Plane Answers: How close are airliners allowed to fly?

Welcome to Plane Answers, where our resident airline pilot, Kent Wien, answers your questions about everything from takeoff to touchdown and beyond. Have a question of your own? Ask away!

This question was submitted by Jim,

Hi Kent,

My question concerns how much advance notice pilots get when there are other planes in their immediate air space. In some of my travels, while at cruising altitude, I’ve seen other planes cross paths just below us. Knowing how many planes fill the sky each day and knowing that your reaction time is minimal, I wonder how pilots and controllers work together to keep all those planes apart. Also, what’s the rule on how much distance must there be between planes when on the same route and at intersection points?Thanks Jim,

I know it can be a little disconcerting to see another airplane cross under or zip by overhead just as you look out the window of an airliner.

Air traffic controllers have rules on how far laterally they must keep airplanes apart as well as how much vertical space needs to be kept between them.

For lateral separation, airplanes that are en route–flying faster and further away from the ATC facility–must have at least 5 nautical miles between them. When the airplanes enter the approach controller’s airspace, that requirement goes down to 3 nautical miles. Finally, when the airplane is in the control of an airport’s tower controller, aircraft can be spaced much closer if that controller has visual contact with the airplanes or if at least one pilot reports they have the other aircraft in sight. A good example of this is the visual approaches to San Francisco where airplanes are lined up on final approach for the parallel runways. You would think the airplanes are flying in formation at times.

This visual separation doesn’t apply when airplanes are in the clouds, in which case the controllers keep airplanes spaced about 2 1/2 NM apart, more if the preceding aircraft is a heavy (over 250,000 pounds–757 or larger) and the following aircraft is not. This limitation is a function of the wake turbulence generated by larger airplanes.

But I suspect the airplanes that you’ve been seeing lately have been even closer laterally than that. Because of some technology improvements to corporate jets and airliners, most of the world has adopted the Reduced Vertical Separation Minima (RVSM) standards. This allows aircraft flying above 29,000 feet to be spaced at 1,000 foot intervals. In the past, that number was 2,000 feet apart.

This has actually had the effect of doubling our airspace above 29,000 feet, which allows for more direct routing and the ability for us to get out of annoying areas of turbulence.

Westbound aircraft are normally put at the even flight levels (altitudes) and eastbound aircraft fly at the odd levels. That doesn’t apply to the North Atlantic, where most of the traffic flies westbound in the morning and eastbound in the afternoon. In that case, airplanes are spaced 1000 feet apart which makes for some great views from our seat as you can see from the following video clip:

Have a question for Kent? Ask away and he’ll pick one to answer here on Friday.