The separation minima in Europe are in general 5NM lateral or 1000ft vertical. If the safety distance is too small, there is a conflict (loss of separation). When both aircraft are on the same track, the adherance to the minimal separation is relatively easy. But if two trajectories cross the same point horizontally, for example over a beacon, the adherance to the separation minima seems not that easy anymore. The same is true for ascending or descending flights which burst through different flight levels.

Is the adherence to the separation minima for IFR flights enough to avoid airborne conflicts for crossing/ascending/descending flights, too?

If not, what is the minimum distance between two flights in space and time, that precludes all the different possible conflict types [page5]? For example there are local conflicts, which occur, when two aircrafts cross a given point from different directions in under 4 minutes.

This questions refers to the european airspace at best. See the linked question How much is the minimum safe distance between two planes in-flight?

  • $\begingroup$ I'm not sure I understand your question. If you always ahere to the separation minima in all phases of flight then there can't be a conflict (ignoring the issue of VFR traffic). And why would it be difficult to keep separation when flights cross the same waypoint? That's what the vertical separation limit is for. I'm probably misunderstanding your question, but it would be great if you could clarify it a bit more. $\endgroup$
    – Pondlife
    Nov 13, 2014 at 14:58
  • $\begingroup$ The separation is 5 miles, independent of directions. But then I don't really understand what the local conflict definition is even about. And on the other hand TCAS may give RA at more than 5 miles (about 9, in fact) if the aircraft are heading in opposite directions, because it is defined by time (40s). $\endgroup$
    – Jan Hudec
    Nov 13, 2014 at 15:04
  • $\begingroup$ I am confused about the question as well. If the question boils down to: Is applying IFR separation enough to avoid airborne conflicts, then the answer is: Yes. That is what IFR separation is intended for... Can you rephrase your question, maybe into simpler terms or clarify what you are asking? $\endgroup$ Nov 13, 2014 at 15:07
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    $\begingroup$ You need to keep in mind that separation is 5nm OR 1,000ft, not AND. ATC is trained to apply either of the two or both, even when giving flight level/altitude changes or headings. $\endgroup$ Nov 13, 2014 at 16:30
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    $\begingroup$ I don't understand what you are asking. Your references are a scientific paper and wikipedia. While they may seem reliable to you, they are not the sources from which Air Traffic Control is defined. Please look at ICAO PANS ATM (Doc 4444) to get an idea of how separation is provided. $\endgroup$
    – DeltaLima
    Nov 14, 2014 at 7:07

3 Answers 3


This might not be the answer to your question, but maybe it will help you understand separation.

Your question specifically asks for how separation is ensured between climbing, descending and crossing traffic over navigational fixes (NAVAIDs).

Flight routes are planned for traffic to use certain flight levels en-route, which follow the semicircular/hemispherical rule in effect in that country or are bound to the airway being used. In my example picture below I will use the German semicircular rule of eastbound flights using odd flightlevels.

Two flights crossing over HMM VOR with vertical separation
(Image Source: Own Work - Program used: Aircraft Situation Editor (ASE))

Both aircraft will cross the HMM VOR roughly at the same time, based on the ground speed displayed. The westbound aircraft (DLH123) is at FL200 (even FL), the eastbound aircraft (BER456) is at FL210 (odd FL). The separation between both aircraft overhead HMM VOR will reduce to 0nm lateral separation, but 1.000ft of vertical separation, and thus, the separation criteria are met.

Two flights crossing over HMM VOR, one intended to descend
(Image Source: Own Work - Program used: Aircraft Situation Editor (ASE))

In this example, I need BER456 to descend to FL100, but I have crossing traffic from the right which would cause a cleared conflict, if I issue a descent clearance directly to FL100 without any conditional instructions. I can use a bit of mathematics here to ensure separation however and issue a clearance nevertheless.

Both aircraft are travelling at 420kt GS. An aircraft will also descend 1.000ft in 3nm, rule of thumb from every flight instruction book available. To ensure 5nm of lateral separation, I would need a buffer of 2.000ft during descent to the crossing traffic, not factoring in that the other traffic will be on a divergent track anyway. Let's just measure the distance to the HMM for simplicity.

