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I was listening to Gander Radio on HF at 5616 kHz today and I've noticed that operator at Gander would often start transmitting reply before the airplane finished transmission, as if trying to interrupt radio operator on the plane.

To me, this was very unusual behavior. From what I've heard and from what I concluded after listening to frequencies, it seems that HF radios in aeronautical mobile service work in traditional simplex mode, so crew normally crew wouldn't be able to hear the part of transmission send before they stop keying their radio.

So this brings me to these questions: Are aviation HF radios capable of receiving while transmitting? If so, how?
Or if not, what's the point of Gander stepping on the plane's transmission?

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    $\begingroup$ Inept operator, perhaps? $\endgroup$
    – Ralph J ♦
    Commented Oct 18, 2015 at 2:02
  • $\begingroup$ I guess Hanlon's Razor applies here. $\endgroup$
    – Peter Kämpf
    Commented Oct 18, 2015 at 7:17
  • $\begingroup$ Could that be the effect of the different propagation times to your location? HF travels by reflexions between the ground and the ionosphere on daytime. I wonder if you didn't hear an apparent overlap that didn't exist. $\endgroup$
    – mins
    Commented Oct 25, 2015 at 3:51
  • $\begingroup$ @mins I myself do not believe that that was the case. Usually, when multiple reflections of similar strength are received, on the receiver, the sound that's made sounds a bit like an echo. I didn't hear any echos, so from this I can conclude that my reception was coming from only one major reflected component of the signal. So if the cause was delay, it would be more-or-less constant within the small time-frame of several relations. So if I was having a delay, it would affect other relations as well, but that didn't happen. $\endgroup$
    – AndrejaKo
    Commented Oct 25, 2015 at 11:25
  • $\begingroup$ If they are operating in simplex mode-No. However 5616 KHz is also used by not Gander but New York, Santa Maria, Shanwick, etc. It is possible that you were listening to another ATC. $\endgroup$
    – Old_Fossil
    Commented Sep 24, 2016 at 8:37

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In general, the HF radios in aviation work in simplex mode.

According to the Annex 10 to the Convention on International Civil Aviation- Aeronautical Telecommunications Volume V Aeronautical Radio Frequency Spectrum Utilization Chapter 3,

3.1.1 In the aeronautical mobile service, single channel simplex shall be used in radiotelephone communications utilizing radio frequencies below 30 MHz in the bands allocated exclusively to the aeronautical mobile (R) service.

Though some aviation radios are capable of semi-duplex operation, virtually all aviation services operate on simplex.

I have no idea why the operator was stepping on the aircraft's transmission though.

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