On April 14, 2006, approximately 2040 mountain daylight time, an experimental Central Copters Lancair IV-P, N671, impacted the terrain during an off-field forced landing about one and one-half miles northwest of the approach end of runway 12 at Gallatin Field, Belgrade, Montana. The commercial pilot and his two passengers were not injured, but the aircraft, which was owned and operated by the pilot, was destroyed by the post-crash fire. The 14 CFR Part 91 personal pleasure flight, which departed Spokane International Airport, Spokane, Washington, about 90 minutes prior to the accident, had been on an IFR flight plan, and was on a night visual approach in visual meteorological conditions at the time of the power loss that lead to the forced landing. There was no report of an ELT activation.

According to the pilot, who failed to meet the reporting requirements of Title 49, Part 830.15 (a), when the aircraft was about three miles from the approach end of the runway, the engine suddenly developed a loud howling sound, the torque became very erratic, and the exhaust gas temperature (EGT) exceeded its normal limits. Soon thereafter the engine lost all power, and though there was no suitable nearby terrain on which to land, the pilot turned toward what appeared to be an open area in order to execute a forced landing. Although the touchdown was successful, the aircraft structure came in contact with the rough terrain during the landing roll, and after it came to a stop, a small fire developed near the engine exhaust. Although all occupants exited the aircraft without injury, they were unable to extinguish the fire, which slowly spread, and ultimately consumed the aircraft structure.

During the investigation, it was determined that the experimental aircraft was powered by a Garrett TPE331-6-252M turbo-shaft engine, driving a 280 centimeter diameter five-bladed variable pitch wood/composite MT Propeller (MTV-27). Engine traceability records indicate that the engine was last overhauled by AlliedSignal Aerospace Services, Repair Station ZN3R030M, on 6/19/97. Records also show that Intercontinental Jet Corporation mounted the engine on a test stand in January of 2001, to comply with the operational checks in accordance with the maintenance manual. While on the test stand, all oil was drained from the engine in preparation for shipment to the owner of N671. On October 10, 2005, the owner/builder of N671 annotated in the engine log book that he had installed the engine on Lancair serial number 423 (N671). Discussions with International Jet Corporation determined that they no longer had process records on file for this engine, and the engine records provided by the owner/builder of N671 did not indicate that any of the monthly or biennial engine preservation/depreservation actions prescribed in Chapter 72 of the TPE331-6/10 Maintenance Manual had been performed.

On May 17, 2006, an owner-directed engine teardown inspection was performed by Ag Air Turbines of Caldwell, Idaho. In addition to the owner and his representatives, both the FAA and Honeywell (Product Integrity) had individuals present to observe the teardown. During this teardown, it was determined that the majority of the structure of the quill shaft forward splines, which showed significant thermal distress, had worn away, resulting in the power section of the engine becoming uncoupled from the reduction gearbox section of the engine. It was also noted that the ball lock coupler and its three locking balls, which ensure the coupling of the quill shaft to the high-speed pinion gear shaft, were found lying in the oil in the bottom of the nose case. The root cause of the disengagement of the coupling assembly was not positively determined.

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