14 CFR § 99.9 - Radio requirements.
(a) A person who operates a civil aircraft into an ADIZ must have a functioning two-way radio, and the pilot must maintain a continuous listening watch on the appropriate aeronautical facility's frequency.
(c) If the pilot operating an aircraft under DVFR in an ADIZ cannot maintain two-way radio communications, the pilot may proceed, in accordance with original DVFR flight plan, or land as soon as practicable. The pilot must report the radio failure to an appropriate aeronautical facility as soon as possible.
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