The 1-2-3 Rule and IFR Alternate Requirements
Learn when 14 CFR 91.169 requires an IFR alternate, how the 1-2-3 rule reads the forecast window, and what the 600-2 and 800-2 numbers actually filter.
An IFR flight plan needs an alternate airport unless two things are true at once. The destination has a published instrument approach, and the forecast holds the ceiling at least 2,000 feet above the airport elevation and the visibility at least 3 statute miles, from one hour before your estimated time of arrival to one hour after it. That is the entire trigger, and it lives in 14 CFR 91.169(b). Fail either half and you file. Pilots remember the figures as the 1-2-3 rule: one hour either side, 2,000 feet, 3 miles.
Filing one is half the job. The airport you name has to clear a separate bar, at a different time, against a different forecast. At your ETA at the alternate, the forecast ceiling and visibility must be at or above that airport’s alternate minimums: whatever the approach chart publishes, or 600 and 2 for a precision approach and 800 and 2 for a nonprecision one where it publishes nothing. Nearly every muddled answer we have heard comes from running those two tests together.
Both are Part 91 floors, not plans. The VFR half of the same question, the airspace-by-airspace table in 91.155, is in our guide to VFR weather minimums by airspace class.
When 91.169 actually requires an alternate
Paragraph (a)(2) of 91.169 puts an alternate on every IFR flight plan; paragraph (b) is the exception, and both of its conditions have to hold. Part 97 has to prescribe a standard instrument approach procedure to the first airport of intended landing, or the Administrator has to have issued the operator a special one. And “appropriate weather reports or weather forecasts, or a combination of them” have to indicate that “for at least 1 hour before and for 1 hour after the estimated time of arrival, the ceiling will be at least 2,000 feet above the airport elevation and the visibility will be at least 3 statute miles.”
Four things in there do most of the work.
It is 2,000 feet above the airport elevation. Not 2,000 MSL, and not 2,000 above the terrain you cross to get there. TAF cloud heights are referenced to the airport they describe, so at a field with its own forecast the arithmetic is direct. It stops being direct when you borrow a forecast from a field 30 NM away that sits 900 feet lower.
It is a two-hour window, not a moment. An ETA of 1300Z puts the test on 1200Z through 1400Z, and the condition has to hold across all of it. A forecast that dips below 2,000 or below 3 SM anywhere inside those two hours has not met the exception, even if the ceiling at your ETA is beautiful.
It is a prediction, not an observation. The regulation accepts reports, forecasts, or a combination, but what it asks them to establish is a two-hour condition that has not happened yet. An observation of 5,000 scattered and 10 miles settles nothing about a landing two hours out, which is the difference between what a METAR observes and what a TAF forecasts. With no TAF at the destination, the Graphical Forecasts for Aviation are usually where you start.
No published approach means no exception. If Part 97 prescribes nothing for your destination, paragraph (b) cannot apply and you file an alternate however good the weather is. The FAA confirmed in a 2015 legal interpretation that you may file IFR to an airport with no instrument approach, provided the flight plan carries an alternate.
Helicopters run a different test under 91.169(b)(2)(ii): from the ETA to one hour after it, a ceiling at least 1,000 feet above airport elevation or 400 feet above the lowest applicable approach minima, whichever is higher, and 2 statute miles.
| Destination test, 91.169(b) | Alternate test, 91.169(c) | |
|---|---|---|
| What it decides | Whether you file an alternate at all | Whether a given airport may be that alternate |
| When it is judged | 1 hour before to 1 hour after ETA at the destination | At ETA at the alternate, a single point in time |
| Ceiling | 2,000 ft above airport elevation | As charted; otherwise 600 ft precision, 800 ft nonprecision |
| Visibility | 3 SM | As charted; otherwise 2 SM |
| Where the numbers come from | The regulation, always | The approach chart first, the regulation as the fallback |
Reading the window in a real TAF
Take a light single with a 1300Z arrival at Springfield, Illinois (KSPI). The forecast issued at 1120Z, valid 1200Z on the 3rd through 1200Z on the 4th, reads like this: wind 140 at 6, visibility better than 6 statute miles, broken at 3,000. Temporarily between 1300Z and 1600Z, visibility 4 statute miles in mist with a broken ceiling at 1,200 feet. From 1700Z, wind 170 at 10, better than 6 miles, scattered at 4,000.
