Revision 4 - Cessna 172SPHUSR05 Pilot Operating Handbook

C ESSNA
MOD EL 1 72S
MASTER SWITCH
The master switch is a split-rocker type switch l abeled MASTER'
and is ON i n the up position and OFF i n the down pos ition . The
right h alf of the switch , labeled BAT, controls the battery power to
the airplane. The left half, labeled ALT, contro ls the alternator.
PRIOR TO T U R N I N G TH E MAST E R SWITCH ON
O R O FF, STARTI N G TH E E N G I N E OR APPLYI N G
AN EXTERNAL POWER SO U RCE, TH E AVIO N I CS
MASTER SWITCH S H O U L D BE TU R N E D O F F TO
P R EVENT AN Y HAR M F U L TRANSI ENT VOLTA G E
FROM DAMAG I N G T H E AVION ICS EQU I P M ENT.
Normally, both sides of the master switch should be used
simu ltaneously; however, the BAT side of the switch cou ld be tu rned
on separately to check equ ipment while on the ground. To check or
use avionics equipment or radios while on the gro u nd , the avionics
power switch must also be turned on. The ALT side of the switch ,
when placed i n the OFF position , removes the alternator fro m the
electrical system . With th is switch i n the OFF positio n , the enti re
electrical load is placed on the battery. Contin ued operation with the
alternator switch in the OFF position will reduce battery power low
enough to open the battery contactor, remove power from the
alternator field, and prevent alternator restart.

Revision 4

&
AI RPLANE
A
CAUTION
SECTION
SYSTEMS DESC R I PTION
I
l
l
7-33