Flight Control Laws

The aircraft is controlled by three primary control computers (Captain's, First Officer's and Standby) and two secondary control computers (Captain's and First Officer's). In addition there are two flight control data computers (FCDC) that read information from the sensors, such as air data (airspeed, altitude). This is fed along with GPS data, into three redundant processing units known as air data inertial reference units (ADIRUs) that act both as an air data reference and inertial reference. ADIRUs are part of the air data inertial reference system, which, on the Airbus is linked to eight air data modules: three are linked to pitot tubes and five are linked to static sources. Information from the ADIRU is fed into one of several flight control computers (Primary and secondary flight control). The computers also receive information from the control surfaces of the aircraft and from the pilots aircraft control devices and autopilot. Information from these computers is sent both to the pilot's primary flight display and also to the control surfaces.

There are four named flight control laws, however Alternate Law consists of two modes, Alternate Law 1 and Alternate Law 2. Each of these modes have different sub modes: ground mode, flight mode and flare, plus a back-up Mechanical Law.

  

Normal law

 

Normal Law differs depending on the stage of flight. These include:

• Stationary at the gate
• Taxiing from the gate to a runway or from a runway back to the gate
• Beginning the take-off roll
• Initial climb
• Cruise climb and cruise flight at altitude
• Final descent, flare and landing.

Normal Law is different depending on the stage of flight. During the transition from take-off to cruise there is a 5 second transition, from descent to flare there is a two second delay and from flare to ground there is another 2 second transition in Normal Law. 

 

Ground mode

 

The aircraft behaves as in direct mode: The auto trim feature is turned off and there is a direct response of the elevators to the side stick inputs. The horizontal stabilizer is set to 4° up but manual settings (e.g. for center of gravity) override this setting. After the wheels leave the ground, a 5 second transition occurs where Normal Law - flight mode takes over from ground mode.


Flight mode

The flight mode of Normal Law provides five types of protection: Pitch attitude, load factor limitations, high speed, high A.O.A and bank angle. Flight mode is operational from take-off, until shortly before the aircraft lands, around 100 feet above ground level. It can be lost prematurely as a result of pilot commands or system failures. Loss of Normal Law as a result of a system failure results in Alternate Law 1 or 2.

Unlike conventional controls, in Normal Law flight mode the side stick provides a load factor proportional to stick deflection which is independent of aircraft speed. When the stick is neutral and the load factor is 1g the aircraft remains in level flight without the pilot changing the elevator trim. The aircraft also maintains a proper pitch angle once a turn has been established, up to 33° bank. The system prevents further trim up when the angle of attack is excessive, the load factor exceeds 1.3g or when the bank angle exceeds 33°.

Alpha protection (α-Prot) prevents stalling and the effects of wind shear. The protection engages when the angle of attack is between α-Prot and α-Max and limits the angle of attack commanded by the pilot's side stick or, if autopilot is engaged, it disengages the autopilot.

High speed protection will automatically recover from an over speed. There are two speed limitations for high altitude aircraft, V_MO (Velocity Maximum Operational) and M_MO (Mach Maximum Operational) the two speeds are the same at approximately 31,000 feet, below which overspeed is determined by V_MO and above 31,000 feet by M_MO.

Flare mode

A380 in take off This mode is automatically engaged when the radar altimeter indicates 100 feet above ground. At 50 feet the aircraft trims the nose slightly down. During the flare, Normal Law provides high AOA protection and bank angle protection. The load factor is permitted to be from 2.5g to -1g, or 2.0g to 0g when slats are extended. Pitch attitude is limited to +30 to -15° which is reduced to 25° as the aircraft slows.

Alternate law

There are four reconfiguration modes for the Airbus fly-by-wire aircraft, two Alternate Law (1 and 2), Direct Law and Mechanical Law. The ground mode and flare modes for Alternate Law are identical to those modes for Normal Law.

Alternate law 1 (ALT1) mode combines a Normal Law lateral mode with the load factor, bank angle protections retained. High angle of attack protection may be lost and low energy (level flight stall) protection is lost. High speed and high angle of attack protections enter alternative law mode.
ALT1 may be entered if there are faults in the horizontal stabilizer, an elevator, yaw-damper actuation, slat or flap sensor, or a single air data reference fault.


Alternate law 2 (ALT2) loses Normal Law lateral mode (replaced by roll direct mode and yaw alternate mode) along with pitch attitude protection, bank angle protection and low energy protection. Load factor protection is retained. High angle of attack and high speed protections are retained unless the reason for Alternate 2 Law mode is the failure of two air-data references or if the two remaining air data references disagree.

ALT2 mode is entered when 2 engines flame out (on dual engine aircraft), faults in two inertial or air-data references, with the autopilot being lost, except with an ADR disagree. This mode may also be entered with an all spoilers fault, certain ailerons fault, or pedal transducers fault.

Direct law

Direct law (DIR) introduces a direct stick-to-control surfaces relationship: Control surface motion is directly related to the sidestick and rudder pedal motion. The trimmable horizontal stabilisator can only be controlled by the manual trim wheel. All protections are lost, but the maximum deflection of the elevators is changed as a function of the aircraft current centre of gravity.
DIR is entered if there is failure of three inertial reference units or the primary flight computers, faults in two elevators, flame out in two engines (on a two engine aircraft) or when the captain's primary flight computer is inoperable.


Mechanical law

In the Mechanical Law back-up mode, pitch is controlled by the mechanical trim system and lateral direction is controlled by the rudder pedals operating the rudder mechanically.