H28519
Stabilizer Trim Circuits  500 
22-11-0 Page 60  Figure 17 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Nov 15/68 

Movement of the pitch computer servo assembly and elevator continues until control wheel force is released or until the mechanical limit in the servo assembly is reached. When control wheel force is released, CWS OD interlock is removed, and the servo assembly is clamped at the new reference position. Output from the CT continues and causes airplane pitch attitude changes until the vertical gyro output results in a CT output null. Switch PS-2, when closed, increases the control wheel steering detent level to hi-detent.
(7)  Altitude Hold Mode
(a)  
The altitude hold mode is selected at the control panel. To engage the pitch channel in the altitude hold mode, the pitch mode select switch must be in the ALT HOLD position and 28-volt dc flag voltage must be available from the air data computer altitude monitor, a glide slope engage NOT signal must be available, a CWS OD NOT and a pitch engage signal must be available (Fig. 18).

(b)  
The mode interlock signals are processed through a logic chain in the pitch channel and fed to one input of an AND gate in the pitch control panel. 28 volts dc from the engage interlock circuit breaker provides a corresponding 1 input to the AND gate which triggers the AND thus providing a 1 output to one input of a second AND gate. The 28 volts dc from the pitch channel circuit breaker through the closed pitch engage switch triggers the second AND gate which energizes the pitch mode select switch solenoid.

(c)  
Selection of this mode disengages pitch attitude hold by unclamping the pitch computer. Altitude error signals from the ADC are applied to summing point 5 past de-energized switch PS-3 (Fig. 15). Altitude rate signals from the ADC are applied to a bus synchronizer through summing point 4. The bus synchronizer removes quadrature components from the rate signals and synchronizes the rate signals to the autopilot 400-Hz reference. The output of the bus synchronizer is fed to easy-on switch PES-2 which applies the altitude rate signals to summing point 5 approximately 3 seconds after engagement of the altitude hold mode. Altitude rate signals are summed with altitude error signals at summing point 5. The altitude rate signals are summed with the error signals to provide mode damping which eliminates "porpoising" about the reference barometric altitude. Output from summing point 5 is fed to a variable gain amplifier, the gain of which is varied as an inverse function of differential pressure to compensate for changes in airplane control dynamics as a function of airspeed. The output of the variable gain amplifier is routed through summing point 8 and the vertical path filter to summing point 6.

542 
Feb 01/76  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  22-11-0 Page 61 

Output from summing point 2 results in rotation in the pitch computer in which the gear changer has selected the numerically higher gear ratio to form an electromechanical integrator. Integrator movement results in CONTROL output which is routed to summing point 6. Output from summing point 6 causes the valve amplifier to command elevator movement. The control wheel steering detent level is increased while in the altitude hold mode. If force transducer output reaches this higher detent level, the altitude hold mode is disengaged. The increased detent level prevents unintentional disengagement of the mode.
(8)  Turbulence Penetration Mode
(a)  The turbulence penetration mode is selected at the Control Panel (pitch mode select switch in TURB position). In this mode, pitch channel and pitch control panel logic is bypassed and 28-volt dc power is fed directly to the pitch mode select switch solenoid (Fig. 18). Selection results in reduction in amplification of the pitch rate and pitch attitude gains because of the effect of the easy-on gain reduction circuit.

H87014
Pitch Mode Select Logic Circuit  542 
22-11-0  Figure 18  Feb 01/76 
Page 62 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

PES-3 on the variable gain amplifier and application of 28 volts dc to clamp the pitch computer (Fig. 15). These changes effectively provide attitude hold mode with softened commands for pitch control movements. Glide slope mode is disabled during use of turbulence penetration mode, but control wheel steering functions normally.
(9)  Automatic Approach Mode
(a)  
In automatic approach mode (mode select switch in AUTO APP position), the pitch channel commands aircraft pitch attitude to follow a glide slope radio beam. This mode is initiated at the control panel (Fig. 19). To engage the mode select switch in the AUTO APP position, 28 volts dc from the pitch engage circuits must be  available, 28 volts dc from the air data computer altitude flag monitor, a pitch control wheel steering not out of detent and a roll engage signal must be available, 28 volts dc from the ILS receiver must be available and the pitch mode select switch must not be in TURB position. The G/S ENG (ILS OC) input state has no effect on the initial AUTO APP mode engage interlock requirements. However, once the vertical beam sensor has changed state (G/S ENG), this circuit is responsible for causing the mode select switch to return to MAN position when pitch control wheel steering is moved out of detent. The pitch CWS OD input is combined with ILS OC input, summed at an effective AND gate, and inverted until a logic 0 is applied to one input of the first AND gate in the roll control panel, thereby causing the mode select switch to return to MAN. Control wheel steering detent level is increased during glide slope mode to prevent unintentional disengagement of the mode. The glide slope automatic mode is executed in three phases: intercept, capture, and track.
　
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