Definition
The minimum weather conditions — specifically ceiling and visibility — that must be forecast at an alternate airport at the estimated time of arrival in order for that airport to be legally listed as the alternate on an IFR flight plan. These minimums are higher than standard approach minimums and are set by regulation, with specific values published for each approach procedure or, where none are published, applied by standard rule.
Plain English
The weather forecast at your backup airport has to be good enough to meet a stricter standard before you're allowed to put that airport down as your alternate.
Context Anchor
Seen when planning an IFR helicopter flight under Part 135 and deciding whether a backup landing site can legally be listed as the alternate.
Derivation
“Alternate” comes from a root meaning “one after the other” or “the other of two.” In aviation, it points to the other place you plan to use if the first landing place is not available. “Minimum” means the lowest allowed amount, which fits the idea of the lowest weather that is still acceptable.
Why Pilots Care
Ensures a legal and safe diversion option remains available, preventing a situation where the aircraft has nowhere to land within regulatory limits.
Grounding Statement
This term is about whether your backup landing site has good enough forecast weather to be a real option, not just a name on the flight plan.
Intuition Check
Do not read “alternate” as merely optional or convenient; here it means the required backup landing place. Do not read “minimums” as the lowest weather you personally feel comfortable using; they are legal weather limits that must be met or exceeded.
Example Sentence 1
The forecast at the alternate was 800 feet and two miles, which met the alternate landing minimums for the ILS approach, so the pilot filed it on the IFR flight plan.
Example Sentence 2
With the destination forecast below minimums, the crew confirmed the alternate landing minimums met Part 135 requirements.