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I have a question - how close are planes allowed to fly when their paths are about to cross? I am sure there are proximity warnings and what not but I also know those accidents happen, even if not that often.

I was on board of a KLM flight from Amsterdam to Madrid and sometimes in the later part of the flight, if I recall correctly, another plane, flew just below us. Of course, I imagine human vision, mine included, is probably not the most reliable source. However, I saw the plane and in less than 1 second it speeded right below us, so I imagine that it was quite close. However, I felt no turbolence after it passed.

Given that description, can you let me know your thoughts - how close in reality it was? Of course if it helps, I can give you the flight number and day.

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  • $\begingroup$ This looks like a dupe of this question, at least if it's about European regulations. However, this question has an answer, the other one doesn't. $\endgroup$
    – Pondlife
    Commented Apr 21, 2018 at 21:39
  • $\begingroup$ @Pondlife I marked the old question as a dupe of this one per numerous answers on meta $\endgroup$
    – TomMcW
    Commented Apr 22, 2018 at 17:36

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Your plane was at least 300 meters above the aircraft you saw.

The limits are determined by the altitude and capabilities of the aircraft.

  • Up to 29,000 ft - 300 meters vertical separation
  • 29,000 ft and above - 600 meters
  • 29,000 - 41,000 ft (where there is RVSM (Reduced Vertical Spacing Minima)), the minimum separation is 300 meters, but the aircraft have to be equipped with specific instruments. Most modern airliners are equipped to fly in RVSM approved airspace.
  • 41,000 feet - 60,000 ft, 300 meter RVSM
  • 60,000 feet and above, 1.5 KM irrespective of RVSM

You won't feel turbulence unless you are flying behind an airplane, or you fly into an airplane's wake. Turbulence happens in disturbed air and this is usually behind the airplane's wingtips.

All modern airliners are equipped with TCAS (Traffic Collision Avoidance System); which consist of visual indicators on the relative position of nearby aircraft.

Here is an image from flightgear wiki that shows how this looks on some modern airliners:

enter image description here

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    $\begingroup$ Hm, that TCAS example does not make much sense. If the plane is 1300 ft above and climbing, it should not be threatening, since that would require you to be climbing by about 2000 ft/min faster and that's extremely unlikely. $\endgroup$
    – Jan Hudec
    Commented Apr 21, 2018 at 21:30
  • $\begingroup$ I believe it is just for illustrative purposes and not an actual screen capture. $\endgroup$
    – Burhan Khalid
    Commented Apr 22, 2018 at 2:59
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No closer than 500 feet above or below, and more likely 1000 feet if both were commercial flights. Side by side, they'd be much farther apart. At airports with parallel runways, both are used by the planes are staggered in time so they don't end up side by side.

You wouldn't feel turbulence from a plane below you. There might have been some turbulence for him from your plane's wingtip vortices descending if they passed behind your plane's wings.

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  • $\begingroup$ 500 feet is about 150 meters, or half the minimum mentioned by Burhan Khalid. Since this is presumably about commercial flight (based on OP's mention of being aboard a KLM flight), it's likely a fairly safe bet that OP's flight was in controlled airspace at the time; can you specify where in controlled airspace a vertical separation of ~500 feet would be allowed, ideally with reference to Spanish or EASA regulations? $\endgroup$
    – user
    Commented Apr 22, 2018 at 11:04
  • $\begingroup$ I cannot. I was thinking VFR traffic flying below IFR traffic, VFR at east/west thousands + 500ft, IFR at east/west thousands. $\endgroup$
    – CrossRoads
    Commented Apr 23, 2018 at 1:00

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