pitot-static system - wikip

Diagram of a pitot-static system including the pitot tube,pitot-static instruments and static port

Pitot-static systemFrom Wikipedia, the free encyclopedia

A pitot-static system is a system of pressure-sensitive instruments that is most often used in aviation todetermine an aircraft's airspeed, Mach number, altitude, and altitude trend. A pitot-static system generallyconsists of a pitot tube, a static port, and the pitot-static instruments.[1] This equipment is used to measurethe forces acting on a vehicle as a function of the temperature, density, pressure and viscosity of the fluid inwhich it is operating.[2] Other instruments that might be connected are air data computers, flight datarecorders, 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 often critical to a successful flight. Several commercial airline disasters have been tracedto a failure of the pitot-static system.[3]

Contents

1 Pitot-static pressure1.1 Pitot pressure1.2 Static pressure1.3 Multiple pressure

2 Pitot-static instruments2.1 Airspeed indicator2.2 Altimeter2.3 Machmeter2.4 Vertical airspeed indicator

3 Pitot-static errors3.1 System malfunctions

3.1.1 Blocked pitot tube3.1.2 Blocked static port

3.2 Inherent errors3.2.1 Position errors

4 Pitot-static related disasters5 References6 See also7 External links

Pitot-static pressure

Pitot-static system - Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Pitot-static_system

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Examples of pitot tube, statictube, and pitot-static tube.

Airspeed indicator diagramshowing pressure sources from

both the pitot tube and static port

The pitot-static system of instruments uses the principle of air pressuregradient. It works by measuring pressures or pressure differences and usingthese values to assess the speed and altitude.[1] These pressures can bemeasured either from the static port (static pressure) or the pitot tube (pitotpressure). The static pressure is used in all measurements, while the pitotpressure is only used to determine airspeed.

Pitot pressure

The pitot pressure is obtained from the pitot tube. The pitot pressure is ameasure of ram air pressure (the air pressure created by vehicle motion orthe air ramming into the tube), which, under ideal conditions, is equal tostagnation pressure, also called total pressure. The pitot tube is most oftenlocated on the wing or front section of an aircraft, facing forward, where itsopening is exposed to the relative wind.[1] By situating the pitot tube insuch a location, the ram air pressure is more accurately measured since itwill be less distorted by the aircraft's structure. When airspeed increases,the ram air pressure is increased, which can be translated by the airspeed indicator.[1]

Static pressure

The static pressure is obtained through a static port. The static port is most often a flush-mounted hole on thefuselage of an aircraft, and is located where it can access the air flow in a relatively undisturbed area.[1]Some aircraft may have a single static port, while others may have more than one. In situations where anaircraft has more than one static port, there is usually one located on each side of the fuselage. With thispositioning, an average pressure can be taken, which allows for more accurate readings in specific flightsituations.[1] An alternative static port may be located inside the cabin of the aircraft as a backup for whenthe external static port(s) are blocked. A pitot-static tube effectively integrates the static ports into the pitotprobe. It incorporates a second coaxial tube (or tubes) with pressure sampling holes on the sides of theprobe, outside the direct airflow, to measure the static pressure.

Multiple pressure

Some pitot-static systems incorporate single probes that contain multiple pressure-transmitting ports thatallow for the sensing of air pressure, angle of attack, and angle of sideslip data. Depending on the design,such air data probes may be referred to as 5-hole or 7-hole air data probes. Differential pressure sensingtechniques can used to produce angle of attack and angle of sideslip indications.

Pitot-static instruments

The pitot-static system obtains pressures for interpretations by thepitot-static instruments. While the explanations below explaintraditional, mechanical instruments, many modern aircraft use airdata computers (ADC) to calculate airspeed, rate of climb, altitudeand mach number. Two ADCs receive total and static pressure fromindependent pitot tubes and static ports, and the aircraft's flight datacomputer compares the information from both computers and checksone against the other. There are also "standby instruments", whichare back-up pneumatic instruments employed in the case of problemswith the primary instruments.


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Diagram of an altimeter

A vertical airspeed indicator

Airspeed indicator

Main article: Airspeed indicator

The airspeed indicator is connected to both the pitot and static pressure sources. The difference between thepitot pressure and the static pressure is called "impact pressure". The greater the impact pressure, the higherthe airspeed reported. A traditional mechanical airspeed indicator contains a pressure diaphragm that isconnected to the pitot tube. The case around the diaphragm is airtight and is vented to the static port. Thehigher the speed, the higher the ram pressure, the more pressure exerted on the diaphragm, and the larger theneedle movement through the mechanical linkage.[4]

Altimeter

Main article: Altimeter

The pressure altimeter, also known as the barometric altimeter, isused to determine changes in air pressure that occur as the aircraft'saltitude changes.[4] Pressure altimeters must be calibrated prior toflight to register the pressure as an altitude above sea level. Theinstrument case of the altimeter is airtight and has a vent to the staticport. Inside the instrument, there is a sealed aneroid barometer. Aspressure in the case decreases, the internal barometer expands, whichis mechanically translated into a determination of altitude. Thereverse is true when descending from higher to lower altitudes.[4]

