Definition
A regulatory shorthand for the IFR alternate airport requirement under 14 CFR 91.169. An alternate airport must be filed on an IFR flight plan if, for at least 1 hour before to 1 hour after the estimated time of arrival at the destination, the forecast weather is less than 2,000 feet ceiling and/or less than 3 statute miles visibility.
Plain English
If the destination's forecast weather, within an hour either side of your arrival, shows the cloud base below 2,000 feet or visibility under 3 miles, you must list a backup airport on your IFR flight plan.
Context Anchor
Seen during IFR preflight planning when deciding whether the flight plan must include an alternate airport.
Derivation
Named for the three numbers it contains: 1 hour (time window before and after ETA), 2,000 feet (ceiling threshold), and 3 statute miles (visibility threshold). The numbering makes the rule easy to recall during preflight planning.
Why Pilots Care
Filing an alternate when required is a legal obligation, not a suggestion. Missing the rule means an illegal flight plan and, more importantly, no planned escape if the destination weather deteriorates. It is one of the first checks an instrument pilot makes after pulling the destination forecast.
Intuition Check
Do not read the 1, 2, 3 Rule as landing minimums or as a guarantee that the destination will be usable. It is only the planning rule for deciding whether an alternate airport must be listed.
Example Sentence 1
The TAF showed a 1,500-foot ceiling around our ETA, so under the 1, 2, 3 rule we filed Lincoln as our alternate.
Example Sentence 2
The pilot used the 1-2-3 rule to confirm that fuel planning could remain standard without extra reserve for an alternate.