Consider Aircraft A cruising at FL400 and Aircraft B cruising at FL410. Both in close proximity, heading in the same direction. Ignore altitude for direction of flight issue. When aircraft B requests to climb to FL430, would allowing him to do that cause a loss of separation?

I'm not sure if my understanding is correct. In the beginning, both aircraft are in RVSM airspace, have 1000ft vertical separation and are therefor correctly separated. When Aircraft B climbs above FL410, they enter non-RVSM airspace, requiring 2000ft. vertical separation and therefor loosing separation. When Aircraft B is between FL410 and FL420, there is less than 2000ft of separation between them (Aircraft B being in non-RVSM airspace).

It seems really counterintuitive that increasing vertical separation between two aircraft can cause a loss of separation.

I'd love to know the answer to this question for both ICAO and FAA land with 4444 and 7110.65 citations if anyone knows them.


  • $\begingroup$ Not sure about this but above certain altitude some equipment may not work accurately as certified. Hence the need for increase separation. $\endgroup$
    – vasin1987
    Jul 12, 2020 at 16:15
  • $\begingroup$ A very good question, although somewhat theoretical, since IRL two aircraft are not flown on top of each other for extended periods of time for safety reasons, even if the vertical separation minimas are met. $\endgroup$
    – Jpe61
    Jul 12, 2020 at 17:12
  • $\begingroup$ Aircraft B would only be given that clearance to climb if doing so maintains separation. See §7110.65 chapter 5 $\endgroup$
    – dawg
    Jul 13, 2020 at 14:51
  • 1
    $\begingroup$ @dawg That's obvious, but the question is whether separation (as outlined in 4-5-1) is maintained in this specific, peculiar scenario. This is purely a theoretical question. $\endgroup$ Jul 13, 2020 at 22:15
  • 1
    $\begingroup$ @dawg I'm sorry, I should have been more clear. I'd like to find the answer to this question from the perspective of the Air Traffic Controller (which is why I mentioned Doc 4444 and JO 7110.65). I am asking whether it is legal for a controller to issue this climb. $\endgroup$ Jul 15, 2020 at 1:33

2 Answers 2


I think the way rules regarding RVSM are worded in ICAO documents leaves some room for interpretation. When RVSM was implemented, it was probably not very common for aircraft to be able to climb above FL400, so this specific problem was probably not considered when writing the rules. But I would say, when in doubt, use the more conservative interpretaion, which would be that: Yes, this would be a loss of separation.

It sounds weird that increasing the distance between two aircraft can cause a loss of separation, but RVSM airspace is not actually the only place where this can happen. I can think of a few other examples:

  • Two aircraft flying on parallel tracks with exactly 3 NM between. They are flying parallel to an airspace border. On one side of the border, the required separation is 3 NM, on the other side, the required separation is 5 NM (this is quite a common setup, actually). Imagine one of them is flying almost directly on the airspace border. If this aircraft now turns slightly away from the other one, increasing the distance between the two, it crosses the border into the airspace where 5 NM of separation is required. That's a loss of separation.

  • Losing lateral separation when no other separation exist. For example, imagine a visual reporting point 1.5 NM from the runway. In the local rules, it is defined that VFR traffic orbiting overhead this reporting point are separated from aircraft taking off and landing. When an aircraft takes off whild a VFR flight is orbiting over this point, that's all well and good. By definition, they are separation. But if the VFR flight now leaves the reporting point, separation may be lost - even if the VFR flight takes a heading directly away from the other aircraft, so the physical distance is increasing. As the VFR flight leveas the reporting point, the rule about lateral separation no longer applies, so if the flights are closer than the prescribed radar minima, that's a loss of separation.

  • Two aircraft taking off after each other. Initially, the tower controller can visually see both aircraft and ensure separation. When the controller can see both aircraft, normal minima do not apply - it is up to the controller to visually make sure there is no risk of collision. Now imagine the first aircraft climbs into a cloud, so the controller can no longer see it. Even if the higher aircraft is climbing faster than the lower one, so physical separation is increasing, if the required vertical separation does not exist when the controller loses visual contact with the first aircraft, that's a loss of separation.

  • Two aircraft flying on opposite, parallel tracks. The required radar separation is 3 NM for aircraft flying less than 250 knots, and 5 NM for aircraft flying faster than that. Both flying at 250 knots, they pass each other with exactly 3 NM in between, and are now flying away from each other. One aircraft now accelerates to 251 knots, while there is still less than 5 NM between them. Although they are rapidly moving away from each other, that would be a loss of separation.

These are all real examples that, although they shouldn't happen, could result in a loss of separation between two aircraft that were initially separated, while distance between them actually increases. I think the key point here is the fact that in each example, the rules regarding separation minima change, which is what causes the loss. A rule that allowed the aircraft to be relatively close together, for one reason or another, no longer applies, and so we have to fall back to another rule, which may require larger minima.

While this may not specifically answer your question regarding RVSM, I think the exact same logic can be applied. In your example, initially, one rule applies: RVSM, which allows 1000 ft vertical separation. As aircraft B leaves RVSM airspace, the RVSM rule no longer applies, so we have to fall back to the general rules for vertical separation, which says we need 2000 ft. Thus, your example would indeed be a loss of separation.


