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Pilot's Handbook of Aeronautical Knowledge
Aircraft Systems
Electrical System

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




Electrical System

Most aircraft are equipped with either a 14- or a 28-volt direct
current electrical system. A basic aircraft electrical system
consists of the following components:
• Alternator/generator
• Battery
• Master/battery switch
• Alternator/generator switch
• Bus bar, fuses, and circuit breakers
• Voltage regulator
• Ammeter/loadmeter
• Associated electrical wiring

Engine-driven alternators or generators supply electric
current to the electrical system. They also maintain a
sufficient electrical charge in the 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. Alternators have several advantages over generators.

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. These are very useful, especially
during cold weather starting. Follow the manufacturer's
recommendations for engine starting using a GPU.

The electrical system is turned on or off with a master switch.
Turning the master switch to the ON position provides
electrical energy to all the electrical equipment circuits
except the ignition system. Equipment that commonly uses
the electrical system for its source of energy includes:
• Position lights
• Anticollision lights
• Landing lights
• Taxi lights
• Interior cabin lights
• Instrument lights
• Radio equipment
• Turn indicator
• Fuel gauges
• Electric fuel pump
• Stall warning system
• Pitot heat
• Starting motor

On this master switch, the left half is for the alternator and the right half is for the battery.
Figure 6-33. On this master switch, the left half is for the alternator
and the right half is for the battery.

Many aircraft are equipped with a battery switch that controls
the electrical power to the aircraft in a manner similar
to the master 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.
[Figure 6-33]

With the alternator half of the switch in the OFF position, the
entire electrical load is placed on the battery. All nonessential
electrical equipment should be turned off to conserve battery