of pressure in the system (vacuum is measured in inches of  mercury less than ambient pressure).  to move to the left. There is a need to turn the handlebars at  low speeds because of the instability of the slowly turning  gyros and also to increase the rate of turn.  Precession can also create some minor errors in some  instruments.  [Figure 8-19]  Precession can cause a freely  spinning gyro to become displaced from its intended plane  of rotation through bearing friction, etc. Certain instruments  may require corrective realignment during flight, such as the  heading indicator.  As shown in  Figure 8-20,  air is drawn into the vacuum  system by the engine-driven vacuum pump. It first goes  through a filter, which prevents foreign matter from entering  the vacuum or pressure system. The air then moves through  the attitude and heading indicators where it causes the gyros  to spin. A relief valve prevents the vacuum pressure, or  suction, from exceeding prescribed limits. After that, the air  is expelled overboard or used in other systems, such as for  inflating pneumatic deicing boots.  It is important to monitor vacuum pressure during flight,  because the attitude and heading indicators may not provide  reliable information when suction pressure is low. The  vacuum, or suction, gauge is generally marked to indicate  the normal range. Some aircraft are equipped with a warning  light that illuminates when the vacuum pressure drops below  the acceptable level.  In some aircraft, all the gyros are vacuum, pressure, or  electrically operated. In other aircraft, vacuum or pressure  systems provide the power for the heading and attitude  indicators, while the electrical system provides the power for  the turn coordinator. Most aircraft have at least two sources  of power to ensure at least one source of bank information is  available if one power source fails. The vacuum or pressure  system spins the gyro by drawing a stream of air against the  rotor vanes to spin the rotor at high speed, much like the  operation of a waterwheel or turbine. The amount of vacuum  or pressure required for instrument operation varies, but is  usually between 4.5 "Hg and 5.5 "Hg.  When the vacuum pressure drops below the normal operating  range, the gyroscopic instruments may become unstable and  inaccurate. Cross-checking the instruments routinely is a  good habit to develop.  One source of vacuum for the gyros is a vane-type engine- driven pump that is mounted on the accessory case of the  engine. Pump capacity varies in different aircraft, depending  on the number of gyros.  Aircraft use two types of turn indicators: turn-and-slip  indicators and turn coordinators. Because of the way the gyro  is mounted, the turn-and-slip indicator shows only the rate of  turn in degrees per second. The turn coordinator is mounted  at an angle, or canted, so it can initially show roll rate. When  the roll stabilizes, it indicates rate of turn. Both instruments  indicate turn direction and quality (coordination), and also  serve as a backup source of bank information in the event an  attitude indicator fails. Coordination is achieved by referring  to the inclinometer, which consists of a liquid-filled curved  tube with a ball inside.  [Figure 8-21]  A typical vacuum system consists of an engine-driven  vacuum pump, relief valve, air filter, gauge, and tubing  necessary to complete the connections. The gauge is mounted  in the aircraft’s instrument panel and indicates the amount  The gyro in the turn-and-slip indicator rotates in the vertical  plane corresponding to the aircraft’s longitudinal axis. A  single gimbal limits the planes in which the gyro can tilt,  and a spring works to maintain a center position. Because of  precession, a yawing force causes the gyro to tilt left or right,  as viewed from the pilot seat. The turn-and-slip indicator  uses a pointer, called the turn needle, to show the direction  and rate of turn. The turn-and-slip indicator is incapable of  “tumbling” off its rotational axis because of the restraining  springs. When extreme forces are applied to a gyro, the gyro  is displaced from its normal plane of rotation, rendering its  indications invalid. Certain instruments have specific pitch  and bank limits that induce a tumble of the gyro.  Plane of Plane of Force Plane of Rotation  FORCE  Figure 8-19.  Precession of a gyroscope resulting from an applied  deflective force.  8-16  Turn-and-Slip Indicator  Sources of Power  Turn Indicators 