If the controller is following FLAS - Flight Level Allocation Scheme, which permits the level, then there's no "loss of separation". FLAS typically takes the form of a diagram or table of the FL's that are allowed in a certain FIR or airspace and also the restrictions on direction, and sometimes further restrictions such as timings or Metric level values etc. It lays down exactly which levels are allowed, in which direction etc. rather than using only a description or formula such as "2000 ft separation". You can check your FL in the table, see what it says, and plan or base your tactical request accordingly.

Here's an example from the UK Flight Planning Guide, CAP 694.


OTS above refers to the Organized Track System in special airspace NAT HLA (earlier MNPSA), ie crossing the Atlantic.

Note also that FL430 is 'traditionally' an eastbound level, but as you can see, this has been modified under FLAS

Here's another 'circular' from the AFI (Africa/ Indian Ocean) area that is quite well aligned with the question we're trying to answer.

CIRCULAR 16-2014


The application of the RVSM FLAS in AFI is restricted to FL290 – FL410 inclusive for all eligible aircraft.

The application of RVSM is adequately documented in ICAO Document 9574, Manual on Vertical Separation Minimum between FL 290 and FL 410 Inclusive.

It has been observed, during the scrutiny of Air Traffic Flow Data, that FL420 is being allocated.

It should be recalled that the application of 1000FT vertical separation above FL290 is only applicable between FL290 and FL410 inclusive. The next available flight level above FL410 is FL430 and not FL420. 2000FT vertical separation is applicable above FL410 due to altimetry system inaccuracies.

In the event that two aircraft cross each other, one at FL410 and the other at FL420, a reduction in vertical separation is deemed to have occurred. This event will thus pose a risk to RVSM and en- route safety.

Air Navigation Service Providers and Aircraft Operators must ensure that FL420 is not allocated or utilised.

Further to this the allocation and use of FL420 should be immediately brought to the attention of ARMA for discussion with the relevant parties.

RVSM and en-route safety is of paramount importance.


This is also the end of my answer, but I'm leaving in place the previous write up which is quite accurate except that it did not answer the question asked as I misunderstood the question. I regret the error. (The moderator may feel quite free to remove the portion below the dashed line.)

Apart from the vertical (flight-level), the required separation can also be taken care of in the lateral (cross-track to the left or right), and longitudinal (along-track, going relatively faster or slower, to achieve the needful) and ATC uses these to ensure separation before clearing the climb within, or through RSVM airspace. Depending on the type of facilities/airspace, ATC may use Radar Control or Procedural Control (position reports) to achieve this.

At no time would ATC compromise separation or have 'doubtful' separation. RVSM based separation would be applied to RVSM capable aircraft, and non-RVSM separation between any non-RVSM aircraft (or one that has become incapable of meeting RVSM requirements)

If, as in the example the two are close proximity same direction traffic such clearance will not be given except as described above.

If the request is made in good time, or planned ahead (even in the filed plan), it is more likely that you will get the level without undue delay. Though RVSM was introduced to accommodate more traffic, nevertheless, every possible airway/airspace slot is not necessarily occupied and ATC generally try and make the most efficient use of the airspace so as to accommodate flights as planned, as well as accommodate tactical real time requests. Fuel saving is the usual benefit and reducing the aviation's carbon footprint is in everyones interest so there is no prohibition on requesting climbs.

Such clearances to climb or descend may typically have a restriction reach the cleared level "by time HHmm", or "best ROC/minimum Vertical Spd XXXXfpm" or AT a WPT or crossing a Longitude or at a distance X nm from some reference point and once accepted it is the crew's responsibility to ensure compliance of the instructions. (will try and include suitable references soon, meanwhile @dawg has given a link above)

  • $\begingroup$ All good points, but you seem to fail to answer the actual question: If two aircraft at roughly the same position, following same route/speed, one at FL400 and the other at FL410, would letting the one at FL410 climb to FL430 result in a loss of separation during the climb? $\endgroup$ Nov 13, 2020 at 8:02
  • $\begingroup$ The question is about a situation where the only form of separation available is vertical separation. Try reading OP's example again, it is pretty clear $\endgroup$ Nov 14, 2020 at 9:54
  • $\begingroup$ OK, I get the point, Flight Level planning/availability in many airspaces, is published as a FLAS - Flight Level Allocation Scheme. This takes the form of a table or diagram of values (numbers) rather than a set of formulae. If the FLAS allows FL430 then it's legal. It's like the International Std Atmosphere is actually defined by a table of values rather than rules like "Lapse Rate = 15degC at SL minus 1.98degC per 1000ft altitude" $\endgroup$
    – skipper44
    Nov 14, 2020 at 15:19
  • $\begingroup$ I still think you are missing the point. The question is not about whether FL400 and FL430 are separated (which they obviously are), but about whether an aircraft in the climb phase from FL410 to FL430 would be separated from an aircraft at FL400. Since, during the climb, there would be less than 2000 FT of separation, and the climbing aircraft would be outside RVSM airspace. $\endgroup$ Nov 15, 2020 at 16:38
  • $\begingroup$ What about FL450 and above according to FLAS? Can’t you use it? $\endgroup$
    – pcfreakxx
    Nov 15, 2020 at 21:05

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