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 G.  Altitude alert uses altitude error developed from the MCP selected altitude and the selected DADC baro-corrected altitude.
 H.  The altitude alert function sounds the aural warning and illuminates the altitude alert lights steady when the airplane approaches within 900 feet of the MCP selected altitude. As the airplane approaches within 300 feet, the lights go off. If the airplane departs more than 300 feet from the selected altitude, the function sounds the aural warning and flashes the altitude alert lights until the airplane either departs more that 900 feet from the selected altitude or returns to within 300 feet of the selected altitude. In either case, the alert lights are then extinguished.
 21. Stabilizer Trim System______________________
 A.  Stabilizer Trim Signals (Fig. 10)
 (1)  The A/P provides trim inputs to the stabilizer at any time that it is engaged. The prime input to the system to determine when trim is required is the amount of command to the elevator. Different trim detection circuits are used depending on the mode of operation. Either the A/P or the speed trim system will provide the commands to trim the stabilizer from the FCC. A stab out of trim warning light is illuminated if the stabilizer is out of trim.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL

ACTUATED A (STA 1256) M924 FLIGHT CONTROL COMPUTER A (E1-3)
Stabilizer Trim SignalsFigure 10
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A
737-300/400/500MAINTENANCE MANUAL
 (2)  
The DADC provides total and static pressure inputs which are combined to generate differential pressure. This signal is combined with feel pressure calculated as a result of stabilizer position and used to calculate total elevator authority as a function of feel pressure. The single channel autopilot authority will range from

 6.0 degrees at low feel pressure to 2.5 degrees at high feel pressure.

 (3)  
There are two prime inputs used to determine when stabilizer trimming is required. These are the elevator command when the flaps are up or the summation of elevator position and neutral shift sensor when the flaps are down. When the required logic to energize switch S1 is present, a flare spring bias is added. This signal is used to provide a nose up trim in the event that the A/P is disconnected during a dual approach. The flap position input is used to vary the bias as a function of the flap position.

 (4)  
The elevator command or the elevator position and NSS input is used to determine the amount of standing error command that is being sent to the elevator. This signal becomes the numerator and the elevator authority is the denominator in the calculation of percentage of authority present. This percent of authority is measured by the trim detectors. If a command of greater than 10% exists for 3 out of the 4 seconds in a sampling period then the CRZ/APP mode, a trim up or trim down detectors provides an output. When the GA mode is in effect, the greater than 10% error sampling time is reduced for 20 seconds. A sampling time of 500 of the 800 milliseconds is used. The trim up or trim down command continues until the less than 2% is sensed. The trim command is applied to gate 2 or 3. These gates are enabled if the local A/P is engaged, not flare and NOT PITCH CWS OD.

 B. Stabilizer Trim Interface (Fig. 11)
 (1)  
The stabilizer trim system maintains proper airplane trim under varying flight conditions. These commands are provided by the FCC using the speed trim system with A/P disengaged or the A/P automatic trim used at all times that the A/P is engaged.

 (2)  
Both stabilizer trim systems require that the clutch voltage to the jackscrew be present at all times. Speed trim or A/P Engaged (ELEV Actuator Detent Pressure) provide the clutch voltage. Trim up and trim down functions then control the direction of motor rotation. The stab trim clutch voltage first passes through the control column forward and aft switches. Pressure opposite to the direction of trim, operate switches in the column switching module, which disengage the clutch. The signal then passes through the A/P trim cutout switch on the control stand. This switch provides removal of all stabilizer signals in the event of stabilizer runaway. The signal then goes to the stab trim servo where it provides the voltage for engaging the clutch.
　
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