US-FAA14 CFR 121.609
Communication and navigation facilities: Supplemental operations
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No person may release an aircraft over any route or route segment unless communication and navigation facilities equal to those required by § 121.121 are in satisfactory operating condition.
Verbatim text of a US Federal Aviation Regulation (public domain), reproduced for reference — not legal advice and not for navigation. A plain-English explainer may be added later; always rely on the current official rule linked above.