Electrical Systems






  • Alternators and/or Generators are engine-driven as an 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



  • 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


Pilot Handbook of Aeronautical Knowledge, Master Switch
Figure 1: Pilot Handbook of Aeronautical Knowledge,
Master Switch

Master/Battery 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:

  • 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


Pilot Handbook of Aeronautical Knowledge, Ammeter and Loadmeter
Figure 2: 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


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


Associated Electrical Wiring:

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


Pilot Handbook of Aeronautical Knowledge, Electrical Schematic
Figure 3: Pilot Handbook of Aeronautical Knowledge, Electrical Schematic


Emergency Procedures:

  • As always, the most important emergency procedure you can ever remember is to aviate, navigate, and then communicate
  • These three steps are really a continuous process which never stops requiring pilot judgment to prioritize steps


All procedures here are GENERALIZED for learning.
Fly the maneuver in accordance with the Pilot Operating Handbook (POH)
and/or current Standard Operating Procedures (SOPs)

  • Aviate:
    • Complete any immediate action procedures that may be required
    • Reduce the electrical load, as required, to buy yourself time
    • After the situation is under control, and while navigating/communicating, open to Chapter 7 of the POH and begin going through the emergency procedure steps, starting back at step 1
  • Navigate:
    • Evaluate the situation and determine if you think the aircraft needs to land as soon as possible, or as soon as practical
    • Depending on your decision and the situation at hand, prepare for arrival
    • Remember that without electrical power to your instruments, you will have to rely on dead reckoning or radar vectors from ATC
  • Communicate:
    • Contact ATC if able
    • If you have not already had to address your passengers, take the time to do so now
    • If you have a hand held radio, break it out and attempt to establish radio communication, as able, with a local agency
    • While less reliable but more predominate, reach for your cell phone and attempt calling ATC
      • With this option in mind, remember that fumbling to find the phone number while in flight is going to be distracting and could make the situation much worse, causing distraction and possibly loss of situational awareness
      • Consider loading your phone with the appropriate telephone numbers a step in preflight