The aircraft emergency frequency (also known as “guard”) on the aircraft radio band is reserved for communications by aircraft in distress. The frequencies are 121.5 MHz for civilian airplanes and 243.0 MHz for the military, and both are used internationally.
In the U.S., 121.5 MHz is monitored by most air traffic control towers and other inflight service functions. Pilots are encouraged to set a secondary communications radio on guard channel to monitor the frequency at all times without transmitting on it.
If an aircraft encounters an emergency situation while not in radio contact with air traffic control, the pilot can transmit on guard channel and in most situations be assured that either a ground station or the pilot of another airplane will hear the transmission and be able to provide some form of assistance.
Another advantage of monitoring the channel is the ability to receive a warning from air traffic control if an aircraft is on a course to inadvertently penetrate restricted or prohibited airspace. More commonly, air traffic control can use guard channel to establish radio contact with an aircraft that has switched to an incorrect radio frequency.
One downside of keeping a secondary radio tuned to receive guard channel is the ever-present possibility of switching to that radio for both receiving and transmitting when you don’t intend to. You think you’re on the correct discrete frequency, when in reality you are transmitting “to the whole world.”
Seldom does this mistake result in anything other than feeling like a fool, but even so, pilots often come up with a way of deflecting criticism. In the following recent example, the tactic involved an attempt to share the blame.
Unknown Pilot #1 (On Guard): “Hey, Greg — are you up?”
Unknown Pilot #2 (On Guard — presumably “Greg”): “You’re on guard!”
Pilot #1: “Really?”
Pilot #2: “Really.”
Pilot #1: “Well, so are you!”