Textron Cessna C172S PIM, Pilot Operating Handbook

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CESSNA SECTION 3
MODEL 172S NAV III EMERGENCY PROCEDURES
GFC 700 AFCS
U.S.
EMERGENCY 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 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
information.
The autopilot will not operate if the AHRS unit fails. The pilot must
manually fly the airplane without AHRS input. Refer to Section 7,
Airplane and Systems Description, 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.
EXECUTING A 180° TURN IN CLOUDS (AHRS FAILED)
Upon inadvertently entering the clouds, an immediate turn to reverse
course and return to VFR conditions should be made as follows:
AHRS FAILURE
1. Note the non-stabilized magnetic compass heading.
2. Using the standby attitude indicator, initiate a 15° bank left turn.
Keep feet off rudder pedals. Maintain altitude and 15° bank
angle. Continue the turn for 60 seconds, then roll back to level
flight.
3. When the compass card becomes sufficiently stable, check the
accuracy of the turn by verifying that the compass heading
approximates the reciprocal of the original heading.
4. If necessary, adjust the heading by keeping the wings level and
using the rudder to make skidding turns (the compass will read
more accurately) to complete the course reversal.
5. Maintain altitude and airspeed by cautious application of
elevator control. Keep the roll pointer and index aligned and
steer only with rudder.
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