National Transportation Safety Board investigators are trying to determine why a Robinson R44 II helicopter (N7531D) crashed on August 2, shortly after taking off from a cleared patch of forest in Washington State, killing the pilot and three passengers. Witnesses reported observing the helicopter takeoff and ascend vertically to an altitude of between 30 to 50 feet above the ground before moving forward. The witnesses stated that as the helicopter began to move forward, it immediately started a left hand turn of approximately 90 degrees. The witnesses further reported after moving forward 100 to 150 feet, they observed the helicopter's tail section moving back and forth from left to right before nosing over and impacting downward sloping clear-cut mountainous terrain in a nose low, left bank attitude. A post-crash fire consumed the helicopter. The fire subsequently spread over several hundred acres. The measured distance from the takeoff point to the point of impact and where the helicopter came to rest was approximately 290 feet. A salvage company subsequently moved the helicopter to a secure location where a detailed inspection of the wreckage was conducted. The crash came six years after another deadly crash involving another R44 operated by Classic Helicopter, which is a large charter operator at Boeing Field in Seattle. The Safety Board found that the helicopter's rear rotor had contacted its tail boom during a maneuver. Three months after the crash, Robinson Helicopter issued a safety notice requiring changes to the rear rotor to prevent the possibility of a rotor hitting the tail boom.