>  I had thought that the VHF air band was simplex. A quick web search
>  didn't yield any answers.
>
>  Anyone know what is going on here?

Airband is simplex, except in rare cases which wouldn't likely be seen
with approach control. Generally the only time we use different
frequencies is if the flight service station is communicating over a
navigation aid (typically a VOR in 108.1MHz-117.95MHz) and the pilot
responds over a standard voice frequency.

What may very well be happening in the Boston case is that you are not
in a location to hear the controller transmit on 133.0 due to antenna
location or other factors, but you can hear the aircraft response in
the air. The 124.4 frequency appears to be another local frequency in
the area that you presumably are in a location to hear. What
controllers typically do when they work more than one frequency at a
time is to transmit on all of them at once so that any aircraft can
tell that the controller is busy even though the aircraft may be on
different frequencies and can't hear each other.

This seems to be an interesting list of how the Boston TRACON is laid out:

http://en.wikipedia.org/wiki/Boston_Consolidated_TRACON

But as I'm not in the Boston area I can't be sure this is what's happening.

-Steve Wormley
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