I think it's in the terminal. Probably in the tunnel.
SOME people (I'm looking at you, loud Essex girl on my flight yesterday) think it's ok to turn it back on as you're coming into land.
I'm sure it's probably not going to cause an almighty crash, but why take the risk eh, when they ask you NOT to? In the case of the lady in question, I'm also pretty sure her fledgling fuck buddy has survived on internet porn and San Marco's pizza in the 3 day absence.