pressure instruments chapter 13. aim to review principals of operation of the pressure instruments

Pressure InstrumentsChapter 13

Aim

To review principals of operation of the pressure instruments

Objectives1. Define International Standard Atmosphere2. Describe the Pitot & Static System3. State what each instrument indicates, name the

power source for each instrument and state the errors associated with each instrument

4. State the effect of system failures on instrument indications

International Standard Atmosphere (ISA)Aircraft performance relies on the atmospheric qualities of temperature, pressure, density and humidityThese qualities vary greatly depending on position within the atmosphere. Therefore, the International Standard Atmosphere (ISA) was developed to provide a referenceStandards set by ICAO for ISA conditions include:• Pressure at sea level 1013.25hPa• Temperature at sea level +15°C• Air temperature decreases at 1.98°C/1000ft• Temperature decreases at -56.5°C at 36,089ft, then remains constant

1. International Standard Atmosphere

Static PressureStatic pressure is the result of the weight of air molecules in the atmosphere and exerts pressure equally in all directionsPressure is higher at sea level and decreases with heightThere is no movement considered in static pressureStatic pressure is measured using a barometric device, units of measurement are hectopascals (hPa) or inches of mercury (inHg)

2. Static and Dynamic Pressure

Dynamic PressureDynamic pressure is the result of relative movement. It exerts pressure in one direction based on the relative airflowPressure is dependent on density and velocity and can measured using a Pitot tube and static portDynamic pressure is expressed as ½ρV² where:ρ air densityV velocity


Velocity

Total Pressure (Pitot Pressure)Static pressure is always exerted, but, for dynamic pressure to be present, there must also be motion of a body relative to the airTotal pressure is the addition of these two pressures and is the pressure measured in the Pitot tube

Static pressure + Dynamic Pressure = Total PressurePs

+ (½ρV²) = Pt (Bernoulli’s Equation)


AltimeterDirectly indicates the aircrafts height above a datumIn Australia below 10,000ft, sea level is set as the datum

Similar to an aneroid barometer, the altimeter is a partially evacuated metal capsule placed in an instrument casing that is completely sealed and connected to the static systemThe capsule expands/contracts as the static pressure changes and this is mechanically transmitted to the gauge indicating altitude

Indirectly indicates pitch attitude if power is known:• Altitude decreasing, nose low• Altitude increasing, nose high

3. Instruments indications and source

The Encoding Altimeter (Mode C)


Some altimeters are equipped with altitude encoders (or digitisers) which provide a coded output signalWhen transmitted via a remote transponder, the coded signal enables the pressure altitude sensed by the capsules to be monitored by air traffic controlThe coded output is always referenced to 1013hPa, showing pressure altitude, and is independent of the pilots’ subscale setting

Altimeter ErrorsBarometric error – Is the error induced in an altimeter when atmospheric pressure at sea level differs from ISA. Below 5,000ft every 1 hPa that the altimeter is in error is equal to 30ft error displayed on the altimeter• If the subscale setting is too low, the altimeter will read low• If the subscale setting is too high, the altimeter will read highTemperature error - Is due to temperature deviations from ISA• The altimeter over-reads in cooler than ISA• The altimeter under-reads in warmer than ISA


Altimeter ErrorsPosition error - Occurs because of static system errors related to airflow sampling. Errors vary with speed and altitude and include manoeuvre-induced errorsInstrument error - Is due to small manufacturing imperfections and the large mechanical amplification necessary for small sensed movements. Instrument error increases with altitudeLag error - Is caused by the response of the capsule and linkage not being instantaneous and so the altimeter reading lags when height is increased or decreased rapidlyBlockages - When the static port is blocked, the altitude indicated upon blockage will remain constant. With partial blockage, lag will be induced Orographic effect - Produces a reduced pressure on the lee side of mountains causing the altimeter to over-read. Combined with likely downdraughts, pilots must be aware of terrain clearance and climb performance


Altimeter ServiceabilityIn Australia in accordance with AIP ENR 1.7 Altimeter setting procedures, whenever an accurate QNH is available and the aircraft is at a known elevation, pilots must conduct an accuracy check of the aircraft altimeter(s) at some point prior to take-offIn order of priority, the pilot should use tarmac, threshold or airfield reference point elevation for the checkTolerances are:• Within 100ft for VFR (110ft if above 3300ft elevation)• Within 60ft for IFR (if more than 75ft, unserviceable)


Vertical Speed Indicator (VSI)Directly indicates the rate of change of altitude

The vertical speed indicator is driven by the aircrafts static vent. It indicates rate of climb/descent in feet per minute

Indirectly indicates pitch attitude if power setting known• VSI decreasing, nose low• VSI increasing, nose high

Useful as a trend indicator


Vertical Speed Indicator ErrorsLag error - The VSI is slow to respond to pressure changes resulting in lag error

