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I'm a plane spotter with only general information about air traffic. I'm aware that airplanes can fly only alongside airways. They should fly on the middle of the air routes. However, each airway is defined by its width which suggests me that an airplane has some flexibility within the airways.

My suspicion was somehow confirmed when I spotted a regular cruise (at 38,000 ft) which is passing more or less just above my house (in Europe). I noticed that sometimes this airplane passes above my location +/- a few kilometers away (usually < 10 km, which I also checked on FR24). Or maybe they just fly using different parallel airways?

I have the following questions:

  1. Can a pilot decide how to fly within a single airway? If not, can a pilot or airline choose one of close, parallel airway?
  2. Is there a way to get a list of passed airways for a particular flight?
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  • $\begingroup$ If passengers could ride up front, some of them would freak out b/c of the 1000 ft vertical separation and GPS nav systems that are super precise. When on a published airway, there is no longer any dispersion of airplanes to either side of the airway from tolerance scatter of the nav equipment; everybody is centered EXACTLY. Airplanes going the other way pass precisely 1000 ft above or below, and when their contrails first come into view a couple miles out, they look like they are coming straight at you until they are less than a mile away. $\endgroup$
    – John K
    Dec 30, 2022 at 13:19

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I'm aware that airplanes can fly only alongside airways.

Your information is slightly out of date. There was a time when all IFR planes were required to stay on defined airways, but that has changed. I'm not sure exactly when it changed, and a quick Google search for the info turned up nothing, but it's been several decades at least. The airways do still exist, and are heavily used whenever possible to make traffic planning easier, but there's nothing wrong with flying between points with no airway connecting them.

Of course, VFR traffic has always been free to fly whichever direction they want, as long as they obey visibility regulations, ask permission for controlled airspace, etc.

But to answer your question: When you're on a route, you're supposed to fly as close to the route centerline as possible. However, it may not always be possible to do that, so the routes do have a defined width that you can maneuver in if needed.

  • Low-altitude VOR routes (V-routes) go between VOR stations. Since VOR navigation gets more imprecise the further you are from the VOR, the width of the airway reflects that. If you're greater than 102nm from the nearest VOR, the width of the airway is based on a calculation of the margin of error for the VOR; the further you are, the wider the airway is. Within 102nm, the width is simply 8nm (i.e 4nm on either side of the centerline)1.

  • High-altitude VOR routes (J-routes) don't have a defined width. You're simply expected to stay as close to the centerline as possible.

  • RNAV routes (T-routes for low altitude, Q-routes for high) are always 8nm wide 2.

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  • $\begingroup$ When I began my career in the 80's a lot of commercial aircraft (probably 50% or so) in the US used some form of RNAV equipment to fly point to point. $\endgroup$
    – RetiredATC
    Dec 30, 2022 at 2:30

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