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A
737-300/400/500MAINTENANCE MANUAL
(3) C456 MACH TRIM AC (P18-1) 115 volts ac to the A FCC for mach trim.
D. 115 Volt AC Electronic Bus 2 provides power to:
(1)
C1042 SNSR EXC B (P6-2), 115 volts ac to the AFC ACC unit or IFSAU. An autotransformer in the AFC ACC unit (or IFSAU) provides 26 volts ac to the B FCC and some of the sensors for the B channel.
(2)
C1037 MACH TRIM AC (P6-2), 115 volts ac to the B FCC for mach trim.
E. 28 Volt DC Electronic Bus 1 provides power to:
(1)
C1044 A MCP DC (P18-1), 28 volts dc to the MCP.
(2)
C1045 A FCC DC (P18-1), 28 volts dc to the A FCC.
(3)
C1048 ENG INTLK A (P18-1), 28 volts dc to the engage interlock circuits for the A FCC.
(4)
C457 MACH TRIM DC (P18-1), 28 volts dc to the A FCC for the Mach Trim Actuator Brake.
(5)
C576 ALTITUDE ALERT AUDIO (P18-2), 28 volts dc to the Aural Alert Unit.
F. 28 Volt DC Electronic Bus 2
(1)
C1046 B FCC (P6-2), 28 volts dc to the B FCC.
(2)
C1047 B MCP DC (P6-2), 28 volts dc to the MCP.
(3)
C716 ENG INTLK B (P6-2), 28 volts dc to the engage interlock circuits for the B FCC.
(4)
C1064 MACH TRIM DC (P6-2), 28 volts dc to the B FCC for the Mach Trim Actuator Brake.
G. 28 Volt DC Switched Hot Battery Bus provides power to:
(1)
C1049 A/P WARN LT-A (P18-1), 28 volts dc to the A DFCS warning circuits in the MCP and the captain's FMA as well as the A FCC for RAM memory.
(2)
C374 A/P WARN LT-B (P6-2), 28 volts dc to the B DFCS warning circuits in the first officer's FMA as well as the B FCC for RAM memory.
18. Mach Trim System (Fig. 7)________________
A. The mach trim system provides nose up commands to the elevator at high mach numbers to compensate for normal tuck tendencies. The output to the mach trim actuator can be provided by FCC A or B.
B. Separate 115v ac and 28v dc inputs are provided to each of the two FCC's for the mach trim system. The 115v ac input provides power for the fixed phase of the motor and the 26v ac input provides the excitation for the feedback synchro. The 28v dc input is used to release the MTA brake when in the mach region. When on the ground, or if the local FCC is not in control, the dc provides an input to the NAND gate which in turn energizes K1 and removes the output from this FCC to the mach trim actuator.
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A 737-300/400/500MAINTENANCE MANUAL
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737-300/400/500MAINTENANCE MANUAL
C. The MTS VALID logic determines that the requirements for satisfactory operation of the MTS from the local FCC are met. These requirements are that the DADC is valid, 26 volts ac is valid, the command and feedback is at a null condition, and that the wraparound logic is valid. The MTS VALID is combined with logic which indicates whether the local channel is operating the MTS. Failure of one channel automatically transfers command to the other. The wraparound checks consist of sending the output signal back into the computer for a comparison to ensure the output agrees with computer output. Wraparound checks are made for the warning circuits, MTS select and the MTA brake.
D. The MTS SEL logic is used to determine which FCC provides the mach trim control. When the FCC's are power up, channel A will be selected to provide mach trim as long as its mach trim system is valid. An input from the squat switch provides a toggle input which selects B if A was used on the last flight and vice versa. However, if the foreign MTS valid logic is not true, then the local channel provides mach trim.
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