On August 9, 2004, at approximately 1700 mountain daylight time, a Eurocopter EC 130 B4, N910U, operated by Air Methods and piloted by a commercial pilot, was substantially damaged during ground hover operations at Centennial Airport, Englewood, Colorado. Visual meteorological conditions prevailed at the time of the accident. The test flight was being conducted under the provisions of Title 14 CFR Part 91 without a flight plan. The pilot reported no injuries. The local flight was originating at the time of the accident. According to the accident report submitted by the pilot, the helicopter had just undergone maintenance and was preparing for a second test flight. The helicopter had departed approximately 15 minutes prior to the accident and returned due to an inoperative HSI. A thorough preflight had been conducted for the first test flight. While the HSI was being repaired, another maintenance technician opened the transmission cowling. The pilot and two avionics technicians boarded the helicopter from the left side, without performing an additional preflight or walk around. The pilot performed a run-up and lifted the helicopter to a hover when an individual on the ground noticed that the cowling was open. The lead mechanic then signaled to the pilot for him to land. As the helicopter set back down, the transmission cowling detached and impacted a main rotor blade and the tail boom. The main rotor blade was damaged at the root and the tail boom was creased and punctured. After the accident, the mechanic reported that it was hot inside and he decided to leave the cowling open.