22-11-01
 ú ú 02 Page 7 ú Feb 25/87
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 F. Altitude Alert System
 (1)  The altitude alert system compares the airplane altitude with the altitude selected on the mode control panel. A visual and aural warning is provided when the airplane approaches (option) or departs from the selected altitude. No warnings are issued while the airplane is within prescribed limits from the selected altitude. The warning is inhibited during approach by flap position logic.
 G. Mach Trim System
 (1)  The mach trim system provides automatic elevator quadrant displacement as a function of mach number in the mach tuck (nosedown) region. Mach data from the air data computer is used to generate a servo position command signal. This signal is applied to the mach trim actuator. The actuator provides elevator pitch-up position to maintain the correct attitude when airplane mach tuck speed is attained. The mach trim actuator changes the position of the column neutral point without altering feel/force characteristics.
 H. Speed Trim System
 (1)  The speed trim system controls the stabilizer trim motor to provide positive speed stability during certain low speed, high thrust flight conditions. The system is automatically engaged when the autopilot is disengaged, flaps are not up, engine speed exceeds a prescribed amount and control column limit switches are not activated.
 I. DFCS Built-In Test Equipment (BITE)
 (1)  The DFCS has three built-in maintenance features:
 -ground maintenance Built-In Test Equipment (BITE), for on ground system checkout
 -continuous monitoring, for system verification
 -maintenance monitoring, for inflight fault storage.
 (a)  
An extensive ground maintenance BITE capability is embedded within the DFCS. BITE can be used for line maintenance fault isolation in less than 5 minutes, and overnight maintenance actions when extensive engineering investigating or trouble shooting is required. A land mode verification feature is available for autoland reinstatement after system LRU replacement. BITE is primarily intended for use by the line mechanic and utilizes software and circuitry which is not flight critical (i.e., does not influence in-flight system operation).

 (b)  
The continuous monitor provides on-line monitoring to determine the health of the FCC's and system input/output operation. When a fault is detected, a fault reaction occurs: either a mode dropout, an autopilot disconnect, or F/D bars bias out of view. In the DFCS, this monitoring covers the computation, the RAM/ROM memories and partly the power supplies. Continuous system monitoring is also provided to detect failures on incoming digital signals, and in the actuator position feedback loops.

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 22-11-01
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 (c)  The maintenance monitor records the causes of DFCS related cockpit effects. These include on-line faults detected by the continuous monitor and system failures that cause an autopilot disconnect, prevent autopilot engagement, cause a F/D bar out-of-view bias, or cause mach trim/speed trim failures. The system records the cockpit effect, the LRU diagnostic, the monitor that tripped, and if the fault is repeated in the flight segment. This data is retrieved on the ground through ground maintenance BITE by selecting INFLIGHT faults.
 (2)  Two BITE printer receptacles are available for use with a portable carry-on printer. One receptacle is located on the P18 panel, the other is located on the left aft station of the E-1 radio rack.
 2.  Flight Control Computer (FCC)_____________________________
 A.  The purpose of the flight control computer (FCC) is to accept input mode and sensor signals, process them, and provide command outputs to the aileron, elevator, and stabilizer control surfaces. The two FCC's are identical and interchangeable.
 B.  Two flight control computers (FCC) designated as A and B are mounted on the E1-3 shelf in the electrical and electronics equipment bay. The units are retained in the rack by cam-loc handles located on the front of the unit which engage adjustable rack-mounted swivel bolts. The flight control computers are standard 1 ATR-long boxes. Electrical connections to the units are provided by four DPX2 connectors located on the rear of the unit which mate with rack-mounted DPX2 connectors. Three capped test connectors and a cooling fan are located on the front panel of the flight control computers.
　
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