Definition
243.0 megahertz (MHz) is the international military aeronautical emergency radio frequency, monitored by military aircraft, search and rescue services, and air traffic control facilities for distress and urgency communications. It is the UHF counterpart to the civilian VHF emergency frequency 121.5 MHz, and is sometimes called the military 'guard' frequency.
Plain English
A specific radio channel set aside for emergencies. Military aircraft and rescue services constantly listen on it, so a pilot in serious trouble can transmit on this frequency and expect someone to hear and respond.
Context Anchor
Seen in emergency communications, lost-aircraft procedures, and discussions of frequencies monitored for distress calls.
Derivation
Megahertz means 'millions of cycles per second' and refers to how many times per second a radio wave oscillates. 'Hertz' is named after Heinrich Hertz, the German physicist who first demonstrated radio waves. Saying 243.0 MHz simply identifies one specific channel on the radio dial.
Why Pilots Care
Tuning this frequency allows immediate contact with military assets or joint rescue centers when civilian frequencies are unavailable or during off-airport emergencies.
Intuition Check
Do not read 243.0 as a heading, altitude, or navigation course here. In this context, 243.0 megahertz is a radio frequency used for emergency communication.
Example Sentence 1
After losing contact with his assigned controller, the pilot tuned 243.0 MHz and broadcast a mayday call.
Example Sentence 2
Many crews leave one radio on 243.0 megahertz for guard monitoring while flying instrument approaches near military training routes.