DAVIS, CA, USA
N91RT
Trickle TR-4
The aircraft was in cruise at 3,500 feet when a slight engine vibration became apparent to the pilot. The engine oil pressure and temperature were normal; however, as a precaution he turned toward a nearby airport. One minute later, a severe vibration began shaking the aircraft. The pilot could not make the airport and landed in a marshy field. The engine ran the entire time from vibration onset until just before ground contact. The engine was factory new 2.5 years and 358 hours prior to the accident. During recovery of the aircraft, no oil film was observed on the aircraft or in the engine compartment. Significant quantities of clean oil were found in the oil filter canister, and the undamaged oil pump. No metal contamination or scoring was observed to the oil pump housing or impeller rotors. Complete engine disassembly revealed that the number 3 connecting rod was separated from the crankshaft journal, with severe peening noted to both the rod end and crankshaft journal. No significant heat distress or scoring signatures were noted on any bearing or journal. The battered number 3 rod end cap and the two associated bolts were found in the oil sump. One bolt was found fractured just below the head. The second bolt was bent about 20 degrees, with severe battering noted to the bolt threads and a portion of the nut. The direction of bend in the distorted bolt was observed to be 180 degrees from the center of the rod cap. Metallurgical examination of the fractured bolt revealed a fatigue crack with multiple plane origins in the radius between the head shoulder and the decreased diameter portion of the bolt. Hardness tests disclosed values in the acceptable range according to Lycoming specifications.
On December 24, 1996, at 1305 hours Pacific standard time, a homebuilt experimental Trickle TR-4 airplane, N91RT, nosed over during a forced landing in a field near Davis, California. The forced landing was precipitated by a loss of engine power during cruise. The aircraft was borrowed by the pilot from the owner for a local area personal flight. Visual meteorological conditions prevailed at the time and no flight plan was filed. The aircraft sustained substantial damage. The certificated commercial pilot and the one passenger onboard sustained minor injuries. The flight originated from the Natomas Airport, Sacramento, California, on the day of the accident at 1230. In both a telephone interview and his written statement, the pilot reported that he was in cruise at 3,500 feet when a slight engine vibration became apparent. The engine oil pressure and temperature were normal; however, as a precaution he turned toward a nearby airport. About 1 minute later, a severe vibration began shaking the aircraft and the pilot reduced the throttle to idle, which lessened the vibration severity. When the pilot realized he would not make the airport, he landed in a marshy field and the aircraft nosed over. According to the pilot, the engine ran the entire time, estimated at 4 minutes, from vibration onset until just before ground contact when he shut the engine down with the mixture control. According to the owner, the aircraft is a prototype for a kit experimental homebuilt airplane. The engine, a Lycoming O-360-A4M, serial number L-34017-36A, was obtained new from the factory 2.5 years ago and had accumulated a total time of 358.9 hours. In an interview, the owner stated that the last work involving removal of the oil system lines was accomplished about 2 years and 300 hours prior to the accident. No unresolved maintenance discrepancies existed at the time the pilot took the aircraft for the accident flight. The owner noted that the only recent maintenance activity was an oil change, about 40 hours prior to the accident; at that time the oil filter was not examined. The FAA did not visit the accident site and the Safety Board allowed the owner to retrieve the aircraft. Following the aircraft's arrival at the owner's facility, the engine was transported to an engine overhaul shop where a detailed examination was conducted by a Safety Board investigator, with the technical assistance of a representative from Textron Lycoming. A detailed report of the examination is appended to this report. At the time of examination, the remotely mounted oil filter, oil cooler, and associated lines were separated from the engine, and, the owner had removed the oil sump and associated induction tubes from the engine prior to it's arrival at the overhaul facility. The owner reported that when he retrieved the aircraft from the marsh, one oil line (engine accessory case out to oil filter) was not attached to the engine accessory case fitting. He further stated that there was no observable oil film on the aircraft or in the engine compartment when the inverted aircraft was recovered. Significant quantities of clean oil were found in the oil filter canister, and the undamaged oil pump. Minor metal particles were observed in the oil filter element. No metal contamination or scoring was observed to the oil pump housing or impeller rotors. A small hole was observed in the bottom of the case at the number 3 cylinder. Complete engine disassembly revealed that the number 3 connecting rod was separated from the crankshaft journal, with severe peening noted to both the rod end and crankshaft journal. No significant heat distress signatures were noted on any bearing or journal on either the crankshaft or engine case halves. Minor scoring was observed on the following bearings: numbers 1, 2, and 4 rod end, and the number 3 main bearing. The numbers 1 and 2 main bearings were nominal in appearance. The battered number 3 rod end cap and the two associated bolts were found in the oil sump. One bolt was found fractured just below the head, with about a 4mm section missing. The second bolt was bent about 20 degrees, with severe battering noted to the bolt threads and a portion of the nut. The bolts were laid out in their correct orientation as they would be installed in the rod and end cap. The direction of bend in the distorted bolt was observed to be 180 degrees from the center of the rod cap. The fractured bolt was submitted to the Safety Board Material's Laboratory for metallurgical examination, and the complete report is appended to this report. The report noted that damage had obliterated most of the fracture features; however, a small area at the fracture's periphery contained "ratchet marks and a crescent shape typical of fatigue cracking." Multiple initiation origins for the fatigue crack were noted in the radius between the head shoulder and the decreased diameter portion of the bolt. Hardness tests disclosed values in the acceptable range according to Lycoming specifications.
The fatigue fracture and separation of one rod end cap bolt. A factor in the accident was the nature of the terrain for the forced landing.
Source: NTSB Aviation Accident Database
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