MBB-BK117 pilot reported the rotor wash of the helicopter while performing a hover taxi over the taxiway caused a nearby aircraft that was fueling to move. The other aircraft did not have a parking brake function and the wheels were not chocked.

Narrative

I approached to land at ZZZ on Runway XX upon reaching a hover I sidestepped left to hover taxi over the taxiway. There was an old crop-dusting plane being fueled at the pump. I passed the plane approximately 80 ft. to the south of the plane over the taxiway hovering between 4 and 8 ft. I proceeded to land next to the ambulance waiting at the ramp. As we passed the plane one of my crew members stated that the small plane began to turn on its own. I landed at the H and shut down. After I shut down; I spoke with the pilot of the plane. I told him that I was sorry for his inconvenience. I told him that I gave him plenty of space when I hover taxied past him. I asked does your plane have a parking brake - he replied no; it is not equipped with one. The plane also did not have wheel chocks in place. I took pictures of the plane and called Person A. I gave the man Person A's contact information and my own. In my professional opinion as a dual rated pilot; the damage to this man's plane occurred purely out of his own negligence; one for not having a parking brake and two not having his wheels chocked. His plane would not have been affected by my rotor wash; if he had secured his plane properly. Have all airplane pilots that are fueling their aircraft with no brakes; to place chocks on the wheels.