TAF KSPI 031120Z 0312/0412 14006KT P6SM BKN030
TEMPO 0313/0316 4SM BR BKN012
FM031700 17010KT P6SM SCT040Our TAF decoder splits those groups apart, and the glossary covers the individual codes.
The window is 1200Z to 1400Z. The base forecast is comfortable at 3,000 and better than 6. The TEMPO group is not: from 1300Z it puts a 1,200-foot ceiling inside the window. Visibility never fails, since 4 SM clears the 3 SM requirement. The ceiling does. That forecast does not hold 2,000 feet across the window, so this flight files an alternate.
Worth noticing what 1,200 and 4 would look like on a moving map. That is marginal VFR by the display categories, a benign enough color on most weather apps and completely irrelevant to 91.169. The regulation does not know what MVFR is.
For the alternate, take Decatur (KDEC), about 40 NM east, with an ETA there of 1325Z. Its forecast for that hour: wind 150 at 6, visibility 4 statute miles in mist, broken at 700.
TAF KDEC 031120Z 0312/0412 15006KT 4SM BR BKN007
FM031500 17008KT P6SM BKN025
FM032100 18008KT P6SM SCT045Decatur has an ILS or LOC Rwy 6. Plan the ILS and the standard filter is 600 and 2, which 700 and 4 clears with room. Decatur qualifies and the fuel gets built to reach it.
Now move the same arrival to a time when the tower is closed. The FAA’s alternate minimums listing for Decatur, in the chart cycle effective 9 July 2026, marks the ILS and LOC not authorized for alternate use when the control tower is closed, except for operators with approved weather reporting services. Take the ILS away and you plan the RNAV (GPS) Rwy 6, a nonprecision line and an 800-2 filter. A 700-foot forecast ceiling no longer qualifies, and the answer has flipped on the clock alone.
What the alternate has to meet
91.169(c) is the filter, written airport by airport. Where an instrument approach procedure is published, the minima are “the alternate airport minima specified in that procedure, or if none are specified” the standard 600 and 2 for a precision approach and 800 and 2 for a nonprecision one. The chart is the authority; the standard numbers are the fallback.
Non-standard alternate minimums are common, and they are published. They live in the alternate minimums section at the front of each Terminal Procedures Publication volume, and the approach chart flags them with an Ⓐ symbol, per chapter 4 of the Instrument Procedures Handbook. The entries carry restrictions as well as raised numbers: not authorized when local weather is unavailable, not authorized when the tower is closed, a higher ceiling for a faster approach category. Memorize 600-2 and 800-2, never open that section, and you are planning against numbers that may not be in force.
Two more things from the same chapter. Approaches with vertical guidance count as nonprecision here, LPV and LNAV/VNAV among them, because they do not meet the ICAO Annex 10 standards for a precision approach; planning to an LPV gets you 800-2. And an approach charted A NA is not available as an alternate at all. A 2006 FAA legal interpretation settled that: you may not list such an airport when an alternate is required, even if the forecast would allow descent from the MEA, an approach, and a landing under basic VFR.
Where no instrument approach exists at the alternate, 91.169(c)(2) sets the bar as the ceiling and visibility “allowing descent from the MEA, approach, and landing under basic VFR.” The MEA feeding that airport is part of the calculation, which is why a no-approach alternate under a 3,000-foot overcast in mountainous country usually fails.
The 600-2 and 800-2 numbers apply at planning time only
This is the part that gets skipped, and the Instrument Procedures Handbook states it plainly: “The 600-2 and 800-2 rules, or any exceptions, only apply to flight planning purposes, while published landing minimums apply to the actual approach at the alternate.”
So when the diversion becomes real, the alternate minimums are done. You fly the approach to the minimums printed on the chart, and under Part 91 nothing stops you starting an approach at an airport reporting below them. An 800-2 filter at planning time and a 250-foot decision altitude on arrival answer different questions. The filter exists to stop you committing fuel to an airport forecast to be as bad as the one you are leaving.
