Definition
121.5 MHz is the international aeronautical emergency radio frequency, monitored by air traffic control, military facilities, and many aircraft for distress calls and emergency locator transmitter (ELT) signals. It is also designated as the VHF guard frequency.
Plain English
It's the radio channel set aside worldwide for emergencies in aviation. If a pilot is in trouble or an aircraft's emergency beacon activates after a crash, this is the frequency people listen on and call out on.
Context Anchor
You may see 121.5 megahertz in emergency procedures, lost-contact procedures, and instructions to monitor the emergency frequency.
Derivation
Megahertz' (MHz) means one million cycles per second and is named after Heinrich Hertz, who first demonstrated radio waves. So 121.5 MHz simply identifies a specific spot on the VHF radio band -- not a name or code, just a precise tuning point.
Why Pilots Care
Provides an immediate, always-monitored channel to declare an emergency or request assistance when primary frequencies are unavailable.
Grounding Statement
Think of 121.5 megahertz as the aviation emergency radio channel that aircraft can use when normal communication is not working or immediate help is needed.
Intuition Check
121.5 megahertz is not a routine chat frequency or a normal flight-following frequency. In aviation use, it is reserved for emergency or urgent communication and emergency monitoring.
Example Sentence 1
After losing contact with Center, the pilot tuned the standby radio to 121.5 and broadcast in the blind on guard.
Example Sentence 2
Controllers monitor 121.5 megahertz continuously for any aircraft declaring an emergency.