Machmeter

Main article: Machmeter

Aircraft designed to operate at transonic or supersonic speeds will incorporate a machmeter. The machmeteris used to show the ratio of true airspeed in relation to the speed of sound. Most supersonic aircraft arelimited as to the maximum Mach number they can fly, which is known as the "Mach limit". The Machnumber is displayed on a machmeter as a decimal fraction.[4]

Vertical airspeed indicator

Main article: Variometer

The variometer, also known as the vertical speed indicator (VSI) orthe vertical velocity indicator (VVI), is the pitot-static instrumentused to determine whether or not an aircraft is flying in level flight.[5]

The vertical airspeed specifically shows the rate of climb or the rateof descent, which is measured in feet per minute or meters persecond.[5] The vertical airspeed is measured through a mechanicallinkage to a diaphragm located within the instrument. The areasurrounding the diaphragm is vented to the static port through acalibrated leak (which also may be known as a "restricteddiffuser").[4] When the aircraft begins to increase altitude, the diaphragm will begin to contract at a ratefaster than that of the calibrated leak, causing the needle to show a positive vertical speed. The reverse ofthis situation is true when an aircraft is descending.[4] The calibrated leak varies from model to model, butthe average time for the diaphragm to equalize pressure is between 6 and 9 seconds.[4]


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Pitot-static errors

There are several situations that can affect the accuracy of the pitot-static instruments. Some of theseinvolve failures of the pitot-static system itselfwhich may be classified as "system malfunctions"whileothers are the result of faulty instrument placement or other environmental factorswhich may be classifiedas "inherent errors".[6]

System malfunctions

Blocked pitot tube

A blocked pitot tube is a pitot-static problem that will only affect airspeed indicators.[6] A blocked pitot tubewill cause the airspeed indicator to register an increase in airspeed when the aircraft climbs, even thoughindicated airspeed is constant. This is caused by the pressure in the pitot system remaining constant when theatmospheric pressure (and static pressure) are decreasing. In reverse, the airspeed indicator will show adecrease in airspeed when the aircraft descends. The pitot tube is susceptible to becoming clogged by ice,water, insects or some other obstruction.[6] For this reason, aviation regulatory agencies such as the U.S.Federal Aviation Administration (FAA) recommend that the pitot tube be checked for obstructions prior toany flight.[5] To prevent icing, many pitot tubes are equipped with a heating element. A heated pitot tube isrequired in all aircraft certificated for instrument flight except aircraft certificated as Experimental Amateur-Built.[6]

Blocked static port

A blocked static port is a more serious situation because it affects all pitot-static instruments.[6] One of themost common causes of a blocked static port is airframe icing. A blocked static port will cause the altimeterto freeze at a constant value, the altitude at which the static port became blocked. The vertical speedindicator will become frozen at zero and will not change at all, even if vertical airspeed increases ordecreases. The airspeed indicator will reverse the error that occurs with a clogged pitot tube and cause theairspeed be read less than it actually is as the aircraft climbs. When the aircraft is descending, the airspeedwill be over-reported. In most aircraft with unpressurized cabins, an alternative static source is available andcan be toggled from within the cockpit of the airplane.[6]

Inherent errors

Inherent errors may fall into several categories, each affecting different instruments. Density errors affectinstruments reporting airspeed and altitude. This type of error is caused by variations of pressure andtemperature in the atmosphere. A compressibility error can arise because the impact pressure will cause theair to compress in the pitot tube. At standard sea level pressure altitude the calibration equation (seecalibrated airspeed) correctly accounts for the compression so there is no compressibility error at sea level.At higher altitudes the compression is not correctly accounted for and will cause the instrument to readgreater than equivalent airspeed. A correction may be obtained from a chart. Compressibility error becomessignificant at altitudes above 10,000 feet (3,000 m) and at airspeeds greater than 200 knots (370 km/h).Hysteresis is an error that is caused by mechanical properties of the aneroid capsules located within theinstruments. These capsules, used to determine pressure differences, have physical properties that resistchange by retaining a given shape, even though the external forces may have changed. Reversal errors arecaused by a false static pressure reading. This false reading may be caused by abnormally large changes in anaircraft's pitch. A large change in pitch will cause a momentary showing of movement in the oppositedirection. Reversal errors primarily affect altimeters and vertical speed indicators.[6]

Position errors


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Another class of inherent errors is that of position error. A position error is produced by the aircraft's staticpressure being different from the air pressure remote from the aircraft. This error is caused by the air flowingpast the static port at a speed different from the aircraft's true airspeed. Position errors may provide positiveor negative errors, depending on one of several factors. These factors include airspeed, angle of attack,aircraft weight, acceleration, aircraft configuration, and in the case of helicopters, rotor downwash.[6] Thereare two categories of position errors, which are "fixed errors" and "variable errors". Fixed errors are definedas errors which are specific to a particular make of aircraft. Variable errors are caused by external factorssuch as deformed panels obstructing the flow of air, or particular situations which may overstress theaircraft.[6]

