(e)  
The heading switch will hold in the heading off (HDG OFF) position only when the roll axis is engaged. When the mode selector switch is set to HDG OFF, the heading computer is unclamped and allowed to synchronize continuously. Therefore, no heading information is provided to the roll control channel when the mode selector switch is in the HDG OFF position.

(5A) OMEGA Steering (NAV) Control
(a)  The roll control channel operates in the NAV mode whenever the roll channel is engaged, the OMEGA system is providing a nav valid and the mode select toggle switch is placed in NAV position. The engage interlocks (Fig. 8) are satisfied when 28 VDC from the NAV switch placed in NAV position, and OMEGA valid are applied to an AND gate (1) to provide a logic 1
5A8
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

output. This output passes through an OR gate (2) as a logic 1 and is applied to another AND gate (3). The other input to AND gate (3) will be a logic 1 level from AND gate (4) when standby power is normal, control wheel steering is not out of detent and radio modes are not engaged. Relay K2, which is only energized in NAV mode, interlocks the autopilot engage logic.
(b)  
The loss of any of the logic signals applied to gates G1 or G4 removes voltage from the holding coil and disengages the NAV mode.

(c)  
The OMEGA steering signal is applied to the roll channel through the same input as the preset heading signal by switching circuits in the autopilot accessory box. OMEGA steering signal processing in the roll channel is the same as preset heading except the OMEGA signal bypasses the heading Q potentiometer and is applied directly to summing point 2.

(6)  Very High Frequency Omnirange (VOR) Control
(a)  
The roll control channel operates in the VOR mode whenever VOR/LOC is selected (mode select switch in VOR/LOC position) and the NAV receiver is tuned to a VOR frequency. In this mode, 28 volts dc is applied through the standby power transfer relay to one input of an AND gate in the control panel (Fig. 9). With interlocks in the roll channel satisfied, and a NAV test NOT signal available, a logic 1 is applied to the other input of the AND gate. An additional logic 1 input is necessary to satisfy the mode select switch interlocks. This input is obtained when any one of the remaining functions on Fig. 9 are available. The logic 1 inputs trigger the first AND gate and apply an input to a second AND gate. The 28 volts dc from the INTLK circuit breaker, through the roll engage switch, triggers the second AND gate to engage the NAV mode select holding coil. The holding coil is held to ground through the VOR/ILS NAV relay (on GJ B-2511 thru B-2520, plus airplanes incorporating SB 34-1184), the NAV transfer relay, the L-NAV INTLK relay, and a circuit interrupter.

(b)  
The CWS OD . ROLL ENG and ILS OC + VOR OC circuit is responsible for disengaging the mode select switch and causing it to return to MAN position when the airplane is in the ILS or VOR on course mode and the control wheel is moved out of detent. When the control wheel is out of detent and the airplane is in ILS or VOR on course, a logic 1 is applied to a gate in the roll channel logic, is inverted to a logic 0 and applied to the first AND gate in the control panel. Operation of the NAV test switch on the NAV control panel, the NAV transfer switch on the captain's or first officer's instrument panel (on GJ B-2511 thru B-2520, plus airplanes incorporating  SB 34-1184) or the NAV select switch on the overhead panel will also cause the mode select switch to return to MAN position thus disengaging the VOR mode. The VOR mode is divided into intercept, capture, on-course, and over-station submodes.

(c)  
The VOR intercept submode can operate in conjunction with other roll axis modes such as heading select or heading hold. In this submode, radio beam deviation signals from the VOR/ILS receiver are modulated and amplified by a modulator amplifier and applied to the lateral beam sensor (LBS). The associated circuitry is armed. At this point, the approach progress display VOR/LOC annunciator illuminates amber. The LBS will automatically change state when the amplitude of the modulated radio beam deviation signal drops below a preset threshold. For the VOR mode, the threshold is established at a level of approximately 30 millivolts by the action of energized switch RS-23 which removes a bias voltage from the LBS. (See figure 10.) When the LBS changes state (LBS drop-out), a signal is fed to the roll interlock circuits which terminates the intercept submode and initiates the capture submode and the VOR/LOC annunciator illuminates green.

5C8 
22-11-0 Page 36  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Oct 20/86 

5C8  Nav/Mode Select Holding Coil Logic Circuit for VOR Mode 
　
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