Circumstances:
The pilot reported that the helicopter was descending through 200 feet on approach to land in a clear area surrounded by trees 15-20 metres tall. Airspeed was 15-20 knots, engine power setting was about 15 inches Manifold Pressure, and the approach was being flown into a north-westerly wind of 5-10 knots. Without warning, as a result of what the pilot thought was a sudden wind change, the helicopter yawed slightly right and began to sink rapidly. The pilot noticed that the airspeed was indicating zero. The pilot managed to avoid the trees but could not prevent the helicopter landing heavily, damaging the main rotor blades and gearbox.
