Cessna 172N Operating Handbook page 139

CESS NA
MODEL 172N
SECTION 7
AIRPLANE & SYSTEMS DESCRIPTIONS
system consists of a vacuum pump mounted on the engine, a vacuum re-
lief valve and vacuum system air filter on the aft side of the firewall be-
low the instmment panel, and instruments (including a suction gage) on
the left side of the instmment panel.
ATTITUDE IND IC A TOR
The attitude indicator gives a visual indication of flight attitude.
Bank attitude is presented by a pointer at the top of the indicator rela-
tive to the bank scale which has index marks at 10", 20o, 30o, 60o, and
90o either side of the center mark.
Pitch and roll attitudes are present-
ed by a miniature airplane in relation to the horizon bar. A knob at the
bottom of the instrument is provided for in-flight adjustment of the
miniature airplane to the horizon bar for a more accurate flight attitude
indication.
DI RECTIO NA T I N DIC A TOR
A directional indicator displays airplane heading on a compass card in
relation to a fixed simulated airplane image and index. The indicator will
precess slightly over a period of time.
Therefore, the compass card should
be set in accordance with the magnetic compass just prior to takeoff, and oc-
casionally re-adjusted on extended flights.
A knob on the lower left edge of
the instmment is used to adjust the compass card to correct for precession.
SU CTI ON GA G E
The suction gage is located on the left side of the instnrment panel
and indicates, in inches of mercury, the amount of suction available for
operation of the attitude indicator and directional indicator.
The desired
suction range is 4.6 to 5.4 inches of mercury. A suction reading below
this range may indicate a system malfunction or improper adjustment,
and in this case, the indicators should not be considered reliable.
S TATL WAR NING S Y S T E M
The airplane is equipped with a pneumatic-type
stall warning
sys-
tem consisting
of an inlet in the leading edge of the left wing, an air-
operated horn near the upper left corner of the windshield,
and asso-
ciated plumbing.
As the airplane approaches a stall, the low pressure
on the upper surface of the wings moves forward around the leading
edge of the wings. This low pressure
creates a differential
pressure in
the stall warning
system which draws air through the warning horn,
resulting
in an audible warning at 5 to 10 knots above stall in all flight
conditions.
7 -33