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A form of longitudinal (pitch) instability. As the aircraft angle of attack increases beyond a certain figure, the aircraft continues to diverge in pitch, thus increasing the angle of attack even further. This tendency normally is found in swept-wing aircraft mainly because of wing-tip stall, which makes the center of pressure move forward. The wing moment is reduced and a nose-up pitching moment results, which aggravates this tendency. In straight-wing aircraft, wing-tip vortices can cause pitch-up if the aircraft has a low aspect ratio and short-span wings.