M Mouse
Showing my inexperience here, 5milesbaby hinted at it but you have confirmed it for me. I never knew that a rate of descent was used only in Maastricht, not having visited too many other centres. It is taught to us through abinitio training and OJT that it is an easy way to assure a certain flight level at a certain time. The fact that we are the only people who use such a clearance would tend to explain the confusion it sometimes causes.
To answer your question, why do we use it? Difficult to explain without a radar picture in front of me. I do not use as a rule, I am aware that you guys have speed/descent profiles to adhere to as much as possible, and although I do not know the intricacies of your job or of aircraft handling, I know that vertical movement affects speed. So I try whenever possible to leave it in the hands of the pilot, using
"descend when ready to be level in x minutes/x miles before xyz"
But it is getting busier up there, and such a clearance is not always possible. Lets take a hypothetical situation. I have an inbound at F380, which I need to be level F260 in 8 minutes, in order to comply with an LOA restriction. He has just cleared traffic at F370, but still has opposite, crossing in 2 minutes at F350. So I give descent F360, expect to be level F260 in 8 mins. Crossing his track are two flights, F300 in 6 mins and F270 in 8 mins.
This would not be an uncommon scenario.
I
could give a step descent, get a lower level, say F230, and after crossing the guy at F300 descent to F230 to cross F260 in 2 mins. Lots of work, lots of monitoring, and several descent clearances for one of maybe 30 aircraft on my frequency.
Or, I could put three aircraft on headings, all needing own nav later, if traffic allowed this, and if military status allowed this. This would require six transmissions for hdg/own nav, and six readbacks.
OR I could simply give the following clearance, after the target clears the opposite tfc at F350
"Continue descent F260(or lower if arranged) rate of descent 2500fpm or greater"
Now, this is the crux of my question.
IF the pilot does as I intend, and descends with a rate never falling below the given rate, then I have my separation with all traffic with just one simple clearance. Efficiency achieved, I can move on to other problems.
However, if the pilot takes option two, ie: 2500fpm for 4 mins = 10,000ft,so I'll do 1500fpm for 2 mins then increase to 3500fpm for 2 mins, there goes my separation, pass me the incident report forms Mr. Supervisor.
And that is why I am asking this question. I realise that the clearance may be restrictive, and
notsofantastic I am reassured to read your last post on this thread, but sometimes we have to be restrictive in order to assure safety. However if my clearance can easily be misunderstood, then maybe I should change the phraseology I use, and suggest the training department do the same. Maybe iso "rod 2000fpm or greater" I should say "continuous rod 2000fpm or greater.
So, clear as mud?
Thx again for your replies...