WPR11LA391

During a training flight, the airplane's occupants heard a hammering sound accompanied by an airframe vibration and reduction in engine power. The certified flight instructor took control of the airplane from the student and began to initiate a return to the airport. He decided that it was unlikely that they would be able to safely return, and he elected to perform a forced landing into a field. The airplane landed hard. Examination of the engine revealed that the exhaust valve head most likely separated from its stem during flight, resulting in catastrophic failure to its associated cylinder components. Discoloration within the rocker area, exhaust valve guide, and cooling fins was consistent with heat damage, most likely caused by a stuck or sticking exhaust valve. The engine was manufactured in 1948, and its serial number did not match any records on file with the manufacturer. The maintenance records did not reveal a complete history of the engine's total flight time.

HISTORY OF FLIGHT On August 17, 2011, about 1845 Pacific daylight time, a Luscombe 8A, N1444K, force landed into a field after a loss of engine power near Cowiche, Washington. Yakima Aerosport LLC operated the airplane under the provisions of Title 14 Code of Federal Regulations Part 91, as an instructional flight. The certified flight instructor (CFI) and student pilot were not injured; the airplane sustained substantial damage during the accident sequence. The local flight departed Yakima Air Terminal/McAllister Field, Yakima, Washington, about 1800. Visual meteorological conditions prevailed, and no flight plan had been filed. The CFI reported that they departed Yakima with the intention of performing routine flight training. During performance of ground reference maneuvers, the CFI heard a hammering sound, accompanied by an airframe vibration. He took control of the airplane from the student, and began to initiate a return to Yakima. He stated that the engine continued to operate, but at reduced power. He decided that it was unlikely that they would be able to safely return to Yakima, and as such, elected to perform a forced landing into a crop field. The airplane landed hard, collapsing the main landing gear, and causing substantial damage to the fuselage. The engine was examined at the accident site by an inspector from the Federal Aviation Administration (FAA). The upper chamber of the number three cylinder exhibited a circumferential crack, which passed through both spark plug bores, separating the upper portion of the head from the engine. Subsequent disassembly revealed that both the exhaust and intake valve heads had separated from their stems, with one of the valves remaining loose within the cylinder chamber. The valve head had sustained crush and bending damage. The remaining valve was not recovered and is assumed to have been ejected from the cylinder during flight. The piston crown and inner cylinder head had become obliterated, and exhibited multiple semicircular, valve-shaped indentations to their surfaces. The damage sustained to the valve stems and heads prevented an examination of their separation surfaces. Examination of the rocker area revealed black deposits and dark discoloration of the exhaust valve guide and surrounding casting. The external cooling fins displayed green-colored discoloration around the area of exhaust valve. According to the FAA inspector, the engine was of the C85-12 type, and manufactured in 1948. The engine data tag revealed a serial number of 30376, and a representative from Teledyne Continental Engines reported that no such engine number existed in their records. Maintenance logbooks revealed that the engine was installed on the airplane in December 2010, about 40 flight hours prior to the accident. The logbooks recorded the total time of the engine at the time of the installation as, "unknown". The engine had undergone an oil change 29 flight hours prior to the accident. Teledyne ContinentalMotors Service Bulletin M77-3 addresses the various grades of fuel approved for aircraft engines, and the potential for sticking valves when utilizing 100 low-lead aviation fuel in older series engines. The SB states that 100 low-lead is an acceptable alternative fuel for the C85 series engine, and goes on to indicate that when using this fuel, exhaust valve sticking can result from lead salt accumulation in the lubricating oil. Under such circumstances, an exhaust leak between the exhaust elbow flange and the exhaust port face is possible, resulting in localized cylinder head overheating and subsequent exhaust valve and guide distress. The SB further states that this condition can be reduced with regular oil changes, and by the replacement of the valves and guides with units which are more tolerant of lead contamination.