Originally Posted by
CONF iture
IF789,
I think you put things far more complicated that they need to be :
When there is a known problem on the data or one is suspected, just stop the Magic and reverse to the conventional aircraft in Direct Law.
Did you say
KISS BOAC ?
That would be far more simple, indeed:
- for the engineering team to implement
- for the pilots to "understand"
Now, Airbus made a different "philosophical" choice. "Why" is a good question.
A guess: Alternate law retains "parts of" automation. What would we say if there was an accident and we found out that the accident could have been prevented by a protection/automation that was made inop because of a reversion to direct law, while the input data needed for this protection were available & reliable.
Exemple : Let's imagine AF447, the aircraft stalls but then the crew engage a recovery : nose down, AoA is lessened, speed come back. At this point, the PF pulls on his stick to go back to level flight
but he pulls too hard.
If reversion to direct law => overstressed the plane => structural failure => irrecoverrable => crash.
If reversion to alternate law => g protection still available => no overstress => plane recovers => continue flight => lands with all souls onboard alive.
See the point? That's about "graceful degradation" if I understand the concept correctly.