Chapter Five

Avionics
Yalew Mekonnen (PhD)
5.1. Defining Avionics

 Avionics are the electronic systems used on aircraft.

 Avionic systems include control, communications, navigation, the display
and management of multiple systems, and the hundreds of systems that
are fitted to aircraft to perform individual functions.
 Control apparatus includes the attitude gyro and any number of
instruments that indicate power, such as the tachometer (in propeller
craft), torquemeter (in turboprops), and exhaust pressure ratio indicator
(in turbojets).
 Performance instruments include the altimeter, Machmeter, turn and slip
indicator, and varied devices that show airspeed, vertical velocity, and
angle of attack.

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 Communications instruments include two-way radios allowing direct
voice communication between the aircraft and the ground as well as
other aircraft; these operate across a wide spectrum, ranging from high
frequency (HF) through very high frequency (VHF) to ultrahigh frequency
(UHF).
 Electronic radio navigation equipment ranges from radar to instrument
landing systems.

Figure 5.1:
Radar and other avionics in the nose
of a Cessna Citation I/SP

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 Figure 5.2:
Core Avionics System

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5.2. Avionics: Its Role in Flight and Historical Context
 Avionics play a crucial role in flight operations, encompassing various
functions vital for safe and efficient aircraft operation.
 Here are some lists of roles of avionics in flight:
1. Communication: Avionics handle all communication between the aircraft,
air traffic control, and other aircraft, enabling clear and reliable
information exchange for safe flight operations.
2. Navigation: Avionics provide accurate and reliable navigational data,
enabling pilots to navigate complex routes and avoid potential hazards.
3. Flight Control: Avionics systems control the aircraft's flight, including
pitch, roll, and yaw, making it possible for pilots to maneuver and
maintain stability in flight.

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4. Monitoring: Avionics continuously monitor critical aircraft parameters
like engine performance, fuel levels, and structural integrity, alerting
pilots to potential issues and ensuring safe flight conditions.
5. Maintenance: Avionics technicians are responsible for ensuring the
proper functioning of all avionics systems, including installation,
maintenance, repair, and troubleshooting of any malfunctions.
 In essence, avionics are the backbone of modern flight operations,
providing the technology and systems necessary for safe and efficient
aircraft navigation and operation.

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Historical Background
 The term "avionics" was coined in 1949 by Philip J. Klass, senior editor at
Aviation Week & Space Technology magazine as a portmanteau of
"aviation electronics".
 Radio communication was first used in aircraft just prior to World War I.
 The first airborne radios were in zeppelins, but the military sparked
development of light radio sets that could be carried by heavier-than-air
aircraft, so that aerial reconnaissance biplanes could report their
observations immediately in case they were shot down.
 The first experimental radio transmission from an airplane was
conducted by the U.S. Navy in August 1910. The first aircraft radios
transmitted by radiotelegraphy.

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5.3. Fundamental Electrical Concepts
 The electrical system is of great importance. Many elements run with
electric energy, e.g: Indicator instruments, navigation and communication
equipments, electro-actuators, electro-pneumatic mechanisms,
illumination, passenger comfort (meals, entertainment, etc).
 The cockpit or, in larger aircraft, under the cockpit of an aircraft or in a
movable nosecone, is a typical location for avionic bay equipment,
including control, monitoring, communication, navigation, weather, and
anti-collision systems.
 The majority of aircraft power their avionics using 14- or 28‑volt DC
electrical systems; however, larger, more sophisticated aircraft (such as
airliners or military combat aircraft) have AC systems operating at 115 volts
400 Hz, AC

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Types of Systems:
 DC Systems: Most common in general aviation, using either 12 or 24 volts.
 AC Systems: Common in larger aircraft, providing AC power to various
systems, often converted to DC for use.
 There are different power generation sources for aircraft. They can be
either for nominal conditions, for redundancy, or to handle emergency
situations. These include:
 Engine driven AC generators.
 Auxiliary Power Units (APU): An independent engine that can provide
power when the main engines are not running.
 External power, also referred to as Ground Power Unit (GPU).
 Ram Air Turbines (RAT): A device that can generate power from the
airflow during an emergency.
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 Transformer Rectifier Unit (TRU) :
The most frequently used method of power
conversion in modern aircraft electrical
system

Auxiliary Power Units (APU)

Figure 5.3: A380 power system components

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5.4. Essential electronic components used in avionics systems

 Essential electronic components in avionics systems include sensors,

flight control systems, navigation systems, communication systems, and
various passive components like resistors, capacitors, and inductors.
 These components work together to manage the electrical and electronic
systems of an aircraft, ensuring safe and efficient flight operations.
Active Components:
1. Integrated Circuits (ICs): These are the "brains" of the avionics systems,
performing complex calculations and controlling various functions, says a
guide to the basics of electronic components.
2. Sensors: These collect data from various parts of the aircraft, including flight
controls, engine performance, and environmental conditions.
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3. Transistors: These are essential for amplifying signals and switching
circuits, says a guide to the basics of electronic components.
4. Diodes: These allow current to flow in one direction, playing a crucial role
in rectifying AC power to DC power.
Passive Components:
1. Resistors: These control the flow of current in a circuit, says VPG Foil
Resistors.
2. Capacitors: These store electrical energy, says Cosmo Films.
3. Inductors: These store energy in a magnetic field, says Justdial.

