[Scan-DC] Should aviation use full duplex for tower comms?

[Andrew Clegg]

I'm listening to the hearing on the DCA crash and air system safety. In reading some of the background on this crash, one of the items that came up is that the helicopter pilots may have had their radio keyed up to transmit to the tower at the same moment the tower was telling them to keep an eye out for the passenger plane, so they did not hear the controller. This is likely not the direct cause of the accident, but it probably didn't help.

Besides this instance, when I listen to DCA tower, there are frequent occurrences when one pilot keys up when the tower is attempting to communicate with a different plane, causing the second pilot (and the first) not able to hear the tower.

It seems to me that if full duplex was used, in which the frequency that the pilots transmit on is different from the one they receive on and they can therefore receive and transmit at the same time, then the interference caused by pilots keying up at inopportune times could be mitigated.

The current simplex mode of operation in the 118-136 MHz aeronautical band reflects decades of established precedent and international agreement, as does the aeronautical radio equipment manufactured for this band. But switching to full duplex comms would seem to be a fairly straightforward option, if not inexpensive. The two frequencies would need to be separated by enough frequency that an effective duplex filter could be employed. There would be challenges with frequency planning, but if this was implemented solely for critical comms, such as tower comms but not approach, departure, and enroute frequencies, the frequency planning might be workable. For example, the tower could transmit at the low side (say, 118-121 MHz), the planes could transmit on the high side (say, 133-136), with a duplex gap from 121-133 MHz, which is where the non-tower comms could be placed.

In the case of DCA, the tower is already using two frequencies: one for communications with planes (119.1) and another with helicopters (134.35). The tower side of the conversation is simulcast on both. I actually receive the tower on the helicopter frequency at my place better than I do the frequency used for airplanes. The antenna patterns might be different (lower elevation for the helicopter frequency perhaps).

Just a thought for what it's worth, which probably isn't much.

73,
W4JE