How close are planes allowed to fly next to each other? 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.
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: