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Vacuum System

Introduction:

  • 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 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 Systems:

  • In order to overcome the major drawback of the venturi tube, that is, its susceptibility to ice, aircraft were equipped with engine driven vacuum pumps and the gyro instruments were driven by air pulled through the instrument by the suction produced by these pumps
  • A suction relief valve maintained the desired pressure (usually about four inches of mercury) on the attitude gyro instruments, and a needle valve between one of the attitude indicators and the turn and slip indicator restricted the airflow to maintain the desired 2 inches of suction in its case
  • Most of the early instruments used only paper filters in each of the instrument cases, but in some installations a central air filter was used to remove contaminants from the cabin air before it entered the instrument case
    • The early vacuum pumps were vane-type pumps of what is called the wet type-one with a cast iron housing and steel vanes
      • Engine oil was metered into the pump to provide sealing, lubrication, and cooling, and then this oil, along with the air, was blown through an oil separator where the oil collected on baffles and was returned to the engine crankcase
      • The air was then exhausted overboard
      • Aircraft equipped with rubber deicer boots used this discharge air to inflate the boots
      • But before it could be used, this air was passed through a second stage of oil separation and then to the distributor valve and finally to the boots (See figure 12-2.)
    • The airflow through the instruments is controlled by maintaining the suction in the instrument case at the desired level with a suction relief valve mounted between the pump and the instruments
      • This valve has a spring-loaded poppet that offsets to allow cabin air to enter the pump and maintain the correct negative pressure inside the instrument case
    • The more modern vacuum pumps are of the dry type
      • These pumps use carbon vanes and do not require any lubrication, as the vanes provide their own lubrication as they wear away at a carefully predetermined rate. Other than the fact that they do not require an oil separator, systems using dry air pumps are quite similar to those using a wet pump. On slight difference, however, is in the need for keeping the inside of the pump perfectly clean. Any solid particles drawn into the system through the suction relief valve can damage one of the carbon vanes, and this can lead to destruction of the pump, as the particles broken off of one vane will damage all of the other vanes. To prevent particles entering the relief valve, its air inlet is covered with a filter, and this must be cleaned or replaced at the interval recommended by the aircraft manufacturer
c. Positive Pressure Systems. Above about 18,000 feet there is not enough mass to the air drawn through the instruments to pro­ vide sufficient rotor speed, and, to remedy this problem, many aircraft that fly at high altitude use positive pressure systems to drive the gy­ ros. These systems use the same type of air pump as is used for vacuum systems, but the discharged air from the pump is filtered and directed into the instrument case through the same fitting that receives the filtered air when the vacuum system is used. A filter is installed on the inlet of the pump, and then, before the air is directed into the instrument case, it is again filtered. A pressure regulator is located between the pump and the in-line filter to control the air pressure so only the correct amount is directed into the instrument case. System Filters. The life of an air-driven gyro instrument is determined to a great extent by the cleanliness of the air that flows over the rotor. In vacuum systems, this air is taken from the cabin where there is usually a good pumps are also subject to damage from in- deal of dust and very often tobacco smoke. gested contaminants, and all of the filters in Unless all of the solid contaminants are re- the system must be replaced on the schedule moved from the air before it enters the instru- recommended by the aircraft manufacturer, ment, they will accumulate, usually in the rotor and more often if the aircraft is operated under bearings, and slow the rotor. This causes an particularly dusty conditions, especially if the inaccurate indication of the instrument and occupants of the aircraft regularly smoke while will definitely shorten its service life. Dry air flying. (See figures 12-3 and 12-4.)

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

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