The helicopter was on the downwind leg of an approach to land at the pilot's private heliport. The pilot said, "I took off from my hangar on my farm at about 5:00 pm with 2 friends to just go for a flight local. After flying about 40 minutes in the area of my farm we started back home. The weather was perfect with hardly any wind under 5 knots out of [the] north. I started a southernly downwind to my landing pad. We had just got over a frozen lake in the downwind when without warning or any reason to believe anything was wrong, the helicopter drop[ped] straight down on the lake without me moving any flight controls." A postaccident examination of the helicopter was conducted. The helicopter came to rest about 300 feet from the initial impact point on the frozen lake. No anomalies were found with respect to the airframe that could be associated with a pre-impact condition. No anomalies were found with respect to the engine. The end of civil twilight occured at 1807 and the phase of the moon was a waxing crescent with 1% of the Moon's visible disk illuminated.

The National Transportation Safety Board determines the probable cause(s) of this accident as follows:

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