Which also means the filed alternate carries no obligation of its own. It is a fuel calculation with an airport name attached.
Fuel, and why the alternate changes the number
14 CFR 91.167 requires enough fuel to complete the flight to the first airport of intended landing, then fly from that airport to the alternate, then fly 45 minutes beyond that at normal cruising speed (30 minutes for helicopters). Paragraph (b) carries the same 1-2-3 exception as 91.169, so no alternate required means no alternate leg in the fuel plan.
Two things get misread. The 45 minutes is at normal cruising speed, not at a lean endurance setting, so it is a bigger number than “45-minute reserve” suggests. And the alternate leg runs from the destination to the alternate, not from your departure point, so an alternate 90 NM past the destination and one 25 NM past it produce very different flight plans out of identical weather. When the legal alternate is a long way off, the fuel stop moves earlier in the route.
Where GPS and WAAS change the arithmetic
A non-WAAS receiver under TSO-C129 or TSO-C196, with fault detection and exclusion and a preflight RAIM prediction for the approach, may file based on a GPS-based approach at the destination or at the alternate, but not both (AIM 1-1-17). Plan an RNAV (GPS) arrival at the destination and the alternate needs a non-GPS approach you are equipped to fly.
A WAAS receiver under TSO-C145 or TSO-C146 may flight plan any approach its avionics are approved for at a required alternate, provided the planning uses the RNAV (GPS) LNAV or circling line, a GPS procedure, or a conventional procedure with “or GPS” in the title, at Part 91 nonprecision weather requirements (AIM 1-1-18). Plan 800-2, then fly the LPV on arrival if the box reports LPV available. The FAA has been removing the alternate-NA symbol from selected RNAV (GPS) and GPS procedures so WAAS aircraft can use them; procedures with no local weather reporting keep it.
Legal is the floor, and the floor has holes
The 1-2-3 rule is arithmetic against a forecast, and you can satisfy it and still be planning something indefensible. AIM paragraph 5-1-10 makes the point: three high-altitude US airports have approved instrument approach procedures where all of the minimum descent altitudes are greater than 2,000 feet and/or the landing visibility minimums are greater than 3 miles (Bishop, California; South Lake Tahoe, California; and Aspen-Pitkin Co./Sardy Field, Colorado). A pilot can decline alternate fuel there on the strength of a forecast, then arrive above the 91.167(b) threshold and below anything that would let them land.
That is the sharpest case. The ordinary one is more common: a forecast that verifies half an hour late, a TEMPO group that settles in and stays, an alternate that was legal at 1120Z and is not by the time you need it. Filing an alternate you were not required to file costs one line on the flight plan. Deciding what the numbers should be for you, rather than what the regulation permits, is our go/no-go framework.
Common questions
- Does a TEMPO group in the destination TAF force me to file an alternate?
- 14 CFR 91.169(b) asks whether the ceiling will be at least 2,000 feet above the airport elevation and the visibility at least 3 statute miles for the whole window, not at one instant inside it. A TEMPO group is part of that forecast. If it puts the ceiling at 1,200 feet anywhere between an hour before and an hour after your ETA, the forecast does not hold 2,000 and 3 across the window, and an alternate goes on the flight plan.
- Can I file an airport whose approach chart is marked A NA?
- Not when an alternate is required. The FAA addressed exactly this in a 2006 legal interpretation: an airport designated NA for alternate use may not be listed as the alternate, even if the forecast at your ETA would allow descent from the MEA, an approach, and a landing under basic VFR. The usual causes are no local weather reporting, an unmonitored NAVAID, or a GPS-based one.
- Do I have to land at the alternate I filed?
- No. Under Part 91 the filed alternate is a planning and fuel commitment, not a destination you owe anyone. When the diversion becomes real, go to whichever airport is the best answer at that moment and tell ATC where you are going. What the filing buys is the fuel: 91.167 makes you carry enough to reach the destination, then the alternate, then 45 minutes at normal cruising speed.