Cessna 182T 2001, Pilot Operating Handbook

Page: 221 / 254

CESSNA SECTION 7
MODEL 182T AIRPLANE & SYSTEMS DESCRIPTION
DIRECTIONAL INDICATOR
The directional indicator is a vacuum/air-driven gyro that 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 with the
magnetic compass just prior to takeoff, and occasionally readjusted
as required throughout the flight. A knob on the lower left edge of
the instrument is used to adjust the compass card to correct for
precession. A knob on the lower right edge of the instrument is used
to move the heading bug.
VACUUM GAUGE
The vacuum gauge is part of the vacuum gauge/ammeter,
located on the lower left corner of the instrument panel. It is
calibrated in inches of mercury and indicates vacuum available for
operation of the Attitude and Directional Indicators. The desired
vacuum 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 Directional
gyros should not be considered reliable. However, due to lower
atmospheric pressures at higher altitudes, the vacuum gauge may
indicate as low as 4.5 in. Hg. at 15,000 feet and still be adequate
for normal system operation.
LOW VACUUM ANNUNCIATION
Each engine-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.
Apr 30/01 7-47