SECTION 7
CESSNA
AIRPLANE & SYSTEMS DESCRIPTIONS
MODEL 172RG
Figure 7-9. Cabin Heating, Ventilating, and Defrosting System
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1 July 1979
CESSNA
SECTION 7
MODEL 172RG
AIRPLANE & SYSTEMS DESCRIPTIONS
indicator and static pressure to the airspeed indicator, vertical speed
indicator and altimeter. The system is composed of either an unheated or
heated pitot tube mounted on the lower surface of the left wing, two
external static ports on the left and right sides of the forward fuselage, and
the associated plumbing necessary to connect the instruments to the
sources.
The heated pitot system (if installed) consists of a heating element in
the pitot tube, a rocker switch labeled PITOT HT and a 10-amp push-to reset
circuit breaker on the left side of the switch and control panel, and
associated wiring. When the pitot heat switch is turned on, the element in
the pitot tube is heated electrically to maintain proper operation in
possible icing conditions. Pitot heat should be used only as required.
A static pressure alternate source valve may be installed beneath the
throttle, and can be used if the external static source is malfunctioning.
This valve supplies static pressure from inside the cabin instead of the
external static ports.
If erroneous instrument readings are suspected due to water or ice in
the pressure line going to the standard external static pressure source, the
alternate static source valve should be pulled on.
Pressures within the cabin will vary with heater/ vents opened or
closed and windows open. Refer to Sections 3 and 5 for the effect of varying
cabin pressures on airspeed and altimeter readings.
AIRSPEED INDICATOR
The airspeed indicator is calibrated in knots and miles per hour.
Limitation and range markings (in KIAS) include the white arc (42 to 100
knots), green arc (50 to 145 knots), yellow arc (145 to 164 knots), and a red
line (164 knots).
If a true airspeed indicator is installed, it is equipped with a rotatable
ring which works in conjunction with the airspeed indicator dial in a
manner similar to the operation of a flight computer. To operate the
indicator, first rotate the ring until pressure altitude is aligned with
outside air temperature in degrees Fahrenheit. Pressure altitude should
not be confused with indicated altitude. To obtain pressure altitude,
momentarily set the barometric scale on the altimeter to 29.92 and read
pressure altitude on the altimeter. Be sure to return the altimeter baromet-
ric scale to the original barometric setting after pressure altitude has been
obtained. Having set the ring to correct for altitude and temperature, read
the true airspeed shown on the rotatable ring by the indicator pointer. For
best accuracy, the indicated airspeed should be corrected to calibrated
airspeed by referring to the Airspeed Calibration chart in Section 5.
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