Cessna 172S Skyhawk - Avionics Master Switch; Ammeter

SECTION 7 CESSNA
MODEL
172S AIRPLANE & SYSTEMS DESCRIPTION
AVIONICS MASTER SWITCH
Electrical power for each Avionics Bus is supplied from a primary
Electrical Bus. For airplane serial numbers 172S8001 through
172S8703, except for certain non-U.S. certified airplanes, both
Avionics Buses are controlled by a single-section rocker-type
Avionics Master switch.
At serial number 172S8704 and on, a two-
section
or "split" rocker-type Avionics Master switch controls power
to each Avionics Bus independently. Placing the rocker in the up
(ON) position provides power to the Avionics Bus. Placing the
rocker in the down (OFF) position removes power from the Avionics
Bus. The Avionics Master switch is located on the lower left side of
the instrument panel.
NOTE
For airplane serial numbers 172S8001 through 172S8703,
aircraft certified outside the United States can have a two-
section
or "split" Avionics Master switch. The two-section
Avionics Master switch
enables independent operation of
Avionics Bus 1 and Avionics Bus 2.
With the Avionics Master rocker in the OFF position, no electrical
power is provided to the avionics, even when the Master switch or
the individual avionics component equipment switches are in their
ON positions. The Avionics Master switch (both sides, if two-section)
should be placed in the OFF position before switching the Master
switch
ON or OFF, starting the engine, or applying an external
power source.
Each avionics bus also incorporates a separate circuit breaker
installed between the primary bus and the avionics master switch. In
the event of an electrical malfunction, this breaker will trip and take
the effected avionics bus off-line.
AMMETER
I The ammeterivacuum gage is located on the lower left side of the
instrument panel. It indicates the amount of current, in amperes,
from the alternator to the battery or from the battery to the airplane
electrical
system. When the engine is operating and the master
switch
is turned on, the ammeter indicates the charging rate applied
to the battery. In the event the alternator is not functioning or the
electrical load exceeds the output of the alternator, the ammeter
indicates the battery discharge
rate.
7-34 Revision 4