Pitot-static related disasters

1 December 1974Northwest Orient Airlines Flight 6231, a Boeing 727, crashed northwest of John F.Kennedy International Airport during climb en route to Buffalo Niagara International Airport becauseof blockage of the pitot tubes by atmospheric icing.6 February 1996Birgenair Flight 301 crashed into the sea shortly after takeoff due to incorrectreadings from the airspeed indicator. The suspected cause is a blocked pitot tube (this was neverconfirmed, as the airplane wreck was not recovered). [7]2 October 1996AeroPeru Flight 603 crashed because of blockage of the static ports. The static portson the left side of the aircraft had been taped over while the aircraft was being waxed and cleaned.After the job was done, the tape was not removed.[8]6 Feb 1999 Boeing 757-225 (B-757) crashed into the sea off the Northern coast of the Dominicanrepublic five minutes after take off. Plane was destroyed and all 189 passengers killed. In final reportthe Dominican investigation team concluded that the airplane had a blocked pitot tube and duringdeparture the flight crew became confused by false indications of increasing airspeed and did notrespond to a stall warning. [9]1 June 2009- Air France Flight 447 is believed to have had a pitot tube error while in flight over theAtlantic Ocean and subsequently crashing, all aboard were lost.

References

^ a b c d e f Willits, Pat, ed (2004). Guided Flight Discovery - Private Pilot. Abbot, Mike Kailey, Liz. JeppesenSanderson. pp. 248253. ISBN 0-88487-333-1.

1.

^ "Pitot Static System Performance" (http://flighttest.navair.navy.mil/unrestricted/FTM108/c2.pdf) (PDF).flighttest.navair.navy.mil. http://flighttest.navair.navy.mil/unrestricted/FTM108/c2.pdf. Retrieved 2008-04-25.

2.

^ Evans, David (1 May 2004). "Safety: Maintenance Snafu with Static Ports" (http://www.avtoday.com/av/categories/maintenance/877.html) . Avionics Magazine. http://www.avtoday.com/av/categories/maintenance/877.html. Retrieved 2008-04-25.

3.

^ a b c d e f g "Pitot-Static Instruments - Level 3 - Pitot-Static Instruments" (http://www.allstar.fiu.edu/aero/PSI.htm) . allstar.fiu.edu. http://www.allstar.fiu.edu/aero/PSI.htm. Retrieved 2007-01-07.

4.

^ a b c "Pilot Handbook - Chapters 6 through 9" (http://www.faa.gov/library/manuals/aviation/pilot_handbook/media/faa-h-8083-25-2of4.pdf) (PDF). FAA. http://www.faa.gov/library/manuals/aviation/pilot_handbook/media/faa-h-8083-25-2of4.pdf. Retrieved 2007-01-07.

5.

^ a b c d e f g h i "Flight Instruments - Level 3 - Pitot-Static System and Instruments" (http://www.allstar.fiu.edu/aero/PSSI.htm) . allstar.fiu.edu. http://www.allstar.fiu.edu/aero/PSSI.htm. Retrieved 2007-01-07.

6.

^ "ASN Aircraft accident description Boeing 757-225 TC-GEN Puerto Plata, Dominican Republic"(http://aviation-safety.net/database/record.php?id=19960206-0) . http://aviation-safety.net/database/record.php?id=19960206-0. Retrieved 2007-01-07.

7.

^ "CVR Database 2 October 1996 Aeroperu 603" (http://www.tailstrike.com/021096.htm) . tailstrike.co.http://www.tailstrike.com/021096.htm. Retrieved 2007-01-07.

8.

9. Flight Safety Foundation Accident Prevention Vol 56 No 10 October 1999. (http://flightsafety.org/ap/ap_oct99.pdf)


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Lawford. J. A. and Nippress, K. R. (1983). Calibration of Air-Data Systems and Flow DirectionSensors (AGARD AG-300 - Vol.1, AGARD Flight Test Techniques Series; R. W. Borek, ed.).Accessed via Spaceagecontrol.com (http://spaceagecontrol.com/AD-CalibrationOfAirDataSystemsAndFlowDirectionSensors.pdf) (PDF). Retrieved on 25 April 2008.Kjelgaard, Scott O. (1988), Theoretical Derivation and Calibration Technique of a Hemispherical-Tipped Five-Hole Probe (NASA Technical Memorandum 4047).

See also

Air Data Inertial Reference UnitAir data boomAustral Lneas Areas Flight 2553Pitot tubePosition error

External links

Macromedia Flash 8-based Pitot-Static System Simulator (http://www.luizmonteiro.com/Learning_Pitot_Sim.aspx)Pitot-Static Tube Installation (MIL-I-5072A) (http://spaceagecontrol.com/MIL-I-6115A-PitotTubeandStaticPortInstallation.pdf)Stainless Steel Pipe for Medical Purpose (http://www.needletube.co.kr)

Retrieved from "http://en.wikipedia.org/wiki/Pitot-static_system"Categories: Aircraft instruments | Speed sensors

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