(b)  
The heading signal is synchronized at summing point (8) until activation of the mode at which time the signal is applied to summing point (9) and an error signal will develop when the airplane deviates from engaged heading. HDG SEL selection applies the signal direct to summing point (9). This signal, heading or HGD SELECT is gain programmed as a function of CAS and applied to point (10) (not on VOR) then to the variable bank limiter. This signal is applied through S3 (engaged) and converted to an analog signal. The signal is routed to summing point (11) which is the input summing point for the analog command section computer. Selection between digital computer and AP approach computer is provided by S4.

(c)  
When other modes are selected the signal path is modified. Heading select mode does not use HDG HOLD (S2 open), when VOR is captured neither HDG HOLD or HDG SELECT are used. The VOR mode also introduces preset course (S13) VOR deviation (S15) and detects VOR capture point within the lateral beam sensor.

591 
Oct 20/83  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  22-11-01 Page 27 

580 
22-11-01 Page 28  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  May 20/82 

591 Roll Channel Block DiagramOct 20/83 Figure 8 22-11-01 Page 29/30BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

T51943
591 Roll Channel Simplified SchematicJun 20/84 Figure 9 22-11-01 Page 31/32BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

(d)  The F/D computer has a parallel computation path similar to the A/P. Preset heading or VOR (22) is combined with attitude at point (27). The signal is applied through bank angle and rate limiters, local select switch, then converted into an analog signal and applied to F/D roll bar. When approach mode is selected, analog signals used by the autopilot are digitized for the F/D. Programmed LOC deviation, filtered accelerometer and lagged roll are combined with preset course at summing point
(26) then applied to summing point (27). Signal processing beyond point (27) is as previously described.
F.  AP/FD Roll Cruise Autopilot Modes
(1)  
The roll axis cruise control consists of the following modes: Heading, Select, VOR and Localizer.

(2)  
Heading Select Mode

(a)  
Heading Select mode provides roll commands to hold the selected heading. The Heading Select (HDG SEL) mode is engaged by pressing the HDG SEL pushbutton on the MCP unless the autopilot is captured in LOC or APP mode. The desired heading is dialed with the heading select knob on the mode control panel (MCP). The heading bugs on both the captain's and first officer's HSIs follow the heading knob. The AP/FD will capture and hold the selected heading. The bank angle command during capture will be limited to the value selected by the bank angle limit knob on the mode control panel. If a heading change is desired, a new heading is dialed which causes commands to capture and hold the new selected heading.

(b)  
The HDG SEL mode can be disengaged by pressing the HDG SEL button, capturing VOR/LOC (VOR/LOC or APPR may be selected while in the HDG SEL mode) or by applying sufficient force to trigger the CWS high detent override. HDG SEL, black on green, is annunciated on the FMA when the mode is engaged.

(3) 
 VOR Mode

(a)  
VOR mode provides roll commands to capture and track the selected VOR radial. The AP/FD is armed in the VOR mode by tuning the appropriate VHF navigation receiver to the desired VOR frequency, selecting the desired radial on the appropriate course knob on the MCP, and pressing the VOR/LOC pushbutton on the MCP.

(b)  
When VOR mode is selected, the mode will be in the VOR ARM phase (FMA white on black) until the VOR capture logic is satisfied. VOR capture logic is satisfied when there is a superflag valid discrete from the NAV receiver, the receiver is tuned to a VOR frequency, and the absolute value of beam error is less than 0.5 degrees or the adaptive capture equation is satisfied. At VOR pushbutton engagement, logic will prevent VOR capture and hold the VOR ARM mode for 3 seconds to allow proper filter initialization. VOR capture will also be inhibited when the beam angle error is greater than 22 degrees.

(c)  
The adaptive capture equation defines the beam angle at which VOR capture should be initiated. The equation accounts for the time required to perform the capture maneuver and by using DME distance data, it also accounts for crosswind impact on the capture of the VOR beam.  When the VOR capture logic is satisfied, the VOR control laws are engaged and the FMA shows VOR capture (green on black).

(d)  
　
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