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There have been serious safety incidents where multiple airplanes using the same frequency caused simultaneous messages to garble each other. For example, at Pearson International Airport (2020-03-07), two planes nearly collided on the runway, because a small Embraer 190's message that it was rejecting takeoff was drowned out by a routine message from a Boeing 777's more powerful transponder. Something similar happened during the 1977 Tenerife disaster, though it likely would not have prevented the incident.

I once worked a job where people communicated by walkie-talkie frequently. Every department had its own channel, so that it could focus on its own most relevant communications. Why don't airports, which are much more safety-critical than my job was, segment communications like that?

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    $\begingroup$ Possibly some day when comms are fully digitized at both ends, say by a system that replaces VHF transmitters/receivers with a digital satellite network, that this can be achieved with software where pilots hear what other pilots and ATC are saying to each other, but are prevented from stepping on each other. Give it 20 yrs. $\endgroup$
    – John K
    Commented Apr 16 at 14:59
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    $\begingroup$ So you lost situation awareness, e.g. other pilots clear to land in your runways (and this extra listening prevented many incidents). It would not be complex to airports to know about talking over (just put antenna and receivers around the airport, and maybe directional: inside/outside perimeter, software can easily see if two aircraft are talking at same time, and plus: ATC can have exact position of who is talking. No need to change infrastructure, just to add few new unconnected systems (and relatively cheap) $\endgroup$ Commented Apr 16 at 15:05
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    $\begingroup$ This makes little sense, the entire point is everyone is supposed to hear everything. $\endgroup$
    – Fattie
    Commented Apr 17 at 22:08
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    $\begingroup$ @Fattie that's a very succinct answer, but an answer nonetheless $\endgroup$
    – Chris H
    Commented Apr 19 at 13:32
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    $\begingroup$ lol thanks @ChrisH . You know, there's a fascinating problem on the few "expert" sites, an example is the physics one… ... ... ... Someone will ask a very basic question for example "Does the Earth go around the Sun, in a circle of square?." The answer to that would be "Circle." But, because the site is packed with basically the world's leading research physicists, someone, indeed everyone, will launch into a spectacularly explanation about the effects of the latest competing quantum gravity theories on the nature spacetime and geometry. But the actual answer was "Circle." !! $\endgroup$
    – Fattie
    Commented Apr 19 at 14:34

3 Answers 3

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Because it is actually critically important that pilots all hear what is going on around them.

In a perfect world, ATC could act as the go-between for everything. But in reality there are times when an airport gets very busy and every second counts. If plane 1 hears that plane 2 is having trouble getting off the runway or that plane 3 is rejecting takeoff, or whatever, that means plane 1 now knows about the problem before ATC contacts them directly.

In some cases, a pilot may notice a problem such as debris on a runway that affects others and by using a common frequency the other planes are notified immediately instead of waiting for ATC.

I am sure there are plenty of specific examples that the professionals can provide.

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    $\begingroup$ I like your answer, but you may want to consider that all these aircraft on "discrete" frequencies are still talking to one controller. If no aircraft can hear other aircraft, there's no reason to not key up and communicate with ATC. Meanwhile, the poor controller has four aircraft talking at the same time, all going into one headset. I controlled aircraft at a very busy facility for many years, and simultaneous transmissions occur, but with the present system and pilot etiquette, it doesn't happen a lot. Additionally, CPDLC will reduce frequency saturation to reduce the problem. $\endgroup$
    – RetiredATC
    Commented Apr 16 at 19:20
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    $\begingroup$ I remember once landing at an airfield with parallel runways. I crossed final for the right runway and looked over to see the airplane cleared to land on the right runway lined up with the parallel taxiway to the right. I alerted ATC about the situation, but in all actuality, I was letting the other pilot know they were lined up with the taxiway. The plane banked left to line up with the runway before they landed without incident. I can only imagine that they were quite thankful for the observation on my part. $\endgroup$
    – wbeard52
    Commented Apr 17 at 21:39
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    $\begingroup$ @paul23, it's going to be a very, very long time before AI is used in such a safety critical and risk averse role as ATC (if it ever happens). $\endgroup$ Commented Apr 18 at 10:18
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    $\begingroup$ @paul23 Because if the AI makes a mistake and there's burning wreckage on the ground, that's the end of that. It has to be pretty much perfect out of the gate, and no AI currently available is anywhere near infallible enough. $\endgroup$ Commented Apr 18 at 12:05
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    $\begingroup$ @paul23 AI can't keep a better overview, just a faster one. Your average AI system beats an untrained human, but a trained human beats any AI system we have yet invented (at ATC, not chess). Worse, an AI system's comprehension would be alien to the humans using it as mediator, so pilots wouldn't be able to easily predict how it'd respond to their communication attempts. People are investigating the use of AI in other areas, but not for handling comms. $\endgroup$
    – wizzwizz4
    Commented Apr 18 at 14:29
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Having separate frequencies all being handled by the same controller isn't going to solve two people trying to talk to the controller at the same time, just the opposite. There's rhythm to the communications on a common frequency, and you generally know when to press the transmit button, even on a busy frequency. For example, I may want to talk to ATC, but they are saying 'G-NWFT turn left to 150, climb and maintain 5000'. I know that FT is going to respond shortly with something like 'left to 150, climb and maintain 5000, G-NWFT' and I will wait for them to finish before I transmit. If everyone's on a separate frequency you'll have frequent instances where several will be talking to a single person at once.

A common frequency sounds like a mess to a layman, but to a pilot it's a wealth of information that gives situational awareness. Listening in will give me an idea of how congested the airspace around me is, and where other aircraft are in the vicinity as well as where they are going. I'll get an idea of aircraft types, and by listening to the pilots I may glean other useful information like their workloads and capabilities. For instance there may be a student pilot, someone with a technical issue, or I may get bad vibes and want to avoid someone.

