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A 737-300/400/500MAINTENANCE MANUAL
M924 FLIGHT CONTROL COMPUTER A (E1-3)
Speed Trim System SchematicFigure 8
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A
737-300/400/500MAINTENANCE MANUAL
G. When the speed trim system goes into operation, the immediate output from SP4 would be zero since the reference and the input would be the same. As the airspeed increases, a command is generated as a result of the difference between the airspeed reference and present airspeed inputs to SP4. This would be compared to the stabilizer position input at SP5 along with any assist from the altitude rate input. This signal passes through relaxed S4, the amplifier, and to polarity and trim error detector circuits.
H. The polarity detector determines whether the command is for nose up or nose down. Gate G1 will be enabled for a nose up command and gate G3 for a nose down command. The trim detector will not put out a command until the error signal is at least equal to 0.072 degrees of stabilizer command. This provides the other required input to either gate G1 or G3 which will provide the nose up or down command. The output of the trim error detector will remain until the error gets less than 0.070 degrees of command error. The output will then go to zero and the stabilizer trim will stop. There is one-half second delay between the output of G1 or G3 and the input to the following gate. Gates G2 and G4 require that the speed trim system is operating and that the local FCC has been selected for command of the system. The signal passes to the nose up or nose down relay in the stabilizer trim servo.
20. Altitude Alert System (Fig. 9)_____________________
A. The altitude alert function provides visual and/or aural indications to both pilots of approach toward or departure from the MCP selected altitude. The altitude alert function is inhibited when making an approach to land as sensed by the glideslope mode capturing or by the flaps being lowered to 20 degrees or greater. Selected baro-corrected altitude is used to determine the distance to the MCP selected altitude.
B. Loss of valid selected baro-correct altitude in the master (1st up) channel during dual channel A/P go-around will result in a steady illumination of the red A/P disengage warning light. This signifies to the flight crew that automatic altitude acquire will not operate and that the pilot should disengage the A/Ps and capture the clearance altitude manually.
C. Loss of valid selected baro-corrected altitude in both channels will cause:
(1) The aural altitude alert to sound once
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A 737-300/400/500MAINTENANCE MANUAL
LIGHT EXTINGUISH
THE ALTITUDE ALERT LIGHTS EXTINGUISH AND THE SYSTEM RESETS WHEN THE AIRPLANE IS 900 FEET FROM THE SELECTED ALTITUDE OR A NEW ALTITUDE IS SELECTED.
Altitude Alert System Operation Figure 9
A53302
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