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2.  Remote Compass Transmitter (Flux Valve)
A.  The flux valve contains a pendulous magnetic detector which senses the direction of the horizontal component of the earth's magnetic field and utilizes this reference to generate an ac signal representative of the airplane's magnetic heading.
3.  Directional Gyros
A.  The directional gyro provides the heading reference for the compass system. Its gimbals may be torqued by two torquemotors, the slaving torquemotor being used to precess the gyro to the magnetic reference generated in the flux valve and the leveling torquemotor to level the gyro with respect to gravity. The former motor is driven by the slaving amplifier and the latter is controlled by means of an electrolytic switch (Fig. 2).
5C8
Jun 20/85 34-21-0 Page 1
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K79413 H24228
Compass System Component Location  5C8 
34-21-0  Figure 1  Jun 20/83 
Page 2 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

513  Directional Gyro Schematic 
Aug 15/68  Figure 2  34-21-0 
Page 3 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

B.  A spin-down brake in the directional gyro prevents nutation of the gyro by holding the outer gimbal steady for 10 seconds after power is applied. At shutdown, the brake is reapplied to the gimbal 30 seconds after power is removed. This prevents the gyro from spinning around its vertical axis. The 30-second time delay of the spin-down brake at shutdown also prevents momentary power interruptions (such as changing inverters) from introducing errors into the heading information.
C.  A logic circuit monitoring the gyroscope wheel speed, gyroscope leveling, nutation brake operation, heading synchro excitation and compass system signal null are used to operate two malfunction relays. The contacts of these relays are used in the flag warning circuits.
D.  Heading information for the autopilot is obtained direct from directional gyro No. 1 or, if the compass transfer switch is in BOTH ON NO. 2 position, from directional gyro No. 2.
4.  Radio Magnetic Indicator (RMI)
A.  Heading information from the directional gyro is displayed on a rotating compass dial which is read against a fixed index at the top of the indicator. Synchro transmitters in the captain's RMI repeat this information to the captain's horizontal situation indicator (HSI) and VHF navigation system No. 1. The first officer's RMI provides a similar service for the first officer's HSI, flight recorder and VHF navigation system No. 2. In addition, on airplanes not incorporating SB 34-1057, the first officer's RMI feeds the captain's RMI when the compass transfer switch is set to BOTH ON NO. 2 or the captain's RMI feeds the first officer's RMI when compass transfer switch is set to BOTH ON NO. 1, on airplanes incorporating SB 34-1057, the RMI's are hand wired to their respective gyro with no switching capability, however switching is provided for the HSI's to display information from the opposite system.
B.  An annunciator is provided to indicate alignment of the compass card with the magnetic reference. It takes the form of a small bar which will point to a cross or a dot whenever slaving of the gyro is taking place (Fig. 1). An indication midway between cross and dot signifies that the system is synchronized.
C.  The synchronizing knob on the face of the RMI is provided to bring about rapid synchronization of the system. This is done by turning the knob in the direction indicated by the annunciator (dot or cross).
D.  The warning flag on the RMI will appear when power is lost to the system or in the event of a malfunction in the system.
E.  The two RMI pointers display information from the two VHF (VOR) navigation systems.
557 
34-21-0 Page 4  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  May 01/76 

5.  Horizontal Situation Indicator (HSI)
A.  One of the synchros in the captain's HSI receives heading information from the F/O's RMI. This synchro is used in conjunction with the instrument amplifier to control the read-out of the HSI compass card. The other two synchros have their stators connected in parallel and are fed from a synchro in the captain's RMI. These two synchros are equipped with set course and set heading knobs to allow presetting of the rotors as necessary. The outputs are fed to steering computer-1 in the flight director system and the autopilot. Two of the synchros in the first officer's HSI are also connected in parallel and are fed from the first officer's RMI. The outputs from the two preset synchros are fed to steering computer-2 in the flight director system (Ref Chapter 34-26-0, Flight Director System).
　
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