The One Two Three Rule, FAR Ninety-One Point One Sixty-Nine, and When an IFR Flight Actually Needs a Filed Alternate

SUMMARY: When an IFR flight needs a filed alternate under the 1-2-3 rule, and the exact weather minimums that make an airport legal to file.

Under FAR 91.169, an IFR flight needs a filed alternate unless the destination forecast holds at least a 2,000-foot ceiling and 3 statute miles visibility for the window from one hour before to one hour after your estimated time of arrival. This is the 1-2-3 rule: 1 hour on either side, 2,000-foot ceiling, 3 miles visibility. If the forecast beats all three across that full two-hour block, no alternate is required - if it fails even one, you file.

When Does an IFR Flight Need a Filed Alternate?

The alternate requirement lives in FAR 91.169, and it hinges on one piece of memory work instrument students have recited for decades. You are required to file an alternate airport unless the destination forecast calls for a ceiling of at least 2,000 feet above airport elevation and visibility of at least 3 statute miles - for the full window of one hour before to one hour after your ETA.

That’s it. One, two, three. One hour on either side, 2,000-foot ceiling, 3 miles.

The window is where pilots get sloppy. It is not just your arrival time - it is a full two-hour block. If you plan to touch down at 3:00 p.m., the forecast has to hold 2,000-and-3 from 2:00 p.m. all the way to 4:00 p.m.

The padding exists because forecasts are promises, not facts. The FAA wants enough margin for you to shoot the approach, possibly miss it, come back around, and try again without the weather quietly collapsing while you’re busy.

So the first job is to read the terminal forecast and ask: does it hold 2,000-and-3, plus or minus an hour around my arrival? If a TEMPO or PROB group drags conditions below the line anywhere inside that window, treat it as a “no” and file.

What Airport Is Legal to File as an Alternate?

Here is the part people fumble on checkrides: the minimums you use to plan to your alternate are not the same numbers you used at your destination. They are a different set entirely, and they depend on the type of instrument approach the alternate has.

Standard alternate minimums:

Precision approach (vertical guidance, a glideslope): forecast of at least a 600-foot ceiling and 2 statute miles - spoken as “600-2.”
Non-precision approach (lateral guidance and step-downs only): 800-foot ceiling and 2 miles - “800-2.”
No instrument approach at all: you must be able to make the descent from the minimum en route altitude, the approach, and the landing all under basic VFR.

These numbers are applied at your estimated time of arrival at the alternate.

Why tighter than the destination? Because the destination test is a generous 2,000-and-3 for the airport you actually want. The alternate is your backup - the place with better weather that gives you a real fighting chance if plan A falls apart.

Planning Minimums vs. Approach Minimums - Don’t Confuse Them

This distinction matters. 600-2 and 800-2 are planning numbers only. They are the forecast conditions an airport needs to qualify for the box back when you’re filing at the kitchen table.

They are not the minimums you fly once you actually divert. If you go missed at your destination and head to the alternate, you fly the published minimums on the approach plate - the MDA, the DA, the charted visibility. The airport does not suddenly require 600-2 when you show up. That number was just the ticket price to get it on your flight plan. Keep the two ideas in separate boxes.

The WAAS and RNAV Alternate Wrinkle

If you fly with modern GPS, note a long-standing quirk with satellite-based approaches. For years, a WAAS-capable GPS (the Wide Area Augmentation System, which delivers LPV approaches with vertical guidance) let you plan an RNAV approach at your alternate - but you had to plan using the non-precision line of minima (800-2), not 600-2, even if the approach could fly like a precision approach on the day.

The FAA has been loosening this, and specific guidance depends on your equipment. This is an area where the rules have moved, so verify against current FAA publications, not memory - read the alternate minimums section of the Aeronautical Information Manual and the current Chart Supplement rules, and know exactly what your navigator is approved for.

Standard vs. Non-Standard Alternate Minimums

Standard minimums (600-2, 800-2) are the default, but many airports carry non-standard alternate minimums - higher numbers, or no authorization as an alternate at all.

You find out by reading the approach plate. On government charts, look for a black triangle with a white letter “A” inside it. When that symbol appears, the alternate minimums there are not standard - go look them up.

You look them up in a separate section in the front of the terminal procedures publication, the alternate minimums section (sometimes called the takeoff and alternate minimums pages). It lists the specific numbers for that airport and approach, often higher because of terrain, out-of-service equipment, or missing weather reporting.

Sometimes you’ll see “NA” - alternate minimums not authorized. That airport cannot be used as your filed alternate, period, no matter how good the forecast is. Often it’s because the field has no weather reporting or a required monitoring facility is unavailable. If you see NA, cross it off and find another airport.

The Full Alternate Workflow, Start to Finish

Check the destination forecast. Does it hold a 2,000-foot ceiling and 3 miles visibility for the full window, one hour before to one hour after arrival? If yes, you’re done - no alternate needed. If no, continue.
Pick candidate alternates near your destination, ideally where the weather is genuinely better, and check what approaches each one has.
Apply the right planning minimum at your ETA at that alternate: precision 600-2, non-precision 800-2, no approach basic VFR to descend and land.
Pull the approach plate and look for the triangle-A symbol. If it’s there, use the non-standard numbers from the alternate minimums section. If you find NA, that field is illegal as an alternate - pick another.
Write it in the box and fuel for it. Under Part 91 IFR, you need enough fuel to fly to your destination, then to the most distant filed alternate, and then still have 45 minutes at normal cruise on top of that. The alternate isn’t paperwork - it’s a place you might really have to fly to.

Why This Matters for Pilots

The 1-2-3 rule is a legal minimum - the floor, not the goal. Nothing stops you from filing an alternate even when the forecast says you don’t need one. On a marginal day, when fronts are moving and forecasters are hedging with PROB and TEMPO lines, filing an alternate you don’t strictly need is one of the cheapest insurance policies in aviation: a few minutes of planning and a little extra fuel.

The pilots who get backed into a corner are rarely the ones who filed an alternate they didn’t need. They’re the ones who read the forecast the way they wanted it to read, skipped the box, and then watched the weather do exactly what the forecaster warned about. Always have somewhere else to go.

For further reading, the regulation itself is FAR 91.169, and a clear breakdown of the 1-2-3 rule and alternate minimums is available at Boldmethod.

Key Takeaways

File an alternate unless the destination forecast holds a 2,000-foot ceiling and 3 miles visibility from one hour before to one hour after your ETA (the 1-2-3 rule, per FAR 91.169).
Standard alternate planning minimums are 600-2 for a precision approach and 800-2 for a non-precision approach; with no approach, you need basic VFR to descend and land.
Planning minimums are not flying minimums - once you divert, you fly the published minimums on the approach plate.
A black triangle with an “A” flags non-standard alternate minimums; “NA” means the airport cannot be filed as an alternate at all.
Carry enough fuel to reach your destination, then your most distant alternate, plus 45 minutes of reserve at normal cruise.