Vacuum System


  • Used for turn coordinator, attitude indicator, and heading indicator
  • Relies on vacuum pressure through a vacuum pump to create suction to spin gyroscopes
  • When the pump is in the beginning of the system, it is referred to as a pressure pump
  • Directional gyros are almost all air-driven by evacuating the case and allowing filtered air to flow into the case and out through a nozzle, blowing against buckets cut in the periphery of the wheel
Pilot Handbook of Aeronautical Knowledge, Fixed Landing Gear
Figure 1: Suction Gauge

Vacuum Pressure:

  • Gyro pressure gauge, vacuum gauge, or suction gauge are all terms for the same gauge used to monitor the vacuum developed in the system that actuates the air driven gyroscopic flight instruments
  • Air is pulled through the instruments, causing the gyroscopes to spin
  • The speed at which the gyros spin needs to be within a certain range for correct operation
  • This speed is directly related to the suction pressure that is developed in the system
  • The suction gauge is extremely important in aircraft relying solely on vacuum operated gyroscopic flight instruments
  • Vacuum is a differential pressure indication, meaning the pressure to be measured is compared to atmospheric pressure through the use of a sealed diaphragm or capsule
  • The gauge is calibrated in inches of mercury
  • It shows how much less pressure exists in the system than in the atmosphere [Figure 1]

Vacuum System Failures:

  • Reduces or eliminates effectiveness of the turn coordinator, attitude indicator, and heading indicator
  • Discovered by a low indication on the vacuum gauge or unusual instrument indicators
  • When the primary air inlet is blocked, the backup inlet automatically opens due to pressure
  • Occurs when the vacuum pump fails or when both intakes are blocked