The aircraft departed Casper, WY at 13:57 following a refueling stop. The Learjet climbed to a altitude of FL450. Around 3 pm control was lost and the airplane crashed near Felt, OK. PROBABLE CAUSE: “A loss of control, possibly initiated by an unexpected encounter with moderate to severe clear air turbulence, which caused the aircraft to depart the narrow flight envelope boundaries in which it was operating and from which recovery was not effected, the flightcrew’s lack of adequate training and experience in the Learjet; and the aircraft’s marginal controllability characteristics near and beyond the boundaries of its flight envelope. Contributing to the accident was the flightcrew’s probable extension of the spoilers in an overspeed situation, a procedure that had been prescribed in the approved aircraft flight manual until 1 year before the accident.”