Blueskiesup
Airbus a/c are 'fly-by-wire' (FBW) designs. This means that a computer sits between the pilot's controls and the various control surfaces/devices.
In a non-FBW a/c there is some direct connection between the levers & knobs that the pilot uses and the surfaces/devices that cause the a/c to move. It could be wire cables, pushrods, hydraulic actuators or electrical actuators or whatever. The pilot has to determine what combination of inputs are needed to cause the a/c to respond in a desired manner. It's possible for the pilot to choose to use too much, too little or even incorrect inputs to try to achieve the desired response from the aircraft.o
FBW a/c have a computer that determines what combination of a/c surfaces/devices must be moved to cause the a/c to respond in a desired manner. The pilot knows what response is desired from the a/c & uses the various control devices to tell the computer what s/he wants the a/c to do. The computer reads those inputs and, using the data inputs & surfaces at its disposal and in accordance with pre-programmed sets of rules, makes a decision about what surfaces must be moved, by how much and how fast. For safety there are multiple computers acting in parallel doing the job and not just one.
Those pre-programmed sets of rules, in Airbus language, are the 'laws'. There are groups of rules for various stages of flight to allow for unique requirements or more desirable behaviour. Some laws provide greater protections against undesirable use of the controls whilst others provide fewer protections but need fewer data sources to do their job.
'Normal law' is - as it implies - the normally operating set of rules the computer abides by. It's not always appropriate for the stage of flight or the mechanical condition of the a/c.
If the a/c loses certain resources then the computer can't provide reliable solutions to provide the a/c response the pilot wants. In this case it falls back to a less protective (but somewhat more directly responsive) 'Alternate law'.
If providing even that level of reliability is doubtful then the system falls back to 'Direct law'. Direct law provides direct control from the pilot's inputs to the controls - much like a non-FBW a/c.
Ground law has its own variant of what the computer does for the pilot vs. what the pilot can directly control.
Take-off law is optimised for that phase of flight & allows for an orderly transition from the more direct law control mode used during the take-off to the normal law mode used in-flight.
Landing law is likewise optimised for that phase. It causes the a/c to pitch nose down slightly because pilots almost invariably will pull on the control column/sidestick during landing to counter (a non-FBW) a/c's pitch down when power is reduced to land.
Any mistakes in the above are the result of my own misunderstandings....
Last edited by Tinstaafl; 3rd October 2006 at 23:41.