IFR alternate required?

The IFR 1-2-3 rule, explained

The "1-2-3 rule" (also written "123 rule") is the memory aid for 14 CFR 91.169's alternate requirement. You may skip filing an alternate only when both are true:

• 1 — from 1 hour before to 1 hour after your estimated time of arrival,
• the destination forecast calls for at least a 2,000 ft ceiling and 3 SM visibility,
• and the destination has a published instrument approach.

Anything less — a TEMPO group dipping to 1,500 ft in your window, no TAF at all, or a destination without an approach — and an alternate goes on the flight plan. Once one is required, the standard alternate minimums apply at the airport you pick: forecast at your ETA of 600-2 with a precision approach or 800-2 with a nonprecision approach, unless the plate publishes non-standard minimums.

Filing an alternate is more than picking the next airport over

The 1-2-3 rule tells you whether you need an alternate. Choosing a legal one is the part that trips pilots up: the alternate needs an instrument approach you're equipped to fly, weather that's being reported, and forecast conditions that meet the alternate minimums. A GPS-only airport doesn't count for a non-WAAS aircraft. This tool checks the destination TAF for the ±1 hr window, then lists nearby airports that actually qualify for your equipment.

How it decides

It parses the live destination TAF, finds the periods covering your ETA window, and flags an alternate as required if the forecast ceiling drops below 2,000 ft or visibility below 3 SM. The candidate list is filtered to airports with a published approach and a reporting weather station — and, for non-WAAS, to those with a ground-based approach.

Always confirm on the plate

Standard alternate minimums are 600-2 (precision) and 800-2 (nonprecision), but many airports publish higher, non-standard minimums (the ⊠ symbol) or are "NA" as an alternate. Verify the approach plate and current NOTAMs before you file.