AIRCRAFT ELECTRICAL SYSTEMS

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AIRCRAFT ELECTRICAL SYSTEMS
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Objectives

• Students will be able to
• Describe the basic components of aircraft
  electrical system
• Explain operation of electrical system
• Interpret aircraft electrical diagram
• Select proper size of wire for installation
• Describe basic causes of electrical system
  malfunctions

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Elements of aircraft electrical systems

• An aircraft electrical system is mainly composed
  of
• Power sources
• Components
• - Control devices
• - Conversion devices
• - Protection devices.
• Power distribution systems
• Electrical loads

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Electrical Power Sources

• Electricity power sources on an aircraft may
  be classified into two groups
• Batteries
• - Lead-acid
• - Nickel-cadmium
• Generators
• - DC generators
• - AC generators

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Battery

• A battery is a device that converts chemical
  energy into electrical energy.
• It is a power reservoir that stores energy in
  chemical form.It does not produce energy.
• Its functions are
• - To provide power when no other power source
  is available
• - To assist in damping transient loads in the
  dc system

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• - To provide a short term source during
  emergency condition
• The capacity of battery is measured in
  ampere-hours.
• Its normal rate is a little over 24vdc in a 28vdc
  system.
• It is automatically recharged when the
  engine-driven generator is operational.
• Two types of batteries are used in aircraft
• - Lead-acid batteries
• - Nickel cadmium batteries

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Lead-acid battery

• It is usually found in piston aircraft.
• It is made up of cells which have
  positive/negative plates of lead and filled with
  electrolyte of sulfuric acid and water.
• Each cell has app. 2.2v, but is simply rated as 2
  v.
• It has corrosive effects.
• Frequent total discharge and remaining battery in
  discharged condition for a long time will shorten
  the life of the battery.

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Battery Cell
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Nickel cadmium battery

• They are the most common type of battery used in
  turboprop and turbojet aircraft.
• They provide electrical discharge at a high rate
  without voltage drop and accept high charge
  rates that shorten recharge time
• They may be subject to thermal runaway condition
  caused by overheating, in which the battery
  destructs itself.

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Nickel-cadmium battery
Cell Terminals
Vent pipe
Container
Main connector
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GENERATORS

• A generator is a machine that converts mechanical
  energy into electrical energy by the process of
  electromagnetic induction
• They are driven by the engine(s), and sometimes
  by APU.
• A generator must be rated at adequate amperage to
  drive all the operating components on its
  circuit(s). The current required to operate each
  electrical component is known as its load.

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• All generators produce alternating current
  naturally.The method which is used to take it
  from the coil will determine if the generator
  provides ac or dc to the circuit.
• If a commutator is used for this purpose it will
  be a dc generator.
• If a slip ring is used it will be an ac generator.

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• DC GENERATORS
• Commutator in the generator converts the ac
  current to dc current.
• DC generator is designed to supply nearly
  constant voltage. Usually the voltage is 28vdc,
  but there are 270vdc systems developed recently.
• They supply current up to 400 A.
• Most aircraft do not have a separate dc
  generator, but ac produced by ac generator is
  converted to dc to power dc systems.

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Basic dc generator
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Operation of a basic dc generator
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Producing steady dc
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AC generator (Alternator)

• On most large aircraft high-load electrical
  devices are usually AC powered.
• AC is produced by ac generator which is usually
  called alternator.
• AC generators use slip rings instead of
  commutators to provide the current to the
  circuits.
• Alternator generates three-phase current, and
  standard aircraft voltage is 115 vac with 400 Hz.

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Basic ac generator
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Modern electrical power generation types

• Mainly two types of electrical power
  generation currently in use on aircraft.
• - Constant frequency Integrated drive
• generators (IDG)
• - Variable speed constant frequency
• (VSCF) generators

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Constant Speed IDG

• It is essential that the alternator output have a
  frequency of 400 Hz with a very small tolerance.
  Thus rotational speed of alternator must remain
  absolutely constant.
• This is accomplished by using a Constant Speed
  Drive (CSD) unit between the engine and
  alternator.
• CSD provides a constant alternator rpm within a
  specified engine rpm range.

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• An alternator and constant speed drive unit
  combination is called integrated drive generator
  (IDG).
• CSD is hydromechanically operated and needs to
  be correctly maintained in terms of oil level and
  oil cleanliness.
• It is the most commonly used power generation
  method on today turbine-powered aircraft.

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Integrated Drive Generator
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Constant speed drive
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Constant Speed Drive
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VSCF

• In this method, the variable frequency power
  produced by the alternator is converted to
  constant frequency 400 Hz, 115 vac by solid-state
  devices electronically.
• Thus there is no need for CSD unit.
• It is a new technique and limited in use.

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Variable speed constant frequency generator
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Power Generation Control

• Power produced by generators is usually
  controlled by generator control unit (GCU).
• The main functions of this device are
• - Act as a voltage regulator
• - Direct current to battery for recharging
• - Provide circuit and generator protection
• by disconnecting the generator from the
• system when electrical abnormalities
• occur.

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Control Devices

• These devices are use to initiate and control the
  operation of the circuits.Control devices include
• Switches
• Rheostats
• Relays
• Solenoids

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Switches

• A switch is used to start, to stop, or to change
  the direction of the current flow in the circuit.
• Toggle switches They are on/off switches and
  extensively used in aircraft electrical system.
• Push Switches They are used primarily for
  operations of short durations
• Rotary switches When it is necessary to select
  several condition for a circuit, a rotary switch
  may be used.

