system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do you communicate with unauthorized/unidentified aircraft?

I've seen movies in which pilots communicate with unauthorized planes in order to force them to go away/land nearby immediately.. etc.

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Do all planes from all countries have an universal radio frequency? (Even ones flying illegally?) How does it work?
gpt
In aviation, 121.5MHz is an international reserved emergency frequency called the "Guard" frequency. Generally plane radios can be tuned to their regular channel and guard at the same time, so you always get heard.

Fun fact, in the Incredibles when helen says she's "Transmitting in the blind guard", she's announcing that she's just speaking on the guard channel to anyone who can hear and help and doesn't know if she's actually being received by anyone.

Also, planes are equipped with a device called a ~~squawkbox~~ transponder, which transmits a four-digit code that can be picked up by air traffic controllers. This is normally used to help ATC identify planes in the air but in the event that a plane's radio is busted, they can set their transponder to squawk a specific number (7700 for general emergency, 7600 for loss of comms, 7500 for a hijacking) to declare what's gone wrong and allow controllers to plan around it.