Pitot Static System

Malaysian Institute of Aviation Technology

Instrument and Avionics System

Topic 2 :

Pitot Static System



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Pitot-Static System

A system in which total pressure created by the

forward motion of the aircraft and static pressure

of the atmosphere surrounding it are sensed and

measured in terms of:

Speed

Altitude

Rate of change of altitude



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Basic Pitot-Static consists of:

Pressure Head– Known as pitot-static probe, consist of static slots,

pitot tube, drain hole and heating element.– If only pitot pressure is collected through the tube, it

is called Pitot Tube or Pitot Head.– It should be covered if the aircraft is park to avoid

foreign object entering the tube.– Warning flag, red in colour, with word ‘REMOVED

BEFORE FLIGHT’ to be attached to the pitot cover.



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Pressure Head



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Static Vent– Also known as static port, it is located at the aircraft fuselage

and are position where there are least disturbance from the air flow.

– Independent static vents, when fitted are always located one on each side of a fuselage and interconnected so as to balance out dynamic pressure effects resulting from any yawing or sideslip motion of an aircraft.

– The static vent mounting plate should not painted and must be cleaned and smooth.

– If aircraft is park the static vent should be blanked and the warning flag “REMOVED BEFORE FLIGHT’ should be attached.



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Static Port



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Drain Trap



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Pipeline– Pitot and static pressures are transmitted through seamless

corrosion-resistant metal (light alloy/or tungum) pipeline.– Flexible pipes (maricon tubing) are used for connection of

components mounted on anti-vibration mountings.– The minimum requirement diameter of the pipeline is 0.25

inch or 6mm. – The piping are marked by a letter P or S.

Drain Trap/Drain Valve– Allow draining of water accumulated in the piping using

spring loaded action.– Drain traps are located at the lowest point of the piping

run/route.



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Pitot-Static Instruments– Airspeed Indicator (ASI)

• Requires pitot and static pressure.• ASI measure the differential pressure between pitot/static.

– Altimeter• Requires static pressure only• Altimeter measures absolute pressure.• If a leak occurs on a pressurised aircraft – under read, unpressurised

aircraft - – Vertical Speed Indicator (VSI)

• Requires static pressure only• VSI measures rate of change of altitude pressure.

– Machmeter• Requires pitot and static pressure.



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Pitot-Static System on Modern Aircraft



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Schematic of a typical pitot-static system on a pressurized multiengine aircraft



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Airspeed Indicator (ASI)Purpose:

to indicate the speed of the aircraft relative to the airflow.to indicate the aerodynamic force acting on the aircraft surface.

Principle of operationuses a differential capsule to measure different in pitot and static pressure.

Constructiontwo detector units which senses pitot and static pressure.a differential capsule as a measuring unit.a mechanical coupling.an indicating element which is a pointer rotating over a scale graduated in knots. – 1 knot = 1.1 miles.

A mechanical airspeed indicator has one differential capsule.A sensitive airspeed indicator has more than one differential capsule.



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Airspeed Indicator (ASI)



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Air Speed IndicatorPITOT PRESSURE = DYNAMIC PRESSURE + STATIC PRESSURE

Dynamic pressure is representative of airspeed, so:

DYNAMIC PRESSURE = PITOT PRESSURE - STATIC PRESSURE

Square Law Compensator

A device that will compensates the non-linear deflection of the capsule when magnify to the pointer become linear.

It will represent linear scale on the ASI.



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Errors of ASIInstrument error– due to imperfection of the instrument mechanism during

manufacturing.Position Error– incorrect position of pressure sources.– measured by trial and error and wind tunnel.

Compressibility Error– results from air being compressed in the pitot tube inlet,

generally at altitudes above 10,000 feet and CAS in excess of 200 knots.

– It generally produces IAS readings that are too high.Density Error– due to decrease in the density of air as altitude increases.



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Reading of ASI

ASIR (Airspeed Indicator Reading)– reading not corrected for any error.

IAS (Indicated Airspeed)– ASI corrected for Instrument error only.

RAS (Rectified Airspeed)– IAS corrected for instrument and pressure errors.

CAS (Calibrated Airspeed)– RAS compensated for non-linear (Square law)

EAS (Equivalent Airspeed)– RAS corrected for compressibility error.

TAS (True Airspeed)– EAS corrected for temperature and density error.



