A320 Flight Control System

Flight Controls System Presentation

In conventional aircraft, the movement of the control column is transferred along cables and pulleys, until it reaches the control surface to be moved.

In the A320 family, the cables and pulleys have been replaced by electrical wires.

This has the advantage of saving weight on the aircraft.

However, there are even greater advantages as the video clip will demonstrate.

The electrical signals, created by sidestick movement, travel through flight control computers  before being passed to the surface hydraulic actuators, also named servo controls.

The pilots use the sidesticks to fly the A/C.

Computers interpret the pilot’s inputs and move the flight controls surfaces as necessary preventing excessive maneuvers and flight outside the safe flight envelope.

Pilot’s input is converted into an A/C control objective.

No direct relationship between stick and surface.

The A/C is servo looped.

This has advantages over conventional systems, it:
- makes the aircraft extremely stable,
- enhances safety,
- reduces the pilot’s workload.

The movements of the flight control surfaces are managed by seven computers. These are:
- two ELevator and Aileron Computers (ELAC),
- three Spoiler and Elevator Computers (SEC),
- two Flight Augmentation Computers (FAC), which manage the rudder movements.

The flight control system is designed to incorporate several redundancy and safety concepts:
- Each computer is able to control the aircraft in flight.
- Each computer is divided into two physical units which are programmed in two different software languages.
- Segregated power supply, control source and signaling lanes.
- Extensive monitoring within each computer allows detection of failures in controls, computers and sensors.
- With the basic configuration, no single failure (computer, electrical system, hydraulic system, sensors …) will cause a degradation of the normal functions.

Two Flight Control Data Concentrators (FCDCs) are installed.

They receive data from ELAC and SEC computers for indication, recording, and maintenance purposes.

The FCDC stores the maintenance data and delivers failure indications (e g.stored failures, failure history, trouble shooting guidance, etc ...) as well as failed LRUs to the Centralized Fault Display Interface Unit (CFDIU).

The FCDC’s interface with ECAM system (surface position, computer status, surface status) and interface with the DFDR (flying parameter recording).

However, the data from both FACs is directly sent to the EIS. 

Three independent hydraulic systems are used to power all the flight control surfaces.

The flight control system incorporates:

• Ailerons:
Each aileron is powered by two servo controls from independent
hydraulic systems.
In normal operation, one servo control is active through the ELAC,
the other one being damped.
Surface position indication is displayed on the ECAM display unit.

• Elevators:

Each elevator is actuated by two independent hydraulic servo controls.
In normal operation one servo control is active through the ELAC
(SEC in backup), the other is damped.

In the event of total electrical control loss, the servo controls are
automatically switched to a centering mode and will hold the surface
in the neutral position.

Surface position indication is displayed on the ECAM display unit.

• A Trimmable Horizontal Stabilizer (THS) for pitch trim:

The THS is actuated by a fail-safe ball screw-jack driven by two
independently supplied hydraulic motors.

Manual control of the THS is achieved through the interconnected
handwheels located on the center pedestal.

On the ground, the THS setting must be performed through the
manual control.

As soon as the aircraft is airborne, the THS automatically engages in
electrical control mode .

The mechanical control can override the electrical control (in case of
auto trim runaway).

Automatic pitch trim is achieved with the THS through the ELAC.

When the auto trim function is engaged, any movement of the
elevators will be followed by a THS movement in order to align the
surfaces for drag reduction (fuel saving).

The THS  position indication is displayed on the ECAM display unit
and is also indicated in degrees on a scale adjacent to each trim
handwheel.

•A rudder:

The single-piece rudder is actuated by three independently supplied
hydraulic servo-controls mechanically signaled from the pedals and
the yaw damper actuators.

The rudder position indication is displayed on the ECAM display unit.

•And finally, ground spoilers/speed brakes:

Each spoiler is driven by a single servo control through one of the
three SECs.

For redundancy, the hydraulic supply from the three systems is
distributed  among the spoilers servocontrols.

In case of failure detected by the SEC, the corresponding spoiler
retracts automatically and is held in that position.

The spoiler position indication is displayed on the ECAM display unit.

Now let’s introduce the ECAM F/CTL page.

