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Aviation Glossary :: Pilot Induced Oscillation  Aviation Glossary :: Pilot Induced Oscillation FAA Written Test Preparation
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Pilot Induced Oscillation
Pilot Induced Oscillation
Normally refers to the rapidly increasing pitch oscillations of a gyro due to improperly applied or improper “phase” pilot control inputs. PIO is the result of pilot actions that cause the natural pitch oscillatory tendencies of an unstable gyro to grow those oscillations to extreme amplitudes. PIO is the result of Positive Feedback from the pilot control inputs that causes the gyro to resonate in increasing amplitude pitch oscillations. PIO can be thought of a Resonance between an unstable gyro and improper pilot control inputs. PIO, when not immediately corrected by the pilot, often rapidly results in a Bunt-Over. PIO occurs with unstable gyros as a result of inadequate pilot skills to stabilize that gyro.
source: Glossary of Gyroplane Terms
Pilot-induced oscillations, as defined by MIL-HDBK-1797A, are sustained or uncontrollable oscillations resulting from efforts of the pilot to control the aircraft and occurs 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. During flight test, pilot-induced oscillation is one of the handling qualities factors that is analyzed, with the aircraft being graded by an established scale (chart at right). 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.

In a controls sense, the oscillation is the result of reduced phase margin induced by the lag of the pilot's response. The problem has been mitigated in some cases by adding lead to the instruments - for example, cause the climb rate indication to not only reflect the current climb rate, but also be sensitive to the rate of change of the climb rate.

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. An increase in power will not result in an immediate increase in airspeed. An increase in climb rate will not show up immediately on the vertical speed indicator.

A pilot aiming for a 500 foot per minute descent, for example, may find himself descending too rapidly. He begins to apply up elevator until the vertical speed indicator shows 500 feet per minute. However, because the vertical speed indicator lags the actual vertical speed, the pilot is actually descending at much less than 500 feet per minute. The pilot then begins applying down elevator until the vertical speed indicator reads 500 feet per minute, starting the cycle over. It's harder than it might seem to stabilize the vertical speed because the airspeed also constantly changes.

Pilot-induced oscillations may be the fault of the aircraft, the pilot, or both. It is a common problem for inexperienced pilots, and especially student pilots, although it was also a problem for the top research test pilots on the NASA lifting body program. The problem is most acute when the wing and tail section are close together in so called "short coupled" aircraft.

The most dangerous pilot-induced oscillations can occur during landing. Too much up elevator during the flare can result in the plane getting dangerously slow and threatening to stall. A natural reaction to this is to push the nose down harder than one pulled it up, but then the pilot ends up staring at the ground. An even larger amount of up elevator starts the cycle over again.

While Pilot-Induced oscillations often start with fairly low amplitudes, which can adequately be treated with small perturbation linear theory, several PIO's will by definition become very large.

In February 1989 a JAS 39 Gripen prototype crashed when landing in Linköping, Sweden. Pilot-induced oscillation as a result of an over-sensitive, yet slow-response steering system was determined to be the cause. Subsequently, the steering system was redesigned.

Pilot-induced oscillation was blamed for the 1992 crash of the prototype F-22 Raptor, landing at Edwards Air Force Base in California. This crash was linked to actuator rate limiting, causing the pilot, Tom Morgenfeld, to overcompensate for pitch fluctuations.

source: Wikitionary / Wikipedia and Related Sources (Edited)

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Disclaimer: While this glossary in most cases is likely to be highly accurate and useful, sometimes, for any number of editorial, transcription, technical, and other reasons, it might not be. Additionally, as somtimes you may have found yourself brought to this page through an automated term matching system, you may find definitions here that do not match the cotext or application in which you saw the original term. Please use your good judgement when using this resource.


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