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True AirspeedAirspeed indicator with temperature and density compensator

will give a True Airspeed (TAS) reading.



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True Airspeed

To get true airspeed on an ASI

without temperature and

pressure compensator, a

correction scale is provided. The dial on the airspeed

indicator can be rotated to find a true airspeed reading.

When the air temperature is lined up with the pressure attitude, the true airspeed can be read from the outside scale.



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Maximum Allowable Airspeed Indicator Aircraft maximum speed that are limited by structural consideration

have their Vne marked by range marking ie RED RADIAL LINE.

Aircraft maximum speed that is limited by critical mach number the RED RADIAL LINE is replaced by RED/WHITE pointer or BARBER POLE driven by an altimeter capsule.



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Machmeter

Installed onboard high-performance aircraft, indicate the

ratio of aircraft speed to the speed of sound at the

particular altitude and temperature existing at any time

during flight.

To indicate to pilot the mach number of the aircraft i.e: the max allowable speed of the aircraft at that altitude.

Mach number = TAS/Speed of Sound

Sonic speed = 660 knot or 760 mph at sea level

Principle of operation

A combination of altimeter mechanism and airspeed mechanism.

It uses aneroid capsule to measure altitude and differential capsule to measure changes in pitot and static pressure.



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Machmeter



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Combine Mach Airspeed IndicatorUse in subsonic aircraft to provide airspeed, mach, and maximum allowable airspeed indications in a single instrument.

It is made up of a machmeter and an airspeed mechanism.

It provides the pilot with:– IAS (Indicated Airspeed)– Vmo (Velocity maximum

operating)– Mmo (Mach maximum

operating)



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Combine Mach Airspeed Indicator

In operation, the airspeed measuring and indicating elements respond to the difference between pitot and static and will gives the airspeed reading.

As altitude increases, mach number will increase and rotates the mach scale anti-clockwise.

When limiting speed is reached, the mach scale will make contact with the V mo pointer.

This will make the V mo pointer rotate anti-clockwise, to lower the max speed operating.



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Vertical Speed Indicator (VSI)It is also called Rate Of Climb (ROC)

Indicator. Purpose:

– To indicate to the pilot the vertical speed of the aircraft or rate of climb or dive of an aircraft in feet per minute.

Principle of operation– It measures the rate at which the

static pressure changes. Construction

– A metering unit – to established second pressure.

– A differential capsule – sense pressure changes.

– An indicating element using pointer which rotates over a scale graduated in feet per minute.



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At level flightPrevailing static pressure is admitted to the interior of the capsule, and also to the instrument case through the metering unit. There is zero differential across the capsule and pointer indicates zero.

When aircraft descendingMetering unit maintains case pressure lower than capsule pressure, changing it at the same rate and thereby creating a constant differential pressure across thecapsule.When aircraft climbMetering unit creates a constant differentialpressure across capsule by maintaining case pressure higher than capsule pressure.



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Instantaneous Vertical Speed Indicator (IVSI)

It is also called Inertia Operated VSI.

It is designed to overcome lag error in the normal VSI using accelerometer or dash pot.

The inertia of the accelerometer piston will move upward (if descent) and downward (if ascent) so as to quicken the expansion and contraction of the differential capsule by pumping air into it.



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Instantaneous Vertical Speed Indicator (IVSI)



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Instantaneous Vertical Speed Indicator



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Altimeter

PURPOSE

To indicate vertical distance from MSL (Mean Sea Level) or AGL (Above Ground Level) from minus 1,000 feet to 50,000 feet.

To indicate pressure altitude when Baroscale is set to QNE (ICAO standard atmospheric pressure at Mean Sea Level)



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PRINCIPLE OF OPERATION

An altimeter measures air pressure, which decreases with increasing altitude, and from the pressure calculates and displays the corresponding altitude.

It follows the aneroid barometric principle – to measure absolute pressure (total pressure).

Construction

A detecting element to collect static pressure.

A measuring element which is the aneroid capsule.

Mechanical coupling.

An indicating element which is a pointer rotating over a scale.



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Types of AltimeterNon-sensitive Altimeter– single pointer/one capsule.

Sensitive Altimeter– multiple pointers/stack

aneroid capsules.Cabin Altimeter– to measure cabin pressure.

Encoding Altimeter– for ATC purposes.

