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DFCS - FUNCTIONAL DESCRIPTION - SPEED TRIM FUNCTIONS
EFFECTIVITY
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737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
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DFCS - FUNCTIONAL DESCRIPTION - SPEED TRIM FUNCTIONS
DFCS - FUNCTIONAL DESCRIPTION - SPEED TRIM FUNCTIONS
EFFECTIVITY
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DFCS - FUNCTIONAL DESCRIPTION - SPEED TRIM FUNCTIONS
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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BOEING PROPRIETARY - Copyright#- Unpublished Work - See title page for details.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 trim signals to the stabilizer when the
autopilot is engaged. The stabilizer trim function calculates
these signals:
* A/P stab trim nose up
* A/P stab trim nose down.
The autopilot calculates how far it can move the elevator
(elevator authority) and how far it commands the elevator to
move (elevator command). If the ratio of the command to the
authority is too large, the autopilot trims the stabilizer to
decrease this ratio. If it did not do this, the elevator movement
may reach its limit and the autopilot could no longer move the
elevator in one direction.
Elevator Authority and Command
If the autopilot is in the pitch go-around (G/A) mode, the
authority is 9 degrees. If the autopilot is not in the pitch goaround
mode then the autopilot uses these signals to calculate
the elevator authority:
* Total air pressure
* Static air pressure
* Stabilizer position.
If the flaps are between 0 and 7 degrees, the authority is limited
to 3 degrees.
If the flaps are up, the autopilot uses the A/P elevator command.
If the flaps are down, the autopilot uses these signals to
calculate the elevator command:
* Elevator position
* Neutral shift sensor position
* Flare spring bias.
Flare Spring Bias
The flare spring bias commands the stabilizer to a nose up
position. The autopilot commands the elevator to move down to
hold the nose in the present attitude. If there is an autopilot
disconnect, the elevators return to the neutral position. This
flare spring bias then causes the airplane to pitch nose up.
These conditions must occur for the autopilot to calculate the
flare spring bias:
* Airplane in dual approach mode
* Radio altitude below 400 feet
* Airplane not in pitch G/A mode.
The autopilot uses the stabilizer position and flap position to
calculate the flare spring bias.
DFCS - FUNCTIONAL DESCRIPTION - A/P STABILIZER TRIM FUNCTION
EFFECTIVITY
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737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
22-11-00-070
Stabilizer Trim Detector
The trim detector looks at the ratio of the elevator command to
the elevator authority. If the elevator command is between 10%
and 25% of the elevator authority for a limited time, it supplies a
trim nose up or trim nose down signal. It stops the signal when
the elevator command is less than 2% of the elevator authority.
If the airplane is in the pitch G/A mode, the detector supplies a
trim signal if the ratio is more than 10% for 500 out of 800
milliseconds.
A/P Stab Trim Commands
The autopilot must meet these conditions before the stab trim
nose up or stab trim nose down signal from the detector
becomes a trim command:
* A/P is engaged
* Airplane is not in flare
* Airplane is not in the pitch CWS out of detent mode.
When the radio altitude is less than 60 feet, the FCC does not
allow a nose down command.
The stab trim nose up command also occurs for 5.5 seconds
after these conditions occur:
* Airplane is in the air and in the flare mode
* Airplane is not in the G/A mode
* A/P G/S is engaged.
This stab trim nose up command causes the airplane to nose up
as it starts the flare maneuver just before touchdown.
The stab trim nose down command also occurs from 0.2 to 5.5
seconds after the airplane is in the A/P G/A mode. Also during
this time, the FCC does not allow any nose up commands.
DFCS - FUNCTIONAL DESCRIPTION - A/P STABILIZER TRIM FUNCTION
EFFECTIVITY
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