The pilot landed the helicopter to allow his passenger to alight. The engine was left running, the cyclic frictioned and collective held fully down. The pilot then felt a low frequency vibration begin and almost immediately the left side of the helicopter lifted and the tail swung to the right. Control imputs by the pilot had no effect and the helicopter rolled onto its side. One of the four belts providing drive to the transmission had jumped off the sheave, for reasons which could not be established. The belt had become wrapped around the clutch shaft and had caused a sudden braking action to the main rotor blades. This braking force was of such a magnitude that the fuselage yawed in an inertia reaction. The right skid dug into the soft ground and a dynamic roll-over resulted.
