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DFCS - FUNCTIONAL DESCRIPTION - SPEED/AUTOPILOT TRIM BLOCK DIAGRAM
DFCS - FUNCTIONAL DESCRIPTION - SPEED/AUTOPILOT TRIM BLOCK DIAGRAM
EFFECTIVITY
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DFCS - FUNCTIONAL DESCRIPTION - SPEED/AUTOPILOT TRIM BLOCK DIAGRAM
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D633A101-SHZ1 Oct 10/2002
BOEING PROPRIETARY - Copyright#- Unpublished Work - See title page for details.
737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
General
The FCC supplies these signals to do the speed trim functions:
* Speed trim nose up
* Speed trim nose down
* Speed trim reference hold
* Speed trim warning.
Speed Trim Commands
These input data and an adjustable gain control make the speed
trim nose up and nose down commands:
* Stabilizer command
* Stabilizer position
* Inertial vertical speed (not used during F/D TO/GA).
The FCC uses computed airspeed (CAS) to calculate the
stabilizer command signal.
This data goes through synchronizers before they are
combined. The combined signals go through these
components:
* Electronic switch
* Adjustable gain amplifier
* Speed trim detector.
Synchronizers
The synchronizer makes a reference signal from the input so
that when speed trim starts, it compares the current value to the
reference value. The synchronizers operate until conditions are
met to calculate the speed trim commands and send them to the
stabilizer actuator. This occurs when all of these conditions are
true:
* Airplane is in the air for more than 10 seconds
* No manual electric trim for at least five seconds
* Autopilot in both FCCs is not engaged
* Flaps are down, gear is up, or CAS is less than 202 knots
* Speed trim function is valid.
The airplane is in the air if the squat switch shows it is in the air
or if the radio altimeter shows an altitude more than 10 feet.
The output signal from the synchronizer is the difference
between the reference signal and the input signals.
Speed Trim Cutout Electronic Switch
The stall detection circuit monitors the flap position and the
angle of airflow. Near stall, the speed trim function trims the
stabilizer to a nose down condition to allow for trim above the
stickshaker AOA and idle thrust. The trim continues until the
stabilizer gets to its limits or the aft column cutout position is
exceeded.
DFCS - FUNCTIONAL DESCRIPTION - SPEED TRIM FUNCTIONS
EFFECTIVITY
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737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
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If the roll angle from the ADIRU is more than 40 degrees, it
opens an electronic switch and stops the speed trim signals.
Gain Control For Adjustable Amplifier
The FCC uses these inputs to calculate the fade out gain of the
output amplifier:
* Reference stabilizer position
* Average of the N1 signals from the DEUs
* Flap position
* Mach.
The gain is 100 percent for a 1.5 degree stabilizer position with
flaps down or -0.75 degree stabilizer position with flaps up. The
gain is zero for a -3.0 degree stab position with flaps down or a -
2.5 degree stab position with flaps up. The gain is 100 percent
for an average N1 more than 80 percent or zero for an average
N1 less than 60 percent. The gain is 100 percent for a Mach less
than 0.5 or zero for a Mach more than 0.68.
Speed Trim Detector
The detector calculates if the trim signal should be a nose up or
a nose down command. It also makes sure that the signal is
there for at least 0.5 seconds before it calculates the command.
Nose Up and Nose Down Commands
The speed trim nose up and nose down commands use these
signals:
* Nose up or nose down signal from the speed trim detector
* Airplane is in the air for more than 10 seconds
* No manual electric trim for at least five seconds
* Autopilot in both FCCs is not engaged
* Flaps are down, gear is up, or CAS is less than 202 knots
* Speed trim function is valid
* FCC is selected to give the speed trim function.
Speed Trim System Selection
The speed trim select circuit looks at these signals to calculate
if the FCC should control the speed trim function:
* Airplane is not on the ground
* Local and foreign FCC speed trim functions are good
* Which FCC is FCC A.
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