Originally Posted by PBL
What you say is of course correct when considering "command" and "monitor" channels inside one of the boxes.
My comment was addressed to discrepancies *between* two (or more) of the ELACs; or SECs; or FACs, and I didn't make that clear.
There are three SECs: they could presumably vote. But there are only two ELACs (FACs are less critical). What happens when they disagree? I don't know.
I can answer that at least partially.
The FACs, SECs and ELACs in the A320 do not in all cases control exactly the same control surfaces, and they never control the same surfaces at the same time through the same hydraulic system. There are three different strategies for dealing with failures:
1/ One computer only controls a fixed set of surfaces, with no overlaps:
- SEC 1 controls spoiler surfaces 3 and 4, SEC 2 control spoiler surface 5 and SEC 3 controls spoiler surfaces 1 and 2, on each side.
If one SEC fails, the respective spoiler surfaces can no longer be controlled and are retracted.
2/ One computer controls the surfaces, and there is a reversion priority if that fails. Different hydraulic systems are also used:
- ELAC 1 normally controls the ailerons, if it fails, ELAC 2 controls the ailerons. If both fail, the ailerons revert to damping mode.
- The reversion priority for the elevators is: ELAC 2, ELAC 1, SEC 2, SEC 1.
- for the rudder it is: FAC 1 -> FAC 2.
3/ multiple computers control the same surfaces, but through different hydraulic systems:
- SFCC 1 controls the slats through Blue hydraulic and flaps through Green hydraulic system, SFCC 2 controls slats through Green and flaps through Yellow hydraulic system.
(Source: FCOM 1.27.10, P 5, SEQ 100, REV 24)
So, for the infamous Lufthansa flight with wrongly wired sidestick, only ELAC 1 will control the ailerons with the wrong sese, and although the spoiler-roll-function will work in the correct sense, at low speeds, the ailerons are more effective than the spoilers, and roll-effect will be reversed.
Bernd