The term control law basically means a set of principles by which the fly by wire controls of the aircraft behave. In Airbus aircraft, the control laws are devided into three:
- Normal Law
- Alternate Law
- Direct Law
The normal law is how the aircraft controls behave when under normal operations with no failures. The alternate and direct laws are reconfiguration laws that becomes active in failure cases, where the latter is the lowest level of control law degradation. Let us look at each of these laws.
Normal Law
When the aircraft is normal law, the bank angle protection, the pitch attitude protection and the high speed protection are marked by green double dashes on the Primary flight display.
The PFD indications in Normal law.
In normal law, there is also a high angle of attack protection.
They are all associated with the high angle of attack protection of the aircraft in normal law. In all Airbus aircraft there is a black and amber band at the lower end of the speed tape. This band is known as the alpha prot. Once the aircraft speed is in this range, the high angle of attack protection activates. At this point, if the autopilot is engaged, it automatically disengages and the aircraft starts a descend until the speed at the top of the amber and black band is reached (this works without any input from the pilot). If there are no inputs from the pilot, the aircraft will maintain this speed all by it self. You can still manually pull back on the stick, but it has to be held back by force as auto trim would not help you out. Nose down trim will still be available if the side stick is pushed forwards. Then we have alpha floor. This is associated with the autothrust system of the aircraft. If the pilot continues to pull back on the side stick after the aircraft reaches alpha prot range, the alpha floor activates. It comes on somewhere between alpha prot and alpha max. It is based on a pre determined angle of attack which is related to the aircraft configuration. i.e. position of flaps and slats. The alpha floor automatically increases aircraft thrust to TOGA thrust in order to add energy into the aircraft, to help it to fight the high angle of attack situation. With full thrust added and with stick fully back, the aircraft usually starts a climb at a very low speed. Once out of the high angle of attack situation, the alpha floor mode is changed to TOGA LK (TOGA thrust locked) and the pilot has to disconnect autothrust to regain control of the engine thrust. According to Airbus A320 FCOM, here is how alpha floor is activated (it can vary between different Airbus models): AIRBUS A320 FCOM DSC-22_40-30 The alpha floor is activated through the A/THR system, when: * Alpha is greater than alpha floor (9.5 degrees in configuration 0, 15 degrees in configuration 1 and 2, 14 degrees in configuration 3 and 13 degrees in configuration FULL), or * Sidestick deflection is greater than 14 ° nose up, with either the pitch attitude or the angle-of-attack protection active. * * The αfloor function is available from lift-off to 100 ft RA before landing.
Lastly, we have alpha max. As the name suggests it is the maximum angle of attack that can be reached by the pilot in normal law. If the high angle of attack protections are ignored and if the pilot keeps the side stick fully back, the aircraft will reach the top of a red band on the speed tape. No matter what the pilot does, alpha max cannot be breached. Alpha max is very close to the stalling angle or the cl max of the aircraft. With the aircraft at alpha max the aircraft can climb at a very high angle of attack without actually stalling. It is quite beneficial to fly at alpha max in escape maneuvers like windshear because it allows the pilot to fly at the very edge of the aircraft envelope where a human pilot will struggle to keep that small margin between safe flight and stall. Note: The high angle of attack protection works in normal law only. If any aircraft failures leads to a degraded law such as alternate or direct, the protections are lost.
In normal law, the pitch control of the aircraft is basically a g-load demand. When the pilot moves the stick, he demands a certain amount of gees and when the stick is released the computers revert to 1 g flight. Also, the trimming is achieved automatically. The roll control, up to 33 degrees of bank shows positive spiral stability, where if a pilot demands a bank angle and release the stick, the aircraft automatically maintains that angle until changed by the pilot.
The yaw control is through the Flight augmentation computer (FAC) and both turn coordination and yaw damping function is available.
Alternate Law
In alternate law, most of the protections are lost except for the load factor protection. The green dashes which mark the bank angle protection and pitch attitude protection are replaced by amber crosses. The high speed protection green dashes are also removed. However, in alternate law 1 (first level of alternate law), there exists a high speed and a low speed stability where if for instance the aircraft were to go above maximum allowable speed (Vmo/ Mmo), a gentle nose up pitch is introduced. In case of a low speed situation a progressive nose down pitch is introduced by the computers. If the aircraft systems are degraded further, an alternate law with reduced protections (alternate law 2) is in effect. Here, only load factor protection is active and the high and low speed stability is lost.
The PFD indications in Alternate law.
The pitch control is same as that of normal law and automatic trimming is available. However, the roll control becomes direct. That is, the spiral stability is lost and if the pilot wants to hold a bank he has to keep that angle with the stick.
The yaw control is degraded and turn coordination is lost. But yaw damping remains active with some limitations.
Direct Law
In direct law, all the protections are lost. On the PFD, you have a message USE MAN PITCH TRIM in amber displayed at all times as auto trimming function is lost. So, the pilot is required to trim the aircraft manually. When in direct law, there is a direct relationship between side stick deflection and control surface deflection. The maximum elevator deflection given to the pilot is a function of the aircraft CG. It ensures that there is enough control authority with a forward CG and makes sure that the controls do not become too sensitive with an aft CG.
The PFD indications in Direct law.
The lateral control or the bank control is such that over control tendency is reduced. In direct law, the aircraft is much more hands on than a conventional aircraft which can lead to over control. So, some roll spoilers might be deactivated by the system, to conserve the handling qualities.
The yaw control becomes fully mechanical. Both the turn coordination and the yaw damping is lost.
Author – Anas Maaz (Airline Pilot )
