Flight Instruments

Remote Indicating Compass Remote indicating compasses were developed to compensate for the errors and limitations of the older type of heading indicators. The two panel-mounted components of a typical system are the pictorial navigation indicator and the slaving control and compensator unit. [Figure 8-29] The pictorial navigation indicator is commonly referred to as an HSI. The slaving control and compensator unit has a push button 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. Figure 8-27. The soft iron frame of the flux valve accepts the flux from A separate unit, the magnetic slaving transmitter, is mounted the Earth’s magnetic field each time the current in the center coil Figure 3-23. The soft iron frame of the flux valve accepts the remotely, usually in a wingtip to eliminate the possibility of reversfleusx. fTrohmis ftlhuex E caaruths’ess m caugrrneentitc t foie flldo wea icnh tthime eth trheee c puircrkeunpt icno tihlse. magnetic interference. It contains the flux valve, which is center coil reverse. This flux causes current to flow in the three the direction-sensing device of the system. A concentration picked coils. As the current reverses between the peaks, it demagnetizes of lines of magnetic force, after being amplified, becomes the frame so it can accept the flux from the Earth’s field. As this flux cuts across the windings in the three coils, it causes current to flow in them. These three coils are connected in such a way that the current flowing in them changes as the heading of the aircraft changes. [Figure 8-28] The three coils are connected to three similar but smaller coils in a synchro inside the instrument case. The synchro rotates the dial of a radio magnetic indicator (RMI) or a HSI. Figure 8-29. Pictorial navigation indicator (HSI, top), slaving meter Figure 8-28. The current in each of the three pickup coils changes (lower right), and slaving control compensator unit (lower left). with the heading of the aircraft. Figure 3-24. The current in each of the three pickup coils changes with the heading of the aircraft. 8-21