Mxsmanic wrote:
Perhaps this is a naive question, but: Why don't voice radio
communications for aviation use FM radio instead of AM radio? I
realize there's substantial inertia in the installed base of AM
equipment, but surely one could allocate some new frequencies to FM
and use them in parallel for some years to ease the transition.

If one were to mandate a replacement technology, it would be far far more
effective to use the packet-based mechanisms that digital cellular phone
technology and 802.11 wireless Ethernet (aka WiFi) rely on. Both these
technologies turn over the job of transmission collision resolution to
chip logic and take humans out of the loop. And it is possible to put
audio over WiFi using Voice over IP (VoIP) technology.

Such a system would be incredibly flexible. If one had, say, ten planes
in the air and they all started to talk to ATC at once, a packet-based
system would make it possible to do any of the following:

1) Clearly deliver only one of the voice signals to the controller and
provide a visual display that indicated 9 other planes had attempted to
speak also. It could even provide audio or visual feedback to the other 9
pilots that their transmissions were not delivered - or it could
automatically sequence the delivery of the transmissions to the
controller if the transmissions were not too lengthy.

2) If multiple controllers were available, the audio from several of the
planes could be routed to multiple controllers with no impact on audio
fidelity as far as the controllers or pilots are concerned.

3) Once you go packetized audio, you can put all sorts of useful stuff in
the packets for presentation to the other end - such as aircraft number,
the location and velocity vector from the aircraft's GPS or
altimeter/DG/airspeed indicator, and so on. A pilot could key the mike
and make a request without needing to ID themselves or their position -
that information would be extracted from the audio packet's header and
automatically presented on either a simple display to the controller or
mapped to a fancy map display.

The technical issues have been pretty much solved and commoditized in
both the WiFi VoIP and digital cellular realms. It is my humble opinion
that the radio technology currently being used for aviation
communications is now less reliable and useful than even that used in
home WiFi networks.

Maybe someday the FAA and/or ICAO will consider replacing analog radios
with a more capable digital system....