EMIDA, ID, USA
N164KA
Kaman K-1200
Shortly after departure, during the initial climb, the helicopter experienced a sudden loss of power and the pilot initiated a forced landing to a nearby road. Post-accident inspection of the transmission assembly revealed that the splined engine adaptor was fractured due to a torsional overstress imposed on the transmission by the engine. It was determined that the adaptor fracture was secondary to another failure further along in the drive train or transmission. Further examination revealed that the helicopter's freewheeling sprag clutch assembly (located at the drive shaft input to the main transmission) exhibited signs of significant wear to include smearing, non-uniform surface wear and cam deformation. Damage or dragging in this assembly could account for the overstress fracture of the adaptor.
HISTORY OF FLIGHT On November 4, 1999, about 1000 Pacific standard time, a Kaman K-1200, N164KA, registered to Long-Line Leasing, LLC, and operated by Mountain West Helicopters, LLC, of Orem, Utah, sustained substantial damage subsequent to a sudden loss of rotor R.P.M. and forced landing. The helicopter was conducting long-line logging operations under the provisions of Tite 14 CFR Part 133 near Emida, Idaho. Visual meteorological conditions prevailed and a company flight plan was filed for the local flight. The commercial pilot, the sole occupant of the helicopter, was not injured. The helicopter had just released a load of logs and was departing the landing area when the accident occurred. There was no report of fire or ELT actuation. The helicopter had been operating in the area for approximately 2 hours prior to the accident. In a written report, the pilot stated that he had just released a load of logs and was climbing out of the landing area. He stated that the helicopter was in a right turn at an airspeed less than 10 knots, and a power setting of approximately 45 pounds per square inch (P.S.I.). Shortly thereafter, about 200-250 feet above ground level (AGL), the aircraft experienced "a mechanical anomaly that resulted in a very sudden loss of power, rotor R.P.M. [revolutions per minute] and altitude", accompanied by a change in engine noise. The pilot initiated a forced landing to a nearby service road. AIRCRAFT INFORMATION The helicopter was manufactured by Kaman Aerospace Corporation of Bloomfield, Connecticut, and was issued a normal category airworthiness certificate on June 10, 1997. Maintenance records indicated the aircraft's last inspection, a zone 3 progressive inspection, was completed on November 1, 1999. The helicopter's total time at inspection was 5,072.1 hours. Maintenance records also indicated that the helicopter's clutch assembly was replaced with a new clutch assembly on September 24, 1999, approximately 41 days prior to the accident. The logbook entry, corresponding to the maintenance, indicated that the clutch was replaced secondary to possible "clutch slippage" that was reported by the pilot. At the time of the accident, the helicopter had accumulated approximately 5,084.9 hours total time. The clutch assembly in the accident helicopter had accumulated approximately 253 hours total time since replacement. WRECKAGE AND IMPACT DAMAGE Personnel from the Federal Aviation Administration (FAA), Flight Standards District Office in Spokane, Washington, conducted the post-accident inspection of the helicopter. The inspectors reported that the helicopter impacted terrain in a relatively level flight attitude and came to rest on a logging service road. Ground impact damage was reported to the helicopter's fuselage, horizontal stabilizer, right and left vertical fins (reference photograph 1A). The helicopter's main landing gear and nose gear were collapsed. The main rotor blades were intact and there was no evidence of a main rotor blade strike. Further examination by FAA inspectors revealed that the splined engine adapter, part number K-974182-001, and the engine adapter bolt, part number K974179-011, were fractured at approximately mid-shank (reference photograph 1B). At the conclusion of the on-site investigation, the helicopter was transported to Timber Choppers, Incorporated, Bonners Ferry, Idaho. The helicopter's clutch assembly was shipped to Kaman Aerospace Corporation, Bloomfield, Connecticut. The fractured engine adapter was shipped to the NTSB materials laboratory in Washington, D.C., and the helicopter's engine and engine accessory were shipped to Honeywell Product Safety, Phoenix, Arizona, for inspection and testing. TESTS AND RESEARCH On November 23, 1999, representatives from Kaman Aerospace and the Boston, MA, FAA Aircraft Certification Office inspected the helicopter's freewheeling sprag clutch assembly. The assembly consists of an input pinion, center input shaft, and sprag assembly. Examination of the sprag assembly revealed that both inner and outer surfaces of the individual sprags were badly damaged and showed signs of smearing, non-uniform wear and cam deformation. Inspection of the center input shaft revealed similar findings. Heavy wear, scrubbing and distortion of the shaft's surface materials were noted. The bevel input pinion, the sprags outer race, also showed signs of heavy wear, to include scuffing, smearing and surface distortion. A Senior Metallurgist from the NTSB Materials Laboratory examined the fractured engine adapter (Materials Laboratory Factual Report No. 99-243, attached). Microscopic examination of the fracture faces revealed "a circumferential fracture pattern consistent with predominately torsional overstress separation." The report also indicated that "the direction of the shear dimples was consistent with the engine side of the adapter overtorquing the drive shaft side." Post-accident inspection and test run of the turbine engine revealed the following: The gas generator rotated freely and continuity was established between the spool and accessory drive train. The power turbine also rotated freely and continuity was confirmed between the spool and accessory drive train. Following a visual inspection of the engine, a normal engine start was conducted and no anomalies were noted. Subsequent to the start, normal engine operations at ground idle, flight idle and takeoff power settings were verified (reference attached report dated February 11, 2000). ADDITIONAL INFORMATION The NTSB did not retain possession of the airframe, which was transported to Timber Coppers, Inc, Bonners Ferry, Idaho. The helicopter's transmission assembly, engine adaptor and turbine engine were retained for further inspection and testing. At the conclusion of testing, the engine, transmission and sub components were released to PAC Northwest, Inc, and Mountain West Helicopters, LLC, (release forms attached).
Failure of the splined engine adaptor as a result of torsional overstress. Factors include a worn freewheeling clutch assembly.
Source: NTSB Aviation Accident Database
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