Definition
A federal regulation governing alternate airport requirements for domestic operations conducted by commercial air carriers under Part 121. It specifies when an alternate airport must be listed on the dispatch or flight release, and the weather conditions that must be forecast at that alternate at the estimated time of arrival for it to qualify as a legal alternate.
Plain English
This is the rule that tells airline dispatchers and pilots when they must pick a backup airport for a domestic flight, and what the weather has to look like at that backup airport for it to count.
Context Anchor
Seen in instrument procedure and dispatch discussions about alternate minimums for commercial air carrier operations.
Derivation
14 CFR means Title 14 of the Code of Federal Regulations, which is the section of US federal law covering aeronautics and space. Part 121 is the chapter of that title dealing with scheduled airline operations. The § symbol means 'section,' so § 121.617 is section 617 within Part 121. Knowing the structure helps you find any FAR quickly: title, part, section.
Why Pilots Care
It guarantees a usable backup airport exists before the aircraft leaves the ground, directly affecting go/no-go decisions and safety margins on commercial flights.
Intuition Check
Do not read this as just a page number or reference label. It is a specific legal rule that controls whether a Part 121 flight may be released when departure weather is below return-to-land minimums.
Example Sentence 1
The dispatcher checked 14 CFR Part 121 § 121.617 to confirm the forecast ceiling and visibility at the proposed alternate met the rule before releasing the flight.
Example Sentence 2
Under 14 CFR Part 121 § 121.617 the captain could not depart until weather at the alternate met the operator's published minimums.