During winter (and associated snow clearence holding) 2000ft seperation in the hold is neccesarry some places up in the far north, because turbulence from terrain cause a lot of vertical movement in the stack from time to time.
'Someone' experienced this on a snowy moring many moons ago when TCAS was just introduced:
5 a/c in the stack (yes not much, but still)
Nr.3 from the bottom gets caught in a downdraft, and decends a lot.
Same time, bottom a/c gets caught in the same downdraft.
Nr.2 from bottom get's RA from Nr.3, and Nr.1 gets GPWS climb from terrain below, and climbs, giving Nr.2 RA climb, and Nr.3 RA climb.
Now, nr.4 did not have TCAS.....so it did not go any further.
Still.....
