The pilot deposited the sling load of delicate instruments on the ground. To avoid dropping the shackle on the load, and because of the proximity of a hangar to the left, he moved the helicopter to the right. The movement, together with type of shackle used and the design of the hook, resulted in the load not being released. Prior to the flight the pilot had not insisted on the fitment of external load observation mirrors. While leaning out of the helicopter to check that the load had been released, he inadvertently caused the helicopter to move further to the right. The resulting tension on the cable connecting the hook and the load induced dynamic rollover. The pilot attempted unsuccessfully to correct the roll and during the subsequent landing the main rotor struck the ground. Use was not made of an available and qualified marshaller.
