Cessna SkyHawk SP 172S NAV III 2005 Pilot Operating Handbook page 97

SECTION 3
EMERGENCY PROCEDURES
CESSNA
MODEL 172S NAV
III
IEMERGENCY OPERATION IN CLOUDS
If the engine-driven vacuum pump fails in flight, the standby attitude
indicator will not be accurate. The pilot must then rely on the
attitude and heading information (from the AHRS) shown on the
PFD indicators. With valid HDG or GPS/NAV and blind turn
coordinator inputs, autopilot operation will not be affected.
If the AHRS unit fails in flight (red X's shown through the PFD
attitude and heading indicators), the pilot must rely on the standby
attitude indicator and non-stabilized magnetic compass for attitude
and heading
The KAP 140 Autopilot will not operate in HDG mode without AHRS
input. With a valid GPS or NAV input and an operating turn
coordinator, the autopilot will operate in NAV, APR or REV modes.
Autopilot altitude hold and vertical speed operating modes are
independent of AHRS operation. Refer to Section 9, Supplements,
for additional details on autopilot operations.
The following instructions assume that the pilot is not very proficient
at instrument flying and is flying the airplane without the autopilot
engaged.
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3-26 U.S.
172SPHAUS-03