The accident occurred as the air medical flight was departing a hospital heliport en route to pick up a patient. In addition to the accident helicopter on the helipad, there was another helicopter parked off to the side of the helipad, but within the fenced helipad area. Prior to boarding the helicopter, the pilot turned the main rotor blades on the parked helicopter to give him more room to maneuver on takeoff. During the takeoff the pilot lifted the helicopter into a hover, and was turning it into the wind and talking to air traffic control (ATC), when he backed the helicopter into the main rotor blades of the parked helicopter. The pilot rolled the throttle to idle and conducted a hover autorotation back onto the helipad. The pilot reported that he knew the parked helicopter was there, but had forgotten about it. The medical crew in the back of the helicopter attempted to verbally alert the pilot through the intercom that he was too close to the parked helicopter; however, the pilot was unable to hear the warning because his microphone was keyed for the conversation with ATC. The accident helicopter received substantial damage to both tail rotor blades and one of the main rotor blades. The operator reported there was no failure/malfunction of the helicopter. The operator’s General Operations Manual states, "Minimum clearances for all helicopters will be at least 1/3 of the rotor diameter between the rotor blade tips on rotor discs of operating aircraft, between the outboard tip of the rotor disc and any obstruction, and at a distance great enough not to create a hazard to persons or property on the ground in non-secured areas. Additional vigilance must be observed when landing in close proximity to another aircraft especially at a hospital helipad or during scene operations."