Pitot-Static System And Instruments - Cessna 172N Pilot Operating Handbook

SECTION 7
AIRPLANE
&
SYSTEMS DEsc
RIPTIONS
EXHAUST---
MUFFLER
SHROUD
FRONT CABIN
AIR OUTLET
ADJUSTABLE
DEFR0'3TER
OUTLET
REAR CABIN
AIR OUTLETS
i\ /#
ADJUSTABLE
~
VENTILATORS
I \
CESSNA
I
MODEL 172N
-----HEATER
¢
~
..
...
VALVE
VENTILATING
AIR DOOR
CABIN AIR
CONTROL
CODE
RAM AIR FLOW
VENTILATING AIR
HEATED AIR
BLENDED Am
MECHANICAL
CONNECTION
Figure 7-8. Cabin Heating, Ventilating, and Defrosting System
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CESSNA
MODEL 172N
SECTION 7
AIRPLANE
&
SYSTEMS DESCRIPTIONS
Front cabin heat and ventilating air is supplied by outlet holes spaced
across a cabin manifold just forward of the pilot's and copilot's feet. Rear
cabin heat and air is supplied by two ducts from the manifold, one
extending down each side of the cabin to an outlet at the front doorpost at
floor level. Windshield defrost air is also supplied by a duct leading from
the cabin manifold. Two knobs control sliding valves in the defroster outlet
and permit regulation of defroster airflow.
Separate adjustable ventilators supply additional air; one near each
.upper corner of the windshield supplies air for the pilot and copilot, and
two ventila"'tors are available for the rear cabin area to supply air to the rear
seat passengers. The airplane may also be equipped with an air condition-
ing system. For operating instructions and details concerning this system,
refer to Section 9, Supplements.
PITOT-STATIC SYSTEM AND INSTRUMENTS
The pitot-static system supplies ram air pressure to the airspeed
indicator and static pressure to the airspeed indicator, rate-of-climb
indicator and altimeter. The system is composed of either an unheated or
heated pitot tube mounted on the lower surface of the left wing, an external
static port on the lower left side of the forward fuselage, and the associated
plumbing necessary to connect the instruments to the sources.
The heated pi tot system consists of a heating element in the pitot tube,
a rocker-type switch labeled PITOT HT on the lower left side of the
instrument panel, a 5-amp circuit breaker on the switch and control panel,
and associated wiring. When the pi tot heat switch is turned on, the element
fa 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 adjacent to
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 port .
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 open cabin ventilators and
windows. Refer to Section 5 for the effect of varying cabin pressures on
airspeed and altimeter readings.
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