My instruction to BER456 would be:

R: BER456, when ready descend FL100, cross HMM VOR at FL230 or above (rate of descend 2.000ft/min or less)

With this instruction, I can issue a clearance to BER456 that will allow the aircraft a continuous descent to FL100, while ensuring minimum separation is kept to the other traffic. Overhead HMM VOR, both aircraft will have 3.000ft of separation. After BER456 has descended a further 2.000ft, it will have 6nm and 1.000ft of separation, making it well clear of a conflict, especially since the traffic is divergent.

The dirty solution would have been to clear BER456 to FL210 and after 5nm issue another clearance to FL100, but this doubles the frequency time and also interrupts the descent for BER456.

  • $\begingroup$ Thank you for this answer and the insight into the conditional clearance! Unfortunately, I can't upvote anybody yet. To be a bit more unspecific, I assume separation between climbing, descending and crossing traffic aside from NAVAIDs has to be guaranteed, too? In your example, BER456 crosses other flight levels too and there could be - theoretically - some more traffic... $\endgroup$
    – JaBe
    Feb 19, 2015 at 14:30
  • $\begingroup$ @JaBe In general, the separation minima need to be maintained, using NAVAIDs is just a tool. There is one possibility for descending and climbing traffic, which depends on jurisdiction, which is during daytime, in VMC: You can transfer responsibility for separation to the pilot and have them climb or descend through another aircraft's level. This is also called visual separation in some documenation. If you cannot upvote, you can still mark as the answer to your question. ;) $\endgroup$ Feb 19, 2015 at 14:35
  • $\begingroup$ I see you edited your comment: ATC uses radar to maintain separation, so when issuing descend or climb instructions, ATC needs to make sure that no traffic will be within 5nm of the descending or climbing traffic on the same level. $\endgroup$ Feb 19, 2015 at 14:44

In the ICAO PANS ATM (Doc 4444) (*) the vertical separation is specified as 1000ft (or 2000ft):

5.3.2 Vertical separation minimum

The vertical separation minimum (VSM) shall be:

a) a nominal 300 m (1000 ft) below FL 290 and a nominal 600 m (2000 ft) at or above this level, except as provided for in b) below; and

b) within designated airspace, subject to a regional air navigation agreement: a nominal > 300 m (1000 ft) below FL 410 or a higher level where so prescribed for use under > specified conditions, and a nominal 600 m (2000 ft) at or above this level.

The minimum horizontal separation mimumum is 5 NM for flights under radar control:

8.7.3 Separation minima based on ATS surveillance systems Unless otherwise prescribed in accordance with, or, or chapter 6 (with respect to independent and dependent parallel approaches), the horizontal separation minimum based on radar and/or ADS-B and/or MLAT systems shall be 9.3 km (5.0 NM).

The approach and departure phases of flight has some small deviations, e.g. small aircraft behind heavy aircraft have to keep 6 NM horizontal separation because of wake vortexes.

(*) Unfortunately I couldn't find the official Doc 4444 on the ICAO website.

  • $\begingroup$ Doc 4444 is a document that you need to buy from ICAO. For $350 you can download it from their website. $\endgroup$
    – DeltaLima
    Nov 14, 2014 at 16:38
  • $\begingroup$ Good to know, thank you! $\endgroup$
    – JaBe
    Nov 14, 2014 at 16:39
  • $\begingroup$ In Europe the standard minimum radar separation is 5 NM en-route. In many Terminal Manoeuvring Areas, it is 3 NM. On final approach it can be 2.5NM or even less when on approach to parallel runways. Of course this increases the requirements on the surveillance systems (higher accuracy, integrity, update rate and probability of detection). $\endgroup$
    – DeltaLima
    Nov 14, 2014 at 16:59

The separation of 5nm (Surveillance system based) or 1000ft vertical is the minimum and safe separtaion between two aircraft in any configuration. Though it needs to be understood that at least one of these separation need to be maintained at all points of time.

If there is likelihood of breach of one kind of separation in near future as in the case of converging traffic at same level, Air Traffic Control has to initiate action well in advance to ensure that other kind of separation is established before the existing separation is breached.

So the question of TCAS warning does not arise.When the process for established other mode of separation is to be initiated is the whole job of ATC. That is what Air Traffic control is all about.


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