surplus1, re #3147.
AFAIK there is no ‘big switch’ enabling the pilot to select Direct Law. However, it appears that the law could be invoked by selecting off an appropriate selection of sensors (e.g. IRS) or flight control computers, but I don’t know the exact details.
Undesirable flight states; stall appears to be prevented in Alternate law (ALTN 1) by the low speed protection, IAS replaces the AOA function, but in this accident it appears that there was no IAS.
AFAIK this gives ALTN 2 law – no speed protections; I don't know about any alerts.
The architecture of the A330 control system provides an adequate level of ‘fail safe’ for aviation, as defined by aviation requirements – based on experience. In this sense Airbus wasn’t the first to test these regulations as there were many test aircraft and military experience beforehand.
Many people appear to have overlooked the point that when the A330 is flown direct law it is essentially the same as the older generation aircraft excepting that, like them on occasions, you might lose VMO/MMO and Stall warnings, and if applicable stick push; in which case you took extra care at low speed.
The one significant difference that I am pursuing is the trim operation. In direct law it is via the hand wheel – the old way, but in ALT law the trim operation appears to be in some quasi state attempting to operate normally (auto-trim / follow-up trim), but not associated with the of the loss of protections, and thus may be ‘auto’ active in a stall. Furthermore, the crew may unaware of this (isolated from trim activity).
Re pilot interface with automation; one of the major issues is the crew’s understanding of the differences in the levels of automation – what are the crew responsible for and when, and then relating these states to a previous or clearly understandable method of operation; but rule 1 still applies – fly the aircraft.
I agree that nothing should be ruled out or in. We all appear to have crossed the bounding rules. The search for understanding is perhaps better achieved by questioning, opposed to statements of ‘fact’ seen elsewhere in this thread.