It refers to a phenomenon in aviation where the pilot unintentionally initiates or amplifies oscillations in the aircraft’s pitch, roll, or yaw axes. These are sustained or uncontrollable oscillations resulting from efforts of the pilot to control the aircraft and occur when the pilot of an aircraft inadvertently commands an often increasing series of corrections in opposite directions; each one is an attempt to control the aircraft’s reaction to the previous input with an overcorrection in the opposite direction. These oscillations can occur due to the pilot’s control inputs that inadvertently resonate with the aircraft’s inherent stability characteristics.
On 2 February 1989, the first prototype JAS 39-1 crashed on its sixth flight, when attempting to land in Linköping.
Footage is from JAS-39 Gripen’s first PIO accident.