On May 23, 2004, approximately 1605 central daylight time, a Bell 412 twin-engine helicopter, N555BA, sustained substantial damage when its tail rotor struck a parked helicopter while hovering prior to takeoff from the University Hospital helipad, near San Antonio, Texas. The commercial pilot, the flight nurse, and flight paramedic were not injured. The helicopter was registered to Bexar County Hospital District, San Antonio, Texas, and operated by Air Methods Corp., Englewood, Colorado. Visual meteorological conditions prevailed, and a company flight plan was filed for the 14 Code of Federal Regulations Part 91 positioning flight. The flight was departing at the time of the accident and was destined for New Braunfels, Texas. According to a statement from the pilot, prior to takeoff, the helicopter was facing the northwest. While in a hover and executing a right pedal turn to align into the wind, the helicopter's tail rotor struck another helicopter that was parked on the helipad. The airborne helicopter entered into right yaw, and the pilot reduced collective and throttles. Subsequently, the helicopter landed hard onto the helipad. In addition, the pilot reported having accumulated a total of approximately 4,640 total rotorcraft flight hours, with 877 in the accident aircraft make and model. Examination of the helicopter revealed the tail rotor drive shaft was separated, the tail rotor blades were damaged, and the landing gear skids were deformed. The parked helicopter sustained damaged to a main rotor blade, and another parked helicopter was damaged by debris. The concrete helipad measured 90 feet in length and 90 feet in width. At 1553, the San Antonio International Airport automated surface observing system (ASOS), located approximately 6 nautical miles north of the accident site, reported the wind from 170 degrees at 14 knots, 10 statute miles visibility, sky broken at 4,000 feet above ground level, temperature 88 degrees Fahrenheit, dew point 70 degrees Fahrenheit, and an altimeter setting of 29.86 inches of mercury.