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5.5. Navigation & Communication
 The air navigation is the process of steering an aircraft in flight from an
initial position to a final position, following a determined route, and
fulfilling certain requirements of safety and efficiency.
 The navigation is performed by each aircraft independently, using diverse
external sources of information and proper on board equipment.
 Besides the primary goal above mentioned (safe and efficient controlled
flight towards destination), three important additional goals can be
mentioned:
• Avoid getting lost.
• Avoid collisions with other aircraft or obstacles.
• Minimize the influence of adverse meteorological conditions.

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 Communication Systems
 The technical means included under the term aeronautical
communication fulfill a mission of spreading any information of interest
to aircraft operations.
 According to the provided service, International Civil Aviation
organization (ICAO) has classified the communications in two main
groups:
Aeronautical Fixed Service (AFS): between terrestrial stations, i.e.,
fixed stations.
Aeronautical Mobile Service (AMS): between terrestrial stations and
aircraft (mobile stations).

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5.6. Flight Instrument & Displays

 The instruments to be installed in an aircraft are, on the one hand, flight

and navigation instruments, and, on the other, instruments of the power
plant.
 Flight instruments are specifically referred to as those instruments
located in the cockpit of an aircraft that provide the pilot with the
information about the flight situation of the aircraft, such as

 The term is sometimes used loosely as a synonym for cockpit

instruments as a whole, in which context it can include engine
instruments, navigational instruments, and communication equipment.

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1. Flight and navigation instruments
ICAO establishes that the minimum required flight and navigation instruments
are:
Airspeed indicator: The airspeed indicator presents the aircraft’s speed (usually
in knots) relative to the surrounding air. It works by measuring the pressure
(static and dynamic) in the aircraft’s pitot tube. The indicated airspeed must be
corrected for air density (using barometric and temperature data) in order to
obtain the true airspeed, and for wind conditions in order to obtain the ground
speed.
Attitude indicator (artificial horizon): The attitude indicator (also known as an
artificial horizon) presents the aircraft’s attitude relative to the horizon. This
instrument provides information to the pilot on, for instance,
.
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 Altimeter: The altimeter presents the altitude of the aircraft (in feet) above a
certain reference by measuring the difference between the pressure in aneroid
capsules inside the barometric altimeter atmospheric pressure obtained
through the static ports.
 Heading indicator (directional gyro): The heading indicator (also known as the
directional gyro) displays the aircraft’s heading with respect to the magnetic
north. The principle of operation is based on a gyroscope.
 Magnetic compass: The compass shows the aircraft’s heading relative to
magnetic north.
 Turn indicator (turn and slip): The turn indicator (also known as turn and slip)
displays direction of turn and rate of turn.

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 Vertical speed indicator (variometer): The vertical speed indicator (also referred
to as variometer) displays the rate of climb or descent typically in feet per
minute. This is done by sensing the change in air pressure.

Figure 5.6: Six basic instruments in a light twin-engine airplane arranged in a basic-T. From top left:
airspeed indicator, attitude indicator, altimeter, turn coordinator, heading indicator, and vertical speed
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2. Power plant instruments
 ICAO also establishes a minimum required set of instruments for the power plant.
We will just mention a few, not going into details:
 Tachometer for measuring the velocity of turn of the crankshaft (or the compressor
in a jet).
 Indicator of the temperature of air entering the carburetor (just for piston aircraft).
 Indicator of the temperature of oil at the entrance and exit.
 Indicator of the temperature at the entrance of the turbine and the exit gases (just
for jet aircraft).
 Indicator of fuel pressure and oil pressure.
 Indicator of tank level.
 Indicator of thrust (jets) and motor-torque (propellers)
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 Figure 5.7: Aircraft cockpit
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A pitot tube is an instrument used to measure fluid flow velocity. A basic pitot
tube consists of a tube pointing directly into the fluid flow, in which the fluid
enters (at aircraft’s airspeed). The fluid is brought to rest (stagnation). This
pressure is the stagnation pressure of the fluid, which can be measured by an
aneroid.

Figure 5.7:
Pitot tube

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 The pressure at the pitot tube can be calculated using the equation:

 with γ = 1.4 the adiabatic coefficient of air, and M the Mach number:

where VTAS is the true airspeed. It is considered as calibrated air speed in the
case of pitot tube analysis, by taking in to account of the mean sea-level
pressure, PMSL, and mean sea-level density, ρMSL.

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5.7. Safety & Surveillance
 Avionics safety focuses on the reliability and security of the electronic
systems used in aircraft, while surveillance systems help monitor and track
aircraft for safety and air traffic management.
 Avionics safety encompasses the measures and technologies used to
ensure the reliability and security of electronic systems used in aircraft
operation, including rigorous testing, maintenance, and innovative solutions.
 Surveillance data is used by air traffic controllers to monitor aircraft,
provide guidance, and ensure that aircraft are departed safely.
 Avionics safety and surveillance systems are often integrated and work
together to ensure safe and efficient air travel.

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5.8. Avionics Integration & Future
 The combination, interconnection and control of theindividual sub-systems
so that the overall system can carry out its tasks effectively are referred to
as integrated system.
 The first major step towards integrating avionic system was taken in 1950s
with the establishment of the weapon system concept.
 The integration of avionic sub-systems in civil aircraft was taken in 1950s
with the adoption of ARINC specifications.
 ARINC defines systems and equipment specifications in terms of
functional requirements and physical dimensions and electrical interfaces.

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Reading Assignment:
 Aircraft Systems

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