121.5 MHz is the civilian aircraft emergency frequency or International Air Distress frequency.
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International Distress/Emergency Frequencies 121.5 MHz: International Aeronautical Emergency Frequency. 156.8 MHz: International Maritime Distress, Calling and Safety Frequency. 243.0 MHz: NATO Combined Distress and Emergency Frequency. 406.0 MHz: Emergency Position Indicating Locator Beacon (EPIRB).
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