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I'm an enthusiast and have only flown flight/atc simulators (on the VATSIM network). Obviously sometimes the airspace you are flying in is not covered by any ATC (so I have to set my frequency to Unicast). And very often there is only the tower of an airport available. How is it in the real world?

  1. Is the whole world's airspace covered by ATC at all times?

  2. Is any airport covered by all its layers of ATC (Delivery, Ground, Tower, ...) at all times?

  3. How often do pilots use the Unicom frequency because there is no ATC available in current airspace?

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    $\begingroup$ These are three mostly separate questions and should be asked that way. Also, you've used two tags about simulators, but your question doesn't mention simulation at all. $\endgroup$
    – David Richerby
    Dec 6, 2018 at 20:58
  • $\begingroup$ Related $\endgroup$
    – Pondlife
    Dec 7, 2018 at 1:50

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The entire world is divided into Flight Information Regions (FIRs), so in theory there is someone responsible for everywhere. However, those FIRs are then subdivided into different classes of airspace.

Class A/B/C/D/E airspace is "controlled", meaning there should always be some sort of ATC available. In remote areas, though, ATC may not have radar coverage, so separation would be based only on position reports (plus ADS-B, for participating aircraft).

Class F/G airspace is "uncontrolled"; any ATC service would be advisory only, and around untowered airports, they'll tell you to use CTAF. Often the reason airspace is uncontrolled in the first place is a lack of radio and/or radar coverage.

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I think you need to understand that airspace is divided vertically as well as horizontally. This image should help you picture it (from Australia, where Class B does not exist).

enter image description here

Glass G airspace has no ATC interaction, and in Class E ATC interaction is only needed for IFR flights. When you picture airspace like this, most of the world is not covered by ATC, with the exception being at airliner altitudes where there is almost universal Class A coverage (note, this is not the same as having universal radar coverage).

The majority of the world's airports only have Unicom/CTAF, I'm sure every pilot has flown to one in their training. However only a handful of these airports are used by airlines. And at some towered airports with low traffic, they might not have a distinct delivery frequency, and on quiet days they can go further and combine ground and tower frequencies.

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  • $\begingroup$ Some towered airports may go even further and only have the tower open at all by request; unless someone files for a flight to it that requires tower services, it operates basically as an uncontrolled airport. One of my nearby airports operates like that, being what is (perhaps informally) knows as class G*, as opposed to class G. You're required to call in entering their airspace and provide position updates, but if there's no answer calling in, you operate as in plain uncontrolled class G with the knowledge that the tower might open at some point while you're in their airspace. $\endgroup$
    – user
    Dec 7, 2018 at 19:29

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