(b)  
The F/D computer has a parallel computation path similar to the autopilot. The F/D computation for cruise modes receive the respective mode signal at the output of the digital CAS gain programmer. This signal is modified by the attitude signal (21) pitch limited, gain programmed modified by elevator surface position (22) and applied to the cruise mode input of the glideslope engage switch S21. The approach (GS engaged) signal is received from the AP approach computer vertical beam sensor, position information from the summed elevator position, neutral shift, elevator LVDT and attitude rate. These signals are converted to digital, summed (24) and applied to the glideslope integrator. The combined glideslope, surface position and attitude rate function form an anticipator circuit for accurate patch axis guidance. The glideslope integrator eliminates steady state errors on the beam to maintain beam centering. The output of the integrator applies the signal to the glideslope engaged input of switch S21. The F/D signal is then limited to proper gradient, converted to analog signal and controls the ADI command bar.

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Oct 20/83  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  22-11-01 Page 35 

H.  AP/FD Pitch Cruise Modes
(1)  
The pitch axis cruise control consist of the following modes: Altitude Acquire, Altitude Hold Capture, Altitude Hold, Vertical Speed, Level Change and PDC Speed. The majority of the cruise modes' computation is accomplished digitally in the FCC.

(2)  
Altitude Acquire Mode

(a)  Altitude Acquire mode is a transition mode between any other pitch cruise mode and the Altitude Hold mode (at the selected altitude) by following a 0.05g circular path to the MCP selected barocorrected altitude until the altitude hold mode engages. This mode is always armed and will automatically engage when the 0.05g circular path is intercepted unless altitude hold capture mode, altitude hold mode, glide slope mode or Go-Around (below 40 feet radio altitude) mode is engaged. ALT ACQ, black or green, is annunciated on the FMA when the mode is engaged.
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H31132
Pitch Channel Block DiagramMay 20/82 Figure 10 22-11-01 Page 37/38BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

H28591
591 Pitch Channel Simplified SchematicMay 20/82 Figure 11 (Sheet 1) 22-11-01 Page 39/40BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

Pitch Channel Simplified Schematic May 20/82 Figure 11 (Sheet 2) 22-11-01 Page 41/42 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

(3)  
Altitude Hold Capture Mode

(a)  Altitude Hold Capture mode is a transition mode between any other pitch cruise mode and the Altitude Hold (not at selected altitude) mode. The altitude hold target altitude is the uncorrected baro altitude existing when the ALT HLD pushbutton is pressed engaging this mode. A 0.09g maneuver toward the target altitude is commanded until a 0.05g circular path to the target altitude is intercepted. At this point, the 0.05g circular path is followed until the altitude hold mode engages. This mode is engaged by pressing the ALT HLD pushbutton on the MCP unless altitude hold mode, glide slope mode or Go-Around (below 400 feet radio altitude) mode is engaged. ALT HLD, black on green, is annunciated on the FMA, when the mode is engaged.

(4)  
Altitude Hold Mode

(a)  
The Altitude Hold mode provides pitch commands to fly at a constant uncorrected baro altitude. This mode automatically engages as the normal termination of either the Altitude Acquire or the Altitude Hold Capture mode. ALT HOLD is annunciated on the FMA, black on green, when the mode is engaged.

(b)  
Although the mode control and mode engagement is the same for the automatic engagement of altitude hold from either the Altitude Acquire or the Altitude Hold Capture mode, the Altitude Hold mode disengagement is different. If the Altitude Hold mode is entered from Altitude Acquire, the uncorrected baro altitude held is equivalent to the Selected baro-corrected altitude on the MCP. In this case, the Altitude Hold mode (at the selected altitude) can only be disengaged by first selecting a new altitude, by capturing the glide slope mode, or by holding the PCWS force out-of-detent until the airplane is outside the Altitude Acquire automatic capture bounds. Altitude Hold mode (at the selected altitude) is indicated by ALT HOLD annunciated on the FMA and the ALT HOLD pushbutton light on the MCP extinguished. Selection of a new altitude on the MCP will result in the reversion to the Altitude Hold mode (not at the selected altitude) (black on green) and V/S armed (white on black) annunciation on the FMA.
　
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