Flight Instruments And Associated Systems; Vacuum System - Cessna 152 Training Manual

Flight Instruments and Associated Systems

The aircraft is equipped with the following standard flight instruments:
Attitude Indicator: or artificial horizon, a gyro which operates on
vacuum pressure and provides a visual indication of flight attitude.
A knob at the bottom of the instrument is provided for adjustment
of the miniature aeroplane to the horizon bar on the case;
Directional Indicator: a gyro which operates on vacuum
pressure, and displays aeroplane heading on a compass card. A
knob on the lower left edge of the instrument is used to adjust the
compass card to the magnetic compass to correct for any
precession;
Airspeed Indicator: operates with dynamic and static pressure
and is calibrated in knots. The instrument has limitation markings
in form of white, green and yellow arcs and a red line;
Altimeter: operates on static pressure and displays aeroplane
altitude in feet. A knob near the lower left edge of the instrument
provides adjustment of the barometric scale to the required setting
– (for example QNH, QNE, or QFE);
Vertical Speed Indicator operates on static pressure and displays
aeroplane rate of climb or descent in feet per minute;
Turn and Slip Indicator a gyro which operates electric power
(providing redundancy for the attitude indicator), for the rate of
turn indication, and a gravity ball for slip indication.
Vacuum
Pump
Illustration 8a Vacuum Pump
A suction range of 3 to 5 inches of mercury below atmospheric pressure is
acceptable. The normal range at cruise rpm is between 4.6 and 5.4 inches.
If the vacuum pressure is too low, the airflow will be reduced, the gyro's rotor
will not run at the required speed, and the gyro instruments will be unreliable.
Low vacuum pressure can have a serious effect on instrument flying, since the
attitude and direction gyros' unreliability is sometimes not picked up until the
aircraft is off course or in an unusual attitude. Beginning with 1983 models, a
low vacuum warning light was fitted which illuminates when the vacuum
pressure drops below 3 inches.
by D. Bruckert & O. Roud © 2004
CESSNA 152 TRAINING MANUAL

Vacuum System

Suction is necessary to operate the main gyro
instruments, consisting of the attitude indicator
and directional indicator. Suction is provided by a
dry-type, engine-driven, vacuum pump. A suction
relief valve, to control system pressure, is
connected between the pump inlet and the
instruments. A suction gauge is fitted on the
instrument panel and indicates suction at the
gyros.
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