(f) VOR Over-Station (OS) Submode  1) The roll channel automatically switches to the over-station submode when the OS portion of the over-station rate sensor senses the rapid fluctuations in radio signal intensity associated with the zone of confusion. (Zone of confusion refers to the area in proximity to VOR stations in which the erratic nature of the radio signals renders them unsuitable for controlling the autopilot.) When the zone of confusion is entered, the roll channel automatically decouples from the radio beam and controls the airplane on the drift-corrected heading existing when the radio signals are decoupled.
2)  The OSS receives radio signals which have been processed by the rate network and an amplifier. Because of the effect of the rate network, the amplitude of these signals is proportional to the time rate of change in the amplitude of the radio deviation signal. During the on-course submode, the sensitivity of the OSS is increased by switch RS-25 which provides a ground return for a diode which acts as part of a voltage-doubler circuit in the OSS. The on-course submode, the threshold sensitivity of the OSS is approximately 8 millivolts per second. When the radio rate exceeds the threshold level, the OSS initiates the over-station mode.
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3)  During the over-station submode, the radio deviation and heading course error signals are removed from summing point 5 by energized switches RS-13 and RS-24. The radio deviation signal is removed from summing point 6 (and summing point 2) by energized switch RS-14. Because the radio signals are disconnected, the roll channel is controlled by heading course error signals while the airplane is in the zone of confusion. It should be noted that the heading/beam integrator is not reset during this submode and therefore stores drift-correction information. The roll channel remains in this submode until the airplane passes out of the zone of confusion. At this time, the radio rate drops below the threshold level and the OSS causes the roll channel to revert to the on-course submode. The bank angle command and roll rate command limits for the over-station submode are the same as those for the on-course submode (Fig. 10).
(7)  Localizer Mode
(a)  The roll channel operates in the localizer mode whenever either VOR LOC or AUTO APP is selected on the control panel and the VOR/ILS receiver is tuned to an ILS frequency. When the mode select switch is in VOR/LOC (localizer) position, the following inputs are required for the mode select switch to remain engaged. The pitch mode select switch must not be in the TURB position. Selection of the TURB (turbulence penetration) mode will prevent selection of the localizer mode or will cause the localizer mode to drop out by providing a logic 0 input to the first AND gate in the control panel (Fig. 11). An additional logic 1 input from any of the functions in the remaining logic circuit on Fig. 11 is necessary to satisfy the mode select switch interlocks. Operation of 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 LOC mode. This circuit operates in the same manner as in the VOR mode.
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Oct 20/86  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  22-11-0 Page 43 

Nav/Mode Select Holding Coil Logic Circuit for LOC Mode  5C8 
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The localizer mode is divided into intercept, capture, on-course, and approach on-course (AUTO APP position only) submodes.
(b)  The localizer intercept submode functions the same as the VOR INTERCEPT SUBMODE EXCEPT THAT SWITCH RS-23 is deenergized and the LBS threshold is changed to approximately 150 millivolts.(See figure 12.)
(c) The localizer capture submode functions approximately the same as the VOR capture submode. The differences between these submodes are as follows: 1) The radio beam deviation signal path to summing point 6 is applied past switch RS-15 of figure 6 instead of switch RS-14. This provides a gain change between the VOR and localizer submodes. 2) Radio rate (cross-beam rate) signals from the rate network are applied to summing point 6 past switch RS-26. As a result of the summation of cross-beam rate signals with radio beam deviation signals, the airplane radio beam intercept angle will remain essentially constant as long as the output from summing point 6 exceeds the limit point of the amplifier limiter. When the output from summing point 6 drops below the limit point, the intercept angle decreases exponentially.
　
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