33 30 24 2I I5 I2 6 3 N S W E X HMR2300 C om:@232#486 ID: no: Y Z Engine-Driven Vacuum Pump 10-49 Figure 10-86.  Solid state magnetometer units.  An advantage of engine-driven pumps is their consistent  performance on the ground and in flight. Even at low  engine rpm, they can produce more than enough vacuum  so that a regulator in the system is needed to continuously  provide the correct suction to the vacuum instruments. As  long as the engine operates, the relatively simple vacuum  system adequately spins the instrument gyros for accurate  indications. However, engine failure, especially on single- engine aircraft, could leave the pilot without attitude and  directional information at a critical time. To thwart this  shortcoming, often the turn and bank indicator operates with  an electrically driven gyro that can be driven by the battery  The advantages of a venturi as a suction source are its  relatively low cost and its simplicity of installation and  operation. It also requires no electric power. But there are  serious limitations. A venturi is designed to produce the  desired vacuum at approximately 100 mph at standard sea  level conditions. Wide variations in airspeed or air density  cause the suction developed to fluctuate. Airflow can also  be hampered by ice that can form on the venturi tube.  Additionally, since the rotor does not reach normal operating  speed until after takeoff, preflight operational checks of  venturi powered gyro instruments cannot be made. For these  reasons, alternate sources of vacuum power were developed. When a vacuum pump develops a vacuum (negative  pressure), it also creates a positive pressure at the outlet  of the pump. This pressure is compressed air. Sometimes,  it is utilized to operate pressure gyro instruments. The  components for pressure systems are much the same as those  for a vacuum system as listed below. Other times, the pressure  developed by the vacuum pump is used to inflate de-ice boots  or inflatable seals or it is vented overboard. Figure 10-85.  A radio magnetic indicator (RMI) combines a slaved  gyro heading indication (red triangle at top of gauge) with magnetic  bearing information to a VOR station (solid pointer) and an ADF  station (hollow pointer).  The vane-type engine-driven pump is the most common  source of vacuum for gyros installed in general aviation,  light aircraft. One type of engine-driven pump is geared  to the engine and is connected to the lubricating system to  seal, cool, and lubricate the pump. Another commonly used  pump is a dry vacuum pump. It operates without external  lubrication and installation requires no connection to the  engine oil supply. It also does not need the air oil separator or  gate check valve found in wet pump systems. In many other  respects, the dry pump system and oil lubricated system are  the same.  [Figure 10-88] 