You can see that all the flight control surfaces we have talked about are displayed. We will now see them in more detail.

The movements of both ailerons and both elevators are symbolized by a green index moving in front of a white scale.

The servocontrol status, displayed on the side of the white scale, will become amber when the corresponding actuator is not available (hydraulic low pressure, electrical control failure, computer failure, etc).

• Ailerons:

The ailerons are shown in neutral position.

NOTE: The indication below corresponds to the new neutral position when the flaps are extended (aileron droop automatic function).

• Elevators:

In case of elevator failure, the deflection of the remaining elevator is limited by the computer to avoid excessive asymmetrical loads on the horizontal tailplane or rear fuselage.

The green rudder symbol is used as an index to display the movements of  the rudder on a white scale.

The servocontrol status is displayed on top of the rudder indication.

The rudder trim is indicated by a small blue line below the scale.

When used, the rudder trim will move the neutral point of the rudder surface and change the rudder pedals position.

The hydraulic systems which actuate each control surface are indicated on the ECAM F/CTL page by G, B and Y.

For example, the rudder is powered by the green, blue and yellow hydraulic systems.

Note that the rudder and the pedal deflections are limited via a rudder travel limiter at high speed.

Rudder travel is limited as a function of airspeed.

The FACs control electric motors coupled with a variable stop mechanism.

The high speed position is indicated by small white ticks on the rudder scale.

The PITCH TRIM position is indicated by THS deflection in degrees up or down.

The pitch trim hydraulic motor status is displayed on top of the THS position indication.

The spoilers have several functions:

- Speed brakes use the 3 central surfaces.
On the ECAM F/CTL page, the spoiler extended position is indicated
by small arrows. This is the case for the speed brakes.

- Speed brakes are used to decrease the aircraft speed and to
increase the rate of descent.

Speed brakes inhibition:
The computers will automatically retract the speed brakes or keep
them retracted in the following conditions:

• Aircraft in high Angle Of Attack condition.

• Flaps full configuration selected (landing).

• Computer failure. 

• Roll spoilers:

To complement the ailerons action, the roll spoilers are deployed automatically by the computers on the same side as the aileron which is deflected upward (they remain retracted on the other wing) and proportionally to this aileron deflection.

The computers are able to mix the speed brakes function with the roll spoilers.

Look at the video carefully.

Roll control uses the four outer surfaces.

On the video, look at the left spoilers as they deploy, then at the right ones as the wings are leveled.

Ground spoilers use all surfaces at their maximum deflection.

The ground spoilers are used to maintain the aircraft on the ground
(ground lift dumper) and to reduce the speed.

Ground spoilers will remain extended during bounces.

Ground spoilers are automatically extended after touchdown or in
case of rejected take off when specific ground conditions are fulfilled.

Pre-selections have to be made through the speedbrake control lever
located on the center pedestal.

They will retract when deselected with the speedbrake control lever
or one engine thrust lever is advanced above idle position.

On the video, watch all the spoilers deploy at touchdown.

The status of the ELACs and SECs is indicated on the ECAM F/CTL page. The other computers are not displayed.

Pilots control pitch and roll through two side sticks.

The side stick advantages are:
- improved crew interface,
- system simplification,
- weight and room saving.

Each side stick sends independent electrical signals to the flight controls computers.

When both side sticks are used at the same time, their inputs are algebraically added by the computers.

When the auto pilot is engaged both side sticks are locked in the neutral position by a solenoid system.

Each side stick is fitted with a red pushbutton which is used for:
- Autopilot manual disconnection.
- Deactivation of the other stick when this pushbutton is kept pressed to take the priority in case of crew incapacitance or during flight training.

For rigging purposes, the side stick can be held in neutral position by a single rigging pin (aileron, spoiler, elevator servo control adjustments).

There are associated side stick priority lights.

Manual control of the THS is achieved through the interconnected handwheels located on the center pedestal.

The trim position is indicated in degrees on a scale adjacent to each trim wheel.

The normal range is marked by a green band.

NOTE: Crew action on the pitch trim wheel does not disconnect the ELACs, they remain synchronized with the manually selected position.

Following touch down, the pitch trim is automatically reset to zero.

