Aviation In Canada

Sunday, July 17, 2005

ATC Radios

A pilot from the Netherlands recently wrote me, asking about a flight experience he had while flying through Gander's airspace. The concern he expressed related to the fact that it was obvious to him the controller was working more than one frequency, and the fact that aircraft on each frequency were unable to hear pilots talking on the other one. This lead to missed readbacks, communications being "stepped on" and so forth. He cited it as an unacceptable safety concern, and wondered how this was possible.

In fact, many ATC units in Canada deal with this on an ongoing basis, as do other units in other countries. Especially in low level sectors where large pieces of airspace must be worked by one controller, it is quite common to have more than one frequency per sector. The higher the airspace worked by a given controller, the less likely they are to need more than one radio since VHF is line of sight and can often reach everywhere in the sector. But down low, terrain and distance often preclude the use of only one radio to reach the entirety of the airspace. Hence, a controller is often "plugged in" to more than one radio. Generally speaking, ATC simulcasts on all of his radios at once, while all are open to receive at the same time when not transmitting.

This leads to some confusion sometimes, as for some strange reason, it seems that dead air is often followed by more than one aircraft calling in at once on separate frequencies. The controller is often able to distinguish who was calling, based on numbers or partial callsigns heard in the mix, but not always. The problems really arise when a clearance readback is stepped on by an aircraft on a separate frequency. There's a little bit of professionalism in a pilot who won't immediately ask for something when ATC just finishes issuing a clearance to an aircraft, even if he doesn't hear the pilot's readback right away. And then, sometimes a well-intentioned pilot doesn't know a clearance was just issued, for whatever reason. Perhaps he just tuned into a new frequency, waits for a second or two to make sure he isn't stepping on someone before calling, and then calls over a readback on a different frequency. Some ATC will get quite impatient with a pilot for such a thing, but hopefully they'll get over it.

Some units have what are called Radio Retransmit Units installed on some or all of their radios. This allows incoming transmissions on one frequency to be rebroadcast on one or more other frequencies, thereby allowing pilots on one to hear pilots on the others. This greatly reduces the chances of readbacks being stepped on. The one thing I like, as a controller working without this, is that while one pilot is calling, I can still take transmissions on other radios. For example, two pilots request altitude changes, and can do so in dramatically different ways. Compare:

"Moncton, JAZZ 8765 request 12,000"
"Moncton Center, JAZZ 8765, it's getting kind of bumpy up here, say, light occasional moderate chop. We're kind of riding the tops here. Is there any chance we might be able to get 12,000 as a final for this leg today?"

In a world with frequency coupling, as it is called, the pilot of an aircraft that suddenly experiences an emergency would be preempted on his frequency by the pilot of another while listening to the altitude request. Or sometimes a VFR pilot with limited experience might begin a long diatribe without realizing he's blocking other aircraft. Without coupling, ATC can mute the radio with the incoming Grammy acceptance speech and listen to the one with the emergency on another radio.

So the RTUs can be a blessing (and normally are), but they can also be a bit of a curse. There is no perfect solution, I'm afraid. It's a matter of knowing what you're up against, and using common sense.