Servo Altimeter– to overcome lag error/more

accurate.Radio/Radar Altimeter – measure vertical distance

Above Ground Level (AGL)– used in GPWS and

autopilot.

Non-Sensitive Altimeter

Sensitive Altimeter

Cabin Altimeter

Encoding Altimeter

Radio/Radar Altimeter



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Encoding Altimeter



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Air Traffic Control Radar Beacon System



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Mode S Operation



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Encoder Altimeter



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Servo Altimeter



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Radar Altimeter Transmitter/Receiver

Uses antenna installed on aircraft belly.Transmitter sends out radio waves at 4.3GHz which strike the earth and bounce back to the receiver antenna.The system measures time taken for the wave to return back and calculation is made of AGL altitude.



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Above 100,000ft– Press 0 psi

37,000 ft– Press 6.39 in Hg– Temp – 56.5°C

30,000 ft– Press 8.88 in Hg– Temp – 44.4°C

10,000 ft– Press 20.57 in Hg– Tem p - 4.8°C

5,000 ft– Press 24.89 in Hg– Temp 5°C

Sea Level (0 ft)– Press 29.92 in Hg– Press 1013.2 Mb– Press 14.7 psi– Temp 15°C

Minus 1,000 ft– Press 31.01 in Hg– Temp 16.98°C



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ALTITUDEThe vertical distance of an aircraft measured from mean sea level (MSL).

PRESSURE ALTITUDEWhen altimeter is set to Standard Mean Sea Level (ICAO) on the barometric window.When an aircraft altimeter is set at this pressure setting, it is flying at Flight Level.

INDICATED ALTITUDEDirect reading of altitude from altimeter when barometric window setting is not set to standard MSL (ICAO).

ABSOLUTE ALTITUDEIt is measured by radio, radar, or laser altimeter.It measured altitude above ground level (AGL).

DENSITY ALTITUDEIt is the pressure altitude corrected for non-standard temperature.

HEIGHTVertical distance of an aircraft from a reference plane (datum).

DATUMA reference line, from which calculations or measurements are taken.

Types of Altitude Measurement



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Q-CodeThe Q code is a telecommunication code. The codes in the range QAA – QZZ (QAA-QNZ are for aeronautical use).

QFESetting aerodrome atmospheric pressure so that an altimeter reads zero on landing and take-off.

QNHSetting Mean Sea Level atmospheric pressure so that an altimeter reads the aerodrome altitude above Mean Sea Level.

QNESetting standard Mean Sea Level atmospheric pressure IAW the ICAO standard atmosphere, i.e. 1013.25 mb or 29.92 in Hg and altimeter will reads pressure altitude.



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Density Altitude

An aircraft altimeter measures only air pressure, nothing else. If the air pressure changes, due to temperature or humidity, then an aircraft altimeter will of course change to indicate the actual air pressure. Nonetheless, the aircraft altimeter is simply measuring air pressure. As odd as it may seem, an aircraft altimeter does not actually measure altitude, it only measures pressure. Hence, the name "pressure altitude" is properly applied to any aircraft altimeter reading. So, the basic idea of calculating density altitude is to calculate the actual density of the air, and then find the altitude at which that same air density occurs in the Standard Atmosphere.



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Error in Altimeter

Instrument error– due to mechanical imperfection during manufacturing.

Position error– due to incorrect location of pressure source.

Blockage error– pitot static piping error.

Lag error– instrument reading responds slower than the actual

altitude of the aircraft.Stiction error– pointer sticking to the dial due to electrostatic effect.– (Friction that tends to prevent relative motion between two movable parts at

their null position.)



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Error in AltimeterDuring ‘calibration check’ in the shop these test are to be complied for IFR flying and any error to be recorded in the altimeter correction card at thousand-foot interval.

Scale error– altimeter must follow reading from master indicator or manometer.

Hysteresis– reading must agree when altitude is in increasing or decreasing.

After effect– indication must return to original reading after performing test.

Friction– reading taken before and after being vibrate.

Case leak– test at 18,000 feet should not leak within tolerance.

Barometric scale error– indication from barometric scale and altimeter reading must tally with

ICAO pressure table.



