On April 26, 2016, about 1500 eastern daylight time, a Eurocopter Deutchland GMBH MBB-BK-117 C-2, N145HN, operated by Air Methods Corp, was substantially damaged during cruise flight to West Penn Hospital Heliport (PN80), Pittsburgh, Pennsylvania. The commercial pilot, three crewmembers and one patient were not injured. The on-demand air medical flight was conducted under the provisions of 14 Code of Federal Regulations Part 135. Visual meteorological conditions prevailed and a company flight plan was filed for the flight that originated from Grafton City Hospital Heliport (22WV), Grafton, West Virginia, about 1430.The pilot reported that the patient transfer flight was the second flight of the day, following maintenance work that was performed on the helicopter earlier that morning. The patient transfer flight was uneventful; however, after landing the pilot noticed that the left vertical fin cowling had partially separated and came in contact with a tailrotor blade. The pilot added that 8 of the 11 fasteners on the left vertical fin cowling were unlocked. Examination of the tailrotor blade by representatives from the helicopter manufacturer revealed that the tailrotor blade had sustained substantial damage. Examination of the helicopter by a Federal Aviation Administration inspector revealed that the Dzus fasteners on the right vertical fin cowling remained secured while the Dzus fasteners on the left vertical fin cowling were unlocked, consistent with them not being properly secured by maintenance personnel following the maintenance work that was performed. Additionally, the operator reported that there were no mechanical failures or malfunctions with the helicopter.