A twin-engine Cessna 500 airplane, N113SH, was destroyed upon impact with terrain following a loss of control shortly after takeoff from runway 17L at the Wiley Post Airport (PWA), in Oklahoma City, Oklahoma. The airplane was registered to the Southwest Orthopedic and Sports Medicine Clinic and operated by Interstate Helicopters. At the time of the accident, Interstate Helicopters was operating the accident airplane in commercial service contrary to its Federal Aviation Administration-issued 14 Code of Federal Regulations Part 135 operating certificate, which, at the time, did not authorize operation of the accident airplane or any other fixed-wing aircraft. The Cessna Citation took off from Wiley Post Airport, OK at 15:13 and climbed to an altitude of 3,100 feet while on a southwesterly heading. While in the airspace near the Lake Overholser dam the airplane started a steep descent. The airplane impacted wooded terrain approximately one mile south of the dam. There were no reported radio calls of distress from the flight crew. Investigation revealed that departure from controlled flight likely resulted from wing structure damage sustained during an in-flight collision with a flock of American white pelicans, which far exceeded the airframe’s design certification limit. Also, the right engine failed as a result of the collision with these birds. PROBABLE CAUSE: “The National Transportation Safety Board determines that the probable cause of this accident was airplane wing-structure damage sustained during impact with one or more large birds (American white pelicans), which resulted in a loss of control of the airplane.”