(11)
SET HDG Knob Function

(a)  
The set heading control is controlled by the SET HDG knob on the compass controller. Since the set heading control is not disabled in the SLAVED mode, the SET HDG knob may be used to check slaving. When the SET HDG knob is rotated to any position other than center, a signal is applied through the set heading control to increase the excitation voltage to the stepper motor and to cause the superflag output to be invalid. The rate at which the stepper motor will rotate (set heading rate) is controlled by the signal which is applied to the frequency control and limiter.

(b)  
When the SET HDG knob is in the first clockwise or counterclockwise (right or left of center) position, the heading output will change at a rate of 40 to 80 degrees per minute. When the SET HDG knob is at either extreme clockwise or counterclockwise position, the heading output will change at a rate of 800 to 1200 degrees per minute. The direction of rotation is controlled by the position of the SET HDG knob from center.

(c)  
The SET HDG knob is spring loaded so that when released, it returns to the center (off) position. In the SLAVED mode, the system will, after the SET HDG knob is released, synchronize back to the aircraft magnetic heading at a rate of 1 to 2 degrees per minute.

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C.  Functional Description, DG Mode (Fig. 3)
(1)  
The compass system operates in the DG mode when the COMPASS switch on the compass controller is placed in the DG position. In this mode, the slaving circuits which control the stepper motor are turned off. The system follows the changes of the heading information from the directional gyro. In this mode the output synchros can be set to any desired heading by using the SET HDG control on the compass controller.

(2)  
Magnetic Field Sensor

(a)  The magnetic flux valve input is not used in the DG mode. The magnetic heading error signal from the heading error amplifier is disabled by the DG mode input to the frequency control and limiter. In the DG mode the output of the SLAVED/DG control disables the excitation monitor, the annunciator, and the slaving circuit.

(3) 
 Superflag Control

(a)  The operation of the superflag control is the same in the DG mode as in the SLAVED mode, except that when magnetic field sensor excitation is lost, the superflag output will remain valid.

(4)  
SET HDG KNOB Function

(a)  The heading outputs of the control transmitters are controlled by the SET HDG knob on the compass controller and the directional gyro heading output signal. The control information supplied by the SET HDG knob in the DG mode is the same as that supplied in the SLAVED mode, except that when the SET HDG knob is released, the heading will remain at the set position until the airplane changes heading. When the airplane changes heading, the servo loop will position itself to the relative heading change.
D.  Controls (Fig. 4)

(1)  Prior to operating the compass system, electrical power must be provided to the airplane and the compass and instrument transformer circuit breakers must be closed. Instrument panel lighting should be adjusted for optimum intensity.
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E.  Switching (Fig. 5)

(1)  
The operation of the compass transfer switch and relays is illustrated in Fig. 5. The relay coils are connected with the dc supply through the relay contacts. The switch is shown in NORMAL position. Assume that the transfer switch is now set in BOTH ON NO. 2 position. This switch puts 28 volts dc on the lower end of the BOTH ON NO. 2 latching relay coil via BOTH ON NO. 2 and BOTH ON NO. 1 relay contacts. The relay will now latch over to the alternate set of contacts and remain until energized once more. Note that the 28 volts dc is removed from the coil by the transfer of BOTH ON NO. 2 relay contacts. In order to energize the coil once more and switch the relay back again, the switch must be returned to NORMAL position. The switching to BOTH ON NO. 1 position is carried out in a similar manner.

(2)  
The switching contacts take care of the transfer of signals within the system. In the normal configuration, the captain's HSI receives its signals from compass coupler No. 1 and the first officer's HSI receives its signals from compass coupler No. 2. AC power for each system is taken from COMPASS-1 and COMPASS-2 circuit breakers respectively. When the transfer relay is switched over to BOTH ON NO. 2, the first officer's system receives its signals and power supplies as before but the captain's system now receives its signals from compass coupler No. 2. A similar transfer takes place on switching to BOTH ON NO.
　
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