Heading indicator Attitude indicator Turn-and-bank indicator Suction gauge Air Oil Air/oil separator Vacuum pump Air filter Restrictor valve Suction relief valve Pressure relief valve Selector valve Gate check valve Figure 10-91.  A typical pump-driven vacuum system for powering gyroscopic instruments.  Electrically-Driven Gyroscopic Instrument Systems Pressure-Driven Gyroscopic Instrument Systems 10-52 It should be noted that electric gyro instruments have some  sort of indicator on the face of the dial to show when the  instrument is not receiving power. Usually, this is in the  At high altitudes, the use of pressure-driven gyros is more  efficient. Pressure systems are similar to vacuum systems  and make use of the same components, but they are  designed for pressure instead of vacuum. Thus, a pressure  regulator is used instead of a suction relief valve. Filters are  Gyroscopic instruments are finely balanced devices with  jeweled bearings that must be kept clean to perform properly.  When early vacuum systems were developed, only oil- lubricated pumps were available. Even with the use of air-oil  separators, the pressure outputs of these pumps contain traces  of oil and dirt. As a result, it was preferred to draw clean air  through the gyro instruments with a vacuum system, rather  than using pump output pressure that presented the risk of  contamination. The development of self-lubricated dry pumps  greatly reduced pressure output contaminates. This made  pressure gyro systems possible.  On many small, single-engine aircraft, electric turn-and-bank  or turn coordinators are combined with vacuum-powered  attitude and directional gyro instruments as a means for  redundancy. The reverse is also possible. By combining  both types of instruments in the instrument panel, the pilot  has more options. On more complex multiengine aircraft,  reliable, redundant electrical systems make use of all electric- powered gyro instruments possible.  Electric gyros have the advantage of being powered by battery  for a limited time if a generator fails or an engine is lost. Since  air is not sent through the gyro to spin the rotor, contamination  worries are also reduced. Also, elimination of vacuum pumps,  plumbing, and vacuum system components saves weight. A spinning motor armature can act as a gyroscope. This is  the basis for electrically driven gyroscopic instruments in  which the gyro rotor spin is powered by an electric motor. A schematic of this twin-engine aircraft vacuum system is  shown in  Figure 10-92 . Note the following components: two  engine-driven pumps, two vacuum relief valves, two flapper  type check valves, a vacuum manifold, a vacuum restrictor  for each turn and bank indicator, an engine four-way selector  valve, one vacuum gauge, and a turn-and-bank selector valve.  Not shown are system and individual instrument filters. A  drain line may also be installed at the low point in the system.  still extremely important to prevent damage to the gyros.  Normally, air is filtered at the inlet and outlet of the pump  in a pressure gyro system.