ENGLEWOOD, CO, USA
N6405U
Mooney M20C
During climb following departure, the engine lost power. The pilot reported the emergency to the tower and attempted to return to the airport. Determining that he would be unable to make it back, he made an emergency landing in a field one and a half miles southeast of the airport. Postaccident examination of the engine revealed that the camshaft gear retaining bolts connecting the camshaft gear to the camshaft had lost torque. As a result, the lobes failed to open their respective valves, creating discontinuity to the gear train. The tread on each of the four camshaft gear bolts exhibited shiny, worn patterns. According to a Textron Lycoming engineer, loss of camshaft retaining bolt torque is indicative of the bolts being improperly torqued during the engine overhaul. According to aircraft maintenance records, the engine was overhauled on March 15, 1985, and had accumulated 833 hours since overhaul.
On October 21, 1998, at 0828 mountain daylight time, a Mooney M20C, N6405U, owned and operated by the pilot, was substantially damaged when it collided with terrain following a loss of engine power during climb after departure from Centennial Airport, Englewood, Colorado. The private pilot, sole occupant aboard, sustained serious injuries. Visual meteorological conditions prevailed, and no flight plan was filed for the personal cross-country flight being conducted under Title 14 CFR Part 91. The intended destination was La Junta, Colorado. According to the pilot, takeoff from runway 10 was normal. While still in a climb, approximately five miles southeast of the airport, the engine lost power. He initiated emergency procedures for loss of engine power, but was unsuccessful in identifying the source of the failure. He reported the emergency to the tower and attempted to return to the airport. Determining that he would be unable to make it back, he made a forced landing in a field one and a half miles southeast of the approach end of runway 28. Postaccident examination of the engine revealed that the camshaft gear retaining bolts connecting the camshaft gear to the camshaft had lost torque. As a result, the lobes failed to open their respective valves, creating discontinuity to the gear train. Three of the retaining bolts, as well as locking tab fragments, were located in the oil sump. The one remaining bolt was fractured and appeared to have backed out before breaking. The tread on each of the four camshaft gear bolts exhibited shiny, worn patterns. The alignment pins had separated from the camshaft gear, and the gear holes were worn and elongated. In addition, the accessory housing revealed shiny wear marks, and the accessory breather shield was separated from the accessory case and found in fragmented pieces. According to the aircraft maintenance records, the engine was overhauled on March 15, 1985, at a tachometer time of 3,734.7 hours. The last annual inspection of the engine was performed on October 23, 1997, at a tachometer time of 4,455.2 hours. The engine had accumulated 833 hours since overhaul, and 112 hours since the last annual inspection. According to a Textron Lycoming engineer, loss of camshaft retaining bolt torque is indicative of the bolts being improperly torqued during the engine overhaul. The engineer stated that if the bolts are improperly torqued, they begin to vibrate and eventually wear, becoming disconnected, as was evident in this accident. If properly torqued, the bolts should have remained torqued until the next scheduled overhaul.
The undertorquing of the camshaft gear retaining bolts during engine overhaul by maintenance personnel. A factor was the unsuitable terrain for a forced landing.
Source: NTSB Aviation Accident Database
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