It's also important for safety. ATC isn't perfect, a common frequency gives the opportunity for pilots to detect mistakes and take action. For instance, if I'm on short final approach and have been cleared to land on runway 18 and I hear 'G-NYLY line up on runway 18, cleared for takeoff' I know that ATC have just made a very dangerous error and told an airplane on the ground to depart on the runway I'm about to land on. I can then go around. Or, if I am the pilot of that airplane on the ground that's just been cleared onto runway 18 and I've been listening on the common frequency I would realize the danger and refuse the clearance.

Yes, occasionally two people will talk on the frequency at the same time, but it's rare and although it's inconvenient when it happens it's worth it to hear everything else.

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    $\begingroup$ "A common frequency sounds like a mess to a layman" is certainly true, but it's pretty easy to get a handle on that mess. The rhythm you refer to helps enormously, as does reading back to give the layman a 2nd chance to pick up the message. Years ago I worked for the UK's NATS timing airport movements for efficiency monitoring, and within a couple of hours of practice (with a good view from half way up the tower, and a few paper reference documents) everything made sense $\endgroup$
    – Chris H
    Commented Apr 17 at 12:11
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    $\begingroup$ How is this handled for on-route controllers? Are the airplanes high enough in the sky that even at opposite ends of the controllers area of control, they can still hear each other. I know in the maritime world with coastguards this can actually cause non-trivial problems, because the coastguards are often using multiple receiver stations where ships can't hear whats going on at other receiver stations. $\endgroup$ Commented Apr 17 at 12:41
  • $\begingroup$ As l long as there is line of sight you can hear airplanes for pretty long distances @user1937198. I'm a GA pilot in the UK, and you get some frequencies which are used over very large areas, like London Information, but that's an information frequency for airplanes operating outside of control areas. There are times I'll know someone is transmitting from a long distance but not be able to hear them, but if they're that far away it doesn't impact me. If there's an airplane that ATC can't hear because of terrain or interference another airplane may relay the message, that's pretty rare. $\endgroup$
    – GdD
    Commented Apr 17 at 12:57
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    $\begingroup$ Not having a common frequency would be a challenge to handle the logistics of managing the frequencies needed, especially in crowded airports. If each plane had a different frequency, they'd need to get new information on which specific frequency to use each time they visit an airport. With a common known frequency, an incoming pilot would know that each time they were on a flight that needed to contact a specific airport tower, they'd already know the frequency as it would be well known and non-changing. Predictability is important, which is why there are standards for communications. $\endgroup$
    – Milwrdfan
    Commented Apr 17 at 16:30
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There are different aspects to assess:

  • Does that solve problems?

  • Is it technically feasible and economically acceptable?

  • Does that create other problems?

Problem solved

It can solve issues like the one you described.

However in that case, the possibility of misinterpretation of ATC directives is already managed by procedures: Important directives must be acknowledged explicitly (read-back) so that both parties can check they have a common understanding. So the added-value is limited, and if an effort should be done, it would likely be first on the proper use of procedures (which is a key factor in Tenerife accident).

Technical and economical feasibility

At the time the communication process was standardized by ICAO, only analog modulations, mostly AM and FM, were really usable. Selective calling (selcal) was used, but only on HF, not on VHF, and selcal doesn't protect against interferences anyway, as the channel is still unique.

AM/FM required different carrier frequencies to work with different channels, meaning a large spectrum occupancy for each airport / ATC unit.

Because the spectrum is limited, it must be shared between airports, different airports will use the same channel, creating interferences between airports (even distant from 200+ km). This issue is likely worse than same unit interferences: Aircraft would listen to different ground stations on the same channel, but each ATC unit would not be aware of the other units.

So technically speaking individual channels were not really feasible.

New problem created

While multiple channels would help solving problems that are supposed to be already solved by procedures, it also creates new ones: Loss of general / common awareness. We could likely find accidents that were avoided by listening to a common frequency.

--> @DeltaLima found already one (thanks). Watch the video and see how a common channel and a professional crew, aware of the overall confusion at the airport, just saved lives.

It's really difficult to say if there is a mean benefit in using individual channels, this requires a serious analysis nobody can do here.

How the problem could be solved today

Today there are digital modulations and spread spectrum technologies (frequency hoping) allowing sharing the same spectrum slice for many channels, resisting to interferences. This mode of access to the channel is used since a few decades by Bluetooth, WLANs, cellular networks, GNSS, etc. There is no doubt the next generation of ATC, currently developed by FAA and Eurcontrol (SESAR) will use spread spectrum communications.

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    $\begingroup$ youtu.be/equVF3ULVw8?si=tO-fs34OK49YHrGK for example. Would have been deadly with separate channels $\endgroup$
    – DeltaLima
    Commented Apr 16 at 18:18
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    $\begingroup$ @DeltaLima. Thanks for the suggestion, and the link (added to my post). Impressive case of a professional crew saving lives. $\endgroup$
    – mins
    Commented Apr 16 at 18:52
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    $\begingroup$ And as a negative example, the runway incursion this morning at DCA (tower cleared an aircraft to depart the same runway that ground cleared another aircraft to cross) was a case where having the two crews on different frequencies worsened the situation, as it meant neither was aware of the conflicting clearances and couldn't hear each other. $\endgroup$ Commented Apr 19 at 5:42
  • $\begingroup$ If you're talking about modern solutions, antenna arrays are also relevant. Modern radio can be directed pretty accurately, which would greatly decrease interference between airports. $\endgroup$
    – MSalters
    Commented Apr 19 at 7:38

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