Alternate Law.

[Flapping_Madly]

On the AF 447 thread there is much talk of different "Laws".
Does this just mean "set of operating instructions" or ----what?

I just find the word "law" strange or odd or is it just aircrew jargon.

Can someone spare a couple of seconds to explain. Please. Thanks:confused:

[Final 3 Greens]

FM

I'm a PPL, but lived next to an Airbus captain for nearly 10 years and he did explain it to me a long time ago.

As I understand it, the 'laws' are the parameters and controls that are used by the fly by wire computers to interpret the pilot's inputs and move the control surfaces.

They mean that the aircraft will respond in different ways, depending on whcih law is being used.

The reason the laws were being discussed (last time I looked. a while ago now) was in speculating how controllable an airbus would be in severe turbulence, under the least helpul 'law.'

With a bit of luck, an airbus pilot will come along and explain it properly.

[deltayankee]

"law" in this context just refers to the relationships between inputs and outputs. It is engineering jargon but not limited to aviation.

[Crusty Ol Cap'n]

In this context Airbus use the word "law" when referring to a computer program. The guidance computers (FMGECs) have different programs or "laws" available depending on the serviceability of the input sensors etc.

[Slopey]

There's a great explanation here (http://www.airbusdriver.net/airbus_fltlaws.htm)

[Mad (Flt) Scientist]

"Law" is a term commonly used by control designers to describe the set of rules under which a control system operates.

Airbus use three main "Laws" to decribe the flight control philosophy - Normal, Alternate and Direct.

In both Normal and Alternate laws the relationship between the pilot inputs and the aircraft response is essentially the same in the normal flight envelope. The main difference is that "Normal Law" includes a whole host of "flight envelope protection" features intended to prevent the aircraft from being stalled, or over-banked, or which provide augmenteed handling in some flight regimes. As the aircraft systems gradually degrade through "Alternate Law" (there are in fact a number of levels within Alternate, depending on what other failures have occurred) these protections are gradually lost.

"Direct Law" is a rather different beast in that the relationship between the pilot inputs and the flight control surfaces are much simpler - hence the term "direct" - they can be considered to behave as if directly, mechanically, linked. The aircraft then flies a little differently as seen by the pilot in terms of aircraft response, but the basics of the control scheme (push/pull sidestick for pitch, side-to-side for roll, rudder pedals for yaw) remains the same.

There is one final level of control degradation which occurs when all the flight guidance computer systems fail, where the aircraft is controlled by, IIRC, using the tailplane trim and the rudder trim. Thats a very degraded "emergency" control scheme, really just intended to give a chance to troubleshoot the computers and hopefully get one or more back online.

edit to add that slopey's link is the detailed version of what i was trying to summarise. In the event of discrepancy, believe him not me