Airbus A320 Flight Controls Laws

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Fly-Through-Computer Flight Augmentation Computers

FAC 2 FAC 1 ELevator Aileron Computers

ELAC 2 ELAC 1 OR

SEC 3 SEC 2

Autopilot (Master FMGC)

SEC 1

Spoiler Elevator Computers

2

Fly-Through-Computer

FAC 2 FAC 1

ELAC 2 ELAC 1

SEC 3 SEC 2 SEC 1 SEC 1 & 2 provide backup THS and elevator control.

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The flight control laws are the manufacturer’s built in computer instructions defining flight control movement, flight characteristics, and aircraft limitations. The flight control laws are a series of instructions that are executed by the flight control computers.

For safety and redundancy there are three flight control laws: • NORMAL LAW • ALTERNATE LAW • DIRECT LAW Mechanical backup control of the THS and rudder is always available. 4

Flight Control Laws LAW: A predetermined set of rules which govern flight characteristics

PITCH

ROLL

YAW

FLIGHT CHARACTERISTICS

PROTECTIONS

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NORMAL LAW The flight control law in use during normal operations when all, or nearly all, aircraft systems are operational.

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=NORMAL=

PITCH

ROLL

YAW


PROTECTIONS

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=NORMAL= Flight Characteristics

PITCH Load Factor Demand

Protections

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LOAD FACTOR DEMAND

Neutral Stick – Requests NO CHANGE in Gs The flight control computers maintain approximately 1G Pitch attitude remains relatively constant.

A specific pitch input Aft Stick – Requests a positive G CHANGE results in the same G The flight control computers order the elevators to move the required amount to provide the requested G change, change regardless of resulting in the nose pitching up. the current airspeed. Forward Stick - Requests a negative G CHANGE

The flight control computers order the elevators to move the required amount to provide the requested G change, resulting in the nose pitching down. 9


PITCH Load Factor Demand Automatic Pitch Trim

Protections

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The Trimmable Horizontal Stabilizer (THS) is trimmed up or down to provide pitch trim.

The THS is a much more active part of the pitch control system on the Airbus than on many other aircraft. When a G change (pitch change) is commanded, the elevators initially move to provide the commanded change. If the desired pitch attitude requires the elevators to remain displaced, THS movement is commanded until the elevators are centered with the THS.

Pilot manual pitch trim inputs are not required. 11


PITCH Load Factor Demand Automatic Pitch Trim Ground Mode Flight Mode Landing Mode

Protections

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GROUND MODE OF NORMAL LAW A direct stick to flight control relationship exists. This enables the pilot to: • Check the flight controls • Rotate the aircraft for takeoff

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FLIGHT MODE OF NORMAL LAW • Load factor demand in pitch • All protections are available

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LANDING MODE OF NORMAL LAW The landing mode was created to give the flare and touchdown a “conventional feel”.

At 50 feet AGL (measured by the RAs) the ELACs memorize the pitch attitude At 30 feet AGL the ELACs add a gentle nose down command to the memorized pitch attitude, which the pilot counters with an aft stick input.

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Protections

Maneuver Protection

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Maneuver protection prevents aircraft damage due to overstressing the aircraft. The flight control computers prevent flight control surface movement that would cause the aircraft to exceed preset G limits.

NOTE: MAX = +2.0G IF FLAPS EXTENDED

NOTE: MAX = 0.0G IF FLAPS EXTENDED 17



Protections

Maneuver Protection Pitch Protection

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NO FLY

30

15

NO FLY 19



Protections

Maneuver Protection Pitch Protection High Speed Protection 20

High speed protection prevents aircraft damage due to excessive speed.

Activates just above Vmo/Mmo A gentle pitch up is produced by the flight control computers to limit further acceleration High speed protection activation

VMO / MMO

Acceleration is limited even if full forward stick is applied. If high speed protection activates, automatic pitch trim in the nose down direction is deactivated and the autopilot, if engaged, disengages. It is always possible to overspeed the aircraft. The fight control computers merely limit the maximum attainable speed to within safe limits. 21



Protections

Maneuver Protection Pitch Protection High Speed Protection Angle of Attack Protection 22

Relative Wind

(ALPHA) = Angle of Attack = the angular difference between the relative wind and the attitude of the aircraft

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= The angle of attack where the flight control computers intervene to prevent alpha from reaching a stall alpha. At Alpha Prot: • Pitch command logic changes from load factor demand to commanding an angle of attack.

VLS

Alpha Prot

• Additional nose up trim is inhibited - nose down trim is still available. • The autopilot, if engaged, disengages. • A lowering of pitch attitude occurs.

If the pilot persists and pulls the stick further aft, eventually the angle of attack will reach . . . . . . . . .