Blockages - When the static port is blocked, pressure inside and outside the capsule will equalise. During climb/decent the VSI slowly return to zero. For S&L, it will remain at zero. Partial blockage will result in sluggish response

Position/Manoeuvre induced error - Is caused by airspeed changes and manoeuvres. The VSI indicates false climb/descents

Instrument Error - is due to small manufacturing imperfections and the large mechanical amplification necessary for small sensed movements. Instrument error increases with altitude


Instantaneous Vertical Speed Indicator (IVSI)


Some aircraft are equipped with an instantaneous vertical speed indicator (IVSI), which incorporates accelerometers to compensate for the lag in the typical VSI

It assists in interpretation by instantaneously indicating the rate of climb or descent at a given moment with little or no lag as displayed in a vertical speed indicator (VSI)During turns, the accelerometer pump may be displaced introducing a turning error and therefore is considered unreliable during steeply banked turns


Directly indicates the aircrafts speed through the air by measuring the difference between total pressure from the Pitot tube and the static pressure from the static source

Total pressure is fed into a diaphragm, static pressure into the sealed instrument case. Mechanical linkage is used to drive the pointer

Indirectly indicates the pitch attitude of the aircraft• Low or decreasing airspeed, nose high• High or increasing airspeed, nose low

Total pressure – Static pressure = Dynamic pressure (½ρV² )

Airspeed Indicator (ASI)

3. Instruments indications and sourceAirspeed indicator (ASI)

Airspeed indicators use markings and colours to designate key airspeeds and ranges

• VS0: Stalling speed max. weight, undercarriage down, flaps down, power off

• VS1: Stalling speed max. weight, undercarriage up, flaps up, power off

• VFE: max. speed flaps extended

• Green arc: normal operating range • Yellow arc: the caution range• White arc: flap operating range• Red line: VNE: never-exceed speed


Density error - is caused by operations other than at ISA conditions.

Blockage - If the static port becomes blocked, on climb the ASI will under-read and on descent it will over-readIf the pitot tube becomes blocked, on climb the ASI will over-read and on descent it will over-read

Instrument Error - is due to small manufacturing imperfections and the large mechanical amplification necessary for small sensed movements. Instrument error increases with airspeed

Compressibility error - corresponds to increase in airspeed but is only applicable above 240KCAS and 10,000ft

Airspeed indicator (ASI) Errors

Position error - is related to static pressure sampling errors induced during varied speed, attitude and configurations. Some manufacturers publish Pressure Error Correction (PEC) charts which are adjusted for Calibrated airspeed (CAS)


IAS (indicated airspeed): airspeed indicated from the pitot/static systemCAS (calibrated airspeed): IAS corrected for position/instrument errorEAS (equivalent airspeed): CAS corrected for compressibility errorTAS (true airspeed): EAS corrected for density error

At 5,000ft TAS exceeds IAS by 8%At 10,000ft TAS exceeds IAS by 17%

For conversion of IAS to TAS, firstly correct for instrument/position error using a table in the aircraft flight manual Correction for density error is achieved by use of a flight computer Alternatively, some ASIs have a calibration card for TAS

Expressions of Airspeed

Pitot Static System4. System failures

• Airspeed indicator

Pitot BlockageInstruments affected

Indicated by:

Troubleshooted by:

• No airspeed increase during takeoff roll• ASI over or under reading if airborne

• If failure is identified during the takeoff roll abort the takeoff if it is safe to do so

• If failure occurs airborne Pitot heat may be required, refer to flight manual

4. System failures

Pitot BlockageAirspeed Indicator

Failure indicted by:• No airspeed increase during take off roll• If Pitot tube blockage occurs during

flight the ASI is only read correctly at blockage altitude- If aircraft climbs above blockage

height ASI over-reads- If aircraft descends below blockage height ASI under-reads

4. System failures

• Airspeed indicator• Altimeter• Vertical speed indicator

Static BlockageInstruments affected

Indicated by:

Troubleshooted by:

• ASI over or under reading• Altimeter not increasing with an increase in altitude• VSI reading zero with an increase in altitude

• Alternate static source may be required, refer to flight manual

4. System failures

Static BlockageAirspeed Indicator

Failure indicted by:• If static source blockage occurs

during flight the ASI is only correct at blockage altitude- If aircraft climbs above

blockage height ASI under-reads

- If aircraft descends below blockage height ASI over-reads

4. System failures

Static BlockageAltimeter

Failure indicted by:• Will continue to read blockage altitude

with an increase or decrease in altitude

Vertical Speed Indicator

Failure indicted by:

• Will continue to read zero with an increase or decrease in altitude

4. System failures

Questions?

pressure instruments chapter 13. aim to review principals of operation of the pressure instruments

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