Transcript Chapter 13 PowerPoint

Pressure Instruments
Chapter 13
Aim
To review principals of operation of the
pressure instruments
Objectives
1. Define International Standard Atmosphere
2. Describe the Pitot & Static System
3. 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
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 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
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)
2. Static and Dynamic Pressure
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
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)
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
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
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
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 manoeuvreinduced 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
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)
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
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
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
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 )
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
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 underread 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
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%
4. System failures
Pitot Static System
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
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
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
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 underreads
- If aircraft descends below
blockage height ASI over-reads
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
Questions?