A320 FLIGHT CONTROL SYSTEM (ATA 27)

Airbus A320 Flight Control System is based on fly-by-wire technology which was designed and certified to render the new generation of aircraft even more safe, cost-effective, and more pleasant to fly. Technically this system of A320 is called the Electrical Flight Control System (EFCS).

What is Fly-by-wire.?

Fly-By-Wire is a system that regulates the flight controls by electrical wires and computers. Fly-by-wire (FBW) replaces all the conventional mechanical linkages like cables, rods, pulleys, etc with an electronic interface.

FLY BY WIRE PRINCIPLE

The pilots use the side sticks to fly the aircraft in pitch and roll (and in yaw indirectly through turn coordination).

• The pilot’s side stick orders are sent to the flight control computers.

• These computers convert the orders into an aircraft objective.

• The computers send surface deflection orders to the surfaces in order to achieve the aircraft objective. Then, the computers monitor the position feedback of the surfaces. This loop is called “inner loop”.

• The computers also receive an aircraft response and compare it to the demand (coming from the orders), this loop is called an “outer loop”. The fly-by-wire design requires the aircraft to be servo-looped.

• However, regardless of the pilot’s inputs, the computer prevents:

• excessive maneuvers,

• flight outside the safe flight envelope.

• Autopilot commands are given directly to the computers.

SURFACES

• All the flight control surfaces are hydraulically operated by actuators which receive electrical signals from the computers. — Electrically controlled and Hydraulically operated.

• The rudder and the Trimmable Horizontal Stabilizer (THS) can also be mechanically controlled.

• All the actuators are hydraulically powered by one of the three hydraulic circuits, except the rudder trim actuator, the rudder travel limitation actuator, and the THS servo-motors which are electrically driven.

• There are two servo controls for each aileron, for each elevator and for the yaw damping function. In normal configuration, one servo control actuates the surface. It is called active servo control. The second, which follows the surface deflection, is in damping mode. When only the mechanical control of the pitch trim is available (all computers are inoperative), the centering mode is applied to the elevators. The actuators are hydraulically maintained in a neutral position.

All flight control surfaces are made of composite materials except for the slats which are made of aluminum alloy.

• All flight control surfaces are electrically controlled and hydraulically operated.

• As a backup, the stabilizer and rudder are mechanically controlled and hydraulically operated.

• Pilots use side sticks to fly the aircraft in pitch and roll.

COMPUTERS

Electrical Flight Control System (EFCS) computers – 

• 2 Elevator Aileron Computers (ELAC) for pitch and roll control,

• Normal elevator and stabilizer control.

• Aileron control.

• 3 Spoiler Elevator Computers (SEC) for pitch and roll control,

• Spoilers control.

• Standby elevator and stabilizer control.

• 2 Flight Augmentation Computers (FAC) for yaw control,

• Electrical rudder control.

• 2 Flight Control Data Concentrators (FCDC) for indication and maintenance tests,

• Flight Control Data Concentrators (FCDC) acquire data from the ELACs and SECs and send it to the electronic instrument system (EIS) and the centralized fault display system (CFDS).

• 2 Flight Management Guidance Computer (FMGC) for autopilot commands,

• 2 Slat Flap Control Computers (SFCC) for slat and flap control which have two channels each. One for Slats and One for Flaps

Function of ELAC : (NNNAAA)

• Normal, Alternate, and Direct Pitch

• Normal and Direct Roll

• Normal and Alternate LAF (Load Alleviation Function)

• Acquisition of Autopilot Order

• Aileron Droop

• Abnormal Altitude

Function of SEC (NAASA)

• Normal and Direct Roll

• Alternate, Direct Pitch (SEC 1 and SEC 2) Only

• Alternate LAF (Load Alleviation Function)

• Speed brake and Ground Spoilers

• Abnormal Altitude

During Normal Law, The FAC provides : (FLY-W)

• Flight Envelope Protection Function: PFD Speed Scale Management Minimum / Maximum Speed computation : (VSW,VLS,VFE, VFE Next, VLE, VMO and MMO)

• Maneuvering Speed Computation (Greendot Speed, S Speed and F Speed)

• Alpha Floor Protection
  

• Low Energy Warning: SPEED, SPEED, SPEED

• Windshear Detection Function

ACTUATORS

• The Aileron surface is powered by 2 servo controls.

