ELECTRICAL SYSTEM
Electrical energy is required to operate navigation and communication
radios, lights, and other airplane equipment. Many airplanes in the
past were not equipped with an electrical system. They were equipped with a
magneto system which supplied electrical energy to the engine ignition system
only. Modern airplanes still use an independent magneto system, but in addition
are equipped with an electrical system. The magneto system does not depend upon
the airplane electrical system for operation. In other words, the airplane
electrical system can be turned off in flight and the engine will continue to
operate efficiently, utilizing the electrical energy provided by the magnetos.
Most airplanes are equipped with a 14- or 28-volt direct-current
electrical system. The 28-volt system provides an electrical reserve capacity
for more complex systems, including additional electrical energy for starting.
A basic airplane electrical system consists of the following
components:
• Alternator or generator • Battery • Master or battery switch •
Alternator or generator switch • Bus bar, fuses, and circuit breakers •
Voltage regulator • Ammeter • Starting motor • Associated electrical
wiring • Accessories Engine-driven alternators or generators
supply electric current to the electrical system and also maintain a sufficient
electrical charge in the battery which is used primarily for starting.
There are several basic differences between alternators and generators.
Most generators will not produce a sufficient amount of electrical current at
low engine revolutions per minute (RPM) to operate the entire electrical system.
Therefore, during operations at low engine RPM’s, the electrical needs must be
drawn from the battery, which in a short time may be depleted. An
alternator, however, produces a sufficient amount of electrical current at
slower engine speeds by first producing alternating current which is converted
to direct current. Another advantage is that the electrical output of an
alternator is more constant throughout the ranges of engine speeds. Alternators
are also lighter in weight, less expensive to maintain, and less prone to become
overloaded during conditions of heavy electrical loads.
Electrical energy stored in a battery provides a source of electricity
for starting the engine and a limited supply of electricity for use in the event
the alternator or generator fails.
Some airplanes are equipped with receptacles to which external
auxiliary power units (APU) can be connected to provide electrical energy for
starting. These are very useful, especially during cold weather starting. Care
must be exercised in starting engines using an APU.
A master switch is installed on airplanes to provide a means for the
pilot to turn the electrical system “on” and “off.” Turning the master switch
“on” provides electrical energy to all the electrical equipment circuits with
the exception of the ignition system. Although additional electrical equipment
may be found in some airplanes, the following lists the equipment most commonly
found which uses the electrical system for its source of energy:
• Position lights • Landing lights • Interior cabin lights •
Radio equipment • Fuel gauges • Stall warning system • Anticollision
lights • Taxi lights • Instrument lights • Turn indicator •
Electric fuel pump • Pitot heat
Some airplanes are equipped with a battery switch which controls the
electrical power to the airplane 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.
With the alternator switch “off,” the entire electrical load is placed on the
battery. Therefore, all nonessential electrical equipment should be turned off
to conserve the energy stored in the battery.
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Figure 2-2.—Electrical system
schematic. | A bus bar is used as a
terminal in the airplane 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. [Figure 2-2]
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 airplane
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 location identify the circuit by name and show the
amperage limit. [Figure 2-3]
An ammeter is an instrument used to monitor the performance of the
airplane electrical system. Not all airplanes are equipped with an
ammeter. Some are equipped with a light which, when lighted, indicates a
discharge in the system as a generator/alternator malfunction. |
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Figure 2-3.—Circuit breaker
panel. |
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An ammeter shows if the alternator/generator is producing an adequate
supply of electrical power to the system by measuring the amperes of
electricity. This instrument also indicates whether the battery is
receiving an electrical charge. The face of some ammeters is designed with
a zero point in the upper center of the dial and a plus value to the right
of center; a negative value is to the left. A vertical needle swings to
the right or left, depending upon the performance of the electrical
system. If the needle indicates a plus value, it means that the battery is
being charged. After power is drawn from the battery for starting, the
needle will indicate a noticeable plus charge value for a short period of
time, and then stabilize to a lower plus charge value. [Figure 2-4] |
Figure 2-4.—Loadmeter and ammeter. |
| If the needle indicates a minus value, it means
that the generator or alternator output is inadequate and energy is being drawn
from the battery to supply the system. This could be caused by either a
defective alternator/generator or by an overload in the system, or both. Full
scale ammeter discharge or rapid fluctuation of the needle usually means
generator/alternator malfunction. If this occurs, the pilot should cut the
generator/alternator out of the system and conserve battery power by reducing
the load on the electrical system.
The loadmeter type of ammeter shows the load being placed on the
alternator. [Figure 2-4]
A voltage regulator controls the rate of charge to the battery by
stabilizing the generator or alternator electrical output. The
generator/alternator voltage output is usually slightly 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.
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