Airspeed Indicator; Rate-Of-Climb Indicator; Altimeter; Vacuum System And Instruments - Cessna 150 COMMUTER 1976 Pilot Operating Handbook

SECTION 7
CESSNA
MODEL 150M
AIRPLANE & SYSTEMS DESCRIPTIONS
must be p.1shed full in.
PilOT-STATIC SYSTEM AND INSTRUMENTS
The pitot-static system supplies ram air pressure to the airspeed in-
dicator and static pressure to the airspeed indicator, rate-of-climb indi-
cator and altimeter. The system is composed of a heated pitot tube
mounted on the lower surface of the left wing, an external static port on
the lower left side of the fuselage, and the associated plumbing necessary
to connect the instruments to the sources.
The heated pUot system consists of a heating element in the pitot tube,
a rocker-type switch labeled PITOT HT on the lower left side of the instru-
ment panel, a 10-amp circuit breaker under the engine controls on the in-
strument panel, and associated wiring. When the pitot heat switch is
turned on, the element in the pitot tube is heated electrically to maintain
proper "' o peration in p()ssible 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 (42 to 85 knots),
green arc (47 to 107 knots), yellow arc (107 to 141 knots), and a red line
(141 knots).
If a true airspeed indicator is installed, it is equipped with a cali-
brated rotatable ring which works in conjunction with the airspeed indica-
tor dial in a manner similar to the operation of a flight computer. To op-
erate the indicator, rotate the ring until pressure altitude is aligned with
outside air temperature in degrees Fahrenheit. Then read true airspeed
on the rotatable ring opposite the airspeed indicator pointer.
NOTE
Pressure altitude should not be confused with indicated
altitude. To obtain pressure altitude, set the baromet-
ric scale on the altimeter to 29. 92 and read pressure
altitude on the altimeter. Be sure to return the altime-
ter barometric scale to the original barometric setting
after pressure altitude has been obtained.
RATE-Of-CLIMB INDICATOR
The rate-of-climb indicator depicts airplane rate of climb or descent
in feet per minute. The pointer is actuated by an atmospheric pressure
7-28
CESSNA
MODE L 150M
SECTION 7
AIRPLANE & SYSTEMS DESCRIPTIONS
change supplied by the static source.
AL TIM ETER
Airplane altitude is depicted by a barometric type altimeter. A knob
near the lower left portion of the indicator provides adjustment of the in-
s tru ment's barometric scale to the proper barometric pressure reading.
VA CUUM SYSTEM AND INSTRU MEN TS
An engine-driven vacuum system (see figure 7 -9) provides the suction
ne cessary to operate the attitude indicator and directional indicator. The
sys tem consists of a vacuum pump mounted on the engine, a vacuum re-
lief valve and vacuum system air filter on the aft side of the firewall
be-
low the instrument panel, and instruments (including a suction gage) on
the left side of the instrument panel.
A TT ITU DE IN DICATO R
An attitude indicator is available and gives a visual indication of flight
a ttitude. Bank attitude is presented by a pOinter at the top of the indicator
r elative to the bank scale which is marked in increments of 10°, 20°, 30",
60", and 90" either side of the center mark. Pitch attitude is presented
by a miniature airplane in relation to the horizon bar. A knob at the bot-
to m of the instrument is p::-ovided for in-flight adjustment of the miniature
ai rplane to the horizon bar for a more accurate flight attitude indication.
DI RECTIONAL INDICATOR
A directional indicator is available and displays airplane heading on
a compass card in relation to a fixed simulated ai rplane image and index.
T he directional indicator will precess slightly over a period of time.
Therefore, the compass card should be set in accordance with the mag-
netic compass just prior to takeoff, and occasionally re-adjusted on ex-
tended flights. A knob on the lower left edge of the instrument is used to
adjust the compass card to correct for any precession.
SU CTION GAGE
A suction gage is located on the left side of the instrument panel and
indicates, in inches of mercury, the amount of suction available for opera-
tion of the attitude indicator and directional indicator. The desired suc-
tion range is 4. 6 to 5.4 inches of mercury. A suction reading below this
range may indicate a system malfunction or improper adjustment, and in
this case, the indicators should not be considered reliable.
7-29