An American Airlines DC-10 on a training flight had just performed a touch-and-go when the Delta DC-9 approached runway 13. The DC-9 suddenly began to roll when passing the runway threshold. The right wing struck the runway and the aircraft crashed in flames. PROBABLE CAUSE: “An encounter with a trailing vortex generated by a preceding “heavy” jet which resulted in an involuntary loss of control of the airplane during final approach. Although cautioned to expect turbulence the crew did not have sufficient information to evaluate accurately the hazard or the possible location of the vortex. Existing FAA procedures for controlling VFR flight did not provide the same protection from a vortex encounter as was provided to flights being given radar vectors in either IFR or VFR conditions.”