Also found in: Acronyms.
mandatory frequency (MF)
A frequency designated at selected airports that are uncontrolled during certain hours only. Aircraft operating within the designated MF area, normally 5 NM radius of the airport, must be equipped with a functioning radio capable of maintaining two-way communications and must monitor and report on the mandatory frequency. Aeronautical charts list the MF frequency and area when it is other than standard 5 NM.