9.alternate-airport-requirements. IFR Alternate Airport Requirements
Under IFR, the alternate airport is a backup destination filed in the flight plan to provide a legal and practical option if weather or other conditions prevent landing at the intended destination. The regulatory framework for when an alternate is required, and the weather minimums that airport must meet, is found in 14 CFR 91.169 and supported by guidance in the AIM and the Instrument Flying Handbook, Chapter 9.
When an Alternate Must Be Filed (the 1-2-3 Rule)
For a flight operated under IFR to a civil airport in the United States, an alternate is required unless:
- For at least 1 hour before to 1 hour after the estimated time of arrival (ETA),
- The forecast ceiling is at least 2,000 feet above the airport elevation, and
- The forecast visibility is at least 3 statute miles.
If any one of these conditions is not met, an alternate must be listed on the IFR flight plan. This is commonly memorized as the "1-2-3 rule."
The applicable forecast is normally the TAF for the destination. If a TAF is not issued for the destination, the area forecast or other approved sources may be used. Note that GPS-only approach destinations and certain operations under Part 121/135 have additional requirements beyond Part 91.
Selecting a Legal Alternate — Weather Minimums
Under 91.169(c), at the ETA at the alternate, the forecast weather must meet the following minimums:
- Precision approach available: ceiling 600 feet and visibility 2 statute miles.
- Nonprecision approach available: ceiling 800 feet and visibility 2 statute miles.
- No instrument approach procedure published at the alternate: the ceiling and visibility must allow descent from the MEA, approach, and landing under basic VFR (typically 1,000 ft ceiling and 3 SM visibility, but actually defined by the airspace at the airport).
These alternate minimums apply only for filing purposes. Once en route, the pilot may proceed to the alternate using the standard published approach minimums on the instrument approach chart for the actual approach being flown.
Standard vs. Nonstandard Alternate Minimums
Many airports have nonstandard alternate minimums, indicated on approach charts by a black triangle containing a white "A" (▲A). This symbol means the airport either:
- Cannot be used as an alternate (notated "Alternate NA", often due to lack of weather reporting or unmonitored facilities), or
- Requires alternate minimums higher than the standard 600-2 / 800-2.
Nonstandard minimums are published in the front of the Terminal Procedures Publication (TPP) under "IFR Alternate Airport Minimums." Always check this section before filing.
GPS and RNAV Considerations
With WAAS-equipped aircraft using LPV, LNAV/VNAV, or LNAV approaches, the FAA permits use of these approaches for filing alternates as long as the nonprecision (LNAV) line of minima is used to evaluate alternate weather (800-2 standard). Non-WAAS RNAV (GPS) users may file an airport with a GPS approach as an alternate, but they must plan to use a non-GPS approach if their destination also relies solely on GPS — i.e., the destination and alternate cannot both require GPS unless WAAS-equipped and the aircraft is operating per AC 90-105 guidance.
Required Fuel
Under 14 CFR 91.167, no person may operate an aircraft in IFR conditions unless it carries enough fuel (considering weather and forecast winds) to:
- Complete the flight to the first airport of intended landing;
- Fly from that airport to the alternate (if one is required); and
- Fly thereafter for 45 minutes at normal cruising speed.
If no alternate is required (because the 1-2-3 rule is satisfied), only the destination plus 45 minutes is required.
Practical Filing Example
Destination KXYZ, ETA 1800Z. Forecast ceiling/visibility from 1700Z–1900Z is 1,800 ft broken, 4 SM. Because the ceiling is below 2,000 ft, an alternate must be filed even though visibility exceeds 3 SM.
The pilot selects KABC, which has an ILS (precision approach). The TAF for KABC at 1800Z forecasts 700 ft overcast, 3 SM. This meets the 600-2 precision minimum and is therefore a legal alternate. Required fuel: KXYZ + KXYZ→KABC + 45 minutes at cruise.
Key Takeaways
- The 1-2-3 rule determines whether an alternate is required.
- The 600-2 / 800-2 / basic VFR rule determines which airports qualify.
- Always check the ▲A symbol and the TPP for nonstandard or NA designations.
- Fuel requirements increase whenever an alternate is filed.
- File minimums apply only at filing time; approach minimums apply at the airport.