Definition
A Federal Aviation Regulation that sets the alternate airport requirements for domestic and flag air carrier operations conducted under 14 CFR Part 121. It specifies when an alternate airport must be listed in 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.
Plain English
This is the rule that tells airline dispatchers and pilots when they need to plan a backup airport for the flight, and what weather that backup airport must be forecast to have when they would arrive there.
Context Anchor
Seen when studying alternate airport requirements for commercial operators, especially departure alternates under Part 121 airline operations.
Derivation
CFR stands for Code of Federal Regulations, the official collection of U.S. government rules. Part 121 covers scheduled airline operations. The numbered section (§ 121.617) points to the specific rule within that part — the symbol § simply means 'section.'
Why Pilots Care
Part 121 operators must meet these standards to remain legal and safe when an immediate return to the departure airport is not possible.
Intuition Check
Do not read this as a weather minimum printed on an approach chart. It is a regulation citation: it tells Part 121 operators when a departure alternate must be listed and how close that alternate must be.
Example Sentence 1
Under 14 CFR Part 121, § 121.617, the dispatcher had to list an alternate because the destination weather was forecast to be below the required ceiling and visibility at the estimated time of arrival.
Example Sentence 2
Before releasing the flight, the captain verified that the chosen alternate satisfied 14 CFR Part 121, § 121.617 weather criteria.