Guard Frequency

From The RadioReference Wiki

Guard Frequency, a.k.a Guard Channel, as defined by Wikipedia, is

The aircraft emergency frequency (also known as guard) is a frequency used on the aircraft band reserved for emergency communications for aircraft in distress. The frequencies are 121.5000 MHz for civilian, also known as International Air Distress (IAD) or VHF Guard, and 243.0000 MHz for military use, also known as Military Air Distress (MAD) or UHF Guard. -- Wikipedia:Aircraft emergency frequency