Pitot-Static System And Instruments; Airspeed Indicator - Cessna 152 1978 Pilot Operating Handbook

CESSNA AIRCRAFT COMPANY FOR TRAINING USE ONLY
MODEL 152 7-25
Heated fresh air and outside air are blended in a cabin manifold just aft of the firewall by adjustment of
the heat and air controls; this air is then vented into the cabin from outlets in the cabin manifold near
the pilot's and passenger's feet. Windshield defrost air is also supplied by a duct leading from the
manifold.
Full ventilation air may be obtained by utilization of the adjustable ventilators near the upper left and
right corners of the windshield, and by pulling the CABIN AIR control knob out. The CABIN HT control
knob must be pushed full in.

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 pitot 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 15-amp circuit breaker under the engine
controls on the instrument 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.

AIRSPEED INDICATOR

The airspeed indicator is calibrated in knots and miles per hour. Limitation and range markings include
the white arc (35 to 85 knots), green arc (40 to 111 knots), yellow arc (111 to 149 knots), and a red line
(149 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 barometric 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. Knowing the calibrated airspeed, read true airspeed on the ring opposite
the calibrated airspeed.