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ELEVATOR
HYD SYS 2
HYD SYS 3
1 4
2
1
4
3
December 01, 1999
747 Operations Manual
Copyright © The Boeing Company. See title page for details.
9.20.4 D6-30151-425
Flight Controls -
System Description
Trim Control
Stabilizer Trim switches on the pilots’ Control Wheels or Alternate Stabilizer
Trim switches on the control stand control stabilizer trim. Pushing both switches
in a pair in the same direction power the actuators, which drive the stabilizer in the
desired direction. The Alternate Stabilizer Trim switches provide trim commands
in the same manner as the Stabilizer Trim switches through a separate control
channel. The Alternate Stabilizer Trim switches also provide an increased range
of stabilizer travel. If the Alternate Stabilizer Trim switches and Control Wheel
switches are held in opposite directions, no trim is commanded. Holding the
Captain’s and First Officer’s Control Wheel switches in opposite directions
commands no trim.
Stabilizer Trim Position Indication and Greenband
Stabilizer position, measured in trim units, displays on trim indicators on both
sides of the control stand. A stabilizer trim indicator OFF flag displays if the trim
indicator is inoperative.
The stabilizer trim indicators incorporate a multiple greenband which indicates
the acceptable range of trim settings for takeoff. There are three possible
greenbands: a mid-band, a nose down band which includes the mid-band plus
additional nose down trim, and a nose up band which includes the mid-band plus
additional nose up trim.
The mid-band segment is a highly visible green paint stripe illuminated by
ambient light and integral panel lighting.
The FMCs use entered center of gravity, gross weight, and takeoff thrust setting to
calculate the correct greenband.
A nose gear oleo pressure switch provides a crosscheck to ensure the correct
greenband has been selected. With either nose up or nose down band selected, the
pressure switch position is compared to the selected greenband.
Stabilizer Trim Cutout
Hydraulic systems 2 and 3 power stabilizer trim. Two guarded Stabilizer Trim
Cutout switches control stabilizer trim. With the guards closed, the switches are
held in AUTO position, allowing automatic cutout of the related hydraulic system
if unscheduled stabilizer trim is detected. With a Stabilizer Trim Cutout switch in
CUT OUT, hydraulic power to the related trim control module is shut off.
Positioning a Stabilizer Trim Cutout switch from CUT OUT to AUTO, activates
the related actuator after a short time delay. Positioning a switch to ON overrides
the automatic cutout function and supplies hydraulic power to the related control
module. If automatic cutout has occurred, hydraulic power remains shut off until
the related cutout switch is placed ON. If one actuator fails to operate, trim
commanded by the flight crew reduces to half the normal scheduled rate.
December 01, 1999
747 Operations Manual
Flight Controls -
System Description
Copyright © The Boeing Company. See title page for details.
D6-30151-425 9.20.5
Trimming with Autopilots Engaged
If a single autopilot is engaged, using the control wheel stabilizer trim switches
causes the autopilot to disengage. If multiple autopilots are engaged, the Control
Wheel Stabilizer Trim switches are inhibited. The Alternate Stabilizer Trim
switches override autopilot trim commands with any number of autopilots
engaged and do not cause disengagement.
Control Column Cutoff
Control column inputs in the direction opposing stabilizer trim will cutoff electric
trim commands to the control modules. The control column trim cutoff function
does not affect alternate trim inputs.
Speed Stability Trim
Speed stability trim uses stabilizer trim to improve handling characteristics of the
airplane in the lower speed range.
Activating the Stabilizer Trim switches or engaging an autopilot inhibits speed
stability trim.
December 01, 1999
747 Operations Manual
Copyright © The Boeing Company. See title page for details.
9.20.6 D6-30151-425
Flight Controls -
System Description
Stabilizer Control Diagram
HYD SYS 3 HYD SYS 2
ACTUATOR
TRIM
ACTUATOR
TRIM
TRIM
STABILIZER
MODULE
CONTROL
TRIM
STABILIZER
O
T
U
A
CUT OUT
2 - ON - 3
STAB TRIM
MODULE
CONTROL
STABILIZER
ALTERNATE
APL NOSE DN
APL NOSE UP
A
N
T
L
A
N
T
L
CUTOFF
SWITCHES
　
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