The aircraft was on an external load logging operation and the pilot in command (PIC) had landed at a field site to refuel. The PIC exited, with the aircraft running at full RPM while it was being refueled, and the copilot remained in the right seat completing load reports. The aircraft controls were unattended. The crew chief was seated in the aft facing hoist operator station, which had the collective pitch stick installed beside the seat. He had become air sick during the flight, and remained seated after the PIC exited. When he attempted to exit, which required that he stand, turn around and step up out of the seat well, the aircraft took off to a hover and rotated to the left. The tail rotor struck a rotor blade box on top of a support trailer parked by the refueling site, fracturing the aft tail boom, pylon and tail rotor from the aircraft. The aircraft pitched nose down, rolled over and impacted the ground.