Flight Controls Flashcards
The flight controls are fly-by-wire,
which is an electronic, rather than a mechanical system for operating the flight controls.
Control wheel & rudder pedals provide conventional control feel and pitch responses to speed and trim changes.
The electronic components
reduce pilot workload and at the same time provide enhanced handling qualities.
Jam override mechanisms allow pilots to maintain airplane control by applying
force to the other column or wheel to overcome the jam.
The primary flight control system is highly redundant, with three operating modes:
normal mode,
secondary mode,
and direct mode.
Roll control uses
two ailerons, 14 spoilers and two flaperons.
The flaperons operate in both
low and high speed flight.
The ailerons assist in roll control during low speed flight only. They are locked out
during high speed flight.
The flaperons provide additional lift by drooping when the flaps extend.
There is no flight deck indication of flaperon droop.
The ailerons also
move down for
flaps 5, 15 and 20
to improve take-off performance.
Two different hydraulic systems power the PCUs for
each aileron and flaperon.
Two different ACEs control each aileron and flaperon.
The hydraulic and control arrangement provides redundancy.
Should a single system be lost,
the ailerons and flaperons are still operational.
Spoilers work asymmetrically when augmenting roll control.
Spoilers 5 and 10 are
only available during low speed operation.
Roll trim is provided by two aileron trim switches. Moving both switches together in the desired direction sends signals to reposition
the ailerons, flaperons and spoilers.
When aileron trim is applied the control wheels are displaced proportionally in the direction of trim switch movement.
Aileron trim is inhibited with the autopilot engaged.
Yaw control is provided by
the rudder pedals,
rudder trim system,
yaw dampers
and the rudder ratio system.
Rudder deflections are proportional to rudder pedal movements.
Hydraulic pressure from all 3 hydraulic systems is used to operate and trim the rudder.
Each PCU is powered by 1 of 3 hydraulic systems and controlled by
right, center and left 1 Actuator Control Electronics (ACEs).
Two rudder trim speeds are available.
Low rate rudder trim is commanded by
High rate rudder trim is commanded by rotating the control
rotating the control to the detent.
past the detent.
The control is spring loaded to the neutral position.
The manual trim input can be cancelled by pushing the MANUAL TRIM CANCEL switch.
The switch has no effect
Thrust Asymmetry Compensation (TAC) inputs.
There are no separate yaw dampers on the aircraft. Command signals from the PFCs to the ACEs and PCUs provide
turn coordination and dutch roll damping.
In the secondary mode, yaw damping is normally degraded,
but may be inoperative for certain multiple failures.
In the direct mode, yaw damping
is not available.
There is no separate rudder ratio unit on the aircraft.
Based on airspeed, the PFCs calculate the amount of rudder deflection
required.
Full rudder deflection is available at
low airspeeds.
As airspeed increases, the PFCs gradually reduce rudder deflection.
This ensures
structural integrity of the rudder.
In the secondary and direct modes, rudder response defaults to fixed ratios
and is determined by flap position.
With flaps up, rudder response is less
than with flaps down.
When a single gust hits the vertical tail,
gust suppression transducers send signals to the ACEs
which in turn send this data to the PFCs
to adjust PCU commands to dampen
the gust side force.
Pitch control is similar to conventional airplanes.
However, the control column does not directly position the elevator in flight.
The control commands the PFCs to
generate a pitch maneuver.
Elevator feel forces vary based on airspeed. Generally, control forces increase
as airspeed increases.
The center and right hydraulic systems power the stabilizer PCUs and they are controlled by all 4 ACEs.
On the ground, primary pitch trim is controlled by dual trim switches
There is a stabilizer trim indicator on each side of the control stand.
The scale is in units of stabilizer trim and the white diamond indicates
on each control wheel.
current stabilizer position.
Stabilizer trim green band indicates the allowable stabilizer range
for take-off as calculated by the FMC,
using take-off weight, CG and take-off thrust data.
not displayed in flight.
The green band is
A nose gear oleo pressure switch provides an automatic cross check of CG
to ensure the correct green band is displayed.
If the FMC computed stabilizer green band conflicts with the nose gear pressure switch,
an EICAS advisory message
STAB GREENBAND displays.
In flight, pitch trim signals from the pitch trim switches and the autopilot do not
position the stabilizer directly.
automatic trimming is with the autopilot. Adjusting the speed sends pitch trim signals to change the trim reference speed in the PFCs.
The PFCs then send signals through the ACEs to reposition the elevators.
The stabilizer moves to a new trim position required by the new reference speed and the elevators streamline with the stabilizer.
When the autopilot is not engaged, as the airspeed changes,
the pitch control system provides conventional characteristics by requiring the pilot to make control column inputs of trim changes to maintain a constant flight path.
Manual trim is necessary only
when changing airspeed.
Manual trim switches are inhibited with the
autopilot engaged.
Pitch trim switches may be overridden by moving the control column in
the opposing direction.
Another way to manually set trim is to use the alternate pitch trim levers on the control stand.
The levers must always be moved together.
They are spring loaded to the
center neutral position.
The Alternate Pitch trim levers are mechanically linked via cables to the
stabilizer trim control modules or STCMs.
The Alternate Pitch Trim levers move the trim reference speed (NORMAL MODE) and also move the stabilizer (ALL MODES)
by setting valves in the STCMs which control hydraulic fluid to move the stabilizer.
With one STCM inoperative,
the stabilizer moves at a reduced rate.
Moving the alternate pitch trim levers with the autopilot engaged moves the stabilizer but will not disengage the autopilot.
Note: the alternate pitch trim levers should not be used
with the autopilot engaged, or during stall or overspeed protection.
Alternate pitch trim commands have priority over wheel pitch trim commands in all flight control modes.
Spoiler panels are numbered 1 to 14
from left to right.
When used as speed brakes, spoilers are paired
and work symmetrically.
Each symmetrical pair of spoilers is controlled by the same ACE, and operated by
the same hydraulic system.
All three hydraulic systems supply the spoilers.
Each hydraulic system is dedicated to a different set of spoiler pairs to provide isolation and maintain symmetrical operation.
Failure of the ACE or hydraulic system
renders the associated pair inoperative.
f the speedbrake lever is moved close to the UP position, spoiler panels
4 and 11 operate symmetrically as speedbrakes.
Moving the speedbrake lever to the UP position signals spoiler panels to extend to the full up position.
Spoilers 5 and 10 are not available in flight as speedbrakes.
The high lift control system consists of flaps and slats.
They have 3 modes of operation; primary, which is hydraulic,
secondary and alternate which
are both electric.
Primary mode operation of the flaps and slats uses the
center hydraulic system.
To prevent inadvertent flap retraction during take-off or go-around there are gates at the
1 and 20 flap positions.
The flap and slat position indicator is displayed on EICAS.
Normally, the indicator is not displayed until
slats begin to extend.
When the flaps and slats are selected UP, the indicator is removed from the EICAS
10 seconds after the flaps and slats reach the up position.
In the normal mode during manual flight,
pilot control inputs through control column,
wheel, rudder pedals and speedbrake lever
are converted to analog signals.
These signals go to
4 Actuator Control Electronics, or ACEs