Definition
An alternate airport is a second airport, named in a flight plan, that a pilot will fly to if landing at the planned destination becomes impractical due to weather, equipment problems, or other operational factors. Under instrument flight rules, an alternate must meet specific weather and facility requirements set out in the regulations.
Plain English
A backup airport you plan to use if you can't land where you originally intended.
Context Anchor
Seen in aviation notices, flight planning text, airport information, and other shortened aviation messages where space is limited.
Derivation
From the Latin alternus, meaning 'one after the other.' In flight planning, the alternate is the next choice in line if the first one doesn't work out.
Why Pilots Care
Regulatory requirements and safe operations often mandate an alternate airport so the flight has a viable landing option if conditions change.
Intuition Check
ALTN does not mean a vague backup idea. In aviation text, it points to a specific alternate choice, such as another airport, route, or procedure.
Example Sentence 1
With a line of thunderstorms moving over the destination, the pilot filed a coastal airport as the alternate.
Example Sentence 2
Low ceilings at the primary airport forced the crew to proceed directly to the ALTN.