AIM 6-5-1 Discrete Emergency Frequency
Chapter 6. Emergency Procedures · Section 5. Aircraft Rescue and Fire Fighting Communications
A reference copy of the official AIM text for study. It is not legal advice or an FAA interpretation; always confirm against the current official source.
a. Direct contact between an emergency aircraft flight crew, Aircraft Rescue and Fire Fighting Incident Commander (ARFF IC), and the Airport Traffic Control Tower (ATCT), is possible on an aeronautical radio frequency (Discrete Emergency Frequency [DEF]), designated by Air Traffic Control (ATC) from the operational frequencies assigned to that facility.
b. Emergency aircraft at airports without an ATCT, (or when the ATCT is closed), may contact the ARFF IC (if ARFF service is provided), on the Common Traffic Advisory Frequency (CTAF) published for the airport or the civil emergency frequency 121.5 MHz.