Separation in holding pattern
 

Hi,

My question is: In holding pattern if A/C A, at FL 130, cleared to FL 120 is currently passing FL 126 can I clear A/C B, currently at FL 140, to FL 130?
My reasoning was, since FL 130 is vacated (A/C A at FL 126, descending to FL 120) there is no problem with descending A/C B from FL 140 to FL 130.
All that in UK ICAO airspace.

It is also correct (I think) according to MATSpt 1, Section 1, Chapter 3, 5B.

Was I right / wrong, and why?

Thank You for any help.

When I was last controlling, the scenario you describe was basically correct. I certainly used it myself, but a/c A had to actually report leaving FL130 before a/c B could be cleared to leave FL140. Your post reads as though you are using Mode C to obtain a/c A's level as FL126. Don't know how modern technology has progressed, but in my day using Mode C in the congested area of a hold was fraught with danger due to garbling etc.

https://cimg8.ibsrv.net/gimg/pprune....bc05a5c2ee.jpg

DOC 4444
5.3.4 Vertical separation during climb or descent 5.3.4.1 An aircraft may be cleared to a level previously occupied by another aircraft after the latter has reported vacating it, except when: a) severe turbulence is known to exist; b) the higher aircraft is effecting a cruise climb; or c) the difference in aircraft performance is such that less than the applicable separation minimum may result; in which case such clearance shall be withheld until the aircraft vacating the level has reported at or passing another level separated by the required minimum. 5.3.4.1.1 When the aircraft concerned are entering or established in the same holding pattern, consideration shall be given to aircraft descending at markedly different rates and, if necessary, additional measures such as specifying a maximum descent rate for the higher aircraft and a minimum descent rate for the lower aircraft should be applied to ensure that the required separation is maintained.

MATS pt 1.
5B. Changing Levels 5B.1 Aircraft may be instructed to change level at a specified time, place or rate. 5B.2 An aircraft may be instructed to climb or descend to a level previously occupied by another aircraft provided that: (1) vertical separation already exists; (2) the vacating aircraft is proceeding to a level that will maintain vertical separation; and (3) either: (a) the controller observes that the vacating aircraft has left the level; or (b) the pilot has reported vacating the level. 5B.3 If severe turbulence is known to exist, instructions shall be delayed until the vacating aircraft is known to be at, or through, another level separated by the required minimum. 5B.4 Controllers shall exercise caution when instructing an aircraft to climb or descend to a previously occupied level. Consideration shall be given to the fact that aircraft may climb or descend at markedly different rates and, if necessary, additional measures such as specifying a maximum or minimum climb or descent rate for each aircraft shall be applied to ensure that the required separation is maintained. This is particularly relevant when the aircraft concerned are established in the same holding pattern.

So there is nothing stating that can not be done.
Only that "extra caution" should be given...

Can anyone confirm or deny that this descent CAN be made?

Cheers

They'd be up there all day if the 400 feet wasn't used...

If you're Aer Lingus, then you might find another company aircraft at the same level with a similar trip number; it happened at BNN back in about '71 but both pilots had the other one in sight so they just spaced themselves to be at opposite ends of the hold before mentioning it to ATC.

Let us hope that that last comment, irrelevant to the original question and questioner, is not misinterpreted.

2 s

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.

Well, the phraseology doesn't! :-)

2 s

Could you point me to documentation that states you are not allowed to use Mode C information to provide vertical separation in a holding pattern?

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.

As long as you make sure you don't loose your 1000' separation minimum.

If you loose it, you're in for a report, cause you were not "cautious" enough.

You could use a descend rate to be dead sure, but I've never used that myself in a holding??

Having aircraft report leaving/reaching is kind of procedural.... only use that one myself when we have no RADAR, though I rarely see any idea in having aircraft reporting leaving... reaching or passing is more convenient.

400 feet and continuing in anticipated direction
 

When using mode C to assertion whether a level is vacant or not 400 feet seems sensible. An aircraft is occupying a level if it’s within 200 feet so to stop a mid air collision adding another 200 feet and continuing in the anticipated direction seems a good idea. Using pilot reports of vacation are inherently fraught with danger as it requires positive action from the pilot who might forget to execute the manoeuvre.
The main thing to remember from an ATC point of view is that you have to monitor the mode C read outs and ensure separation is never less then 1000 feet.

In what context?

2 s

For those advocating using Mode C in a holding pattern: go back to Post#3 and tell me how you can make sense of each aircraft's level in that jumble.

By reading the FL column in the centre column (albeit derived from Mode S rather than Mode C), perhaps ?