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B.  The captain's and first officer's HSI flags obtain their dc power from instrument amplifiers No. 1 and 2 and are dependent on an output from these amplifiers. The flag will be biased out of view when a dc output is obtained from the instrument amplifier and a logic circuit receives the proper ac and dc power inputs to complete a ground for the flag circuit.
6.  Rack Mountings
A.  The compass racks (No. 1 and 2) provide "plug-in" mountings for the servo-amplifiers and slaving amplifiers (Fig. 1). Each rack is wired into the associated compass system.
7.  Instrument Amplifiers
A.  Instrument amplifiers No. 1 and 2 contain the servo-amplifiers used to power the captain's and first officer's HSI flag and servo drive circuits.
8.  Flight Instrument Accessory Box
A.  The flight instrument accessory box contains the logic circuitry used to control the captain's and first officer's RMI and HSI flag warning circuits. The logic circuitry also controls flag warning circuits in the flight director system.
9.  Operation
A.  Both systems are made operational by closing the circuit breakers shown in Fig. 4 and 5. After a brief period of time the gyros will run up and the compass cards will reflect the magnetic heading of the airplane (Fig. 3).
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B.  Assume that the system is activated and that the airplane heading is magnetic north. This heading will be displayed upon the RMI and HSI compass cards. Assume now that the airplane turns 10 degrees to the left. The synchro in the directional gyro responds immediately to the change in direction, and puts out a signal which is now related, in both phase and amplitude, to the new heading. This signal is fed to the servo-amplifier via the heading synchro in the RMI's (Fig. 3). The servo-amplifier then drives the servomotor in the RMI (which is geared to the compass card) towards the new heading, and at the same time turns the rotor of the servocontrol synchro in such a direction as to null out the error in the servo circuit. Thus, when the motor stops, the signal from the synchro in the directional gyro has been nulled out in the RMI servocontrol synchro, and the compass card in the RMI reflects the new airplane heading.
C.  The sensing element in the flux valve will also reflect this change in heading, in the form of a signal which is fed to the slaving amplifier via the slaving synchro in the RMI. However, the error in the slaving amplifier circuit is also nulled out by the servomotor in the RMI since both synchro rotors are driven simultaneously by the same servomotor.

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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

D.  Random drift in the gyro is overcome by means of the flux valve and servo slaving circuits. Assume there are no error signals in the system and that the airplane is headed towards magnetic north once more. The gyro now drifts slightly off course. This results in a signal being generated in the gyro synchro which is passed to the servo-amplifier via the heading synchro in the RMI. As in the case of the heading change, the servo-amplifier drives the servomotor to null out the error in the heading synchro and automatically turns the rotor of the slaving synchro in the RMI along with it. However, in this case the stator of the slaving synchro is still referenced to north by the flux valve; hence, an error signal is now generated in the slaving synchro which is amplified by the slaving amplifier and fed to the torquemotor to precess the gyro back to a northerly heading. The error in the gyro synchro is thus nulled out and the system returns to its original zero error condition with the gyro again "pointing" north.
E.  In practice, when the compass system is first activated, some disagreement will exist between the airplane heading and that of the directional gyro. The system will immediately start to precess the gyro to take out this error, but the precession rate for the slaving circuits is a relatively slow one, and the time lag involved could not always be tolerated. Therefore, the heading synchronization knob on the RMI (ADF) is used to introduce an artificial error signal into the heading servo circuit, which rapidly produces the desired synchronization. An annunciator indication of a dot or cross, will show that the slaving circuits are operating, and will also serve to indicate which way the HDG/SYNC knob should be turned to synchronize the system. When the knob is turned, it turns the stator of the heading synchro, thus generating an error voltage in the rotor which is amplified by the servo-amplifier and drives the servomotor and compass cards to the correct airplane heading (and nulls out the error in the heading synchro as before). Since the servo-amplifier circuit is able to respond far more rapidly to an error signal than the slaving circuit, the "manual" method of synchronizing the system is preferred.
　
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