The gyroscopic instruments can be operated either by the vacuum system or the electrical system. In some aircraft, all the gyros are either electrically or vacuum motivated; in others, vacuum systems provide the power for the attitude and heading indicators, while the electrical system drives the gyro for operation of the turn needle. Both systems have advantages and disadvantages.

 Vacuum (Suction) System. The vacuum system spins the gyro by sucking a stream of air against the rotor vanes to turn the rotor at high speed, essentially as a water wheel or turbine operates. Air at atmospheric pressure drawn into the instrument through a filter or filters, drives the rotor vanes and is sucked from the instrument case through a line to the vacuum source, and vented into the atmosphere. Either a venturi or a vacuum pump can be used to provide the suction required to spin the rotors of the gyro instruments.

 Vacuum values vary with differences in gyro design for optimum rotor speed, ranging approximately from 8,000 to 18,000 rpm in different instruments. The suction for the three indicators is given in inches of mercury (Hg) as follows:

                          Minimum   Desired   Maximum
                              (inches of mercury)
   Turn Indicator.......    1.8       1.9       2.1
   Attitude indicator...    3.5       4.0       5.0
   Heading Indicator....    3.5       4.0       5.0

 Venturi Tube. Although the venturi tube is not as common as it was in the past, it can still be used when low cost, and simplicity of installation and operation are desired. A light single-engine airplane suitable for limited instrument training can be equipped with a 2" venturi (2" Hg vacuum capacity) to operate a turn needle. With an additional 8" venturi, power is available for the attitude and heading indicators. A line from the throat of the venturi is connected to the gyros. Throughout the normal range of operating airspeeds, the velocity of air through the venturi creates sufficient suction to spin the gyros. The limitations of the venturi system are clearly evident. The venturi is designed to produce the desired vacuum at approximately 100 mph under standard sea-level conditions. Wide variations in airspeed or air density, or restriction to airflow by ice accretion, will affect the pressure at the venturi throat and thus the vacuum driving the gyro rotors. Further, since the gyro rotors do not reach normal operating speed until after takeoff, preflight operational checks of venturi-powered gyro instruments cannot be made. For this reason, the system is adequate only for light plane instrument training and limited flying under instrument weather conditions. Aircraft flown throughout a wider range of speed, altitude, and weather conditions require a more effective source of power independent of airspeed and less susceptible to adverse atmospheric conditions.