Every IFR flight plan in the United States must name an alternate airport, unless the destination weather is good enough to meet the so-called 1-2-3 rule. If you do need an alternate, the alternate’s own forecast must meet alternate minimums. These are two separate tests that pilots often mix up. This guide covers the general aviation rules in 14 CFR 91.169 and 91.167. Check destination weather fast with the METAR and TAF Decoder.
Test 1: do you need an alternate at all?
Under 14 CFR 91.169(b), for aeroplanes, you do not need to file an alternate if both of these are true:
- The first airport of intended landing has a published standard instrument approach procedure (or a special one you are authorised to use), and
- From 1 hour before to 1 hour after your estimated time of arrival, the forecast ceiling will be at least 2,000 ft above the airport elevation and the visibility at least 3 statute miles.
That is where “1-2-3” comes from: 1 hour, 2,000 ft, 3 miles. If either condition fails, an alternate is required.
Test 2: is this airport good enough to be your alternate?
Under 14 CFR 91.169(c), unless the approach chart publishes nonstandard alternate minimums, the forecast at the alternate at your estimated time of arrival there must be at least:
| Approach available at the alternate | Ceiling | Visibility |
|---|---|---|
| Precision approach | 600 ft | 2 SM |
| Non-precision approach | 800 ft | 2 SM |
| No instrument approach | Weather allowing descent from the MEA, approach and landing under basic VFR | |
Chart symbols change this. A triangle with an “A” on an approach chart means the airport has nonstandard alternate minimums: look them up in the chart supplement section listing them. “A NA” means the airport cannot be used as an alternate at all, often because it has no weather reporting.
Fuel follows from the answer
Under 14 CFR 91.167, an IFR flight must carry enough fuel to fly to the destination, then to the alternate if one is required, and then for 45 minutes at normal cruise speed. So the alternate decision directly sets your minimum fuel. See also our guide to fuel reserve rules.
Worked example
You plan to land at 1500Z. The destination TAF shows:
- 1300Z to 1600Z: BKN018, visibility 4 SM
Test 1: The window is 1400Z to 1600Z. The ceiling of 1,800 ft is below 2,000 ft, so the 1-2-3 rule fails. You must file an alternate.
Test 2: A nearby airport with an ILS forecasts OVC008, visibility 3 SM at your ETA there. The precision standard is 600 ft and 2 SM. 800 ft and 3 SM meet it, so the airport can be your alternate, provided its approach chart shows no “A NA” and no higher nonstandard minimums.
Fuel: you now need fuel to the destination, plus to the alternate, plus 45 minutes.
Traps worth knowing
- “Ceiling” means broken or overcast. A scattered layer below 2,000 ft is not a ceiling. Vertical visibility (VV) is.
- The whole two-hour window counts. A TEMPO or PROB group with lower weather inside the window can break the 1-2-3 rule. Many instructors treat any forecast that dips below the limits in the window as a fail.
- The alternate’s forecast is for your ETA there, not for when you file and not for your destination ETA.
- GPS-only approaches at the alternate. Equipment and procedure restrictions can affect which approaches you may plan to use at an alternate. Check the current AIM and your avionics limitations.
- Air carriers use different rules. Operators under Parts 121 and 135 follow their own alternate requirements and operations specifications, not just Part 91.
FAQ
Can I file to an airport with no instrument approach?
Yes, but then an alternate is always required, because the first condition of the 1-2-3 exception cannot be met.
Does the 1-2-3 rule use the TAF or the METAR?
It uses the forecast for the period around your arrival, normally the TAF. A current METAR shows what is happening now, not what will be there at your ETA.
Are the rules the same outside the US?
No. EASA, the UK and other authorities set their own destination-alternate and planning-minima rules. Use the rules of the authority your flight operates under.
Sources and further reading
14 CFR 91.169 on eCFR, 14 CFR 91.167, the FAA Instrument Procedures Handbook and the Aeronautical Information Manual. Rules can change: always check the current regulations and charts before flight.
Sources checked November 2026.