Definition
The set of regulatory conditions under 14 CFR 91.169 that determine when an IFR flight plan must list an alternate airport, and the weather minimums that airport must meet to be legally usable as that alternate. In general, an alternate must be filed unless the destination has an instrument approach procedure and the forecast weather at the destination, from one hour before to one hour after the estimated time of arrival, calls for a ceiling of at least 2,000 feet above the airport elevation and visibility of at least 3 statute miles. When required, the filed alternate must have forecast weather at the ETA that meets either the standard alternate minimums (600-2 for a precision approach, 800-2 for a non-precision approach) or any non-standard minimums published for that airport.
Plain English
The rules that tell a pilot when they must name a backup airport on an IFR flight plan, and how good the weather has to be at that backup airport for it to count.
Context Anchor
Seen when preparing and filing an IFR flight plan, especially when checking destination weather before departure.
Derivation
Alternate' comes from the Latin alternatus, meaning 'one after the other' or 'a second choice.' In aviation it carries that exact sense: a second airport held in reserve in case the first one cannot be used.
Why Pilots Care
Ensures a usable diversion airport exists if the destination weather drops below landing minimums, preventing an unsafe situation or regulatory violation.
Intuition Check
Do not read alternate as “any airport I can decide on later.” In this context, it means a specific backup airport planned in advance and checked against FAA weather rules.
Example Sentence 1
Because the forecast at the destination showed a ceiling of 1,500 feet at our ETA, we had to file an alternate to meet IFR alternate requirements.
Example Sentence 2
Because the destination was forecast below minimums, IFR alternate requirements forced selection of a second airport with adequate weather.