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= The highest angle of attack that the ELACs will allow. ALPHA MAX is a lower alpha than ALPHA STALL; therefore, it is nearly impossible to stall the aircraft in normal law. NOTE: Although the FACs continuously compute the airspeeds corresponding with the various alpha protection values and display them on the airspeed tape, these speeds are for pilot reference only.

VLS

Alpha Prot Alpha Max

The actual angles of attack that correspond with Alpha Prot and Alpha Max are computed by the ELACs based on angle of attack, not airspeed.

If autothrust is available, the airspeed is unlikely to reach Alpha Max because Alpha Floor will probably activate prior to reaching it. 25

Alpha Floor: Is a predictive function of the autothrust system. It activates based on the current trend if it predicts thrust will be required. Is normally available from immediately after takeoff throughout the flight down 100 feet RA in configuration 1 or greater.

Is only available in normal law.

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Alpha floor uses the autothrust system to automatically provide TOGA thrust if Alpha is predictive and can be triggered at any airspeed; therefore it is any ofFloor the following conditions occur: NOT depicted on the airspeed tape.

• Excessive high angles of attack.

is independent of Alpha Prot and Alpha Max. In other words, Alpha Floor •ItIndirectly as a result of windshear. can be triggered at airspeeds significantly higher than Alpha Prot.

• Excessive nose up attitudes combined with specific sidestick inputs. Excessive high angles of attack

Indirectly as a result of windshear

Excessive high nose up attitudes combined with specific sidestick inputs

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If Alpha Floor activates, TOGA thrust is automatically applied, regardless of thrust lever position. Alpha Floor activation DOES NOT require that autothrust be ENGAGED, but it must be AVAILABLE (i.e., operational). A. FLOOR

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=NORMAL= FLIGHT CHARACTERISTICS

PITCH

ROLL

YAW

Load Factor Demand Automatic Pitch Trim Ground Mode Flight Mode Landing Mode

PROTECTIONS

Maneuver Prot Pitch Protection Hi Speed Prot  of Attack Prot

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ROLL Roll Rate

Protections

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ROLL RATE Neutral Stick – Requests zero roll rate. The aircraft essentially maintains the current bank angle.

Full Stick Deflection – Requests 15 / sec roll rate.

Stick Slightly Left / Right of Center - Requests a between 15 and 0 / sec roll rate. 31


ROLL Roll Rate Pitch Trim < 33 

Protections

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Automatic pitch trim is available up to 33 of bank to assist the pilot in maintaining altitude during turns.

Beyond 67 NO FLY Automatic pitch trim is available

33 to 67

0 to 33

33 to 67

Automatic pitch trim is available

0 to 33

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ROLL Roll Rate Pitch Trim < 33  Bank Angle Hold < 33 

Protections

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If the stick is released to neutral, a 0 / second roll rate is commanded. The aircraft essentially maintains a constant bank angle, unless commanded otherwise.

Beyond 67 NO FLY “Bank angle hold” 0-33°

33 to 67

0 to 33

33 to 67

“Bank angle hold” 0-33°

0 to 33

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ROLL Roll Rate Pitch Trim < 33  Bank Angle Hold < 33  Positive Spiral Stability > 33 

Protections

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If the bank angle exceeds 33 and the stick is released, bank angle returns to 33. This called positive spiral stability. Positive spiral stability

Beyond 67 NO FLY 33 to 67

0 to 33

33 to 67

0 to 33

NOTE: Positive spiral stability returns the bank angle to 0° if high speed protection is active. 37


ROLL Roll Rate Pitch Trim < 33  Bank Angle Hold < 33  Positive Spiral Stability > 33 

Protections

BANK ANGLE PROTECTION

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In normal, the flight control computes will not allow bank angles in excess of 67.

Bank angle protection

Beyond 67 NO FLY 33 to 67

0 to 33

33 to 67

0 to 33

NOTE: Bank angle is limited to 45° if high speed protection or angle of attack protection is active (the limit marks do not move). 39


=NORMAL= FLIGHT CHARACTERISTICS

PITCH

ROLL

Load Factor Demand

Roll Rate

Automatic Pitch Trim

Bank Angle Hold

Ground Mode

YAW

Pitch Trim

Pos Spiral Stab

Flight Mode Landing Mode

PROTECTIONS

Maneuver Prot Pitch Protection

Bank Angle Protection

Hi Speed Prot  of Attack Prot

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YAW

Turn Coordination

Protections

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ELAC 2 ELAC 1

FAC 2 FAC 1

Yaw orders associated with bank are… …processed by the ELACs then... …transmitted to the FACs. The FACs direct the rudder to rudder’s hydraulic servos to move the rudder. Turn coordination is automatic, requiring no pilot rudder input. There is no rudder pedal movement resulting from turn coordination. 42


YAW Turn Coordination Yaw Damping

Protections

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YAW DAMPING

Yaw Damp Actuators

Two yaw dampers are installed. A yaw damper moves the entire rudder as necessary to dampen yaw oscillations.

