Standby Battery Switch; Avionics Switch - Cessna 172S NAV III Pilot Operating Handbook

CESSNA
MODEL 172S NAV III
SECTION 7
AIRPLANE AND SYSTEMS DESCRIPTION
ELECTRICAL SYSTEM
(Continued)
STANDBY BATTERY SWITCH
The STBY BATT master switch is a three position (ARM-OFF-TEST)
switch that tests and controls the Standby battery system. The
energy level of the battery shall be checked before starting the
engine (Refer to Section 4) by placing the switch in the momentary
TEST position and observing the correct illumination of the TEST
lamp found to the right of the switch. Energy level tests after
starting engine are not recommended.
Placing the switch in the ARM position during the engine start cycle
allows the Standby battery to help regulate and filter Essential Bus
voltage during the start cycle. The switch is set to the ARM position
during normal flight operation to allow the Standby battery to charge
and to be ready to power the Essential Bus in the event of
alternator and main battery failure. PlaCing the switch in the OFF
position disconnects the Standby battery from the Essential Bus.
Operation with the STBY BA TT switch in the OFF position prevents
the Standby battery from charging and from automatically providing
power should an electrical system failure occur.
AVIONICS SWITCH
The AVIONICS switch is a two-pole rocker-type switch that controls
electrical power to AVIONICS BUS 1 and BUS 2. Placing either
side of the rocker switch in the ON position supplies power to the
corresponding avionics bus. Both sides of the AVION ICS switch
should be placed in the OFF position before turning the MASTER
switch ON or OFF, starting the engine, or applying an external
power source.
(Continued Next Page)
172SPHAUS-OO U.S. 7-49