When Do You Need an Alternate Airport for an IFR Flight?

Quick Answer

Under 14 CFR 91.169 you must file an alternate unless, from 1 hour before to 1 hour after your ETA, the destination forecast shows a ceiling of at least 2,000 feet and visibility of at least 3 SM — the "1-2-3 rule" — and the destination has a published instrument approach. No instrument approach at the destination means an alternate is always required, regardless of forecast.

Applying 1-2-3 to a Real TAF

The oral version of this question is never abstract. The DPE hands you a TAF and your ETA and watches you work: does any group covering the ETA-plus-or-minus-one-hour window drop the ceiling below 2,000 or the visibility below 3? Conditional groups count against you — a "TEMPO 1SM -SHRA BKN008" inside your window busts the 1-2-3 test even though the prevailing line looks fine. Applicants who only read the first line of the TAF hand the examiner an easy escalation.

And remember the second prong: the exemption exists only when the destination has a published instrument approach procedure (91.169(b)(1)). A perfect forecast at a no-IAP field still requires an alternate, because without a procedure you have no instrument way down.

What Makes an Airport Legal to File as the Alternate

This is the layer most applicants blur, and DPEs drill it deliberately. To file an airport as your alternate, its forecast at your ETA must meet the alternate minimums: the standard values are ceiling 600 feet and 2 SM for a precision approach, or 800 feet and 2 SM for a nonprecision approach — unless the airport has nonstandard published alternate minimums, flagged on the chart by the triangle-A symbol (91.169(c)). An airport whose approach chart carries the "A NA" symbol cannot be used as an alternate at all — often because it lacks local weather reporting.

If the alternate has no instrument approach, it may still be filed only if the forecast allows a descent from the MEA and landing under basic VFR.

GPS wrinkles are fair game too: under AIM 1-1-18 rules, WAAS-equipped aircraft may plan RNAV (GPS) approaches at both destination and alternate (flight-planning uses the LNAV line for non-baro-VNAV aircraft); non-WAAS aircraft face restrictions on relying on GPS approaches at both airports.

Filing vs. Flying: the Distinction That Wins the Question

The 600-2 and 800-2 numbers are planning minimums for filing — they exist to make sure the airport you name is likely to be usable. If you actually divert there, you fly the minimums published on the approach chart, whatever they are. Applicants who say they would "fly down to 600 and 2 at the alternate" reveal they memorized the rule without understanding it. And regardless of everything above, 91.167 fuel must cover destination, then alternate, then 45 minutes at normal cruise.

The judgment layer closes it out: an alternate that is legal but sits behind the same weather system is paper compliance. Name an alternate with genuinely different weather, and say so to the examiner.

The IFR Oral Exam Guide ($24) works the full alternate drill — TAF reading, nonstandard minimums, A-NA traps, fuel math — through cited Q&As and scenario drills, part of 269 examiner questions across 155 pages aligned to FAA-S-ACS-8C.