During the 16 glide flights of the M2 -F2 lifting body vehicle, severe lateral pilot-induced oscillations occurred on three occasions in the low - angle of -attack, final-approach, preflare situation. These oscillations were analyzed qualitatively to determine the type and similarity and by a systems analysis to determine the root cause. The analysis was complemented by a piloted simulator study, which verified the results. The systems analysis revealed the presence of a coupled roll-spiral mode which caused the pilots to generate a closed-loop lateral instability in the low-angle -of -attack, preflare flight region. A systems analysis, a piloted simulator study, and flight data showed that the addition of a fixed center fin lessened the pilot-induced-oscillation tendencies in the critical flight region.