How are multiple frequencies monitored at once?

Question

In a tiny single-engine aircraft like a Cessna, all the way up to a modern jetliner like a 747, it seems the cockpits of modern aircraft have at least 4 separate airband radios, for monitoring up to 4 different frequencies at once.

However, the cockpit has only one up to (rarely) three occupants wearing headsets.

When Ground calls you and says, "monitor Tower, on 121.65", Tower wants to tell you something from that frequency, but Ground also wants you to stay on their frequency.

By which mechanism are multiple frequencies monitored by one or more (fewer than the number of radios/frequencies) crew members?

• all the running cockpit radios are heard at the same time, mixed into the same audio

  • how to know which transmissions came from which freqs?

• switches that cause exactly one of the radios to be "active" in the headsets at once, and the other ones are not heard in the headsets

  • how to monitor multiple freqs this way, without missing transmissions or switching back and forth so quickly it would be unreadable?

I've looked around, but I can't understand how multiple frequencies are easily monitored at once, without running into the problems I mentioned in my guesses.

So how can a pilot alone, or a pilot with a first officer, listen for their callsign on multiple frequencies at once?

Answer 1

First let's get rid of some misunderstandings:

• Many smaller aircraft have just one single radio.

• Being told to monitor a frequency does not mean you have to listen to two frequencies at once. See: Being told to monitor a frequency, when are you allowed to transmit on that frequency? Any controlled flight will always be under the control of exactly one controller, never more at the same time.

Now, as for actually listening to multiple radios at once, that is certainly possible, and quite normal on large airliners. In the cockpit of an airliner there will often be up to somewhere around 10 different audio inputs, each with their own volume control. This can include 2-3 VHF radio, an HF radio, NAV radios, communication with ground crew (pushback/handling), as well as audio from the cabin (the PA) and probably a few others. All these audio streams are merged into one, based on the individual volume controls.

Recently having had the opportunity to join a crew on a flight on the jumpseat, I was quite amazed at how good the pilots were at distinguishing between the multiple audio inputs, even when different people spoke at once. But with practise, it's actually not hard to see how it's done. Each audio channel will have a different "topic", using different key phrases and words. An ATC ground unit will be talking about taxiing, whereas the tower will be talking about taking off and landing. And the cabin PA will be explaining how to fasten your seatbelt to a bunch of passengers not paying attention. And of course then there is the fact that people all have different voices. Sometimes it is easy (a female on one frequency and a male on another) and sometimes it is more tricky, but I have never heard two people with the exact same voice.

Answer 2

The expectation is that when ground instructs you to monitor tower, 118.5, you will leave the ground frequency.

If instructed, hold short of 10, then monitor tower, they mean when you are holding short, you should monitor tower. Except that tower will call you. If they don't and you think they should have, check in.

Many aircraft have more complex audio panels, and it is possible for one crew member to be talking to dispatch, while another crew member is talking with ATC. That, however, is different from an ATC instruction to "monitor xxx."