Textron Cessna 172S Skyhawk SP, Information Manual

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CESSNA
MODEL 172S
VACUUM GAGE
SECTION 7
AIRPLANE & SYSTEMS DESCRIPTION
I
The vacuum gage is part of the vacuum gage/ammeter, located.
on the lower left corner of the instrument panel. It is calibrated in
inches of mercury and indicates vacuum air available for operation
of the attitude and directional indicators. The desired suction
I
range
is 4.5 to 5.5 inches of mercury. Normally" a vacuum reading out of
this range may indicate a system malfunction or improper
adjustment, and in this case, the attitude and directionai (heading).
indicators should not be considered reliable. However, due to lower
atmospheric pressures at higher altitudes, the vacuum gage
may.
indicate as low as 4:0 in. Hg. at 20,000 feet and still be adequate
for normal system operation.
LOW VACUUM ANNUNCIATION
Each engin'e-driven vacuum pump is plumbed to a common
manifold, located forward of the firewall. From the tee, a single line
runs
into the cabin' to operate the various vacuum system
instruments. This tee contains check valves to prevent back flow
into a pump if it fails. Transducers are located just upstream of the
tee and measure vacuum output of each pump' ,
If output of the left pump falls below 3.0 in~ Hg., the amber L VAC
message will flash on the annunciator panel for approximately 10
seconds before turning steady on. If output of the right pump falls
below 3.0 in. Hg., the amber VAC R message
will flash on the
annunciator panel for approximateiy 1
o
seconds before turning
steady on. if output of both pumps falls below 3.0 in. Hg" the. amber
L VAC R message will flash on the annunciator panel for
approximately 10 seconds before turning steady on.
Revision 4
7-45