Pilot-induced oscillation

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Pilot-induced oscillations (PIOs), as defined by MIL-HDBK-1797A, are sustained or uncontrollable oscillations resulting from efforts of the pilot to control the aircraft. They occur when the pilot of an aircraft inadvertently commands an often increasing series of corrections in opposite directions, each an attempt to cover the aircraft's reaction to the previous input with an overcorrection in the opposite direction. An aircraft in such a condition can appear to be "porpoising" switching between upward and downward directions. As such it is a coupling of the frequency of the pilot's inputs and the aircraft's own frequency. In order to avoid any assumption that oscillation is necessarily the fault of the pilot, new terms have been suggested to replace pilot-induced oscillation. These include aircraft-pilot coupling, pilot–in-the-loop oscillations and pilot-assisted (or augmented) oscillations. The physics of flight make such oscillations more probable for pilots than for automobile drivers. An attempt to cause the aircraft to climb, say, by applying up-elevator, will also result in a reduction in airspeed. Another factor is the response rate of flight instruments in comparison to the response rate of the aircraft itself. For example, an increase in power will not result in an immediate increase in indicated airspeed, nor will an increase in climb rate show up immediately on the vertical speed indicator.