Dexter, OR, USA
N133KA
Kaman K-1200
While approaching the collection point with a load of logs, the helicopter's power turbine and rotor system experienced a steady-state overspeed due to the failure of the splines on the shaft that provides turbine speed input to the turbine governor. Without this input, the governor erroneously sensed a turbine underspeed, and therefore provided fuel in excess of that required to maintain the correct speed. The splines failed due to a significant reduction in the flow of lubricating oil resulting from the partial blockage of an oil filter outflow orifice. The outflow orifice became partially blocked because the oil filter screen was aligned in its housing in a manner that allowed some contaminants to bypass it. Although the Kaman K-1200 Flight Manual contains an emergency procedure that allows for the pilot to take manual (non-governed) control of the fuel input to the engine in the event of a governor failure, the pilot did not perform the procedure because he was not able to correctly identify the cause of the sequence of events.
On March 12, 2006, approximately 1100 Pacific standard time, a Kaman K-1200, N133KA, impacted the terrain during an attempted precautionary landing in a forest clearing approximately 10 miles southeast of Dexter, Oregon. The airline transport pilot, who was the sole occupant, received minor injuries, and the aircraft, which is owned and operated by Grizzly Mountain Aviation, sustained substantial damage. The 14 CFR Part 133 long-line lumbering flight, which had been airborne for about 20 minutes, was being operated in visual meteorological conditions. No flight plan had been filed. According to the pilot, as he was approaching the collection point with a load of logs, the warning horn started sounding, and he heard a repetitive loud banging/popping sound coming from the area of the rotors. At the same time, the helicopter started vibrating and became hard to control. Because there were people on the ground at the collection point, and because the pilot was not sure what was causing this sequence of events to occur, he flew past the collection point and dropped the load of logs. He eventually jettisoned the long-line, and maneuvered to an uneven stump-covered open area about 700 yards from the collection point. Upon reaching that area, he attempted to land the helicopter, but just prior to the touch-down, the rotors impacted a stump, resulting in their separation from the aircraft, and substantial damage to the helicopter's structure as it impacted the terrain in an upright position. During the investigative process, recorded data from the aircraft, and an FAA monitored teardown of the engine, determined that the engine had experienced an overspeed/over-fueling event. This event resulted in a turbine speed (N1) of over 107 percent, and a rotor speed (Nr) that had reached 130 percent for a period of 54 seconds. A teardown of the engine sub-components determined that the splines on the drive end of the shouldered shaft between the overspeed and tachometer drive gearbox assembly and the power turbine governor had worn away to the point where drive power was not being received by the turbine governor. Without drive input, the turbine governor sensed a continuous underspeed, and therefore increased fuel flow, resulting in an increase in turbine speed, and a continuous N1/rotor overspeed. A further inspection determined that the overspeed governor and tachometer drive gearbox oil supply filter (finger screen) had, for an undetermined reason, become located too deep into its housing. This allowed the gearbox oil supply to partially bypass the filter (approximately 50% by orifice area) at the filter's retainer plug end. It was further determined that the orifice at the outlet end of the filter screen (measured diameter of 0.013 inch) was partially blocked (approximately 65%) by a piece of metallic debris (aluminum), and by two smaller pieces of non-metallic debris. The partial blockage of the filter outlet orifice restricted the flow of lubricating oil to the governor drive shaft and its splines. Although the helicopter experienced an overspeed of both the power turbine and the main rotors, the pilot stated that he was too busy moving the load of logs away from the people on the ground, and to consumed with finding a reasonable place to land the helicopter, to take the time to look back inside to analyze the rpm gauges. He further stated that because of the noise and control problems that he was encountering while trying to maneuver the helicopter away from the ground crew and toward a landing site, he was not able to accurately determine if the warning horn was sounding continuously (rotor overspeed) or intermittently (rotor underspeed). He ultimately was not able to accurately determine what was causing the difficulty he was encountering, and therefore elected to land the helicopter instead of trying to take any in-flight emergency action. A review of the Kaman K-1200 Flight Manual revealed that there is an explanation of how to recognize the situation where a fuel control unit exceeds its maximum governed N1/Nr limits (Fuel Control Malfunction - High Side Failure), and a pilot emergency procedure to follow should the event occur. According to the Flight Manual, "This condition will be recognized as an increasing Nr accompanied by increasing Ng (N1) and EGT (exhaust gas temperature)". This condition would also be accompanied by a steady audio warning tone. The emergency procedure described in the Flight Manual is as follows: Collective --------------------------------Increase as required/available to control Nr. Throttle -----------------------------------Reduce as required to control N2/Nr. Load ------------------------------------- Jettison as required. GOV (governor switch) -------------EMERG (to emergency position). Throttle ----------------------------------Adjust as required. Rotor speed ---------------------------Maintain 98 - 102%. Aircraft ---------------------------------- Land as soon as practical. The execution of the aforementioned emergency procedure would provide the pilot manual control over the fuel input to the engine and allow the rotor speed to be maintained within the design limits of 98 to 102 percent.
The pilot's failure to identify the overspeed of the helicopter's main rotors due to a turbine governor malfunction, and his failure to take the appropriate actions described in the Flight Manual's emergency procedures section. Factors include the malfunction (partial bypass) of a lubricating system filter screen, which led to the partial blockage of an oil supply orifice, resulting in the failure of the splines on the power turbine governor speed input shaft, and ultimately to the loss of the governor's ability to correctly control fuel input to the engine. Additional factors include no suitable landing terrain and tree stumps in the area the pilot elected to land in.
Source: NTSB Aviation Accident Database
Aviation Accidents App
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