Omak, WA, USA
N69PF
Hughes 369D
At the time of the accident, the pilot was descending into a hover over a pond that he was using for an aerial fire bucket refill site. As he began to level off in the hover, he heard a loud noise and felt an "accelerated vibration." Almost immediately thereafter, the helicopter began to spin to the right, so the pilot closed the throttle and made an autorotational landing in about three to four feet of water. A post-accident inspection of the helicopter revealed that the tail rotor driveshaft had failed in torsional overload, and that one of the tail rotor blades had experienced delaminating as a result of contact with the water over which the helicopter was hovering.
On July 21, 2007, approximately 1740 Pacific daylight time, a Hughes 369D helicopter experienced the torsional failure of a tail rotor drive shaft while hovering over a pond about 15 miles south of Omak, Washington. The commercial pilot, who was the sole occupant was not injured, but the aircraft, which is owned and operated by Panhandle helicopters, Inc., sustained substantial damage to a flight control system (tail rotor drive shaft). The Public Use aerial fire suppression flight, which was being conducted under contract to the United States Department of Interior, was being flown in visual meteorological conditions. No flight plan had been filed. There was no report of an ELT activation. According to the Department of Interior, the pilot was descending into a hover over a pond that he was using as an aerial fire bucket refill site when he heard a loud noise and felt an "accelerated vibration." Almost immediately thereafter, the helicopter began to spin to the right, so the pilot closed the throttle and made an autorotational landing in about three to four feet of water. After a successful touchdown, the pilot shut off the engine, applied the rotor brake, and turned off all the electrical switches. A post-accident inspection of the helicopter revealed that the tail rotor driveshaft had failed about four and one-half inches from its forward end. The driveshaft failed in a twisting/spiral manner that appeared consistent with a torsional overload. The tail rotor driveshaft was removed from the helicopter and sent to the manufacturer's facility for an FAA monitored inspection. That inspection determined that there were no material anomalies or deficiencies in the tail rotor driveshaft structure, and confirmed that the driveshaft had failed in torsional overload. A further inspection of the tail rotor driveshaft system did not reveal any evidence of an anomaly or malfunction that would have produced a binding force leading to the overload of the driveshaft. Although there were no obvious impact marks on the tail rotor blades, an inspection of the blades by the manufacturer reveled that the leading edge strip on one of the blades was delaminated. According to the manufacturer, the delaminating of the leading edge strip without evidence of impact marks was consistent with the tail rotor blade coming in contact with the water. According to the operator, "After examination of the tail rotor gearbox during overhaul, and inspection of the tail rotor blades by the manufacturer, it is now evident that the aircraft did sustain a tail rotor strike (water) with subsequent sudden stoppage of the tail rotor drive train."
The pilot's failure to maintain clearance from the surface of a pond that he was using as a water bucket refill site, which resulted in a tail rotor strike to the water's surface and a torsional overload failure of the tail rotor drive shaft.
Source: NTSB Aviation Accident Database
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