"Bucky"

I'm not a pilot, just your typical passenger. I don't fly United often,
so I just discovered that they put the ATC on the inflight audio. I was
totally fascinated and want to learn some more. Is there a website that
has some basic ATC protocols? Not some huge reference manual, just the
common phrases that you hear over a commercial flight's radio.

Google +ATC +"radio communications" 15,600 hits.

http://bathursted.ccnb.nb.ca/vatcan/...rentTopic.html

- What happens if 2 people try to talk at the same time? Do you hear
both voices or neither? How do they avoid talking over each other?

They just get used to it. You can't hear anything on a radio that's
transmitting and if two stations are transmitting at the same time, they
tend to block out each other's tranmissions. It can be confusing and even
nerve-wracking to the point that many amateur pilots avoid talking to ATC.

- It seems that there is no authentication/security, the parties just
identify themselves. Have there been cases of troublemakers making
false impersonations?

Never heard of that. But it could easily happen

- They ask pilots their speed, altitude, and heading a lot. Can't the
transponder send that information?

It does. But ATC is required to confirm this. If there's a transponder
equipped plane being mentioned by ATC, but not talking to them, you'll hear
something like: "Air XXX 123 your traffic is at 2:00, 3 miles, 6500 feet
unconfirmed".

- Do any other airlines put ATC on the inflight audio?

Dunno. I wonder if they turn it off if there's a problem... Some fairly
innocuous things could be disconcerting to non-aviation folks.

moo