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Pilot-induced oscillation rating scale, start position at bottom left

Pilot-induced oscillations, as defined by MIL-HDBK-1797A,[1] 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.[2]

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.[3]

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.

## References

1. ^ DEPARTMENT OF DEFENSE INTERFACE STANDARD, Flying qualities of piloted airplanes, Washington, D.C.
2. ^ Witte, Joel B, An Investigation Relating Longitudinal Pilot-Induced Oscillation Tendency Rating To Describing Function Predictions For Rate-Limited Actuators https://research.maxwell.af.mil/papers/ay2004/afit/AFIT-GAE-ENY-04-M16.pdf
3. ^ McRuer, Duane T. "Pilot-Induced Oscillations and Human Dynamic Behavior". NASA. Dryden Space Flight Research Center. Retrieved 16 September 2011.
 9185 videos foundNext >
 Pilot Induced Oscillations during Enterprise landing on slow motion - 10/26/1977 - Pilot Induced Oscillation at Big Bear Airport (L35)Pilot induced oscillation happens even to Space Shuttle pilots. In this case it was caused by a bad flare, I just didn't slow down the sink rate enough, this being a warmer day at Big Bear,... F-16 Pilot Induced Oscillation during First Flight TestDuring a high speed taxi test, the F-16 prototype experience severe PIO resulting in becoming airborne. JAS 39 Gripen crash (rare alternate angle) due to pilot-induced oscillation on February 2 1989.On 2 February 1989, the first prototype JAS 39-1 crashed on its sixth flight, when attempting to land in Linköping. The pilot, Lars Rådeström escaped with a fractured elbow and some minor... PIO leads to crash landing and loss of aircraftRtishchevo Russia Student pilot on first solo flight in Let L 410 another student as right seat observer with guidance from his Instructor Pilot heard on radio. The students lack of experience... C-17 Pilot Induced Oscillation in Air Refueling Saab Gripen Pilot Induced Oscillation during Flight Test - Landing CrashThe Saab Gripen had a unique flight control system that contributed to this PIO experienced by the test pilot during a landing task in early fight test. The aircraft was destroyed by the crash... Chase plane view of F-8 DFBW pilot-induced oscillationsThis 38 second movie clip shows pilot-induced oscillations of the F-8 DFBW aircraft taken from a chase plane. From 1972 to 1985 the NASA Dryden Flight Research Center conducted flight research... Pilot Induced Oscillation. CrashCloud Hopper club member Mat 'Flymo' Howard's unfortunate accident. Please subscribe to our main channel: http://www.youtube.com/subscription_center?add_user=lookupparagliding Main DIY web. Hang glider Pilot Induced Oscillation (PIO) whilst landing at Stanwell Oct 2013on final and low to the ground Pilot Input Ossicilation (PIO) the glider starts to ossilate, nearly end up in the trees but I get it back in control and land on the sand.
 9185 videos foundNext >
 29 news items
 Motorcycle-Control Skills For Safer Street Riding Practice teaches true ... Cycleworld Wed, 11 Mar 2015 06:41:35 -0700 In addition to the pilot-induced oscillation, the bar moving back and forth opened and closed the throttle in my hand. Luckily, it was a 1/6 turn throttle on a CBR1000RR in 3rd gear, so small angular changes didn't make a big difference, right? Of ... Edwards Air Force Base Test Pilot School grads to embark on new horizons Edwards Air Force Base Wed, 18 Jun 2014 13:26:15 -0700 TPS designed a flight test technique to replicate pilot induced oscillation in a safe and controlled environment called switch-induced simulated PIO. This project tested SISPIO to determine if it was a viable way of reproducing actual PIOs. HAVE POSIT ... Lateral thinking for simulators Flightglobal (blog) Tue, 01 Jul 2014 10:19:28 -0700 With a bit of over-controlling you can get into a pilot induced oscillation in a real aeroplane, but in a simulator it's difficult not to get into one even if you play it gently. Some years ago I tried flying a classic 737 simulator doing landings with ... Heroes or villains? The Economist (blog) Tue, 10 Jun 2014 04:36:05 -0700 FINANCIAL MARKETS have been celebrating since June 5th when the European Central Bank (ECB) announced a new battery of measures to counter low inflation in the euro area. Stockmarkets around the world have been hitting new highs. In Frankfurt the ... Crawford Bay plane crash claims one life Nelson Star Sun, 31 Aug 2014 12:08:05 -0700 “We heard later it was porpoising,” she said, referring to pilot-induced oscillation, which is when a pilot inadvertently corrects in opposite directions, causing the plane to violently switch between upward and downward directions. The plane ... Plane crash claims life Castlegar News Thu, 04 Sep 2014 15:07:30 -0700 “We heard later it was porpoising,” she said, referring to pilot-induced oscillation that occurs when a pilot inadvertently corrects in opposite directions, causing the plane to violently switch between upward and downward directions. The plane ... Flying high (really high) St. George Daily Spectrum Tue, 20 May 2014 23:16:26 -0700 The bicycle landing gear and the rear wheel must touch down at the same time to avoid a bouncing motion called "pilot-induced oscillation" that can end in disaster. After touching down, one of the wings may fall to the ground and skim along the runway. AVweb Used Aircraft Guide: Citabria and Decathlon AVweb Thu, 05 Jun 2014 18:49:59 -0700 One potential handling trouble spot is PIO (pilot-induced oscillation) during landings. Although not unique to Champions, the spring-steel main gear can bounce the airplane if the pilot dumps it too hard or if he or she fails to go around. More bounces ...
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