Cessna 172R, Operating Handbook

Page: 238 / 500

SECTION 7
AIRPLANE
&
SYSTEMS DESCRIPTION
CESSNA
MODEL
172R
IGNITION AND STARTER SYSTEM
|
Engine ignition is provided by
two
engine-driven
magnetos,
and
two
spark plugs in each cylinder. The right magneto
fires
the lower
right and
upper
left spark plugs, and the
left
magneto
fires the lower
left and
upper right spark plugs. Normal operation is conducted with
|both
magnetos
due to the
more complete burning of the fuel/air
mixture with dual ignition.
|
Ignition and starter
operation is controlled by a
rotary-type
switch
located on the left switch and control panel. The switch is labeled
clockwise, OFF,
R, L,
BOTH,
and START. The engine should be
operated
on
both magnetos (BOTH position) except for magneto
checks. The R and
L positions are for checking purposes and
emergency
use only. When the switch is rotated
to
the spring
loaded START position, (with the master
switch in the ON position),
|the
starter contactor is
closed and the
starter,
now energized, will
crank the engine.
When the switch is released, it will automatically
return to
the BOTH position.
AIR INDUCTION SYSTEM
The
engine air
induction
system
receives
ram
air
through an
intake on the lower front portion of the engine cowling. The intake is
covered by an air filter which
removes dust and other foreign
matter
from the induction air. Airflow
passing through the filter
enters
an air
box. The air box
has a spring-loaded alternate
air door. If the air
induction filter should become
blocked, suction created by the
engine will
open
the door
and draw unfiltered air from inside the
lower cowl
area.
An
open alternate air door
will
result
in an
approximate
10%
power loss
at
full throttle. After passing through
the air
box, induction air enters a fuel/air
control unit under the
engine, and is then ducted to
the engine
cylinders through intake
manifold tubes.
7-22
Revision
4