Textron Cessna C172S PIM, Pilot Operating Handbook

Page: 281 / 322

CESSNA SECTION 7
MODEL 172S NAV III AIRPLANE AND SYSTEM DESCRIPTION
GFC 700 AFCS
U.S.
CLOCK/O.A.T. INDICATOR
A numerical time or clock window, based on GPS time, and an outside
air temperature (O.A.T.) indicator window are provided along the lower
edge of the PFD. The O.A.T. indicator uses an air temperature sensor
located on top of the cabin.
STALL WARNING SYSTEM
The airplane is equipped with a pneumatic-type stall warning system
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
associated 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 a audible warning at 5 to 10 knots above stall
in all flight conditions.
The stall warning system should be checked during the preflight
inspection by applying suction to the system either by placing a clean
handkerchief over the vent opening and applying suction or using some
other type of suction device to activate the warning horn. The system is
operational if the warning horn sounds when suction is applied.
172SPHBUS-00 7-67