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Recently (2018-08-19 17:40 UTC) two planes flew above my city (Poznań, Poland). They were heading in the same direction, kept really close distance (side by side) - both flights to Riga:

Is this common? Why would they do that - are there any benefits of it?

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    $\begingroup$ Related if not duplicate: aviation.stackexchange.com/questions/36782/… $\endgroup$
    – Sanchises
    Aug 19 '18 at 17:58
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    $\begingroup$ Were they at the same altitude within, say, 300 hundred feet? $\endgroup$
    – Terry
    Aug 19 '18 at 18:30
  • $\begingroup$ @Terry I'm not sure but they seemed pretty close on the altitude. $\endgroup$
    – mcjlnrtwcz
    Aug 19 '18 at 18:42
  • $\begingroup$ @Sanchises thanks. Well one of those flights was from Rome, the other one from Barcelona. Both the same airline. $\endgroup$
    – mcjlnrtwcz
    Aug 19 '18 at 18:43
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    $\begingroup$ Closely related, maybe a dupe? $\endgroup$
    – Pondlife
    Aug 19 '18 at 19:14
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BT634 BT684

The top one is Baltic 634, the bottom one BT684.

As you can see they had 2000ft seperation. That's 600m for us europeans not accustomed to aviation.

Furthermore, at 17:40:19 BT634 reported in at 52.409599,16.981899. 17:40:25 BT684 reported in at 52.423222,17.019314. While those are 6 seconds apart, the planes were north-eastern course at the time, so I guesstimate that they had 1-2km horizontal seperation as well.

As to why - they're going the same place. The separation is big enough that there's almost certainly no benefit to it.

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    $\begingroup$ Just for context for the OP: minimum separation in RVSM airspace is just 1000ft, so 2000ft is plenty. $\endgroup$
    – Sanchises
    Aug 19 '18 at 19:53
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    $\begingroup$ @Sanchises well, one could add at that point that if the separation would have been 1000ft, the two aircrafts would have been travelling in opposite directions $\endgroup$
    – Federico
    Aug 19 '18 at 20:09
  • $\begingroup$ @vidarlo thanks for the thorough answer :) $\endgroup$
    – mcjlnrtwcz
    Aug 20 '18 at 5:42
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Flights generally do not travel directly from origin to destination along the shortest (great circle) path; they do estimate that but then adjust it so the actual route filed passes over various airports and navaids along the way in a giant game of "connect the dots". There are only so many dots to connect, though, so the closer you get to a destination, the more likely any two planes' routes will converge--even though they started from completely different places.

It sounds like these two planes'' routes converged and by coincidence they just happened to get to that point at roughly the same time, but since they were at (or ATC moved them to) different altitudes, that's totally fine. Closer to their destination, ATC will start to give them slightly different turns to separate them properly for safe descent.

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