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).
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.
H31437
Basic Compass System Circuit 541
34-21-0 Figure 3 Feb 1/74
Page 6
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
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