PORTAGE, AK, USA
N4672U
CESSNA 206
DURING THE CRUISE CLIMB, THE ENGINE BEGAN TO EMIT METALLIC SOUNDS AND LOOSE POWER. A SUCCESSFUL OFF AIRPORT EMERGENCY LANDING WAS PERFORMED. DISASSEMBLY OF THE ENGINE REVEALED THAT THE NO. 2 CYLINDER CONNECTING ROD BEARING HAD SHIFTED WITHIN ITS JOURNAL RACE WHICH PRODUCED AN OIL STARVATION/THERMAL DISTRESS EVENT. THE CAUSE OF THE BEARING SHIFT COULD NOT BE DETERMINED.
On June 13, 1993, at 1010 Alaska daylight time, a float equipped Cessna 206 airplane, N4672U, owned and operated by Jim Air of Anchorage, Alaska, experienced a loss of engine power during an enroute cruise climb. The commercially certificated pilot and the two passengers, the sole occupants, were not injured and the only damage sustained to the airplane was to the powerplant. At the time of the mishap, the commercial revenue flight was being conducted under the on-demand requirements contained in 14 CFR Part 135. The flight departed the Lake Hood Seaplane Airport at approximately 0950 and was enroute to Prince William Sound when the incident occurred. Visual meteorological conditions prevailed and a VFR flight plan was on file with the Anchorage Flight Service Station. The pilot in command told the NTSB investigator in charge that prior to commencing the flight, he preflighted the airplane and noted that the engine crankcase contained 11 quarts of oil. The pretakeoff engine runup, which included a check of the magneto system and the cycling of the propeller, indicated that all engine monitoring instrumentation was in the normal range. After takeoff, the engine rpm and manifold pressure controls were reduced to 2,400 and 24 inches, respectively, for the cruise climb. At approximately 3,800 feet, the pilot thought that he detected a slight vibration in the engine. He checked the engine gauges and determined that they were all as they should be. About 20 to 30 seconds later, the engine began to emit a metallic sound and loose power. The pilot set up for best rate of glide and landed the airplane on a small lake by a highway. A preliminary examination of the engine, a Teledyne Continental Motors Model IO 520F, S/N 286240R, by the NTSB investigator in charge on June 15, 1993, revealed a failure of the No. 2 cylinder connecting rod assembly. The crankshaft end of the No. 2 cylinder connecting rod - including the rod cap in two pieces, both bolts (fractured), and the fractured lower ends of the rod body (the bearing straps) were forwarded to the NTSB's Materials Laboratory Division in Washington, D.C. for examination. Optical examinations established that both bolt fractures and the rod cap fracture were bending overstress separations with no indications of preexisting cracking. The cap and rod pieces were darkly discolored indicative of exposure to elevated temperatures. The rod cap pieces also evidenced a considerable amount of post fracture mechanical damage. The bearing surfaces of the strap pieces displayed some light to moderate rotational scratching, light circumferential fretting and had areas of galling and transferred metal. Further details of the examination are contained in the Metallurgist's Factual Report No. 94-07 (See Attachment). The engine assembly was shipped to Teledyne Continental Motors for examination under the direction of an FAA Aviation Safety Inspector/Mfg. Disassembly of the engine revealed the number two connecting rod bearing had suffered an oil starvation/thermal distress event. Damage to the number two main bearing and its support structure prevented any meaningful dimensional inspection. The cause of the bearing shift could not be determined. Further details of the examination are contain in the FAA inspectors notes on the examination (See Attachment).
THE FAILURE OF THE NUMBER TWO CYLINDER CONNECTING ROD BEARING DUE TO THE SHIFTING OF THE BEARING WITHIN ITS JOURNAL RACE WHICH PRODUCED AN OIL STARVATION/THERMAL DISTRESS EVENT.
Source: NTSB Aviation Accident Database
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