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positions.
The greenband is calculated using the FMC inputs of CG, gross weight, and
takeoff thrust. A nose gear oleo pressure switch provides an automatic
cross–check of the CG to ensure that the correct greenband has been selected.
When either the nose up or nose down band is selected, the pressure switch
position is compared to the computed greenband. The EICAS advisory message
STAB GREENBAND is displayed if the pressure switch and the greenband
disagree. If the stabilizer signal is not present or is invalid, the greenband and the
pointer are not displayed.
Normal Mode Roll Control
Roll control characteristics are similar to conventional airplanes. Unlike
conventional airplanes, the control wheel does not directly position the lateral
surfaces in flight. Control wheel input commands a roll maneuver with the rate
dependent on wheel displacement. The flight control system automatically
positions the ailerons, flaperons and spoilers to generate the commanded
maneuver. Airplane roll response to turbulence is automatically minimized by
automatic control surface commands. Control wheel forces increase as control
displacement increases. Control wheel forces do not change with airspeed
changes. The ailerons are locked out at high speeds.
787 Flight Crew Operations Manual
Flight Controls -
System Description
9.20.14 Boeing Proprietary
Spoilers 4 and 11 are controlled through direct electric wiring from the control
wheel. These spoilers are available for roll control until the speedbrake lever is
moved to near the UP position, when they function as speedbrakes only.
Roll Envelope Bank Angle Protection
Bank angle protection reduces the likelihood of exceeding the bank angle
boundary due to external disturbances, system failures, or inappropriate pilot
action.
Bank angle protection provides roll control wheel inputs when airplane bank angle
exceeds the bank angle protection boundary of approximately 35°. If the boundary
is exceeded, the control wheel force rolls the airplane back within 30° of bank.
This roll command can be overridden by the pilot. Maximum control wheel
deflection always commands maximum control surface deflection. The autopilot
disengage bar disables bank angle protection.
Excessive bank angles are indicated on the PFD bank indicator. The indicator
changes color to amber at bank angles exceeding 35°. Refer to Chapter 10, Flight
Instruments, Displays, for PFD indications.
Secondary and Direct Mode Roll Control
Roll control in the secondary and direct modes is similar to a conventional
airplane. The control wheel now commands a proportional lateral surface
deflection instead of a maneuver command. Bank angle protection is not available
in either the secondary or direct mode. Spoilers 5 and 10 are always locked out.
Normal Mode Yaw Control
Yaw control operation is similar to a conventional airplane. Rudder surface
deflections are proportional to rudder pedal movements.
Pedal forces increase as pedal displacement increases. Pedal forces do not change
with airspeed changes.
The rudder ratio changer automatically reduces rudder deflection (for a given
pedal input) as airspeed increases. This protects the vertical tail structure from
stresses resulting from large rudder surface deflections at high airspeeds.
Sufficient rudder authority is provided at all airspeeds to maintain airplane control
in engine–out conditions, as well as during takeoffs and landings in crosswinds.
787 Flight Crew Operations Manual
Flight Controls -
System Description
Boeing Proprietary 9.20.15
Thrust Asymmetry Compensation
The thrust asymmetry compensation (TAC) system significantly reduces
uncommanded flight path changes associated with an engine failure. TAC is an
inherent part of the normal mode roll and yaw control laws. On the ground above
65 KTS, the yaw control law will command rudder to minimize yaw due to engine
failure. The pilot can still recognize the initial onset of an engine failure through
the the airplane yaw cue. The yaw cue is greatly reduced when compared to an
airplane operating without TAC. In the air, the roll and yaw control laws will
command the rudder and lateral surfaces to minimize roll and yaw due to engine
failure. Rudder movement is back–driven through the rudder pedals and the
rudder trim indicator to provide rudder control awareness to the pilot.
TAC is available except:
• when airspeed is below 70 knots on the ground, or
• when reverse thrust is applied, or
• when automatically disengaged due to system malfunction or loss of
engine thrust data.
TAC automatically disengages if engine thrust data is lost. Also, if the engine is
　
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