A common and feared phrase in the Infinite Flight world is the “on guard” phrase. Everyone knows what it means, but does anyone actually know why it’s called “on guard”?
In the real world, all nearby stations have a unique frequency that they transmit on. However, some frequencies are reserved. The most popular one is the “unicom” frequency (122.8 MHz). However there is one more that is not so well known to non-pilots – the guard frequency.
The guard frequency (121.5 MHz) is a frequency reserved for emergency transmissions. Some airlines are required to monitor the guard frequency in cruise, in case someone is in trouble (this is often done on VHF 2, the secondary radio channel).
This means that a lot of aircraft listen to this frequency at all times. Sometimes in real life, ATC can encounter a scenario IF ATC’s deal with a lot. An airplane is in or near your airspace, but not on your frequency. A solution to that is that the controller can try to contact the airplane via the guard frequency. To let everyone know that it is a transmission from the guard frequency, they can sometimes start the transmission with something like this:
“Delta 2 6 Tango Foxtrot, Atlanta Center on guard, contact me on 134.80”
This can also be done, and actually has been done in the case of a pilot having a stuck mic.
Now you know what the phrase really means! :)