 3, 2000. Review of the engine maintenance logbooks revealed that the engine was then overhauled and tested on June 9, 2005, and installed on the accident airplane on August 11, 2009. The number 2 cylinder was removed and replaced on August 28, 2012, at 423.3 hours since overhaul. The number 1 cylinder was removed and replaced on September 4, 2013, at 764.1 hours since major overhaul. The most recent maintenance performed on the engine was a 100-hour inspection and an oil and oil filter change, which was conducted on August 19, 2014 at an engine time of 1,071.8 hours since major overhaul. According to the tachometer reading, from the time of the accident to the last recorded oil change was 95.5 hours. The time between the last recorded oil changes were 105.7 and 95.8 hours.
The application of insufficient torque to the engine cylinder through bolts during the replacement of the No. 1 cylinder, which resulted in the displacement of the No. 2 bearing, oil starvation, and a catastrophic engine failure.
Source: NTSB Aviation Accident Database
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