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When a passenger aircraft travels at higher altitudes will it be in constant touch with air traffic controller?

Are the altitudes or flight levels assigned by ATC or does the pilot file them as part of flight plan?

While flying over regions with no radio communication, how does one ensure that there are no other aircraft nearby?

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marked as duplicate by J. Hougaard, SMS von der Tann, aeroalias, J Walters, rbp Oct 22 '16 at 17:40

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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Generally speaking airliners are always able to be in contact with ATC, either by VHF (regular radio), HF (usable over longer distances, e.g. on oceanic crossings), or satellite communication (possibly relayed through their company dispatch). It is also possible for one aircraft to relay messages for another (for example if one aircraft's HF radio has failed they may contact a nearby aircraft on VHF to relay messages to ATC).

Because they can usually talk to ATC even when they are out of radar contact airliners can and will provide position reports, and ATC updates their position information the old fashioned way (which we talked about over here). The controllers can then assign altitudes, speeds, and headings/routes in such a way that no two aircraft attempt to occupy the same piece of sky at the same time. (This is generally the way oceanic routes are handled. There are also air-to-air frequencies for aircraft flying these routes so they can coordinate with each other if necessary.)

In the event an aircraft is out of contact with ATC - whether through a radio blind spot, an equipment failure, or for some other reason - they rely on a combination of lost communication procedures (here's a quick reference from the FAA) and "see-and-avoid" (if you see another aircraft avoid hitting it) until contact can be re-established.
The use of standard lost communication procedures also allow the controllers to continue updating the aircraft's position in a non-radar environment (they can estimate progress based on the standard procedures and the aircraft's prior position reports/speed/wind data). A larger block of airspace will be reserved in such situations to account for any error.

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  • $\begingroup$ You know, "if you see another aircraft avoid hitting it", just seems so intuitively obvious that it shouldn't even need to be said, but, human nature being what it is... $\endgroup$ – FreeMan Apr 9 '16 at 13:58
  • $\begingroup$ @FreeMan Don't forget it is also human nature to start panicking when something unexpected happens. Avoiding another aircraft requires staying in control of oneself in the first place. $\endgroup$ – Rob Vermeulen Apr 10 '16 at 14:52
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At high altitudes one in never out of radio range for ATC. The usual VHF airband frequencies only work within line-of-sight, but if you're too far away for that, one can switch to HF radio which follows the curvature of the Earth.

Most parts of the oceans are outside radar coverage, so the controller cannot see where you are. In these areas ATC depends on position reports from pilots for separating flights.

Flight plans contain a requested cruising altitude, which ATC will generally clear the flight for if possible, but when traffic requires it pilots have to take what ATC assigns.

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    $\begingroup$ "At high altitudes one in never out of radio range for ATC" There are a few oceanic routes where there is no ATC contact. $\endgroup$ – Ron Beyer Apr 8 '16 at 20:17
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    $\begingroup$ Alaska also has high altitude routes that have no ATC or radar $\endgroup$ – rbp Apr 8 '16 at 20:35

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