There's is s combination of things going on with a 1 & 4 failure.
Firstly though a single system 4 failure requires the speed brake to NOT be armed as without the inboard spoilers (5,6,7 & 8) the outboards will cause a pitch up upon touch down / activating.
Now a double failure 1 & 4 leaves a risk of the aircraft going into ground mode in flight as there's no hydraulics keeping the gear tilt in check.
This is where the nose gear gets involved, if it's compressed then it must be on the ground (there's no tilt to worry about) The speed brake is limited to flight mode because of the possible faulty main gear tilt indications, so that's why it won't go past half way on touch down but is free to go the rest of the way upon nose gear compression.
Reverses are linked to the nose as well to ensure they aren't unlocked because the main gear tilt attempts to say the aircraft is in the ground.
So to put it simply without 1 & 4 gear tilt is no longer used for safety critical ground sensing some other things may still go into ground mode in flight like flight idle may go to ground idle.
Last edited by CCA; 14th Sep 2020 at 06:03.