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Centralized Air Data UnitCADU is introduced to minimized the ‘plumbing’ arrangement of pitot/static system. Pressures are detected and centralized in one unit (ADC-air data computer) and then transmitted electrically to the system/indicators that requires it.A system that utilizes the pitot static pressure and total air temperature to get altitude, true air speed, vertical speed, mach, SAT, TAT etc. Aircraft that operate at high speeds and high altitudes can have significant errors in the pitot-static system instruments with simple probes used on smaller aircraft.These aircraft will use an air data computer (ADC) to operate the ASI, altimeter, VSI, and any other systems that require the data.



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Air Data Computer (ADC)



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Air Data Computer (ADC)ADC has three inputs:

Pitot pressure from the pitot probesStatic pressure from the static portsTotal air temperature from the TAT probe

ADC outputs are:Altitude Indicated AirspeedTrue airspeedMachmeterVertical speedStatic air temperatureTotal air temperature



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Air Data Computer (ADC)

Electro-mechanical ADC

Force balance transducer as a measuring device.

Uses shaft rotation from transducers .

Electrical ADC (called CADC)

Force balance transducer as a measuring device.

Uses electrical signal from transducers.

Digital ADC (DADC)

Uses pressure sensor as a measuring element– Diaphragm with small magnet in centre and pickup coil at the bottom of

diaphragm– Silicon diaphragm contains diffused piezo resistors.– Capacitance transmitter.

Input are fed to microprocessor via ARINC 429.



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ADC – Electro Mechanical



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Computation of Mach Number

Mach number = (P-S)/S

Since analogue computer cannot compute division, then; log (P-S) – log (S)

Hence, log cam and anti-log cam is used.



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Computation of Vertical Speed

The output from tacho generator is proportional to the rate of change of static pressure.ICAO law cam represent ICAO pressure height.Feedback signal is via a follow up pot to null off signal to the amplifier.



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Computation of Height

ICAO law cam provide correct pressure/heightPE error cam corrected errors due to incorrect pressure to transducer.PE due to speed is from the mach computation



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Computation of Airspeed

P-S is dynamic pressure and this is equal to ½ρV²

The cam on the output shaft modifies the shaft rotation to supply square law compensation.



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TAS ComputationTrue airspeed is computed according to the formula TAS = aM (a = local speed of sound. M = Mach number)

Local speed of sound = C√T (T = True airspeed temperature (SAT). C = Constant)

TAS = C √ T X M



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ADC – Electro Mechanical



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CADC – Electrical



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CADC – Electrical



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DADC – Digital Air Data Computer



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DADC – Digital Air Data Computer



75ADCs receive input from the pitot-static sensing devices and process them for use of numerous system



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Pitot-Static leak Test

To determine the rate of leak of a pitot-static system.

Rate of leak is measured in feet per min or knot per minute.

Different aircraft has different leak rate – always refer to aircraft MM.

Leak test on pitot-static system is carried out when;– The pitot static piping and instrument are disturbed

(remove/install).– Suspect of a leak in the pitot static system/instrument.– It is called for in the task card when aircraft comes in for a

maintenance visit. (Every 24 month) – When the drain holes of the pressure head is disturbed.



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ProceduresPitot Leak Test

Connect up Pitotstatic Tester to the pitot head using adaptor.Pump in positive pressure slowly to make airspeed indicator pointer reads V ne – red radial line.– Caution: about ½ PSI is required to make ASI read

150 knots.Retain pressure for 1 minute.If ASI reading drop more than 10 knots means there is a leak in the pitot pipelines.Trace leak from the indicator to source.



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Pitot-Static Leak Tester



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ProceduresStatic Leak Test

Connect up Pitotstatic Tester to the static port using adaptor.Pump in negative pressure slowly to make altimeter reads more than 1,000 feet above current reading.– Caution: When applying suction to the system do not

exceed the vertical speed indicator maximum limit.Retain pressure for 1 minute.If altimeter reading drop more than 100 feet means there is a leak in the static pipelines.Trace leak from the indicator to source using soap water.



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Caution when working on Pitot Static System

The amount of pressure required for 150 MPH indication is less than ½ PSI .– Avoid high pressure as instruments damage will result.

Do not apply suction to pitot pressure lines. When applying or releasing suction, take care not to exceed

rate range of vertical speed indicator. Do not apply pressure to static lines with instruments

connected. Never blow through the pitot or static lines toward the

instruments.– Doing so may damage them.

Pitot Static System

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