Hi PA 18 151
Originally Posted by
PA 18 151
AlphaZuluRomeo:
Consistent AND valid = Correct
My understanding of the ALT2 being latched is that as soon as you've got an ADR disagree, the system cannot be sure that the equation you write above is true.
Why ? Well, imagine an aircraft in an ash cloud :
- at first, pitots (front end) are clogged => speeds decreases, but not exactly at the same time => ALT law
- then, out of the ash cloud, the ashes move in the pitots. The front end became unclogged, but then the remaining ashes clogg the drain hole of, say, two of the three pitots => those two pitots give consistent and valid speed. But that speed is uncorrect (too high, because of the drain hole clogged).
If the system goes with this (incorrect value) to revert back to normal law, it's kind of dangerous, cause it may trigger the overspeed protection (climb order) when not needed/relevant.
Therefore I think the logic is to latch the ALT law, until the aircraft/system (probes, computers...) can be fully checked, on the ground.
The rule would be some kind of : ("you" being the computers)
"As soon as you've got a problem with the probes/ADR/speed calculation, you cannot be sure the speed you sense/vote/choose/know is indeed correct. Then you have to let the human crew in unrestricted control, via the ALT law being latched".