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A
737-300/400/500MAINTENANCE MANUAL
 (2)  
The altitude alert lights to flash
 (Reset by rotating the altitude select knob)

 (3)  
The Selected Altitude display window to jump to and stay at 50,000 feet.

 D.  At least one of the channel's selected baro-corrected altitudes becoming valid will also reset the altitude alert lights and enable the selected altitude display window to change with altitude select knob rotation in a CCW direction.
 E.  The altitude alert function (STD option) warns with a single aural alert and steady altitude alert lights between 900 feet and 300 feet from the selected altitude approaching and with a single aural alert and flashing altitude lights between 300 feet and 900 feet from the selected altitude when departing.
 F.  Altitude alert uses altitude error developed from the MCP selected altitude and the selected DADC baro-corrected altitude.
 G.  Channel "A" provides the altitude alert function unless the "B" selected baro-corrected altitude is valid and:
 (1)  
"B" autopilot engaged, or

 (2)  
No autopilot engaged and only the "B" flight director is ON, or

 (3)  
None of above and "A" selected baro-corrected altitude is invalid

 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 than 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. The functional block diagram of the altitude alert is presented in Fig. 30.
 21. Stabilizer Trim System______________________
 A.  Stabilizer Trim Signals (Fig. 10)
 (1)  The prime input 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 an engaged A/P or the speed trim system will provide the commands to trim the stabilizer from the FCC.
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A 737-300/400/500MAINTENANCE MANUAL

ACTUATED A (STA 1256) M924 FLIGHT CONTROL COMPUTER A (E1-3)
Stabilizer Trim Command SchematicFigure 10
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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

 10.64 degrees at low feel pressure to 0.85 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 greater than 10 percent detectors. If a commmand of greater than 10% exists for 3 out of the 4 seconds in a sampling period the CRZ/APP trim up or trim down detectors will provide an output. When not in the GA mode, the signal will pass through logic gates 1, (9), 2(10) and on to 3(11). As long as the stab trim disable signal is low the signal will be passed through 4(12), 5(13). The column forward or aft signal is used to prevent a trim signal if the pilot is applying column force in the opposite direction.
　
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