Cessna 172 - Static Pressure Alternate Source Valve

vided through a split bus bar having all electronic circuits
on one side of the bus and other
electrical circuits on the
other side of the bus.
When an external power source is
connected, a contactor automatically opens the circuit to
the electronic portion of the split bus bar as a protection
against damage to the
transistors in the electronic equip-
ment by transient voltages from the power source. There-
fore, the external power
source can not be used a,s a source
of power when checking electronic components.
Just before connecting an external power source (generator type or bat-
tery cart), the master switch should be turned on.
The ground
service plug rec~ptacle circuit incorporates ~polarity
reversal protection. Power from the external power solirce will flow only
if the ground service plug is correctly connected to the ~irplane. If. the
plug is ace identally connected backwards, no power will flow to the air-
plane's electrical system, thereby preventing any damage to electrical
equipment.
The battery and external power 'Circuits have been designed to -com-
pletely eliminate the need to
"jumper" across the battery contactor to
close
it for ~h~rging a completely "dead" battery. A speCial fused circuit
in the_external power-- system suppJies the needed "jumper" across the
contacts so that with a !'dead" _
battery and an external power source ap-
plied, -
tl!-rning on -the m~ster switch will ~lose the Qattery contactor.
STATIC , ~RESSURE ALTERNATE SOURCE VALVE.
A static pressure alternate source valve may be installed in the static
system for use when the external
static source is malfunctioning.
If erroneous instrument readings are suspected due to water or ice
irl the static pressure lines, the static pressure alternate source -valve
control
knob located below the wing flap switch should be opened, thereby
supplying static pressure from the cabin; Cabin- pressures will vary,
however,
with open cabin ventilators or windows. The most adverse com-
binations will result in airspeed and -
altimeter variations of no more than
2
MPH and 15 feet, respectively. -
7-2