580 
22-11-01 Page 18  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Oct 20/83 

Autopilot Engage Circuit Schematic (Typical)Oct 20/84 Figure 7 (Sheet 1) 22-11-01 Page 19/20BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

553 Autopilot Engage Circuit Schematic (Typical)Jun 20/87 Figure 7 (Sheet 2) 22-11-01 Page 21/22BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

Autopilot Engage Circuit Schematic (Typical)Jun 20/87 Figure 7 (Sheet 3) 22-11-01 Page 23/24BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

(2)  A bar bias circuit and a flag circuit monitor the flight directors. Mode and signal interface valids form combined logic, which controls bar bias. When a fault is detected the respective bar is biased out of view. The ADI flag is out of view when ADI driver power is correct.
D.  AP/FD Mode Priorities
(1)  
The autopilot approach mode has the highest control priority. The approach mode, when armed, will automatically capture from any of the cruise modes. Flight Director only approach mode before approach-on-course (AOC) can be exited by turning off both flight directors, deselecting APP or pressing go-around (GA) pushbutton (which will select F/D G/A if the G/A conditions are met on config. III F airplanes). After AOC, only turning off both flight directors or pressing GA pushbutton will cause F/D APP to be exited.

(2)  
The autopilot approach mode (LOC and GS Engage on the FMA and MCP APP pushbuttons not being lit) can be exited only by disengaging the autopilot, pressing the GA pushbutton or exceeding either the pitch or roll CWS "Hi Detent" force during a single channel approach. Pressing the GA pushbutton during a single channel approach will disengage the Autopilot and reset APProach mode. If the F/D is ON, it will go to G/A, if G/A conditions met. Pressing the GA pushbutton during a dual channel approach will engage the dual channel G/A mode. The F/Ds will also go to G/A if they are ON (one or both).

(3)  
The second priority in the AFCS control is altitude control. The altitude selected is assumed to be the ultimate target in cruise operation and is the reference for the Altitude Alert function and the Altitude Acquire mode (which is always armed in cruise operation when not in Altitude Hold mode). All pitch cruise modes, except Altitude Hold (at the selected altitude) are assumed to be used to transition toward the selected altitude. As a result, if an altitude higher than the existing altitude is selected; the A/T is commanded to use the climb EPR limit rating from the PDCS and the AP speed-on-elevator modes (LVL CHG and PDC SPD) are not allowed to command a loss of altitude to acquire the selected speed. If an altitude lower than the existing altitude is selected; the A/T is commanded to move the thrust levers to the aft limit (if in an EPR mode) or to use the Climb EPR limit (if in a speed mode) and the AP speed-on-elevator modes are not allowed to command an increase in altitude to acquire the selected speed. The A/T is commanded to use Climb EPR limit, if in a speed mode, because the non-speed-on-elevator modes are not restricted as to the direction of altitude change and climb EPR limit is required if climbing.

(4)  
Altitude hold (at the selected altitude) has the highest priority of the cruise modes. Altitude hold (at the selected altitude) is indicated by ALT HOLD being annunciated on the FMA and the MCP ALT HOLD pushbutton not being lit. No cruise mode pushbutton selection will cause disengagement of altitude hold (at the selected altitude). A new altitude selection will result in being in altitude hold (not at selected altitude) mode (indicated by the altitude hold pushbutton lighting [indicating that it can now be disengaged] and the vertical speed ARM annunciation appearing on the FMA indicating that rotation of the V/S Wheel will automatically engage V/S mode.

(5)  
Pressing the altitude hold pushbutton in any cruise mode, except altitude hold, will cause an altitude hold capture of the existing altitude. The altitude hold capture mode has higher priority than any other pitch cruise mode, except altitude hold. The altitude hold (not at selected altitude) mode is a transition mode and any subsequent pitch cruise mode selection will be allowed.

(6)  
Speed control is third in the AFCS control priority. Once a speed mode is engaged, either manually or automatically (except for A/T Minimum [Alpha floor] Speed in certain instances), the AFCS control function will keep speed under control until specifically deselected. This deselection can only be accomplished by pressing the speed pushbutton on the MCP when lit or by disengaging the A/T. Pressing the lit pushbutton causes the A/T to go to the ARM mode. In this situation, the next mode change which requires engagement of A/T Speed or EPR modes will cause the A/T to change to that required mode. A disengaged A/T causes the AFCS control to assume that compatible thrust control is now provided with the engaged AP or F/D modes. Speed modes on both AP and A/T simultaneously are not allowed, except for the special case of alpha floor speed control.
　
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