What’s the design justification for even having altenate law? Wouldn’t it be better for the pilot simply to know that the normal law envelope protections are gone, rather than have to remember which subset of them are still in effect? In some ways, partial envelope protection (if that’s what alternate law provides) is worse than none at all – especially if the transition occurs at a difficult moment, as seems to be inherently the case.
This highlights what I have seen a lot on this forum: the lack of differentiation between automation, control systems and flight protection systems. Unless you have the autopilot on, the airplane is not making any flight decisions for you. The flight envelope protections LIMIT your control inputs but they do not make control inputs for you. These limits are calculated on the fly (haha) by the computers using the available flight data, according to some extremely complex models about the aircraft, and then padded a little bit for stability (as in, derated to be on the safe side). Other than that, the inputs made on the sidestick (or the yoke on a Boeing aircraft) are modified by a gain and then transmitted on.
The autopilot is another matter, being a feedback system that trys to keep a control variable at a given value. This system GENERATES flight control inputs. FEP limits flight control inputs, but does not generate them.
As for why you'd want alternate laws, not all the protections rely on every peice of sensor data. Why give up ALL protections if you only need to give up one? For whatever reason, the concepts of modes of operation are very common in many industries. The idea being that a person can associate a number of states with just a single name. I'd assume that Boeing would have a similar mechanism for degrading the flight protections but I can't find any verification for this right now.
As for the "interface", there is no real interface. Someone in this thread has mentioned this multiple times. The sidestick functions exactly the same in every law. The only difference with FEP is that if you slam the sidestick to one side then you won't crash the aircraft. The FEP system reduces pilot workload by making it so s/he doesn't have to concentrate strictly on keeping the airplane aloft, but on providing it a course.
I don't understand why these concepts do not seem to stick.