Loss of separation or not?

[woocash]

Airspace:

GND-FL095: class G
FL095-UNL: class C

a)
ABC123: maintaining FL100, IFR
DEF456: maintaining FL95, IFR

b)
ABC123: maintaining FL100, IFR
DEF456: maintaining FL95, VFR

No horizontal separation in both cases.

[Andy Mayes]

Which state are you referring to?

[eagleflyer]

a) Not so sure since we don´t have this particular problem here. Since G is uncontrolled (do you know which IFR traffic is using it??) it should be no loss of seperation, shouldn´t it?

b) definitely no loss of seperation

[woocash]

If it helps, i'm referring to Polish airspace.

I understand that in b) we can say "no loss", because even in theory, in class G, there can be anything flying even without a transponder.
But if the VFR traffic has the transponder, and IFR is TCAS equipped - RA will occur, right?

[woocash]

a) Not so sure since we don´t have this particular problem here. Since G is uncontrolled (do you know which IFR traffic is using it??) it should be no loss of seperation, shouldn´t it?

I do understand the logic, but on the other hand we explicitly have to provide 1000ft of separation for an IFR traffic in class C.

[2 sheds]

For the traffic at FL95, the rules of the lesser class of airspace apply, i.e. it is considered to be in Class G. (However, if IFR it should not be in level flight at FL95, surely?)

2 s

[ron83]

As said above, according to IFR FL table DEF456 shouldn't be at FL95 in a).
But I'd say that it is loss of separation in both cases from ABC123 perspective,since he is in "C" and shall be separated both from IFR and VFR.:rolleyes:

[Plazbot]

Sheds and Ron, depends on the country. I can think of three different rules in three different countries and one has the answer as both yes and no.

[ron83]

Absolutely,no doubt in that. But should it be that way?
We have same boundary between G and C in our CTA,so as seen in this topic, it leaves place for different interpretation.
In our APP sector, however, MRVA's introduced. We have G at 1500' and 3000', MRVA' set at 2500' and 4000' accordingly to keep at least 1000ft from traffic in G.

Not to open separate topic another example:

CTR ( class C): SFC-2500 ft. MRVA-1500 ft. Around CTR class G up to 1500 ft.

Aircraft (A) inside CTR at 1500 ft flying along boundary of CTR, Aircraft (B) flying at 1500 ft on the other side of boundary in G.
Loss of separation? Or separation is the thickness of boundary line on Controllers display?:E

[renard]

From a pilot point of view, if both aircraft are in level flight then there won't be an RA. We would get a TA "Traffic Traffic".

Flying out of London City this morning at 3000' IFR, we had a light a/c at 2,200' VFR (?) pass pretty close to use horizontally. I think the base of the London TMA there is 2,500'.

We had TA but no RA.

[Spitoon]

If we approach this from first principles the first point to think about is what the rules are and ICAO Annex 11 in the Note to para 2.6.3 says...Where the ATS airspaces adjoin vertically, i.e. one above the other, flights at a common level would comply with requirements of, and be given services applicable to, the less restrictive class of airspace. In applying these criteria, Class B airspace is therefore considered less restrictive than Class A airspace; Class C airspace less restrictive than Class B airspace, etc.
Next comes what traffic has to be separated. Here, PANS-ATM (in para 5.2.1) says 5.2.1 Vertical or horizontal separation shall be provided:
a) between all flights in Class A and B airspaces;
b) between IFR flights in Class C, D and E airspaces;
c) between IFR flights and VFR flights in Class C airspace;
d) between IFR flights and special VFR flights; and
e) between special VFR flights, when so prescribed by the appropriate ATS authority....Just in case there was any doubt, note that there is no requirement to separate anything from traffic that is flying in class G.

It's a strange thing - and nearly 35 years later I can still clearly remember the college lesson where I learned it - but all traffic inside controlled airspace is separated from anything outside. This is an axiom that appears to be commonly held by controllers wherever I go. Of course, in all my years as an operational controller I usually tried to build in some distance between aircraft in such situations, even if not quite the same distance as standard separation - and, once or twice, I have to admit that I used the inside vs. outside point when things got a bit closer than intended.

So, to answer the OP's question - in both examples a) and b) there is no loss of separation (because there is no separation required). Whether it is a good idea to allow such a situation to occur is a different matter entirely - at the very least I would pass traffic information to the two aircraft involved.

