On United's Channel 9, why are there certain times when you can't hear other pilots?

Question

For the uninitiated, Channel 9 is a channel on United Airlines planes that you can plug your headphones into and listen to Air Traffic Control (ATC).

You can hear just about everything the pilots hear on ATC, but there are certain times when the only voices you can hear are the the air traffic controllers, themselves, and your own pilots. You can't hear other pilots.

Here's a made-up example of what you might hear:

ATC: United 1194, climb and maintain FL310.

<silence>

<silence>

ATC: Continental 728, deviation approved. Advise when resuming navigation.

<silence>

In those silences, you should've heard the first pilot complying to ATC, then you should've heard a request for deviation, and finally a confirmation that the deviation is approved. Sometimes you can go through several centers without hearing other pilots besides your own. Then, miraculously, at some point the other pilots return.

What's the cause of this? Is this a glitch in Channel 9, or are other forces at work, here?

Answer 1

You&#039;re hearing what the pilot hears. Controllers frequently handle more than one sector, or they handle more than one frequency. In either case when they transmit they may be transmitting on several frequencies. So if you&#039;re tuned into 132.8 and the controller is handling 132.8 and 134.05, you won&#039;t hear the airplanes talking to him on 134.05 but you will hear the controller talking to them because he&#039;s simultaneously transmitting on both frequencies.

Answer 2

I have often wondered the same thing...interesting responses from all...thank you.

Answer 3

Craig R has the right answer.

Answer 4

it could be a duplex radio system. there&#039;s an input and output frequency and youre listening to the input. otherwise you might not be picking up the other radio because it&#039;s far away, if it&#039;s a cheap radio installed just for the AV system

Answer 5

To further expand on the second answer which is 100% correct.  A controler might very well be controlling a sector that is something in the area of 100 miles wide and 175 miles north to south at its peak points.  Simply put, radio waves only travel so far.  I don&#039;t pick up FM stations that far away and thus, channel 9 doesn&#039;t either.  If the controler only needs to speak to at most someone 90 miles away and you&#039;re 150 miles away trying to listen to a pilot you won&#039;t hear them.

Answer 6

To further further expand, ground-based radios inevitably have far more power than airborne radios, which is why you sometimes hear the controller but not the relatively weak reply from the aircraft.  The controller will, though, he has a large aerial array at his disposal.

Yes, controllers often transmit on all the frequencies they&#039;re handling simultaneously, it saves them having to continually switch channels and also gives all aircraft in the area an idea of what&#039;s going on around them, even if it&#039;s not directly relevant to them.

For instance I have heard an aircraft querying the landing details they&#039;d just been given, as it contradicted what they&#039;d just overheard being given to another aircraft - and quite rightly so, as it turned out . . .