This is a key point, since if the turbulence is predictable and the continuous underspeed / overspeed problems are predictable, it can be time for an advisory / warning, I think.
I have to admit I'm a bit lost here. What exactly are you proposing to advise/warn of? And how would the turbulence be predictable?
If I know well, the risks of turbulence / gusts can depend on weight and altitude (since the difference between underspeed and overspeed is bigger). And if there are risks (coffin corner aviation + weather + icing) some "early warning" could be helpful.
I don't really see what this has to do with fly by wire? What you are describing is basic airmanship 101. If you can see weather on your radar, you suspect turbulence or potential icing you try to avoid it, and if you can't you select turbulence speed and if necessary descend to a lower altitude to improve your stall and overspeed margins. Again, this is standard procedure on any aircraft, not just FBW aircraft, and a warning is no more necessary on a FBW than on a conventionally controlled aircraft.
If I remember well there was an accident where the pilot flying tried to use rudder extensively in a turbulence. One of the early acars messages are related to "RUDDER AND PEDAL TRAVEL LIMITING ACTUATION". I am not sure if operation of rudder travel limiter is different in normal / alternate law, and if unrelaible speed data and switching to alternate law could lead to unsafe operation of rudder.
If its the accident I'm thinking of it occurred to an A300, which is not FBW and has an entirely different rudder feel system. That is beside the point as in that accident they shouldn't have been using rudder nor is it likely they'd have been using rudder in the Air France accident. Use of the rudder is not an appropriate or necessary technique in turbulence, and unless the aircraft lost an engine then rudder inputs would not have been necessary at all. The failure or the rudder limiter is more than likely due to the failure of it's airspeed input. The purpose of the limiters are to stop you damaging the rudder through excessive travel at high speed - no airspeed input means you have no idea how much to limit the travel, although I suspect the failure mode of the A330 is to reduce the rudder travel available and impose a reduced crosswind limit for landing. It's unlikely the AF crew needed to use the rudder (unless they'd had an engine failure), and I'd say entirely implausible that they'd have cycled opposing inputs to the rudder, which was what caused the failure in the A300 accident.
But if we see icing, turbulence, and when some instruments are lose the pilot flying is surprised by "unusual behavior" of the rudder, and as he tries to recover he loses some altitude, and descends into a CB they tried to avoid... Now that is flying in a CB with many instruments failed.
It's not impossible, but rather unlikely. If you are suggesting some uncommanded rudder hardover like in the B737 it's a possibility, but as far as I know there has been no recorded incident of a rudder hardover of that nature in an Airbus.
May I ask how much experience you have of flying large aircraft, as some of your suggestions seem to be aimed at warning pilots of fairly basic things that should be very obvious to them.
Originally Posted by patrickal
Suppose you own a high end passenger sedan. It has computer controlled handling characteristics which define their "modes" as primary and alternate law. In primary law, the amount of pressure you have to place on the accelerator is somewhat standard with other cars, as is the amount of pressure you need to place on the brakes to achieve a full stop. The steering characteristics are what would be described as "tight", meaning you do not have to move the steering wheel very far in order to achieve a relatively tight turn. In alternate law, all of these change.
Patrickal - your analogy is incorrect, but a common misconception about the Airbus. In alternate law those characteristics do not change. The car would handle the same, except the cruise control wouldn't work any more, the top end electronic speed limiter would be disabled and the device which stops you stalling the car at the traffic lights would quit. In short, it would still be easy to drive.
Whilst your story is indeed colourful, it's not a realistic scenario in the flight deck. There may be a lot going on, but as the aircraft autopilot drops out you'll get a loud, unmistakable aural warning which won't go away until you acknowledge it. As the other systems fail, you'll get a loud warning and an indicator light right in front of you that won't go away until you acknowledge it. As the aircraft drops into alternate law you'll get another warning. If your even half competent it'll be no surprise to you when the aircraft goes into alternate law as you'll have trained the scenario in the simulator a number of times and be expecting it.
So after all of that, my point is, mode should not change without pilots concurrence, especially when it will effect how the controls react to input and what they will do with that input. If the pilot is assuming that limits are being controlled by the computer (especially where it pertains to rudder breakout or throttle settings), he may very quickly find himself further confused and with even less time to react. More importantly, he may further exacerbate the situation, if not cause damage to the aircraft.
The mode does not change from Normal to Alternate without pilot concurrence for fun, it changes because the aircraft is physically unable to continue operating in normal. It means it has lost the inputs or systems required to maintain normal law. You cannot offer the chance for the pilot to continue in normal law because the ship can't do it! What the aircraft does provide is ample warnings to the pilots of what it's done and what systems are degraded. For emphasis, even after this, the aircraft flies just fine with the same handling characteristics as before. And once again, you don't use the rudder in flight unless you've lost an engine, and even then you always use it with caution, not reckless abandon like in the A300 accident.