Either yaw damper is capable of providing full yaw damping authority. There is no rudder pedal movement resulting from yaw damping.

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YAW Turn Coordination Yaw Damping Rudder Trim

Protections

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RUDDER TRIM Rudder trim commands are sent from the FACs to one of two rudder trim motors which move the entire rudder surface. Trim tabs are not used. When an autopilot is engaged, rudder trim needs are computed by the FACs and automatically carried out. Manual rudder trim is deactivated. With the autopilots off, the rudder may be trimmed using the RUD TRIM knob on the pedestal.

As the rudder is trimmed (either manually or automatically), the rudder pedals ARE symmetrically displaced. 46


YAW Turn Coordination Yaw Damping Rudder Trim Manual Rudder (HYD)

Protections

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Mechanical Connection

Rudder Pedals

Backup mechanical control of the rudder is always available via cables from the rudder pedals to the rudder servos if at least one hydraulic system is available.

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YAW Turn Coordination Yaw Damping Rudder Trim Manual Rudder (HYD) Rudder Limiting

Protections

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RUDDER LIMITING The FACs, using airspeed inputs from the ADRs, limit rudder surface movement at high speeds to prevent excessive airframe loads and yaw responses. The two rectangular symbols on the rudder travel indication represent the unrestricted rudder travel limit at low speeds.

The maximum rudder travel available at the current speed is indicated by small L shaped indications next to the rudder travel arc. Rudder PEDAL travel is never restricted. Only rudder SURFACE travel. NOTE: On some Spirit aircraft the high speed rudder travel limit depiction on the F/CTL page is fixed and does not move with changes in the rudder travel limit. 50

Flight Control Laws LAW: A predetermined set of rules which govern flight characteristics x=NORMAL= ALTERNATE x FLIGHT CHARACTERISTICS

PITCH

ROLL

YAW

Load Factor Demand

Roll Rate

Turn Coordination

Pitch Trim

Yaw Damping


Bank Angle Hold Rudder Trim

Ground Mode

Pos Spiral Stab

Flight Mode

Manual Rudder Rudder Limiting

Landing Mode

PROTECTIONS




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x ALTERNATE LAW x A single failure WILL NOT cause the flight control system to degrade from NORMAL to ALTERNATE LAW.

A minimum of TWO or MORE failures must occur for the flight controls to degrade from NORMAL LAW, usually in the following systems: Flight Control Computers ELAC 1

SEC 3

FAC 2

ELAC 2 FAC 1

SEC 2 SEC 1

Hydraulic Systems

ADIRUs

A D

I R

A D

I R

A D

I R

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x ALTERNATE LAW x Two failures may not cause the flight control system to degrade. For example: ELAC 1

SEC 3

FAC 2

ELAC 2 FAC 1

SEC 2 SEC 1

The aircraft remains in normal law and the autopilots are operational. 53

x ALTERNATE LAW x X

X

F / CTL ALTN LAW (PROT LOST) MAX SPEED . . . . . . . . . . . . . . 320 KT

This statement is misleading.

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Flight Control Laws LAW: A predetermined set of rules which govern flight characteristics x ALTERNATE x FLIGHT CHARACTERISTICS

PITCH

ROLL

YAW

Load Factor Demand

Roll Rate

Turn Coordination

Pitch Trim

Yaw Damping



Ground Mode

Pos Spiral Stab

Flight Mode


Landing Mode

PROTECTIONS




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x ALTERNATE x

Flight Characteristics

PITCH Load Factor Demand Automatic Pitch Trim Flight Mode

Protections

Maneuver Protection High Speed Protection Stability

 AttackStability Protection LoofSpeed

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Stabilities are flight control commands that smoothly attempt to change pitch to prevent an overspeed or a low speed (stall) condition. Pilots can manually override either stability with stick movement, causing an or a occur.

to

activates at a speed slightly below VMO/MMO. activates if the speed decreases to within a few knots of the stall warning.

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PITCH

ROLL

YAW

Load Factor Demand

Roll Rate

Turn Coordination

Pitch Trim

Yaw Damping



Flight Mode

Pos Spiral Stab


PROTECTIONS

Maneuver Prot

Hi Speed Stab


Lo Speed Stab 58



ROLL

YAW

ROLL DIRECT

Turn Coordination

A direct stick to flight control surface relationship in roll

Rudder Trim

Yaw Damping


PROTECTIONS

Maneuver Prot

Hi Speed Stab Lo Speed Stab 59

Flight Control Laws LAW: A predetermined setMAN of rules which govern USE PITCH TRIM flight characteristics x ALTERNATE x DIRECT x x FLIGHT CHARACTERISTICS


PROTECTIONS

ROLL

YAW

*

ROLL DIRECT

Yaw Damping


Manual Rudder

Rudder Trim

Rudder Limiting

Maneuver Prot

Hi Speed Stab

*Assuming a double FAC failure was not the cause of the degradation.

