Instruments - Complete Summary
Table of Contents
The basic T
Pitot static system
ADC driven systems
Units and accuracy
Line blockages and leaks
Altimeter pressure settings
Vertical Speed Indicator
Instantaneous VSI’s (IVSI)
Displaying the information
Errors and blockages
Measuring Air Speed
Air speed indicators
Calibrated Air Speed (CAS)
Pitot static blockages and leaks
Pitot blockages and leaks
Air Data Computer
Compass and errors
Direct reading compass
Rate integrating gyros
Ring laser gyros
Principle of operation
Measuring turn rates
Air driven gyro erection
Electrically driven attitude indicators
Servo driven attitude indicators
Vertical gyros and the AHR’s
DI Drift calculations
Remote reading compass and errors
Remote reading compasses
IRS and INS
Inertial Navigation Principles
Alignment and level ing
Latitude and longitude
Controls and indicators
INS – normal operation
Stable platform configuration
Wander angle INS
Inertial system errors
IRS outputs and inputs
Flight Management Systems
The navigation database
The performance database
LNAV and VNAV
Changing the routing
Flight envelope protection
There are two types of indicators:
- Direct indicators. The input, like pressure, is directly fed to the instrument.
- Servo-driven indicators. Input data is fed to a computer and converted to electrical signals,
which then drive needles in the cockpit.
The basic T
- Airspeed indicator
- Attitude indicator
These are the primary flight instruments. Others are useful
extras. The arrangement aside is standard.
When a turn indicator and a vertical speed indicator are added, you’l get the so-called ‘six-pack.’
The image above il ustrates these two instrument (bottom left and right).
When the Electronic Flight Instrument System (EFIS) is used to display primary information on a
LCD-screen, the basic T arrangement is no longer used. Most of the time, standby instruments are
fitted into the cockpit to make sure there is a back-up when the electronic system fails.
a. Freezing point of water à 0 degrees
b. Boiling point of water à 100 degrees
a. Freezing point of water à 273 degrees
b. Boiling point of water à 373 degrees
c. Absolute zero is equal to -273 degrees Celsius
a. Freezing point of water à 32 degrees
b. Boiling point of water à 212 degrees
Degrees Fahrenheit = (degrees Celsius * 9 / 5) + 32
- Bimetallic sensors.
Two metals bound together. While heating, one of the two wil expand more than the other,
causing a flat strip to bend or a coil to uncoil.
- Resistance thermometers/ Resistance Temperature Detector (RTD).
Relies on the change of electrical resistance of a pure metal such as platinum, nickel or
copper as temperature changes. Wide range of temperatures possible: -200 up to +600
degrees Celsius. The R1, 2 and 3 are of a known value. If the value of RX,
depending on the temperature, is the same, there wil not be a
voltage between D and B. Whenever there is a difference
between RX and R1 etc., the difference wil be measured and
converted to a temperature. The equipment used to measure the voltage is cal ed a galvanometer.
Used for very high temperatures.
A known temperature is created at the ‘measuring’
junction (TSense). Due to the use of two different metals
the two sides between the galvanometer, a potential
difference occurs. This difference stabilises at the
reference unit (TREF). From there, it is fed to the
instrument via a coper wire. This instrument, the
galvanometer, converts it to a temperature. The electromotive force (E or emf) can be
calculated: ! = # ∗ %& where K is a constant, and Th is the hot junction temperature. Can be used between -200 and 1250 degrees Celsius.
- Radiation pyrometers.
For an even higher temperature, an optical radiation pyrometer can be used. This type of
sensor measures the frequency of the emitted radiation from the area to be examined, and
is normal y used to measure turbine blade temperatures at the turbine inlet area of a jet
Air temperature probes
Two types of probes available for measuring air temperature:
- Expansion type. Utilises a bimetallic strip, also referred to as a direct reading probe.
- Electrical wire/resistance type. Relies on change of electrical resistance with change of
temperature, also referred to as the remote reading type.
Speed and temperature rise
When the speed is high, the temperature wil rise too due to ram rise/ kinetic heating. This is
caused by frictional heating. Therefore, the temperature sensing wil be distorted. Therefore, there
are two temperatures commonly used to distinguish the two:
- Static air temperature (SAT). The temperature you would measure when in a hot air
bal oon, so without forward speed.
- Total air temperature (TAT). The temperature sensed with the appearance of the kinetic
%'% = ('% ∗ (* + ,. ./.)
Temperatures in Kelvin!
The sensing systems used are not 100% efficient. Therefore, the above formula needs a
correction factor, which is a constant. A value of 0.9 is typical. This constant recovery factor is
called the recovery factor.
Temperature sensing on faster aircraft
First, the air passes a heater, equipped to the system to
prevent icing from building up throughout the flight. Then, the
air is accelerated through a Venturi. The reason for this, is to
speed up water and dust in the air, which wil move forward
to the exit of the system due to momentum.
The ‘clean’ air then bends off, and moves towards the
sensing element. The temperature is measured by a
resistive element, usual y nickel.
Temperature measurement errors
Three sources of error:
1. Instrument error. Caused by imperfections in the manufacturing and can be compensated
for by fine calibration of the instrument.
2. Environmental error. Solar heating or ice accretion. Prevented by creating a ‘shadow’ so
the instrument is not heated by solar radiation any more.
3. Heating error. Comes from ram rise or frictional heating. It is only an error when you want
to find the SAT. The TAT is the higher temperature, caused by the ram effect. So, not an
The time used in aviation worldwide is UTC. Time is an important parameter in engine and systems
maintenance. Therefore, all modern aircraft are equipped with an electrical clock.
Used in smal er aircraft to measure hours, and tenths of hours. It is not related to UTC or
something like that, it just records time flown. It may be activated:
- Electrically from the moment the aircraft is powered up
- By oil pressure signifying engine running
- By a weight-on-wheels switch or occasional y
- By an airspeed sensing vane to record flight time
Used to determine load factors. The simplest form is just a weight mounted on a spring.
Acceleration extends or compresses the spring and a suitable scale records the value of the
It makes use of:
- I-bar. The weight itself.
- E-bar. Coils.
The bobweight is centred, so that the magnetic flux of the E-
bars is cancelling each other out. When the I-bar moves, the
flux measured wil not be equal anymore; an output signal
wil be generated.
This is the smallest type available. Stands for Micro Electro Mechanical System accelerometers.
An IRS wil have three accelerometers mounted at right angles to each other, to sense
acceleration in all planes.