Airplanes are fitted with an engine driven vacuum pump to provide suction, which drives the artificial horizon and directional gyro. As air is moved through the two instruments by the pump, small rotors inside the instruments cause their internal gyroscopes to spin at high speed.
At these high speeds, the gyroscopes become rigid, refusing to move. Once they take on this rigidity, they are able to provide usable references to the pilot, such as the pitch and bank provided by the artificial horizon and the heading information provided by the directional gyro.
Powering these two instruments by engine driven vacuum pump means that they are not dependent on the electrical system for their operation. If the engine is running and the vacuum pump is operating properly, the pilot should have two good gyroscopic instruments.
The Suction Gage
A suction gage is provided to the pilot to verify proper vacuum system operation. The gage should be scanned during flight operations and checked during the run-up to assure proper suction exists to drive the vacuum powered flight instruments.
The Gyroscopic Instruments
The attitude indicator (or artificial horizon), the directional gyro, and the turn coordinator are the three gyroscopic instruments.
The attitude indicator is used by reference to the attitude of the instrument's miniature airplane versus its artificial horizon. The relationship of the miniature airplane to the artificial horizon is the same as the relationship of the real airplane to the actual horizon.
The directional gyro displays the airplane's heading directly.
The turn coordinator provides an indication of movement of the airplane about the yaw and roll axis. The movement of the instrument's miniature airplane is proportional to the roll rate of the airplane. When the bank is held constant, no roll rate exists, and the miniature airplane indicates rate of turn. This instrument does not indicate bank angle or airplane attitude.
Vacuum System Failure
A vacuum system failure will result on the failure of the attitude indicator and directional gyro, since these two instruments rely on suction for their operation.
These instruments will not fail suddenly after a loss of suction. Instead, they will slowly spool down over time. This can be deceptive, since they continue to display information to the pilot as they spool down. This information becomes increasingly inaccurate until it becomes obvious that the instruments are not working correctly.
On many small airplanes, the vacuum system is powered by a single, engine driven vacuum pump. On these airplanes, an engine failure would result in a vacuum system failure and the loss of these two flight instruments.
The turn coordinator is electrically powered. It is not compromised by the loss of suction.
Electrical System Failure
Should the electrical system fail the turn coordinator will fail. The turn coordinator is the only electrically powered flight instrument. The loss of electrical power to the turn coordinator is indicated by the appearance of a small red flag in the instrument.
Like the other two gyroscopic instruments, it will spool down over a period of time. During this period, it will provide the pilot increasingly inaccurate indications.
The attitude indicator and directional gyro are not affected by the loss of the airplane's electrical system. While the engine is running and the vacuum pump is operating, these instruments are still powered by suction from the vacuum system.
Gyroscopic Precession
The gyroscopic instruments are affected by precession, which causes the gyros to move in a direction other than that from which the outside force was applied. After a steep turn or a series of maneuvers, the attitude indicator will erroneously indicate a slight bank angle where none exists due to precession.
The primary impact of precession for our purposes is its affect on the directional gyro. As you fly along, the directional gyro will drift from the correct heading over time. It must be checked against the magnetic compass and reset whenever it is in error. The magnetic compass is the pilot's source of magnetic heading information.
The directional gyro should be set against the magnetic compass before takeoff and at intervals during the flight.
The gyroscopic instruments are the attitude indicator, the directional gyro, and the turn coordinator.
The attitude indicator and directional gyro are powered by an engine driven vacuum pump, while the turn coordinator is electrically powered.
Many airplanes equipped with modern avionics utilize an attitude-heading reference system (AHRS) in place of traditional gyroscopes.