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737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
General
The FCC sends a speed trim warning signal to the flight control
module if it finds a failure in the speed trim function. The FCC
sends a stab out of trim signal to the stab out of trim warning
light if a stabilizer mistrim condition occurs.
Speed Trim Warning
The speed trim warning circuit supplies a warning signal if any
of these conditions are not present:
* Air/ground sensors valid
* Stabilizer move in 10 seconds when commanded
* Engine N1s valid
* Stab trim position sensors valid
* Data from the air data inertial reference unit (ADIRU) valid
* Clutch valid and trim valid signals present.
The air/ground sensors are valid if these conditions are
present:
* Engines N1s are more than 18%
* Computed airspeed (CAS) is less than 80 knots
* Angle of attack (AOA) less than 15 degrees
* The squat switch shows the airplane is on the ground.
The speed trim warning circuit also supplies a warning if the
FCC commands a trim up and trim down signal at the same
time.
Speed Trim Fail Warning Light
If the function fails in only one FCC, the light does not come on.
However, if you push either master caution recall switch when
there is one failure, the speed trim fail light comes on. If you
push the master caution reset switch, the light goes off.
Stab Out Of Trim Warning
The stab out of trim warning circuit looks at these conditions
and if any occur, the warning may be set:
* Stabilizer does not move in 10 seconds when commanded
* Too much A/P actuator movement for 10 seconds
* Too much elevator command for 10 seconds.
Too much actuator movement means the difference between
the elevator A/P actuator position and the elevator position
sensor is greater than 3 degrees. If in single channel operation,
the difference between the elevator A/P actuator position and
the neutral shift sensor position must be less than 0.5 degrees
to reset the warning.
Too much elevator command occurs when the difference
between the elevator A/P actuator position and the neutral shift
sensor position plus a bias is more than 5 degrees. The bias is
zero unless these conditions are present and then the bias is 3
degrees nose up:
* Flare is armed
* Radio altitude is less than 400 feet
DFCS - FUNCTIONAL DESCRIPTION - SPEED AND STABILIZER TRIM WARNINGS
EFFECTIVITY
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* A/P is engaged
* A/P G/S is engaged.
These conditions cause the stab out of trim annunciator to come
on:
* Warning ready to set
* A/P engaged
* Radio altitude more than 50 feet or G/S not engaged or stab
out of trim warning already set.
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EFFECTIVITY
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DFCS - FUNCTIONAL DESCRIPTION - SPEED AND STABILIZER TRIM WARNINGS
DFCS - FUNCTIONAL DESCRIPTION - SPEED AND STABILIZER TRIM WARNINGS
EFFECTIVITY
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737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
General
The mach trim system moves the elevators to prevent a nose
down maneuver at high speeds. This occurs when the speed is
between Mach 0.615 and Mach 0.84. The mach trim system also
moves the elevators to increase a nose up maneuver at takeoff.
The air data inertial reference unit (ADIRU) sends the mach
value to the FCC. The FCC calculates mach trim command
signals. It sends the signals through the integrated flight system
accessory unit (IFSAU) to the mach trim actuator. The FCC
calculates the mach trim signals anytime it receives power.
The mach trim actuator is on the elevator feel and centering
unit. When the actuator moves, it turns the feel and centering
unit.
Autopilot Disengaged
When the autopilot is disengaged, the feel and centering unit
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