Definition
A radio frequency (121.5 megahertz) reserved internationally for emergency communications in aviation. It is monitored by air traffic control facilities, many airline aircraft, and search and rescue services, and is the standard frequency a pilot uses when lost, in distress, or unable to establish contact on a normal working frequency.
Plain English
A special radio channel set aside for emergencies. If a pilot is lost or in trouble and cannot reach anyone on the frequency they were using, they switch to 121.5 because someone is almost always listening.
Context Anchor
Seen in lost procedures, emergency radio procedures, and cockpit radio use when a pilot needs help and normal communication is not working.
Derivation
Megahertz (MHz) means millions of cycles per second, describing the radio wave's frequency. The number 121.5 was chosen and protected by international agreement so that aircraft anywhere in the world have a single, common frequency for emergencies.
Why Pilots Care
It gives pilots a direct way to reach air traffic control or other aircraft for rescue coordination when normal frequencies are unavailable.
Grounding Statement
If you are lost and normal radio calls are not working, 121.5 MHz is the frequency to try for help.
Intuition Check
Emergency frequency does not mean any frequency a pilot happens to use during an emergency. In this context, it means the specific aviation radio frequency 121.5 MHz set aside for emergency and urgent help.
Example Sentence 1
Unable to raise approach control after several attempts, the pilot tuned 121.5 MHz and declared they were lost.
Example Sentence 2
The controller advised all aircraft in the area to monitor emergency frequency 121.5 MHz for a missing flight.