Lo Speed Stab 60

x DIRECT LAW x It is possible for the flight control system to degrade from NORMAL law straight to DIRECT law (e.g., dual radar altimeter failure). The primary reason that DIRECT law is reached is because the aircraft is in ALTERNATE law and the landing gear is lowered in preparation for landing. Why? Because ALTERNATE law does not provide a landing mode.

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x DIRECT LAW x DIRECT law results in a direct stick to flight control surface relationship. What does that really mean?

Stick inputs are still processed by the flight control computers and transferred to the flight control surfaces. The computers, however, carry out pilot orders exactly as they are signaled. Unlike NORMAL or ALTERNATE laws, the computers have no authority to modify or override the stick inputs, thus NO protections OR stabilities are provided.

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x DIRECT LAW x USE MAN PITCH TRIM

This statement is more accurate. F / CTL DIRECT LAW (PROT LOST) MAX SPEED . . . . . . . . 320 KT/ .77 MANEUVER WITH CARE USE SPEED BRAKE WITH CARE

No pitch or roll protections are provided in DIRECT law.

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Flight Control Laws LAW: A predetermined setMAN of rules which govern USE PITCH TRIM flight characteristics x DIRECT x FLIGHT CHARACTERISTICS


PROTECTIONS

ROLL

YAW

ROLL DIRECT

Yaw Damping


Manual Rudder

Rudder Trim

Rudder Limiting

Maneuver Prot

Hi Speed Stab Lo Speed Stab 64

Flight Control Laws LAW: A predetermined setMAN of rules which govern USE PITCH TRIM flight characteristics x DIRECT x FLIGHT CHARACTERISTICS

PITCH

ROLL

YAW

PITCH DIRECT

ROLL DIRECT

Yaw Damping

A direct stick to flight control relationship in pitch.


Manual Rudder

Rudder Trim

Rudder Limiting

Manual THS Input PROTECTIONS

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Flight Control Laws LAW: A predetermined setMAN of rules which MAN PITCH TRIMgovern ONLYflight characteristics USE PITCH TRIM

Mech. Backup x DIRECT x FLIGHT CHARACTERISTICS

PITCH

ROLL

YAW

PITCH DIRECT

ROLL DIRECT

Yaw Damping

A direct stick to flight control relationship in pitch.


Manual Rudder

Rudder Trim

Rudder Limiting


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Mechanical Backup Mechanical backup control of the THS and rudder is always available. Mechanical backup is the only way to maintain aircraft control if all the flight control computers fail or there is a TOTAL loss of electrical power. It either occurs, the sidesticks are inoperative and the aircraft is controlled using the trim wheels, rudder pedals, and engine thrust. Mechanical backup provides a means of aircraft control until a higher law can be restored.

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Mechanical Backup

X

F/CTL L + R ELEV FAULT MAX SPEED. . . . . . . . . . . 320/.77 - MAN PITCH TRIM . . . . . . . . USE - SPD BRK . . . . . . . . DO NOT USE

X

No ECAM message specifically indicates that mechanical backup is active.

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Flight Control Laws LAW: A predetermined set PITCH of rules which MAN TRIMgovern ONLYflight characteristics

Mech. Backup FLIGHT CHARACTERISTICS

PITCH

ROLL

YAW

PITCH ROLL MECHANICAL DIRECT A Direct Stick to Flt Control Surface Relationship in Roll

Manual Rudder


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Flight Control Laws LAW: A predetermined set PITCH of rules which MAN TRIMgovern ONLYflight characteristics

Mech. Backup

PITCH

YAW YAW


PITCH MECHANICAL

YAW MECHANICAL Manual Rudder


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If extreme conditions cause the aircraft to leave the protected envelope (e.g. severe turbulence), and exceed a normal law limit, abnormal attitude law becomes active.

Abnormal attitude law is alternate law without protections and stabilities except for load factor protection. This is a safety feature to ensure that the flight control computers never prevent the pilots from recovering from an abnormal attitude. After recovery, the flight controls remain in alternate law without protections but with auto trim. There is no reversion to direct law when the gear is extended. 71

Flight Control System Page

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Flight Control System Page

X X X X X

X X X X X

XX

XX

1

1

2

2

3

XX

XX

XX

XX

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Airbus A320 Flight Controls Laws

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