Pilot-induced oscillation

Last updated
Pilot-induced oscillation rating scale, with start position at bottom left. Pilot-induced oscillation rating scale.svg
Pilot-induced oscillation rating scale, with start position at bottom left.

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

Contents

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. A pilot aiming for a 500-foot per minute descent, for example, may find themselves descending more rapidly than intended. They begin 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 aircraft 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. In this way, stabilizing vertical speed can be difficult due to constantly variable airspeed. 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 a latency term to the instruments – for example, to 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.

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. 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).

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 PIOs will incrementally increase in amplitude. [3]

Notable examples

On 20 January 1974, a YF-16 (a development prototype for what was to become the General Dynamics F-16 Fighting Falcon) was on a high-speed taxi test when PIO caused the aircraft to veer off to the left of the runway. The test pilot decided to take off and landed safely after six minutes. [4] After that unintentional maiden flight, the development team reduced the roll gain of the fly-by-wire computer to eliminate similar PIO during takeoff or landing.

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 flight control system was determined to be the cause. Subsequently, the flight control 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.

See also

Related Research Articles

For fixed-wing aircraft, ground effect is the reduced aerodynamic drag that an aircraft's wings generate when they are close to a fixed surface. During takeoff, ground effect can cause the aircraft to "float" while below the recommended climb speed. The pilot can then fly just above the runway while the aircraft accelerates in ground effect until a safe climb speed is reached.

<span class="mw-page-title-main">Flight instruments</span> Aircraft instrument that gives information during flight

Flight instruments are the instruments in the cockpit of an aircraft that provide the pilot with data about the flight situation of that aircraft, such as altitude, airspeed, vertical speed, heading and much more other crucial information in flight. They improve safety by allowing the pilot to fly the aircraft in level flight, and make turns, without a reference outside the aircraft such as the horizon. Visual flight rules (VFR) require an airspeed indicator, an altimeter, and a compass or other suitable magnetic direction indicator. Instrument flight rules (IFR) additionally require a gyroscopic pitch-bank, direction and rate of turn indicator, plus a slip-skid indicator, adjustable altimeter, and a clock. Flight into instrument meteorological conditions (IMC) require radio navigation instruments for precise takeoffs and landings.

<span class="mw-page-title-main">Stall (fluid dynamics)</span> Abrupt reduction in lift due to flow separation

In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack exceeds its critical value. The critical angle of attack is typically about 15°, but it may vary significantly depending on the fluid, foil – including its shape, size, and finish – and Reynolds number.

<span class="mw-page-title-main">Variometer</span> Flight instrument which determines the aircrafts vertical velocity (rate of descent/climb)

In aviation, a variometer – also known as a rate of climb and descent indicator (RCDI), rate-of-climb indicator, vertical speed indicator (VSI), or vertical velocity indicator (VVI) – is one of the flight instruments in an aircraft used to inform the pilot of the rate of descent or climb. It can be calibrated in metres per second, feet per minute or knots, depending on country and type of aircraft. It is typically connected to the aircraft's external static pressure source.

<span class="mw-page-title-main">Landing</span> Transition from being in flight to being on a surface

Landing is the last part of a flight, where a flying animal, aircraft, or spacecraft returns to the ground. When the flying object returns to water, the process is called alighting, although it is commonly called "landing", "touchdown"a or "splashdown" as well. A normal aircraft flight would include several parts of flight including taxi, takeoff, climb, cruise, descent and landing.

<span class="mw-page-title-main">Wing loading</span> Total mass divided by area of wing

In aerodynamics, wing loading is the total weight of an aircraft or flying animal divided by the area of its wing. The stalling speed, takeoff speed and landing speed of an aircraft are partly determined by its wing loading.

<span class="mw-page-title-main">Spin (aerodynamics)</span> Aviation term for a corkscrew downward path

In flight dynamics a spin is a special category of stall resulting in autorotation about the aircraft's longitudinal axis and a shallow, rotating, downward path approximately centred on a vertical axis. Spins can be entered intentionally or unintentionally, from any flight attitude if the aircraft has sufficient yaw while at the stall point. In a normal spin, the wing on the inside of the turn stalls while the outside wing remains flying. It is possible for both wings to stall, but the angle of attack of each wing, and consequently its lift and drag, are different.

<span class="mw-page-title-main">Flight control surfaces</span> Surface that allows a pilot to adjust and control an aircrafts flight attitude

Aircraft flight control surfaces are aerodynamic devices allowing a pilot to adjust and control the aircraft's flight attitude.

The CarterCopter is an experimental compound autogyro developed by Carter Aviation Technologies in the United States to demonstrate slowed rotor technology. On 17 June 2005, the CarterCopter became the first rotorcraft to achieve mu-1 (μ=1), an equal ratio of airspeed to rotor tip speed, but crashed on the next flight and has been inoperable since. It is being replaced by the Carter Personal Air Vehicle.

<span class="mw-page-title-main">Northrop X-4 Bantam</span> American experimental jet aircraft

The Northrop X-4 Bantam was a prototype small twinjet aircraft manufactured by Northrop Corporation in 1948. It had no horizontal tail surfaces, depending instead on combined elevator and aileron control surfaces for control in pitch and roll attitudes, almost exactly in the manner of the similar-format, rocket-powered Messerschmitt Me 163 of Nazi Germany's Luftwaffe. Some aerodynamicists had proposed that eliminating the horizontal tail would also do away with stability problems at fast speeds resulting from the interaction of supersonic shock waves from the wings and the horizontal stabilizers. The idea had merit, but the flight control systems of that time prevented the X-4 from achieving any success.

