Air flow to vacuum pump Air flow from vacuum pump Spring Gate check valve open Gate check valve closed 10-51 Air filter—A master air filter screens foreign matter from the  air flowing through all the gyro instruments. It is an extremely  import filter requiring regular maintenance. Clogging of the  master filter reduces airflow and causes a lower reading on  the suction gauge. Each instrument is also provided with  individual filters. In systems with no master filter that rely  Restrictor valve—Since the turn needle of the turn and bank  indicator operates on less vacuum than that required by the  other instruments  the vacuum in the main line must be  reduced for use by this instrument. An in-line restrictor valve  performs this function. This valve is either a needle valve or  a spring-loaded regulating valve that maintains a constant   reduced vacuum for the turn-and-bank indicator. Selector valve—In twin-engine aircraft having vacuum  pumps driven by both engines  the alternate pump can be  selected to provide vacuum in the event of either engine or  pump failure  with a check valve incorporated to seal off  the failed pump. Twin-engine aircraft vacuum systems are more complicated.  They contain an engine-driven vacuum pump on each engine.  The associated lines and components for each pump are  isolated from each other and act as two independent vacuum  systems. The vacuum lines are routed from each vacuum  pump through a vacuum relief valve and through a check  valve to the vacuum four-way selector valve. The four-way  valve permits either pump to supply a vacuum manifold.  From the manifold  flexible hoses connect the vacuum- operated instruments into the system. To reduce the vacuum  for the turn and bank indicators  needle valves are included  in both lines to these units. Lines to the artificial horizons  and the directional gyro receive full vacuum. From the  instruments  lines are routed to the vacuum gauge through a  turn and bank selector valve. This valve has three positions:  main  left turn and bank (T&B)  and right T&B. In the main  position  the vacuum gauge indicates the vacuum in the lines  of the artificial horizons and directional gyro. In the other  positions  the lower value of vacuum for the turn and bank  indicators can be read. Figure 10-91  shows a typical engine-driven pump vacuum  system containing the above components. A pump capacity  of approximately 10"Hg at engine speeds above 1 000 rpm  is normal. Pump capacity and pump size vary in different  aircraft  depending on the number of gyros to be operated. Suction/vacuum pressures discussed in conjunction with the  operation of vacuum systems are actually negative pressures   indicated as inches of mercury below that of atmospheric  pressure. The minus sign is usually not presented  as the  importance is placed on the magnitude of the vacuum  developed. In relation to an absolute vacuum (0 psi or 0 "Hg)   instrument vacuum systems have positive pressure.  Suction gauge—a pressure gauge which indicates the  difference between the pressure inside the system and  atmospheric or cockpit pressure. It is usually calibrated in  inches of mercury. The desired vacuum and the minimum and  maximum limits vary with gyro system design. If the desired  vacuum for the attitude and heading indicators is 5 inches  and the minimum is 4.6 inches  a reading below the latter  value indicates that the airflow is not spinning the gyros fast  enough for reliable operation. In many aircraft  the system  provides a suction gauge selector valve permitting the pilot  to check the vacuum at several points in the system. only upon individual filters  clogging of a filter does not  necessarily show on the suction gauge. Figure 10-89.  A vacuum regulator  also known as a suction relief  valve  includes a foam filter. To relieve vacuum  outside air of a  higher pressure must be drawn into the system. This air must be  clean to prevent damage to the pump. Figure 10-90.  Gate check valve used to prevent vacuum system  damage from engine backfire. Twin-Engine Aircraft Vacuum System Operation