• Each Spoiler surface is powered by a single servo control.

• The Flaps and Slats surfaces are powered by their dedicated PCU.

• The Rudder surface is powered by 3 servo controls.

• The THS is moved by one actuator.

• The Elevator surface is powered by 2 servo controls.

PITCH CONTROL

Pitch control is achieved by – 2 Elevators and THS

• Elevators are used for short-term activity.

• THS is used for long-term activity.

ROLL

Roll control is achieved by

• 1 Aileron and 

• Spoilers 2 to 5 on each wing. — (Roll Spoilers)
  

• Numbered from wing root to wing tip.

YAW

Yaw control is fulfilled by the rudder.

• The rudder is used during crosswind take-off and landing, and in case of engine failure (thrust asymmetry).

• The yaw damper function controls the rudder for Dutch roll damping and turn coordination.

What is Dutch Roll.?

It is a combination of yawing and rolling motions. When the aircraft is yaw, it will develop into a roll. The yaw is not significant but the roll it does and more noticeable and unstable. This is so because the aircraft suffers from continuous reversing rolling action. Dutch roll is oscillatory instability associated with swept-wing jet aircraft.

SPEED BRAKES

The speed brake function is used in flight to increase the aircraft drag.

• Spoilers 2, 3, 4 are used.

• Roll orders and speed brake orders are added with priority given to the roll function.

• The speed brake lever controls the position of the speed brake surfaces and the pre-selection of the ground spoiler function.

• For the speed brake function, the lever has to be pushed down and placed in the required position.

• To arm the ground spoilers for an automatic extension, the lever must be pulled UP when in the retracted position.

• Speed brake extension is inhibited, if one of the following occurs:

• SEC1 and SEC3 both are faulty, or

• An elevator (L or R) is faulty, or

• Angle-of-attack protection is active, or

• Flaps are in configuration FULL, or

• Thrust levers are above the MCT position, or

• Alpha Floor is activated.

• When the speedbrake fails on one wing, the symmetric one on the other wing is inhibited

• If inhibition occurs when the speedbrake is extended, they retract automatically and stay retracted until the inhibition condition disappears and the pilots reset the lever. (The speedbrake can be extended again 10 seconds or more after the lever is reset)

• In-flight if the spoiler fault is detected or electrical power is lost the spoiler automatically retracts.

• With Flaps FULL for landing selected the speedbrake is inhibited.

  • During RTO the speedbrake will automatically extend when GRD SPOILER ARMED and the speed exceeds 72 knots when both thrust levers Idle

  • During RTO speedbrake will automatically extend when GRD SPOILER is NOT ARMED and the speed exceeds 72 knots when REV is selected on one engine (the other thrust lever remains at IDLE)

• The GRD SPOILER automatically extends when in ARMED :
  

  • Both Main LDG GEAR on the GROUND

  • Both Thrust Levers at or below IDLE, or REV on at least one engine (and the other Thrust Lever below MCT)
    

• The GRD SPOILER automatically extends when in NOT ARMED

  • Both main LDG GEAR on the Ground

  • REV selected on at least one ENG (and the other thrust lever below MCT position)

• The GRD spoiler will extend Partially if a touchdown with REV is selected and ONLY One LDG Gear is compressed. It will go to full extension when both main LDG GEARS are compressed

• The Ground Spoiler will automatically retract when :
  

  • After LDG RTO : When Ground Spoiler Disarmed

  • During Touch and Go : When at least On Thrust Lever is advanced above 20 deg

THS CONTROL

THS is automatically trimmed during flight.

• After the touchdown, the THS is automatically trimmed to the neutral position.

• THS mechanical control is used to set the pitch trim before take-off or when the automatic pitch trim is not available.

