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column back pressure.
As airspeed changes, the PFCs provide conventional pitch control characteristics
by requiring the pilot to make control column inputs or trim changes to maintain
a constant flight path.
Primary Pitch Trim Control
Primary pitch trim is controlled by the dual pitch trim switches on each control
wheel. Both switches must be moved to command trim changes. The primary
pitch trim switches are inhibited when the autopilot is engaged. Pitch trim does not
move the control column.
In the normal mode, primary pitch trim operates differently on the ground than it
does in flight. On the ground, the stabilizer is directly positioned when the pilot
uses the pitch trim switches. In flight, the pitch trim switches do not position the
stabilizer directly, but make inputs to the PFCs to change the trim reference speed.
The trim reference speed is the speed at which the airplane would eventually
stabilize if there were no control column inputs. Once the control column forces
are trimmed to zero, the airplane maintains a constant speed with no column
inputs. Thrust changes result in a relatively constant indicated airspeed climb or
descent, with no trim inputs needed unless airspeed changes.
787 Flight Crew Operations Manual
Flight Controls -
System Description
9.20.10 Boeing Proprietary
When pilot trim inputs are made, the PFCs automatically move the elevators to
achieve the trim change, then move the stabilizer to streamline the elevator.
Stabilizer motion may also automatically occur to streamline the stabilizer and
elevator for thrust and configuration changes.
Alternate Pitch Trim
Alternate pitch trim is controlled by the dual alternate pitch trim switches on the
control stand. Both switches must be moved to command trim changes. These
switches move the trim reference airspeed (normal mode) and also move the
stabilizer (all modes). The alternate pitch trim switches are linked electrically to
the horizontal stabilizer trim actuator, and then mechanically to the stabilizer.
Alternate pitch trim commands have priority over wheel pitch trim commands in
all flight control modes.
Moving the alternate pitch trim switches with the autopilot engaged does not
disconnect the autopilot, but does move the stabilizer. Moving the alternate pitch
trim switches during stall or overspeed protection does move the stabilizer, but
does not remove column forces.
Note: The alternate pitch trim switches should not be used with the autopilot
engaged, or during stall or overspeed protection.
Pitch Envelope Protection
The pitch envelope protection functions include:
• stall protection
• overspeed protection.
Stall Protection
Stall protection reduces the likelihood of inadvertently exceeding the stall angle
of attack by providing enhanced crew awareness of the approach to a stall or to a
stalled condition.
Stall protection limits the speed to which the airplane can be trimmed. At
approximately the minimum maneuvering speed, stall protection limits the trim
reference speed so that trim is inhibited in the nose up direction. The pilot must
apply continuous aft column force to maintain airspeed below the minimum
maneuvering speed.
Use of the alternate pitch trim switches does not reduce the column forces. When
flying near stall speed, the column force increases to a higher level than would
occur for an equivalent out-of-trim condition above the minimum maneuvering
speed.
787 Flight Crew Operations Manual
Flight Controls -
System Description
Boeing Proprietary 9.20.11
The autothrottle can support stall protection if armed and not engaged. If speed
decreases to near stick shaker activation, the autothrottle engages in SPD mode
and advances thrust to maintain minimum maneuvering speed (approximately the
top of the amber band) or the speed set in the mode control panel speed window,
whichever is greater. The EICAS message AIRSPEED LOW is displayed.
Note: When the pitch mode is FLCH or TOGA, or the airplane is below 400 feet
above the airport on takeoff, or below 100 feet radio altitude on approach,
the autothrottle will not automatically engage.
Refer to Chapter 10, Flight Instruments, Displays, for PFD indications.
Refer to Chapter 4, Automatic Flight, for an explanation of PFD flight mode
annunciations, and for mode control panel and autothrottle operation.
Overspeed Protection
Overspeed protection limits the speed to which the airplane can be trimmed. At
VMO/MMO, overspeed protection limits the trim reference speed so that trim is
inhibited in the nose down direction. The pilot must apply continuous forward
column force to maintain airspeed above VMO/MMO. Use of the alternate pitch
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787机组操作手册Flight Crew Operations Manual 2(71)