<span class="mw-page-title-main">Flap (aeronautics)</span> Anti-stalling high-lift device on aircraft

A flap is a high-lift device used to reduce the stalling speed of an aircraft wing at a given weight. Flaps are usually mounted on the wing trailing edges of a fixed-wing aircraft. Flaps are used to reduce the take-off distance and the landing distance. Flaps also cause an increase in drag so they are retracted when not needed.

In aviation, a phugoid or fugoid is an aircraft motion in which the vehicle pitches up and climbs, and then pitches down and descends, accompanied by speeding up and slowing down as it goes "downhill" and "uphill". This is one of the basic flight dynamics modes of an aircraft.

Aircraft flight mechanics are relevant to fixed wing and rotary wing (helicopters) aircraft. An aeroplane, is defined in ICAO Document 9110 as, "a power-driven heavier than air aircraft, deriving its lift chiefly from aerodynamic reactions on surface which remain fixed under given conditions of flight".

<span class="mw-page-title-main">Rate of climb</span> Aircraft vertical velocity during flight

In aeronautics, the rate of climb (RoC) is an aircraft's vertical speed, that is the positive or negative rate of altitude change with respect to time. In most ICAO member countries, even in otherwise metric countries, this is usually expressed in feet per minute (ft/min); elsewhere, it is commonly expressed in metres per second (m/s). The RoC in an aircraft is indicated with a vertical speed indicator (VSI) or instantaneous vertical speed indicator (IVSI).

<span class="mw-page-title-main">Pitot–static system</span> System of pressure-sensitive instruments used to determine an aircrafts speed, altitude, etc.

A pitot–static system is a system of pressure-sensitive instruments that is most often used in aviation to determine an aircraft's airspeed, Mach number, altitude, and altitude trend. A pitot–static system generally consists of a pitot tube, a static port, and the pitot–static instruments. Other instruments that might be connected are air data computers, flight data recorders, altitude encoders, cabin pressurization controllers, and various airspeed switches. Errors in pitot–static system readings can be extremely dangerous as the information obtained from the pitot static system, such as altitude, is potentially safety-critical. Several commercial airline disasters have been traced to a failure of the pitot–static system.

<span class="mw-page-title-main">LTV XC-142</span> Experimental military tilt-wing aircraft

The Ling-Temco-Vought (LTV) XC-142 is a tiltwing experimental aircraft designed to investigate the operational suitability of vertical/short takeoff and landing (V/STOL) transports. An XC-142A first flew conventionally on 29 September 1964, and completed its first transitional flight on 11 January 1965 by taking off vertically, changing to forward flight, and finally landing vertically. Its service sponsors pulled out of the program one by one, and it eventually ended due to a lack of interest after demonstrating its capabilities successfully.

<span class="mw-page-title-main">AeroVironment Helios Prototype</span> Type of aircraft

The Helios Prototype was the fourth and final aircraft developed as part of an evolutionary series of solar- and fuel-cell-system-powered unmanned aerial vehicles. AeroVironment, Inc. developed the vehicles under NASA's Environmental Research Aircraft and Sensor Technology (ERAST) program. They were built to develop the technologies that would allow long-term, high-altitude aircraft to serve as atmospheric satellites, to perform atmospheric research tasks as well as serve as communications platforms. It was developed from the NASA Pathfinder and NASA Centurion aircraft.

Throughout a normal flight, a pilot controls an aircraft through the use of flight controls including maintaining straight and level flight, as well as turns, climbing, and descending. Some controls, such as a "yoke" or "stick" move and adjust the control surfaces which affects the aircraft's attitude in the three axes of pitch, roll, and yaw. Other controls include those for adjusting wing characteristics and those that control the power or thrust of the propulsion systems. The loss of primary control systems in any phase of flight is an emergency. Aircraft are not designed to be flown under such circumstances; however, some pilots faced with such an emergency have had limited success flying and landing aircraft with disabled controls.

The minimum control speed (VMC) of a multi-engine aircraft is a V-speed that specifies the calibrated airspeed below which directional or lateral control of the aircraft can no longer be maintained, after the failure of one or more engines. The VMC only applies if at least one engine is still operative, and will depend on the stage of flight. Indeed, multiple VMCs have to be calculated for landing, air travel, and ground travel, and there are more still for aircraft with four or more engines. These are all included in the aircraft flight manual of all multi-engine aircraft. When design engineers are sizing an airplane's vertical tail and flight control surfaces, they have to take into account the effect this will have on the airplane's minimum control speeds.

<span class="mw-page-title-main">Aeroflot Flight 1912</span> 1971 aviation accident in the Soviet Union

Aeroflot Flight 1912 was a scheduled domestic Aeroflot passenger flight on the Odessa-Kiev (Kyiv)-Chelyabinsk-Novosibirsk-Irkutsk-Khabarovsk-Vladivostok route that crashed on 25 July 1971, making a hard landing at Irkutsk Airport. It touched down 150 metres (490 ft) short of the runway, breaking the left wing and catching fire. Of the 126 people on board the aircraft, 29 survived.

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://apps.dtic.mil/sti/pdfs/ADA424366.pdf
  3. McRuer, Duane T. (July 1995). "Pilot-Induced Oscillations and Human Dynamic Behavior". NASA. Dryden Space Flight Research Center. hdl:2060/19960020960.
  4. Mizokami, Kyle (23 January 2020). "That Time When the F-16 Accidentally Had Its First Flight". Popular Mechanics . Retrieved 31 July 2021.