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.

Worked example

Destination TAF, ETA 2100Z:

TAF KABC 261720Z 2618/2718 24012KT P6SM SCT035 BKN050
  FM262000 25010KT P6SM BKN018
  TEMPO 2621/2623 4SM -RA BR OVC012
  FM270200 26006KT P6SM SCT025

The window is 2000Z to 2200Z. The FM2000 group gives a 1,800 ft broken ceiling, already below 2,000 ft, so the exemption fails on the base forecast alone. Even if that line had read BKN025, the TEMPO group starting at 2100Z forecasts 4 SM and a 1,200 ft overcast inside the window, which fails it again. Either way, an alternate goes on the flight plan.

Now flip it. Same TAF but the FM2000 line reads 25010KT P6SM BKN025 and there is no TEMPO until 2300Z: 2,500 ft and better than 6 SM for the whole 2000Z–2200Z window, and the field has a published approach. No alternate is required. Note that a TEMPO beginning at 2300Z is outside the window by one hour and does not count; a TEMPO beginning at 2200Z would.

The alternate you then pick needs its own forecast: at your alternate ETA, 600 ft and 2 SM if you will fly a precision approach there, 800 ft and 2 SM for a nonprecision one, unless the plate lists other numbers. And under 91.167 the fuel plan grows to destination, then alternate, then 45 minutes at normal cruise. The E6B fuel panel takes the reserve as a number of hours.

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. Each SkyReady airport page lists the field's approaches and links its FAA alternate minimums page under Charts.

FAQ

What is the 1-2-3 rule?

Under 14 CFR 91.169, you must file an IFR alternate if, from 1 hour before to 1 hour after your ETA, the forecast at the destination is below a 2,000 ft ceiling or 3 SM visibility. This tool reads the destination TAF for that window and tells you whether an alternate is required.

Do I need an alternate if the destination has no instrument approach?

Yes — always. The 1-2-3 exemption only applies when the destination has a published instrument approach. No approach means you must file an alternate no matter how good the forecast is.

What if my destination has no TAF?

The 1-2-3 test needs an appropriate forecast for your ETA window. Without one you generally cannot show the exemption is met, so file an alternate. (An area forecast can sometimes serve, but a TAF is the clean answer.)

Why does WAAS matter for an alternate?

When filing an alternate, a GPS/RNAV approach may only be used as the alternate if you have a WAAS receiver. A non-WAAS aircraft needs a ground-based approach (ILS, LOC, VOR, NDB) that is operational, so the tool filters candidates accordingly.

Does it use the actual alternate minimums?

It finds airports that qualify structurally (an approach you can use + weather reporting). Use the standard alternate minimums (600-2 precision / 800-2 nonprecision) unless the approach plate publishes non-standard minimums (the ⊠ symbol) or "NA". Always confirm on the plate.

What is the 0-1-2-3 rule?

The same rule with a zero added as a memory aid: "0" for zero instrument approaches at the destination, which means an alternate is required regardless of the forecast. Then 1 hour either side of ETA, 2,000 ft ceiling, 3 SM visibility.

Does a TEMPO or PROB30 group count against the 1-2-3 test?

A TEMPO group that forecasts below 2,000 ft or 3 SM inside your ETA window means the forecast does not meet the exemption, so file an alternate. PROB30/PROB40 groups are treated the same way by the FAA for planning: if the low condition is forecast in your window at all, plan the alternate. This tool evaluates every forecast group that overlaps the window, TEMPO and PROB included, and uses the worst ceiling and visibility it finds.

How much fuel does an alternate add?

Under 14 CFR 91.167 you need fuel to the destination, then to the alternate, then 45 more minutes at normal cruise. If carrying that lands you in your reserve, the alternate decision just became a fuel-stop decision.