As Plazbot points out, the interpretation of the rules can differ in different countries and I, too, have seen three different ways that this is handled. In each case I have seen the differences are not in what is separated (or not) but rather the national procedures about what levels can be allocated in relation to the airspace boundary (or, horizontally, how close an aircraft can be vectored).

And that's before you consider the UK rules which allow aircraft to be 'controlled' outside CAS.

One last thought - although I'm sure it's fully understood. renard talks about TAs and RAs - ACAS/TCAS has little to do with whether separation exists but is intended only to try to prevent a collision between aircraft which are on converging trajectories.

[Not Long Now]

Just to add spice to the mix, what if the lower were at FL97, assumed to be at FL95 +/- tolerance off 200', and the upper at FL98, equally within tolerance? 100' is enough then...

[Spitoon]

It's legal. Spicey, but I'd still say legal. But if it were me I'd have done a bit more - some spices are just too hot.

[1Charlie]

No separation required in either. But IFR vs IFR requires tfc info when they're cruising within 2000' of each other or tracks laterally separated by less than 20nm.

[mcstiofan]

Here's a question since the topic is at hand...and hopefully this isn't too far off it :)

I never understood how the heck these splits are meant to work.

Such and such AIP says "Class blah1 till FL such and such and class blah2 from such and such onward". So who owns "such and such"? Is there some very legal definition that's hidden someplace or that most folk aren't aware of. Does one own it by definition, unless stated otherwise?

Same goes for sectors. For the sake of the example, LO SFC-FL245 and HI FL245 - So WHO owns FL245?

By my interpretation, in your example, even if FL95 is also meant to be class G, if CTA starts at FL95, isn't the guy within it at the same time...and shouldn't he thus be talking to you?;)


I'm very eager to hear what those in the industry have to say about this, cause I sure as heck can't find the definition anywhere(If there is one, I'd love a reference).

Multiple thanks!

[woocash]

In one of the LoA's between my ACC and another FIR you can even find things like:

The airspace is divided vertically into sectors:
Low FL095-FL280
Middle FL290-FL330
High FL340-FL460
Now You tell me who is responsible for traffic climbing through FL 335 :)

[mcstiofan]

From my understanding, unless otherwise agreed upon, the lower sector always goes up to his limit and the higher one has everything above, starting immediately above, but not including the top of the lower sector's ceiling. Essentially, the low guy is given an upper limit and the high is above, rather than the high having a low limit and the low going up to, but not including that limit. Of course there are exceptions.

So in your case, the high would have FL335.

The way this seems to work, is that the strata values are defined based on aircraft separation, rather than being perfectly specific. It's a funky way, but it seems to work....until you throw a wrench in the works, but having to chunks of the sky have the same level :)

[theresnospeed]

This is not a loss of separation. Anyone is allowed to cross at the base of an airway where the base is defined as a Flight Level. So it's Class G, so no sep required.

Similarly, where the base of CAS if defined as an altitude, say 2.5A, it's perfectly fine to be in Class G at 2.4 only 600ft away from an aircraft at 3.0A in Class A.

[10W]

Just to add spice to the mix, what if the lower were at FL97, assumed to be at FL95 +/- tolerance off 200', and the upper at FL98, equally within tolerance? 100' is enough then...

Then the lower has infringed Controlled Airspace by 200'.

For the UK, I would argue the tolerance is for confirming a reported level when checking Mode C, or for level occupancy using Mode C for validated and verified aircraft, not an accuracy tolerance for assuming someone is clear of Controlled Airspace. If the aircraft is unknown and unvalidated/unverified, I now have to treat it as an infringer and try to acheive 5NM or 5000' separation against it. I can't say 'Oh, he was within +/-200' of the base on Mode C, so I'll just ignore it' unless I want to get a few weeks holiday on suspension .... mmmm, with this weather, there's an idea ;)

[frontlefthamster]

Separation is not quite like pregnancy. It depends upon circumstance.

The terms under which separation should be achieved are defined, and by those terms, it may be achieved.

There is a crucial difference between separation and the assumed minimum miss distance, which will be less for an aircraft under IFR in class A at FL100 where the base of CAS is FL 95 than the aircraft 1000 ft above it.

Spitoon, the relevant CATC lecture hadn't changed when I went through, albeit not quite the glorious 35 years ago that you mention.

Either way, what is the relevance of the OP's question, in the operational world?