Lopez Island, WA, USA
N2939J
Piper PA-28-181
While en route to the destination airport, the pilot heard the engine generate a loud "bang," as though something had shattered internally. The engine subsequently experienced a total loss of power. The pilot maneuvered the airplane to a pasture where the airplane collided with a fence during the landing roll. According to the engine logbooks, the last engine annual inspection had been performed about 6 months and 124 hours prior to the accident. The left magneto had been replaced about 3 months and 54 hours prior to the accident. A teardown inspection of the engine disclosed that the left idler gear was damaged with several teeth missing and one tooth wedged between the forward face of the gear and crankcase. A detailed metallurgical examination of the left idler gear revealed that it failed due to fatigue. Fatigue features were observed at two locations, and both had initiated in the radius between a gear tooth flank and the root. The gears that were engaged with the idler gear were intact and mostly undamaged. The fracture surface at one fatigue location on the left idler gear was much more reflective than the other. A reflective surface, indicative of a rubbing motion of one fracture surface against another, indicates a misalignment of the meshing gears to produce the motion. The fracture surface at the other location was duller, indicating that little or no rubbing occurred, consistent with the fracture occurring after the first fracture with the rubbed surface. The second fracture propagated until the material below the gear teeth could no longer support the stresses of operation and the portion of the gear located between the two fatigue fractures departed the left idler gear. The fatigue was probably initiated by abnormal contact between the left idler gear and the other mating gears, with the relatively recent installation of the left magneto gear being suspect as the likely source of the initiating stressors.
HISTORY OF FLIGHT On August 24, 2007, about 1730 Pacific daylight time, a Piper PA-28-181, N2939J, experienced a loss of engine power and collided with a fence during a forced landing on Lopez Island, Washington. The pilot was operating the airplane under the provisions of 14 Code of Federal Regulations Part 91. The airline transport pilot and passenger were not injured; the airplane sustained substantial damage. The personal flight departed Snohomish County Airport (Paine Field), Everett, Washington, about 1710, with a planned destination of Nanaimo Airport, Vancouver Island, Canada. Visual meteorological conditions prevailed, and a visual flight rules (VFR) flight plan had been filed and activated. In a written statement, the pilot reported that while en route, about 20 minutes after departure, the airplane was at an altitude of 4,500 feet. He heard the engine generate a loud "bang," as though something had shattered internally. The engine experienced a total loss of power as the airplane was over the ocean and adjacent to Lopez Island. The pilot maneuvered the airplane to a cow pasture on the south end of the island that contained grass about 3 feet in height. During the landing roll, the pilot noticed a wire fence ahead and attempted to veer in between two posts. The airplane collided with the fence, sustaining damage to the wings. AIRPLANE INFORMATION A Lycoming O-360-A4M engine, serial number L-27011-36A, was the original Piper factory installed engine in the airframe and had accumulated the same total time in service as the airframe. According to logbooks provided by the pilot, the engine had a reported total operating time of 3,326 hours, of which 1,271 hours were accumulated since a field overhaul, and 124 hours since the last inspection. The logbook reflected that the engine had been overhauled in 1996, and that the gears in the accessory casing had been inspected at that time. The last engine annual inspection had been performed in February 2007, at 1,130.8 hours since the field overhaul; the left magneto had been replaced in May 2007, at 1,217.7 hours since the field overhaul. There was no record of any gears being replaced. The engine was engineered for the crankshaft gear to drive the left and right idler gears. Thereafter, the left idler gear drives the camshaft gear, the left magneto gear, and the fuel pump idler gear. A cam on the rear face of the left idler gear drives a rod that drives a diaphragm type fuel pump. The gear train diagram from the Lycoming Overhaul manual illustrating the gear positions is contained in the public docket for this accident. TESTS AND RESEARCH A National Transportation Safety Board investigator performed a teardown inspection of the engine. The engine's crankshaft was rotated by means of the investigator turning the propeller. The internal mechanical continuity was established during rotation of the crankshaft and upon attainment of thumb compression in each cylinder. Removal of the accessory housing disclosed that the left crankshaft idler gear was damaged with several teeth missing and one tooth wedged between the forward face of the gear and crankcase. The idler gear would not rotate when the propeller was turned. The drive key additionally did not rotate and the key cable was examined with no anomalies found. Metal remnants were found in the oil pan, consistent in size and shape with the missing gear teeth. The crankshaft gear, the camshaft gear and the left magneto gear were relatively undamaged. The left idler gear and tooth pieces were sent to the Safety Board Materials Laboratory for a detailed examination. Examination of the left idler gear revealed the absence of two gear teeth and a portion of the gear disc that originally contained those gear teeth. Circumferentially oriented smear marks were observed on the gear face, consistent with a piece of tooth being trapped between the gear and the adjacent engine casing. Further examination of the idler gear revealed fracture faces that were relatively smooth and displayed arced crack arrest marks, consistent with fatigue progression. Ratchet marks were observed with slight vertical steps in the fracture that linked slightly offset planes of fatigue cracking at a fatigue origin area. The fatigue initiation areas were located in the root radius, adjacent to the tooth flank. The examination additionally revealed that a portion of the gear had been deformed. The mechanical damage to the gear teeth were gradually reduced, from left to right, counter to the direction of rotation; the remaining gear teeth were mostly intact. The surfaces of the gear teeth flanks had been smeared, consistent with operation during the failure process. Corrosion was observed on a gear tooth, with corrosion pits on the flat side of the tooth. A corrosion product was additionally observed in the radius between the flank and the root. Examination of the six gear pieces found in the sump revealed that the two largest pieces were severely damaged the full width of the gear teeth. The smaller of the two pieces consisted of two flanks and a crest. The larger of the two pieces consisted of a root radius, two flanks, a crest, and a partial root radius. Examination of the gear piece found trapped between the left idler gear and the crankcase revealed that it was severely damaged, and consisted of a full width gear tooth with a portion of the gear below it. The complete examination report is contained in the public docket for this accident.
A loss of engine power due to the fatigue failure of the left idler gear. Inadequate maintenance installation of the recently installed left magneto was also causal.
Source: NTSB Aviation Accident Database
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