When the pitch trim is engaged in electrical mode, a feedback movement is sent to the wheels via the mechanical channel

There are two sets of conventional rudder pedals.

These pedals are adjustable for pilot comfort.

The two sets of pedals are mechanically interconnected.

When the autopilot is engaged, the rudder pedals are locked in neutral

position.

A RUD TRIM panel is located on the pedestal.

Manual trim orders are received from a switch located on the center pedestal.

An automatic reset function is initiated through a pushbutton switch and allows the trim position to be nulled.

The position indicator displays the rudder trim direction (L or R) and value.

The rudder trim rotary switch is not active when the autopilot is engaged as the rudder trim is controlled by the autopilot system.

A speed brake lever is located on the left side of the pedestal.

To select speed brakes, the lever has to be pushed down and set to the required position.

To arm the ground spoilers the lever must be pulled up when in retracted position.

Upon ground spoiler deployment, there is no movement of the lever, a it is an electrical control.

In addition, there are two panels, located on the overhead panel,  to control the flight control computers.

Switching OFF then ON resets the corresponding computer.

The ELAC 1 and SEC 1 are normally supplied by the ESSential BUS
but can be supplied by the HOT BUS in case of electrical failure.

Now, we will introduce the lift augmentation devices.

High-lift control is achieved on each wing by:
- five leading edge slats,
- two trailing edge flaps,
- one aileron (aileron droop function).

Slats and flaps are driven through similar hydromechanical systems consisting of:
- Power Control Units (PCU),
- differential gearboxes and torque shafts,
- rotary actuators.

There are 5 slats on each leading edge ...

… and 2 flaps on each trailing edge.

NOTE: the A321 has double slotted flaps.

The slats and flaps Power Control Units are hydraulically actuated.

They are electrically controlled via  two Slat Flap Control Computers (SFCC).

The SFCCs monitor the slats and flaps operation through Position Pick-off Units (PPUs) located on the PCUs and at the end of the transmission torque shafts.

The slats and flaps operate with protection functions such as:
- asymmetry,
- runaway,
- overspeed,
- uncommanded movement.

Each SFCC has  two channels, one for the flaps and one for the slats.

Each channel can drive its associated surfaces.

The flaps and slats information is shown on the EWD.

The System Data Acquisition Concentrators (SDACs) receive slat and flap positions from feedback PPUs  through SFCCs to generate appropriate ECAM displays.

The flap and slat positions are indicated by white dots. Here, the surfaces are extended to the full position.

This is the flap 0 indication. Notice there is no labeling with this setting.

The flap lever, located on the right side of the pedestal, operates the slats and flaps.

Before selection of any position, the lever must be pulled out of the detend.

Moreover, balks are provided at position 1 and 3 to avoid excessive flap/slat travel demand by a single pilot action.

The flap lever has the following positions: 0, 1, 2, 3 and FULL.

ELAC 1, SEC 1, FAC 1 and FCDC 1 are located in rack  83 VU.

ELAC 2, SEC 2, FAC 2 and FCDC 2 are located in rack  84 VU.

SFCC 1 is located in rack 85 VU.

SFCC 2 is located in rack 86 VU.

SEC 3 is located in rack  93 VU.

The ELACs, SECs and SFCCs BITE memories can be read and tests
triggered on the MCDU through F/CTL key.

The FACs send data to the CFDIU. Their BITE memories can be read
and tests triggered through the MCDU AFS menu.

The ELACs and SECs send data to the CFDIU through the FCDCs.

The SFCCs send data directly to the CFDIU.

When you work on flight controls, make sure that you obey all the AMM safety

procedures. This will prevent injury to persons and /or damage to the

aircraft. Here is an overview of main safety precautions relative to the

flight control system.

Make sure that the controls agree with the position of the flight control surfaces

before you pressurize hydraulic system.

Put safety devices and warning notices in position before you start a task

 on or near flight controls.

Make sure that the travel ranges of the flight control surfaces are clear.

Movement of flight controls can cause injury to persons and/or damage

to the aircraft.

WARNING: The flight control tests are dynamic.

Movement of flight controls can cause injury to persons and/or
damage to the aircraft.