TC pretty much covered it,
6k alt limit is a hard ceiling, the terrain has an inbuilt cloud layer at this point. If you go over it your out of the round as you broke that rule.
This can be achieved in one of two ways the pilot admits his error (his plane starts getting pushed and pulled all over by the wind vortex that is set >6k) and bails out. A quick "I went over" on vox would help everyone understand what happened.
The host grounds you because you were over the limit for what is deemed to be long enough to cause concern to your opponent. IE you come hurtling down out of the cloud at a tremendous speed and continue to fight. I was watching this fight in god mode and did ground you as explained at the time. Don't feel bad about this because it does happen often when people aren't used to the rules and get lost in the moment and go over the cap. The grounding was removed instantly after so you can continue to play in the following rounds.
As for the Euro KOTH I wish Nef hadn't referred to it as such as it has caused 2 different players to question whether they are allowed to play. ANYONE can play KOTH whatever time it is held!
Look forward to seeing you all this Saturday for MrKrabs first KOTH as host. The evening slot at 8pm EDT / 1am BST. The fights were excellent thanks to some good plane choices and I'll post up the official results this afternoon when I have my notes infront of me.
