Originally Posted by
Dr.Triax
In holding patterns radar separation is not used, rather procedure control. So if an A/C is cleared to FL120 from (let's say) FL160, unless he reports (asked or not by ATC) that a FL is vacated, ATC cannot clear another traffic to that FL.
In a holding:
ATC: BAW123, descent FL120, report leaving FL160.
BAW123: Cleared FL120, will report leaving FL160. BAW123.
[...]
BAW123: Passing now FL160 down to FL120, BAW123.
ATC: Roger, BAW123. Break. AEA456, descent now FL160.
Hope the example helps.
I don't know where Triax operates but in the UK Mode C is definitely used. When I'm holding I wait for the aircraft to have vacated the level by 400ft + some common sense before clearing the next aircraft to that level. I.e. I know the rate of descent when descending 1000ft within a hold is going to be pretty low so I'll either check the rate is reasonable enough once 400ft has passed or wait for 5-600ft to have passed before clearing the next one down. I certainly never ask a pilot to report leaving/passing a level, that's way to time consuming not to mention relies on him/her remembering.