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

• Mechanical control of the THS is available from the pitch trim wheel at any time, if either the green or yellow hydraulic system is functioning. The THS requires a hydraulic system

• Mechanical control from the pitch trim wheel has priority over electrical control. When the manual input is given, the mechanical trim has priority over the computer input, the autopilot will disconnect.

• The Trim wheel will be reset to zero when the aircraft pitch attitude becomes less than 2.5 deg for more than 5 seconds and it will reset to zero position. The functioning of this condition will be available when the green and yellow hydraulic system is pressurized.
  

RUDDER PEDALS

• Two sets of pedals enable the rudder's mechanical control.

• The pedals can be individually adjusted.

RUDDER TRIM

• The rudder trim control switch operates the electrical trim actuator to move the rudder to a new neutral position.

• The rudder trim RESET switch resets the trim position to zero.

• The rudder trim indicator displays the rudder trim position in degrees

FLAP CONTROL LEVER

• The flap control lever selects the simultaneous operation of the slat and flap.

• It has five positions.

• WTB (Wing Tip Brake On) The Wing tip brake is installed on slats and flaps in case of uncommanded movement of surfaces, such as runway, asymmetric, or overspeed. They can not be released in flight

• Using Blue and Green power for slats and right-wing flaps

• Using Blue and Yellow power for left-wing flaps

• if the slat wingtip brake is on the pilot can still operate the flap

• if the flap wing tip brake is on the pilot can still operate the slat

• if one SFCC is inoperative , slats and flaps both will operate at half speed.

• If one of the hydraulic systems is inoperative the corresponding surfaces (slats or flaps_ operate at half speed.

• There are two SFCCs installed in the system

• If One SFCC fails the slats and flaps will operate at 1/2 speed

• CONFIG 1 + F means the slat extend and Flap extend function on the ground, the F speed will not display inflight unless the aircraft speed is below 100 knots and when you are in the air using flap 1 will extend the Slats only.

• ARS (Automatic Retraction System) will operate when the speed of the aircraft exceeds 210 knots, Only the flaps will retract if you are taking off with CONFIG 1 + F.

• The ALPHA LOCK function is inhibits Slat Retraction when at High angle of attack and low speed.

• The Aileron droop is extended about 5 deg when the Flaps are extended.

ACTUATION

ELEVATORS

• Two electrically controlled hydraulic servo jacks drive each elevator.

• Each servo jack has three control modes

• Active: The jack position is electrically controlled.

• Damping: The jack follows the surface movement.

• Centering: The jack is hydraulically retained in the neutral position.

STABILIZER

• A screwjack driven by two hydraulic motors drives the stabilizer.

• The two hydraulic motors are controlled by :

• One of three electric motors, or

• The mechanical trim wheel.

AILERONS

• Each aileron has two electrically controlled hydraulic servo jacks.

• One of these servo jacks per aileron operates at a time.

• Each servo jack has two control modes :

• Active: Jack position is controlled electrically

• Damping: Jack follows the surface movement.

• The system automatically selects damping mode, if both ELACs fail or in the event of blue and green hydraulic low pressure.

SPOILERS

• A servo jack positions each spoiler.

• Each servo jack receives hydraulic power from either the green, yellow, or blue hydraulic system, controlled by the SEC1, 2 or 3.

• The system automatically retracts the spoilers to their zero position, if it detects a fault or loses electrical control.

RUDDER

• Three independent hydraulic servo jacks, operating in parallel, actuate the rudder. In automatic operation (yaw damping, turn coordination) a green servo actuator drives all three servo jacks. A yellow servo actuator remains synchronized and takes over if there is a failure.

RUDDER TRIM

• The two electric motors that position the artificial feel unit also trim the rudder.

• In normal operation, motor N° 1 (controlled by FAC1), powers the trim, and FAC2 with motor N° 2 remains synchronized as a backup.

• In manual flight, the pilot can apply rudder trim via the rotary RUD TRIM switch on the pedestal.

• The pilot can use a button on the RUD TRIM panel to reset the rudder trim to zero.

• With the autopilot engaged, the FMGC computes the rudder trim orders. The rudder trims rotary switch and the rudder trim reset pushbutton are not active.