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The pilot of a BK117 helicopter reported that while in a gentle climb at about 4,600 feet during a post maintenance test flight, the helicopter suddenly pitched nose-up. The indicated airspeed decreased to zero and the helicopter then pitched nose-down to a slightly inverted attitude. After descending about 2,000 feet, the pilot regained control and landed safely.

Inspection of the helicopter systems by the company maintenance personnel could not find any reason for the sudden loss of control.

A Bureau of Meteorology area forecast, issued on the day of the incident, indicated severe turbulence below 10,000 feet. The forecast also included increasing westerly wind speeds ranging from 30 knots at 2,000 feet to 45 knots at 10,000 feet. The actual weather report for the area that the helicopter was operating in, indicated westerly wind speeds increasing from 35 knots at 2,000 feet to 50 knots at 7,000 feet. These conditions are conducive to mountain wave and rotor activity.

A rotor is a large air mass rotating about a substantially horizontal axis. It is generated in the lee of a mountain or sharp ridge in strong wind conditions.

The helicopter was operating on the lee side of a mountain range when the pilot experienced the rapid loss of control. It is probable that the helicopter encountered a rotor.

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