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supplies an input to the elevator power control units (PCU). This
moves the elevators. A signal from the mach trim actuator tells
the FCC how much it moves.
Autopilot Engaged
When the autopilot is engaged, the feel and centering unit
cannot supply an input to the elevator PCUs. This is because the
autopilot elevator actuators lock the elevator input torque tube.
This will not let the input arms of the PCUs move. However, the
mach trim actuator will turn the neutral shift sensor. The signals
from the neutral shift sensor and the elevator position sensor go
to the FCC. The FCC knows that the neutral shift position
changes and the elevator position does not move. The FCC
calculates an autopilot signal which then causes the autopilot
elevator actuators to move the input to the PCUs.
FCC Selection For Mach Trim
The proximity switch electronics unit (PSEU) sends air/ground
signals to the FCCs. When in the air, only one FCC supplies the
mach trim signal to the mach trim actuator. FCC B controls
which FCC to use and sends a signal to the IFSAU to make the
selection.
Mach Trim Failure Display
The mach trim fail annunciator on the flight control module
shows when the mach trim function in the FCC meets certain
failure conditions.
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EFFECTIVITY
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FCC Controlled Neutral Shift Enable (FCNSE) Function
The mach trim system also moves the elevators to do an auto
neutral shift function when the airplane is in the FCNSE region.
The FCNSE region is when the flaps are not up and either
engine N1 is more than 18%. The movement of the elevators
depends on the trailing-edge flap position and the horizontal
stabilizer position.
This function allows the airplane to have a larger nose up
attitude during takeoff.
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737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
General
The FCC supplies these voltages and calculates these mach
trim signals to operate the mach trim actuator:
* 26v ac mach trim excitation power
* 28v dc brake release power
* 115v ac motor excitation power
* Mach trim command signals
* Mach trim select signal
* Mach trim warning signal.
Mach Trim Excitation Power
The FCC power supply gets 115v ac from the electronics bus
and changes this to the 26v ac, 400Hz position sensor excitation
power.
Brake Release Power
The FCC supplies the 28v dc brake release power to the IFSAU
if the FCC is in BITE or if it is in the FCC controlled neutral shift
enable (FCNSE) region. The brake release power will stay for 20
seconds after the airplane exits the FCNSE region. The FCC
also supplies the brake release power if all of these conditions
are true:
* Airspeed is more than mach 0.60
* Airplane is in the air
* Mach trim select status signal is a ground
* Input 28v dc is good.
The FCC also supplies the 28v dc brake release power to the
IFSAU if the airplane is in the FCNSE region. These conditions
set the FCNSE region:
* Flaps not zero
* Either engine N1 more than 18 percent.
Motor Excitation Power
The FCC supplies the 115v ac motor excitation power to the
IFSAU if the FCC is in BITE, in the FCNSE region or all of these
conditions are true:
* Airspeed is greater than mach 0.60
* Airplane is in the air
* Mach trim select status signal is a ground
* Input 115v ac is good.
Mach Trim Commands
The FCC gets the mach from the ADIRU and calculates an
elevator command from the mach speed. The FCC also gets the
mach trim position signal from the mach trim actuator. The FCC
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