At about 1030 Pacific time, the helicopter was substantially damaged when its left engine cowling door opened while airborne. The commercial pilot and six passengers were not injured. Visual conditions prevailed for the Part 135 air-tour flight. During cruise, the pilot heard a “thumping” noise followed by slight feedback 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 the left engine cowling appeared to be open and partially separated. The pilot initiated a precautionary landing to a dirt road and landed without further incident. Examination revealed 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 its root. A portion of the left engine cowling was separated and not located.
