Electrical Systems


  • The primary function of an aircraft electrical system is to generate, regulate, and distribute electrical power throughout the aircraft. There are several different power sources on aircraft to power the aircraft electrical systems. These power sources include: engine-driven alternating current (AC) generators, auxiliary power units (APUs), and external power. The aircraft's electrical power system is used to operate the flight instruments, essential systems, such as anti-icing, and passenger services, such as cabin lighting
  • Electricity is produced in two types depending on their use:
    • Direct Current (DC): battery, generator, transformer-rectifier
    • Alternating Current (AC): generator, inverter
  • Most aircraft are equipped with either a 14- or a 28-volt direct current electrical system
  • The electrical system is composed of numerous components which power various systems on the aircraft

Electrical Components:

  • Alternator/Generator
  • Battery
  • Master/Battery Switch
  • Alternator/Generator Switch
  • Bus bar, fuses, and circuit breakers
  • Voltage regulator
  • Ammeter/Loadmeter
  • Static Wicks
  • Associated electrical wiring
  • Pilot Handbook of Aeronautical Knowledge, Electrical Schematic
    Pilot Handbook of Aeronautical Knowledge, Electrical Schematic
  • Alternator/Generator:

    • Alternators and/or Generators are engine-driven power source accessories which supply electric current to the electrical system for in-flight operations while maintaining a sufficient electrical charge on the battery
    • Alternators:

      • Alternators rotate a magnetic field inside stationary coils of wires
      • Alternators produce sufficient current to operate the entire electrical system, even at slower engine speeds, by producing alternating current, which is converted to direct current
      • The electrical output of an alternator is more constant throughout a wide range of engine speeds
      • Some aircraft have receptacles to which an external ground power unit (GPU) may be connected to provide electrical energy for starting which can be very useful, especially during cold weather starting
    • Generators:

      • In the generator, the conductors are copper wires that are wound around an armature that is bolted to the drive pulley
      • As the armature rotates, the copper wires move through a magnetic field that is produced by permanent magnets which produces electrical power
      • Generators don't produce rated output until engine rpm is up in the midrange of operation - typically above 1,400 rpm
      • Pilots who have experienced the rapid dimming of a landing light as they reduce engine rpm on short final will understand one of the drawbacks of a generator-powered system
      • Disadvantages:
        • Heavy
        • Lower electrical output
        • Electrical noise and static that radiate to other avionics
        • Require more maintenance than alternators
      • Advantages:
        • Not sensitive to errant electrical spikes or reversed polarity
        • an produce electrical power even if the battery is dead
    • Learn more about alternator maintenance, read the AOPA's articles on caring for your alternator and 500-hour inspections
  • Battery:

    • Electrical energy stored in a battery provides a source of electrical power for starting the engine and a limited supply of electrical power for use in the event the alternator or generator fails
    • Most direct-current generators will not produce a sufficient amount of electrical current at low engine RPM to operate the entire electrical system
    • During operations at low engine RPM, the electrical needs must be drawn from the battery, which can quickly be depleted
  • Master/Battery Switch:

    • Pilot Handbook of Aeronautical Knowledge, Master Switch
      Pilot Handbook of Aeronautical Knowledge,
      Master Switch
    • The electrical system is turned on or off with a master switch
      • This would be the equivalent of turning your car keys to run electrical components without actually starting the car
    • Turning the master switch to the ON position provides electrical energy to all the electrical equipment circuits except the ignition system
    • Many aircraft are equipped with a battery switch that controls the electrical power to the aircraft in a manner similar to the master switch
  • Alternator/Generator Switch:

    • In addition, an alternator switch is installed which permits the pilot to exclude the alternator from the electrical system in the event of alternator failure
    • With the alternator half of the switch in the OFF position, the entire electrical load is placed on the battery
    • All non-essential electrical equipment should be turned off to conserve battery power
  • Bus Bar, Fuses, and Circuit Breakers:

    • A bus bar is used as a terminal in the aircraft electrical system to connect the main electrical system to the equipment using electricity as a source of power
    • This simplifies the wiring system and provides a common point from which voltage can be distributed throughout the system
    • Fuses or circuit breakers are used in the electrical system to protect the circuits and equipment from electrical overload
    • Spare fuses of the proper amperage limit should be carried in the aircraft to replace defective or blown fuses
    • Circuit breakers have the same function as a fuse but can be manually reset, rather than replaced, if an overload condition occurs in the electrical system
    • Placards at the fuse or circuit breaker panel identify the circuit by name and show the amperage limit
  • Voltage Regulator:

    • Voltmeter
      Amazon, Voltmeter
    • A voltage regulator controls the rate of charge to the battery by stabilizing the generator or alternator electrical output
    • The generator/alternator voltage output should be higher than the battery voltage
    • For example, a 12-volt battery would be fed by a generator/alternator system of approximately 14 volts
    • The difference in voltage keeps the battery charged
    • Consider backup solutions to panel gauges that can plug into cigarette lighters (if available)
      • Ensure the digital voltmeter matches the electrical system (i,e., 12V or 24V)
    • Voltmeter
      Amazon, Voltmeter
  • Ammeter/Loadmeter:

    • Pilot Handbook of Aeronautical Knowledge, Ammeter and Loadmeter
      Pilot Handbook of Aeronautical Knowledge,
      Ammeter and Loadmeter
    • Ammeter:

      • Ammeters are designed with the zero point in the center of the face and a negative or positive indication on either side [Figure 2]
      • An ammeter is used to monitor the performance of the aircraft electrical system which shows if the alternator/generator is producing an adequate supply of electrical power
      • When the pointer of the ammeter is on the plus side, it shows the charging rate of the battery
      • A minus indication means more current is being drawn from the battery than is being replaced
      • A full-scale minus deflection indicates a malfunction of the alternator/generator
      • A full-scale positive deflection indicates a malfunction of the regulator
        • In either case, consult the AFM or POH for appropriate action to be taken
      • Not all aircraft are equipped with an ammeter-some have a warning light that, when lighted, indicates a discharge in the system as a generator/alternator malfunction
        • Refer to the AFM or POH for appropriate action to be taken
      • It also indicates whether or not the battery is receiving an electrical charge
    • Loadmeter:

      • The loadmeter reflects the total percentage of the load placed on the generating capacity of the electrical system by the electrical accessories and battery [Figure 2]
      • When all electrical components are turned off, it reflects only the amount of charging current demanded by the battery
    • Pilot Handbook of Aeronautical Knowledge, Ammeter and Loadmeter
      Pilot Handbook of Aeronautical Knowledge,
      Ammeter and Loadmeter
  • Static Wicks:

    • Static wicks control electrical discharge into the atmosphere, isolating noise and preventing it from interfering with aircraft communication equipment
    • This discharge prevents buildup which allows for satisfactory operation of on-board navigation and radio communication systems

Electrical Equipment:

Aircraft Lights:

  • Aircraft lighting systems include both interior and exterior lighting arrangements
  • Externally, lighting consists of anti-collision lights,
  • Anti-Collision Lights:

    • An aircraft anti-collision light system can use one or more rotating beacons and/or strobe lights, be colored either red or white
    • They assist others in gaining attention of an operating aircraft's position, particularly at night where their use is mandatory
    • Care should be used during their operation at night when in the vicinity of others while on the ground
      • This means their use on the ground should be limited to when required (when an engine is running) if their use otherwise risks blinding pilots operating nearby
      • In this way, when conducting a preflight, quickly check the function of the lights and then turn them off for the remainder of the walk-around

Associated Electrical Wiring:

  • This type of gauge has a scale beginning with zero and shows the load being placed on the alternator/generator

Common Training Aircraft Electrical System Characteristics:

  • Cessna-172:

    • 28 Volt DC electrical system
    • Powered by 60-amp alternator (belt-driven) and a 24-volt battery (left forward side of firewall)
    • Power distribution module (J-box) located on the left forward side of the firewall houses all relays, the alternator control unit, and the external power connector within the module
  • Piper Arrow:

    • 14 Volt DC electrical System
    • Powered by a 60-amp alternator (belt-driven) and a 12-volt, 35 ampere hour battery


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