On October 18, 2014, about 1215 central daylight time, a BAE Hawk Mk 67 airplane, N509XX, departed off the side of the runway at the Tinker Air Force Base (KTIK), Oklahoma City, Oklahoma. The airline transport rated pilot was not injured, and the airplane sustained substantial damage. The airplane was registered to and operated by Air USA, Inc., Henderson, Nevada, under the provisions of 14 Code of Federal Regulations Part 91 as a public airplane flight. Visual meteorological conditions prevailed, and an instrument flight plan had been filed. The flight was originating at the time of the accident and was en route to Cherry Point Marine Corps Air Station, (KNKT) Cherry Point, North Carolina. The pilot landed at KTIK for a refueling stop, and the airplane was fueled to a full load of 747 gallons. The pilot reported that during the takeoff roll, as he reached about 90 knots, the airplane made a hard left turn. He applied right rudder and brake to stop the left turn; however, the airplane continued left, off the runway surface. The airplane came to rest in the grass in an upright position. Examination of the airplane revealed the airplane landing gear had collapsed, with the left landing gear strut penetrating the wing. The airplane was configured with two external fuel tanks and two CBLS-200 bomb racks. The airplane and landing gear was inspected by engineers from BAE Systems. The airplane tires, wheel brakes, and anti-skid systems were visually inspected and then disassembled. The airplane rudder system was also inspected; the inspections did not identify any abnormalities that would have accounted for the airplane's departure from the runway. The Nose Wheel Steering Digital Control Unit (DCU) was sent to the manufacturer, SAFRAN Electronics, facility in Canada for further investigation. The DCU showed no faults when run through the automated test program. During environmental testing, the unit failed 2 hours into the test at minus 40 deg C. The unit was disassembled and the fault was identified as a short in the power supply module. It was determined that the power supply failure would cause a system shut down, resulting in a castering nose wheel, rather than un-commanded steering input. Examination of the airplane did not find any abnormalities that would account for the un-commanded turn