system are the pictorial navigation indicator and the slaving  control and compensator unit.  [Figure 5-25]  The pictorial  navigation indicator is commonly referred to as an HSI. There are a number of designs of the remote indicating  compass; therefore, only the basic features of the system are  covered here. Instrument pilots must become familiar with  the characteristics of the equipment in their aircraft. As instrument panels become more crowded and the pilot’s  available scan time is reduced by a heavier flight deck  workload, instrument manufacturers have worked toward  combining instruments. One good example of this is the  RMI in  Figure 5-26 . The compass card is driven by signals  from the flux valve, and the two pointers are driven by an  automatic direction finder (ADF) and a very high frequency  omnidirectional range (VOR). The slaving control and compensator unit has a pushbutton  that provides a means of selecting either the “slaved gyro”  or “free gyro” mode. This unit also has a slaving meter  and two manual heading-drive buttons. The slaving meter  indicates the difference between the displayed heading  and the magnetic heading. A right deflection indicates a  clockwise error of the compass card; a left deflection indicates  a counterclockwise error. Whenever the aircraft is in a turn  and the card rotates, the slaving meter shows a full deflection  to one side or the other. When the system is in “free gyro”  mode, the compass card may be adjusted by depressing the  appropriate heading-drive button. A separate unit, the magnetic slaving transmitter is mounted  remotely; usually in a wingtip to eliminate the possibility of  magnetic interference. It contains the flux valve, which is  the direction-sensing device of the system. A concentration  of lines of magnetic force, after being amplified, becomes  a signal relayed to the heading indicator unit, which is also  remotely mounted. This signal operates a torque motor in  the heading indicator unit that processes the gyro unit until  it is aligned with the transmitter signal. The magnetic slaving  transmitter is connected electrically to the HSI. Flight without reference to a visible horizon can be safely  accomplished by the use of gyroscopic instrument systems  and the two characteristics of gyroscopes, which are rigidity  and precession. These systems include attitude, heading,  and rate instruments, along with their power sources. These  instruments include a gyroscope (or gyro) that is a small wheel  with its weight concentrated around its periphery. When this  wheel is spun at high speed, it becomes rigid and resists tilting  or turning in any direction other than around its spin axis. Figure 5-26. Driven by signals from a flux valve, the compass card  in this RMI indicates the heading of the aircraft opposite the upper  center index mark. The green pointer is driven by the ADF. The  yellow pointer is driven by the VOR receiver. Figure 5-25. The pictorial navigation indicator is commonly  referred to as an HSI. Gyroscopic Systems Attitude and heading instruments operate on the principle  of rigidity. For these instruments, the gyro remains rigid  in its case and the aircraft rotates about it. Rate indicators,  such as turn indicators and turn coordinators, operate on the  principle of precession. In this case, the gyro precesses (or  rolls over) proportionate to the rate the aircraft rotates about  one or more of its axes. 33 30 24 2I I5 I2 6 3 Slaving meter  Slaving control compensator unit Pictorial navigation indicator (HSI)  33 30 24 2I I5 I2 6 3 N S W E 5-16