STEVENSVILLE, MD, USA
N515TN
PITTS S-1S
THE AIRPLANE'S ENGINE LOST TOTAL POWER JUST AFTER TAKEOFF WHEN THE PILOT REDUCED THE THROTTLE. THE PILOT RE-APPLIED FULL THROTTLE WITH NO RESULT. DURING THE FORCED LANDING IN A FIELD, THE AIRPLANE'S WING STRUCK A TREE, AND THE AIRPLANE NOSED OVER. EXAMINATION OF THE ENGINE REVEALED THAT THE CRANKSHAFT GEAR (13S19646) BECAME DISCONNECTED FROM THE CRANKSHAFT DUE TO A BROKEN BOLT. METALLURGICAL EXAMINATION OF THE ACCESSORY GEAR RETAINING BOLT (P/N STD-2213), AND DOWEL PIN (P/N STD-1065) REVEALED THAT THEY SEPARATED AS A RESULT OF HIGH CYCLE FATIGUE MECHANISM, MOST LIKELY CAUSED BY INSUFFICIENT TORQUE OF THE BOLT.
On July 15, 1994, at 1445 eastern daylight time, a Pitts S-15, N515TN, registered to and piloted by Teresa Nagy, sustained substantial damage when it impacted the terrain at the Kentmore Airport, Stevensville, Maryland. The pilot received minor injuries. Visual meteorological conditions prevailed, and no flight plan was filed. The flight was operated under 14 CFR Part 91. The airplane's engine lost partial power just after takeoff. The pilot was attempting to return to the airport, when the engine lost total power. The pilot made a forced landing in a field, the airplane's wing struck a tree, and the airplane nosed over. According to the pilot's statement on the NTSB Form 6120.1/2: I applied power and began climb at 100 IAS...at approximately 500 feet I began to gently reduce power...just prior to the power reduction I thought I heard a "surge" but no vibration or any indication of problem. When I reduced the power the engine stopped, I immediately re applied full throttle-no result.... The engine was removed from the airplane, and retained for further examination. On October 6, 1994, the engine from N515TN, was disassembled under the supervision of NTSB Investigator, Margaret Napolitan, at Lycoming Engine's facilitates, Williamsport, Pennsylvania. The engine disassembly revealed that engine continuity could not be established as result of the crankshaft gear (13S19646) becoming disconnected from the crankshaft. The accessory gear retaining bolt (STD-2213) was fractured, and a crack was observed in the root of the adjacent thread. According to Lycoming, "...this is consistent with overtorqueing of the bolt." The head of the bolt was worn from coming in contact with the oil pump drive after it was fractured and while it was encapsulated in the crankshaft gear counterbore. The lockplate (LW-18639) was worn, a portion of it was missing, and was not recovered. The dowel (STD-1065), used to position the gear for internal timing, was fractured. The fracture surface of the dowel displayed crack arrest lines (beach marks). The engine disassembly also revealed that the connecting rods were not torqued to the specified 40 foot pounds. Several bolts used in various locations were not the correct parts, and safety wire was not used were required. The following parts to include, two pieces of the accessory gear retaining bolt, P/N STS-2213; the separated dowel pin. P/N STD-1065; and lockplate, P/N STD-1065, were sent to the NTSB Materials Laboratory, Washington, DC. Examination of the accessory gear retaining bolt, revealed that the fracture on the head portion of the bolt and the bolt identification markings were completely obliterated by post separation mechanical damage. The fracture face on the threaded portion of the bolt, revealed features typical of fatigue cracking over approximately 90 percent of the bolt cross section. A secondary crack was found located in the root of the third tread from the fracture surface. The examination of the bolt revealed no material defects at the fracture origin. Material of the bolt and its hardness were found to be within the specified requirements. Examination of the dowel pin revealed that most of the fracture contained well defined crack arrest positions consistent with a fatigue fracture mechanism. The fracture features in the final fracture zone were typical of overstress separations. A band of fretting wear was found in the area adjacent to the fracture surface.
Inadequate maintenance by which undertorquing of the accessory gear retaining bolt resulted in fatigue failure of the bolt and the subsequent loss of engine power.
Source: NTSB Aviation Accident Database
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