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737-300/400/500MAINTENANCE MANUAL
H. The polarity detector determines whether the command is for nose up or nose down. 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 also provides the other required input to either gate 2 or 3 which provides the nose up or down command. The output of the trim error detector remains until the error gets down to less than 0.070 degrees of command error. The output then goes to zero and the stabilizer trim stops. Gates 2 and 3 also require that the speed trim system is in operation and that the local FCC has been selected for command of the system. The signal then passes to the nose up or nose down relay in the stabilizer trim servo. Selection of FCC A or B is altered at each squat or selection of the operational system, if one system fails.
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-corrected altitude, with neither Glideslope nor Go-Around mode engage, will cause that channel's autopilot to disengage if in the CMD position and will cause that channel's flight director to bias out of view and reset all modes. This is to emphasize to the pilot that the automatic MCP altitude acquire mode and the local altitude alert will not operate and that he is now required to control and maintain the clearance altitude. 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
(2)
The altitude alert lights to flash (Reset by rotating the altitude select knob)
EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. AIRPLANES WITH EFIS ú
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A 737-300/400/500MAINTENANCE MANUAL
LIGHT 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
A53360
EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. AIRPLANES WITH ALT ALERT OPTION STD ú
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737-300/400/500MAINTENANCE MANUAL
(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. 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
F. The altitude alert function warns with a single aural alert and steady altitude alert lights between 900 feet and 300 feet from the selected altitude when approaching and with a single aural alert and flashing altitude lights between 300 feet and 900 feet from the selected altitude when departing.
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