On Sept. 8, 1994, a Boeing 737-300 passenger airplane was on a downwind approach to the Pittsburgh International Airport at an altitude of 5000 ft above ground level. While in a shallow left turn onto a downwind-approach heading, the airplane crossed into the vortex trail of a Boeing 727 flying in the same approach pattern about 4 miles ahead. The B-737 airplane rolled and turned sharply to the left, exited the vortex wake, and plunged into the ground. The airplane was equipped with a 11+ channel digital flight data recorder (FDR) and a multiple-channel cockpit voice recorder. Both recorders were recovered from the crash site and provided excellent data for the development of an accident scenario. Radar tracking of the two airplanes as well as the indicated airspeed perturbations clearly visible on the B-737 FDR recordings indicate that the upset was apparently initiated by the airplane's crossing into the wake of the B-727. A 6-degree-of-freedom simulation program for the B-737 airplane was constructed. The simulation was initialized at the stabilized flight conditions of the airplane about 13 s prior to its entry into the vortex trail of the B-727 airplane