Bus
Bar
Avionic
Bus Avionics Relay
On
Off
Avionics Master
Switch
Lights
Panel
Position
Beacon
Trim
Instr
Lndg Lt
Radio
Xpdr
Clutch
Ammeter
Mag Switch
Left
Magneto
Right
Magneto
Starter
Relay Engine
Starter Bus Bar
Battery
Relay
Battery
Battery
Switch
Starter
Switch
M/R Gearbox
Press Switch
Release
Hold
Engage
Clutch
Switch
Alternator
Alternator
Switch
Alternator
Control Unit
Clutch Actuator
(Internal Limit Switches
Shown in Full
Disengage Position)
24V
– +
F2 F1
–
+
Starting
R Vibrator
R
Both
L L
Off
Ret
Adv
Adv
G
(Optional Avionics)
Figure 5-12. An electrical system scematic like this sample is
included in most POHs. Notice that the various bus bar
accessories are protected by circuit breakers. However, you
should still make sure all electrical equipment is turned off
before you start the engine. This protects sensitive components,
particularly the radios, from damage which may be
caused by random voltages generated during the starting
process.
Filter To Carb
Carb Heat
Collector
Manifold
Pipe
Door
Filter To Carb
Carb Heat
Collector
Manifold
Pipe
Door
Heated Air
Carb Heat Off
Carb Heat On
Figure 5-11. When you turn the carburetor heat ON, normal
air flow is blocked, and heated air from an alternate source
flows through the filter to the carburetor.
5-9
incorporated to prevent excessive voltage, which may
damage the electrical components. The bus bar serves
to distribute the current to the various electrical components
of the helicopter.
A battery is mainly used for starting the engine. In
addition, it permits limited operation of electrical
components, such as radios and lights, without the
engine running. The battery is also a valuable source
of standby or emergency electrical power in the event
of alternator or generator failure.
An ammeter or loadmeter is used to monitor the
electrical current within the system. The ammeter
reflects current flowing to and from the battery. A
charging ammeter indicates that the battery is being
charged. This is normal after an engine start since
the battery power used in starting is being replaced.
After the battery is charged, the ammeter should stabilize
near zero since the alternator or generator is
supplying the electrical needs of the system. A discharging
ammeter means the electrical load is
exceeding the output of the alternator or generator,
and the battery is helping to supply electrical power.
This may mean the alternator or generator is malfunctioning,
or the electrical load is excessive. A
loadmeter displays the load placed on the alternator
or generator by the electrical equipment. The RFM
for a particular helicopter shows the normal load to
expect. Loss of the alternator or generator causes the
loadmeter to indicate zero.
Electrical switches are used to select electrical components.
Power may be supplied directly to the component
or to a relay, which in turn provides power to the
component. Relays are used when high current and/or
heavy electrical cables are required for a particular component,
which may exceed the capacity of the switch.
Circuit breakers or fuses are used to protect various
electrical components from overload. A circuit breaker
pops out when its respective component is overloaded.
The circuit breaker may be reset by pushing it back in,
unless a short or the overload still exists. In this case,
the circuit breaker continues to pop, indicating an electrical
malfunction. A fuse simply burns out when it is
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