787 Flight Crew Operations Manual
DO NOT USE FOR FLIGHT
Flight Controls -
System Description
Copyright © The Boeing Company. See title page for details.
D615Z003-TBC 9.20.15
Secondary and Direct Mode Roll Control
Airplane roll control is different in the secondary and direct flight control modes.
The flight control system continues to receive and process pilot control inputs
using simplified computations to generate flight control surface commands. Roll
envelope bank angle protection is not available in the secondary or direct modes.
Normal Mode Yaw Control
In the normal mode, airplane yaw control characteristics are similar to
conventional airplanes. Unlike conventional airplanes, the rudder pedals do not
directly position the rudder surface in flight, they instead command a sideslip
maneuver with the rate dependent on pedal displacement. The flight control
system automatically positions the rudder to generate the commanded maneuver.
It constantly monitors airplane response to pilot inputs and repositions the rudder
to carry out these commands. Airplane sideslip response to turbulence is
minimized by automatic control surface commands.
Pedal forces increase as pedal displacement increases. Pedal forces do not change
with airspeed changes. The rudder ratio changer automatically reduces sideslip
(for a given pedal input) as airspeed increases. Sufficient rudder authority is
provided at all airspeeds to maintain airplane control in engine failure conditions,
and during takeoffs and landings in crosswinds.
On the ground above 60 knots groundspeed, the flight control system attempts to
maintain a yaw rate near zero by commanding rudder to counter the majority of
the yawing moment due to an engine failure. The capability to counter a thrust
asymmetry is an inherent part of the flight control system. The pilot can still
recognize the initial onset of an engine failure through the airplane yaw cue and
rudder pedal movement.
In flight, when a roll/yaw asymmetry condition exists for any reason (not only
thrust related), the flight control system initially attempts to maintain zero roll rate
and sideslip. An automatic rudder input counters the yawing moment and an
automatic lateral control input counters the rolling moment. After the initial
inputs, the system off loads any steady state lateral control input to the rudder,
resulting in a small sideslip. The rudder pedals move to give the pilot awareness
of the automatic function and the resulting rudder trim indication is automatically
shown on the EICAS display.
This off load increases sideslip on the airplane, therefore the off load has limits
based on the severity of the asymmetry. For large asymmetry conditions a greater
sideslip is permitted, but some automatic lateral control input may remain after the
off load stops. For large asymmetries the EICAS caution message ROLL/YAW
ASYMMETRY is displayed.
August 14, 2009
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