On December 6, 2009, about 1030 Pacific standard time, an Aerospatiale AS 355F1 helicopter, N548SA, was substantially damaged during cruise flight when the left engine cowling door opened in flight near Temple Bar, Arizona. The helicopter was registered to and operated by Heli-USA Airways, Inc., Las Vegas, Nevada, under the provisions of Title 14 Code of Federal Regulations Part 135. The commercial pilot and six passengers were not injured. Visual meteorological conditions prevailed and a company flight plan was filed for the cross-country flight. The air tour flight originated from the McCarran International Airport (LAS), Las Vegas, Nevada, about 1000, with an intended destination of Grand Canyon, Arizona. The pilot reported that the morning of the accident, he was late to work due to unforeseen traffic and local events within the area. The pilot stated he was about 40 minutes late to work and upon his arrival, he started his pre-flight inspection. As the pilot was inspecting the area of the left engine, the fuel truck arrived to refuel the helicopter. The pilot “lowered the cowling door [and] walked around to the pilot seat to get the gas cap keys.” After the helicopter was refueled, the pilot “continued the pre-flight, not realizing that I left the engine cowling ‘unlatched’.” The pilot further reported that his passengers were dropped off on the ramp and he proceeded to board them “still not realizing that I left the cowling door unlatched.” The pilot departed the airport and while en route to his destination, he heard a “pop” followed by “shake feed” within the cyclic control. The pilot contacted a second company helicopter to have them visually inspect the helicopter in flight. The pilot of the second helicopter informed the pilot that the left engine cowling appeared to be open and partially separated. The pilot initiated a precautionary landing to the desert surface and landed without further incident. Examination of the helicopter by a Federal Aviation Administration (FAA) inspector revealed that two of the three main rotor blades were damaged. One main rotor blade exhibited a one-inch long gouge about one-quarter of an inch in depth near the blade root. A portion of the left engine cowling was separated and not located.