Separation in stack
 

One for the experts: what's the minimum distance allowed between planes in a stack?

I ask because this evening, as I was watching the little specks whizzing past on the other side of our holding pattern I noticed a Lufthansa flight, in a layer a little above us really sprinting round. A couple of laps later and it passed over, perpendicular to us. It was close. I could clearly see the details on the wings, the windows, I could even make out a face pressed agaisnt one of them. Spreading two fingers on my window to measure, I'd say i took up 1.5 inches at a close viewing angle. How close is too close?

1000ft vertically, 3 miles horizontally.

I can't judge the vertical, but the horizontal was definitely a bit dodgy... thanks for the info :)

Its 1000ft OR 3 miles in the UK - doesnt have to be both. If you are holding further out though and talking to an Area Controller rather than a Terminal or Approach controller, it's 5 miles rather than 3. I know what you mean though, i've seen aircraft in the stack before that seem a lot closer than they really are especially if you're both in the turn in a 747 and what seems like horizontal separation is really vertical if you understand what I mean...