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Rotary Switch
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Toggle switch
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• Rheostats control the amount of current that
  flows throughout the circuit and used as dimmer
  devices for instrument and cockpit lights.
• Relays Electromagnetic switching devices which
  are used to remotely control electric circuits
  carrying large amount of current.
• Solenoids Remote control devices quite similar
  to relays but they are designed to move a shaft
  over a short distance. Thus, solenoids are used
  as mechanical control devices to operate
  hydraulic or pneumatic valves, locking pins, etc.

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Rheostat
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Schematic of a relay
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Relay
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Conversion Devices

• There are many occasions within an aircraft
  electrical system where it is required to convert
  power from one form to another.
• Typical examples of power conversion are
• - Conversion from dc to ac power
• - Conversion from 115vac to 28 vdc
• - Conversion from one ac voltage level to another
• - Battery charging (from 115vac to 28vdc)

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• Following devices are used for these purposes
  
• Transformers
• Rectifiers
• Transformer-rectifier units (TRUs)
• Inverters
• Transistors

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• Transformers Used to change ac voltage level.
• Rectifiers Used to convert ac into
  high-amperage, low-voltage dc.
• TRUs Combination of transformers and rectifiers
  and used as main unit to convert alternator
  output to dc in an aircraft
• Inverters convert 28 vdc power to 26 vac power
  particularly for flight instruments or 115 vac.
• Transistors Electronic devices that control
  electron flow and convert ac to dc.

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Transformer
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Analogy to TRU (ac adapter)
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TRUs
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Inverter
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Analogy to inverter (ups for computer)
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Protection Devices

• These devices are used to protect circuits,
  cables and system components from damage due to
  failures.
• Fuses
• Circuit breakers
• Diodes
• GCUs

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• Fuses They are designed to protect the cables
  against the flow of short-circuit and excessive
  current. They break the circuit and stop the
  current flow when the current exceeds a
  predetermined value.
• Circuit breakers They have same function with
  the fuses.The difference from the fuse is that
  they are resettable, while a fuse must be
  replaced.
• Diodes They are the electronic equivalent to
  the check valves in hydraulic system. They allow
  electricity to flow in one way only.

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Circuit Breakers
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Circuit Breakers
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Primary Causes of Electrical Circuit Failures

• Open circuit
• It is a circuit that is not complete or
  continuous. This is an uncommanded interruption
  of electrical power tosome components or systems.
  When an open occurs the affected component stop
  to operate, but the other components still remain
  in operative condition.

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Common causes of open circuit
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• Short Circuit
• It occurs when electricity is allowed to take
  a shortcut through or around a component or
  system. This is the most serious problem. It has
  two effects
• - Affected components have no power
• and fail to operate
• - Since current will not flow through
• affected components, the other
• components will be subjected to higher
• level of current causing them to burn
• out.

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Common causes of a short
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Power Distribution

• An electrical distribution system is required
  in order to convey the electrical power to the
  equipments and systems that need it.
• Busbar systems
• Wire and cables

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Busbar System

• In most types of aircraft, output from the
  generators is sent to one or more conductors
  before distribution throughout the system.
• These conductors are called busbars and they act
  as distribution centers for electric power.
• A busbar system is set up so that each power
  source supplies one or more specific buses.

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Analogy to bus
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• A variety of electrical items are hooked up to
  each bus for power.
• The buses are interconnected via circuit
  protection devices which are called bus ties.
• Bus ties are switches or relays used to connect
  or disconnect buses from one another.
• They serve to isolate failed buses from working
  ones and/or send electrical power to buses that
  lost their normal power sources.

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• Each engine-driven generator, for example,
  normally drives its own generator bus.
  High-current drawn items are connected to these
  buses.
• Items like fixed fire extinguishers and emergency
  lights are usually powered off battery bus. That
  way they are powered for use even when no
  generator power is available.

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Typical 28vdc system
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WIRES AND CABLES

• Wires and cables conduct electrical power in its
  various forms and quantities to and between
  equipments.
• There are various types of wires used in aircraft
  electrical system. The conductor is made of
  copper or aluminum. The insulation material may
  be nylon, PVC, or fiberglass.

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Aircraft wires
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Wire size selection

• The wires installed in an aircraft electrical
  system are chosen on the basis of ability to
  carry the required current
• - without overheating and
• - without producing an excessive voltage
• drop.
• Electrical wire charts may be used for this
  purposes.

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Maximum voltage drop in load circuits
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Wire chart
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Electrical wire size measurement
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Aircraft wiring diagrams

• There are different types of electrical diagrams
  available to understand electrical systems. These
  diagrams may be in the following forms
• - Block diagrams
• - Pictorial diagrams
• - Schematic diagrams
• Many symbols are used in these diagrams.

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Pictorial electric diagram
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ELECTRICAL DIAGRAM
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Aircraft electrical system diagram
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Electrical loads

• Once the aircraft electrical power has been
  generated and distributed then it is available to
  the aircraft services.
• These electrical services are distributed
  throughout the aircraft and may be broadly
  subdivided into following categories.
• Motors
• Lighting services
• Heating services
• Avionics

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Aircraft Lighting System

• Lighting system represent an important element
  of the aircraft electrical services.
• External lighting systems
• Internal lighting systems

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External lighting system

• Provides illumination for such operation as
  landing at night, inspection of icing condition,
  and safety from midair collision.
• Most common aircraft exterior lights
• Position lights (navigation lights)
• Landing/taxi lights
• Anti-collision lights
• Inspection lights

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• Position lights
• - Red at left wing tip
• - Green at right wing tip
• - White at vertical stabilizer
• Anti-collision lights
• - Rotating beam lights
• - Usually at the top of fuselage or tail
• Landing and taxi lights
• - Landing lights at the leading edge of wings
• - Taxi lights at nose landing gear

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Exterior lights
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Aircraft exterior lights
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Exterior lights
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Exterior lights
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Exterior lights
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Exterior lights