1. Pressure Instruments
Chapter 13

2. Aim
To review principals of operation of the pressure instruments

3. Objectives Define International Standard Atmosphere
Describe the Pitot & Static System
State what each instrument indicates, name the power source for each instrument and state the errors associated with each instrument
State the effect of system failures on instrument indications

4. 1. International Standard Atmosphere
International Standard Atmosphere (ISA)
Aircraft performance relies on the atmospheric qualities of temperature, pressure, density and humidity
These qualities vary greatly depending on position within the atmosphere. Therefore, the International Standard Atmosphere (ISA) was developed to provide a reference
Standards set by ICAO for ISA conditions include:
Pressure at sea level hPa
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

5. 2. Static and Dynamic Pressure
Static Pressure
Static pressure is the result of the weight of air molecules in the atmosphere and exerts pressure equally in all directions
Pressure is higher at sea level and decreases with height
There is no movement considered in static pressure
Static pressure is measured using a barometric device, units of measurement are hectopascals (hPa) or inches of mercury (inHg)

6. 2. Static and Dynamic Pressure
Dynamic pressure is the result of relative movement. It exerts pressure in one direction based on the relative airflow
Pressure is dependent on density and velocity and can measured using a Pitot tube and static port
Dynamic pressure is expressed as ½ρV² where:
ρ air density
V velocity
Velocity

7. 2. Static and Dynamic Pressure
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 air
Total pressure is the addition of these two pressures and is the pressure measured in the Pitot tube
Static pressure + Dynamic Pressure = Total Pressure
Ps + (½ρV²) = Pt (Bernoulli’s Equation)

8. 3. Instruments indications and source
Altimeter
Directly indicates the aircrafts height above a datum
In Australia below 10,000ft, sea level is set as the datum
Indirectly indicates pitch attitude if power is known:
Altitude decreasing, nose low
Altitude increasing, nose high
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 system
The capsule expands/contracts as the static pressure changes and this is mechanically transmitted to the gauge indicating altitude

9. 3. Instruments indications and source
The Encoding Altimeter (Mode C)
Some altimeters are equipped with altitude encoders (or digitisers) which provide a coded output signal
When transmitted via a remote transponder, the coded signal enables the pressure altitude sensed by the capsules to be monitored by air traffic control
The coded output is always referenced to 1013hPa, showing pressure altitude, and is independent of the pilots’ subscale setting

10. 3. Instruments indications and source
Altimeter Errors
Barometric 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 high
Temperature error - Is due to temperature deviations from ISA
The altimeter over-reads in cooler than ISA
The altimeter under-reads in warmer than ISA

11. 3. Instruments indications and source
Altimeter Errors
Position error - Occurs because of static system errors related to airflow sampling. Errors vary with speed and altitude and include manoeuvre-induced errors
Instrument error - Is due to small manufacturing imperfections and the large mechanical amplification necessary for small sensed movements. Instrument error increases with altitude
Lag 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 rapidly
Blockages - 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

12. 3. Instruments indications and source
Altimeter Serviceability
In 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-off
In order of priority, the pilot should use tarmac, threshold or airfield reference point elevation for the check
Tolerances are:
Within 100ft for VFR (110ft if above 3300ft elevation)
Within 60ft for IFR (if more than 75ft, unserviceable)

13. 3. Instruments indications and source
Vertical Speed Indicator (VSI)
Directly indicates the rate of change of altitude
Indirectly indicates pitch attitude if power setting known
VSI decreasing, nose low
VSI increasing, nose high
Useful as a trend indicator
The vertical speed indicator is driven by the aircrafts static vent. It indicates rate of climb/descent in feet per minute

14. 3. Instruments indications and source
Vertical Speed Indicator Errors
Lag error - The VSI is slow to respond to pressure changes resulting in lag error
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
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

15. 3. Instruments indications and source
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

16. 3. Instruments indications and source
Airspeed Indicator (ASI)
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
Indirectly indicates the pitch attitude of the aircraft
Low or decreasing airspeed, nose high
High or increasing airspeed, nose low
Total pressure is fed into a diaphragm, static pressure into the sealed instrument case. Mechanical linkage is used to drive the pointer
Total pressure – Static pressure = Dynamic pressure (½ρV² )

17. 3. Instruments indications and source
Airspeed indicator (ASI)
Airspeed indicators use markings and colours to designate key airspeeds and ranges
Green arc: normal operating range
Yellow arc: the caution range
White arc: flap operating range
Red line: VNE: never-exceed speed
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

18. 3. Instruments indications and source
Airspeed indicator (ASI) Errors
Density error - is caused by operations other than at ISA conditions.
Compressibility error - corresponds to increase in airspeed but is only applicable above 240KCAS and 10,000ft
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)
Instrument Error - is due to small manufacturing imperfections and the large mechanical amplification necessary for small sensed movements. Instrument error increases with airspeed
Blockage - If the static port becomes blocked, on climb the ASI will under-read and on descent it will over-read
If the pitot tube becomes blocked, on climb the ASI will over-read and on descent it will over-read

19. 3. Instruments indications and source
Expressions of Airspeed
IAS (indicated airspeed): airspeed indicated from the pitot/static system
CAS (calibrated airspeed): IAS corrected for position/instrument error
EAS (equivalent airspeed): CAS corrected for compressibility error
TAS (true airspeed): EAS corrected for density error
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
At 5,000ft TAS exceeds IAS by 8%
At 10,000ft TAS exceeds IAS by 17%

20. 4. System failures
Pitot Static System

21. 4. System failures Pitot Blockage Instruments affected
Airspeed indicator
Indicated by:
No airspeed increase during takeoff roll
ASI over or under reading if airborne
Troubleshooted by:
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

22. 4. System failures Pitot Blockage Airspeed 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

23. 4. System failures Static Blockage Instruments affected
Airspeed indicator
Altimeter
Vertical speed indicator
Indicated by:
ASI over or under reading
Altimeter not increasing with an increase in altitude
VSI reading zero with an increase in altitude
Troubleshooted by:
Alternate static source may be required, refer to flight manual

24. 4. System failures Static Blockage Airspeed 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

25. 4. System failures Static Blockage Altimeter 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

26. Questions?