IFR 1-2-3 rule: is an alternate required?

The short answer, and the part that costs you something

An alternate goes on every IFR flight plan unless you can show two things: the destination has a published instrument approach, and the forecast is at or above a 2,000 foot ceiling and 3 statute miles for the whole hour either side of your ETA. That is the 1-2-3 rule, and it is the easy half.

The half that changes the flight is what follows once an alternate is required. The airport you name has to meet its own forecast minima at its own arrival time, under 91.169(c), using an approach you are actually equipped to fly. Your fuel plan grows by the destination to alternate leg plus 45 minutes, under 91.167(a). Those two consequences are where a legal flight plan turns into a fuel stop, a later departure or a different destination, and they are what this page works through rather than restating a memory aid.

The regulation, both halves of it

The 1-2-3 rule is not one rule. The same test appears twice, once for what you file and once for what you carry, in identical words. 14 CFR 91.169(a)(2) requires an alternate airport in the flight plan "except as provided in paragraph (b) of this section". 14 CFR 91.167(a)(2) requires fuel to "fly from that airport to the alternate airport", with the same exception. Both paragraph (b)s then read:

(b) Paragraph (a)(2) of this section does not apply if:

(1) Part 97 of this chapter prescribes a standard instrument approach procedure to, or a special instrument approach procedure has been issued by the Administrator to the operator for, the first airport of intended landing; and

(2) Appropriate weather reports or weather forecasts, or a combination of them, indicate the following:

(i) For aircraft other than helicopters. 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.

(ii) For helicopters. At the estimated time of arrival and for 1 hour after the estimated time of arrival, the ceiling will be at least 1,000 feet above the airport elevation, or at least 400 feet above the lowest applicable approach minima, whichever is higher, and the visibility will be at least 2 statute miles.

14 CFR 91.169(b), word for word the same as 14 CFR 91.167(b)

Three things repay a close reading. The requirement is the default and the 1-2-3 test is the exception, so the burden is on you to show the exception applies. The ceiling figure is measured above the airport elevation, which happens to be how a TAF already gives cloud heights, so a TAF reads straight across while an MSL figure from another source does not. And the standard is what the forecast says the condition "will be" across the whole window, which is why one TEMPO group inside the window is enough to cost you the exemption.

When no alternate is required at all

Only when both parts of (b) are satisfied. Miss the approach and the forecast is irrelevant. Miss the forecast and the approach is irrelevant. Even when you clear both, the 45 minute reserve in 91.167(a)(3) does not move, and for a helicopter that reserve is 30 minutes.

The alternate has its own weather test

This is where most of the real planning work lives, and it is a different test on a different time basis: a single point in time, your arrival at the alternate, rather than a two hour window.

(c) IFR alternate airport weather minima. Unless otherwise authorized by the Administrator, no person may include an alternate airport in an IFR flight plan unless appropriate weather reports or weather forecasts, or a combination of them, indicate that, at the estimated time of arrival at the alternate airport, the ceiling and visibility at that airport will be at or above the following weather minima:

(1) If an instrument approach procedure has been published in part 97 of this chapter, or a special instrument approach procedure has been issued by the Administrator to the operator, for that airport, the following minima:

(i) For aircraft other than helicopters: The alternate airport minima specified in that procedure, or if none are specified the following standard approach minima: (A) For a precision approach procedure. Ceiling 600 feet and visibility 2 statute miles. (B) For a nonprecision approach procedure. Ceiling 800 feet and visibility 2 statute miles.

(2) If no instrument approach procedure has been published in part 97 of this chapter and no special instrument approach procedure has been issued by the Administrator to the operator, for the alternate airport, the ceiling and visibility minima are those allowing descent from the MEA, approach, and landing under basic VFR.

14 CFR 91.169(c), abridged only by running subparagraphs (A) and (B) onto one line and omitting (c)(1)(ii), the helicopter case, which is in the FAQ below

Note the order inside (c)(1)(i). The charted alternate minima come first. The 600-2 and 800-2 figures everybody quotes are the fallback for a procedure that specifies nothing, so quoting them at an airport whose procedure publishes something else is simply the wrong number. Note (c)(2) as well: an airport with no published procedure can be an alternate, on the condition that the forecast allows descent from the MEA, approach and landing under basic VFR. Demanding in practice, but a permission rather than a prohibition.

GPS, WAAS, and whether the approach counts

The common shorthand, that an RNAV (GPS) approach only counts at an alternate if you have WAAS, is not what the AIM says. The baseline in AIM 1-2-3 d. is that a required alternate must have an available approach that does not require the use of GPS, including through a substitute means of navigation based on GPS, and that this restriction "does not apply to RNAV systems using TSO-C145/-C146 WAAS equipment". Two carve outs then matter:

• Without WAAS. AIM 1-1-17 b.5(c) permits a TSO-C129 or TSO-C196 user whose system has fault detection and exclusion, who performs a preflight RAIM prediction for the approach, and who has the required knowledge, training or approval, to file on a GPS based approach "at either the destination or the alternate airport, but not at both locations". At the alternate such a pilot may plan for the LNAV or circling minimum descent altitude, and for an LNAV/VNAV decision altitude only with approved barometric VNAV equipment. Where those conditions cannot be met, 1-1-17 b.5(d) requires the alternate to have an approved approach other than a GPS based one, anticipated to be operational and available at the estimated time of arrival, that the aircraft is equipped to fly.
• With WAAS. AIM 1-1-18 c.9(a) lets you plan any procedure your WAAS avionics are approved for at a required alternate, but the planning "must be based on flying the RNAV (GPS) LNAV or circling minima line", and "part 91 non-precision weather requirements must be used for planning", which is the 800-2 line rather than 600-2. Vertical guidance is still yours on arrival if the receiver reports LNAV/VNAV or LPV service then.

The tool's WAAS switch filters candidates on exactly this distinction, conservatively: with the switch off it keeps airports that have a ground based approach, because that choice is sound whether or not you have run a RAIM prediction.

NA and nonstandard alternate minimums on the plate

Some procedures may not be used to qualify an airport as an alternate at all. AIM 1-1-18 c.9(a) names the symbol: "the NA (Alternate Minimums Not Authorized) symbol". The same paragraph explains that the FAA "has begun removing" it from selected RNAV (GPS) and GPS procedures so that approach approved WAAS receivers can use them at an alternate, while "some approach procedures will still require the NA for other reasons, such as no weather reporting". Other procedures carry nonstandard alternate minimums, which send you to the alternate minimums pages at the front of the Terminal Procedures Publication instead of to the 600-2 and 800-2 defaults. Either way the published procedure decides and the default does not. Every SkyReady airport page lists the field's approaches and links its FAA alternate minimums page under Charts.

Worked examples against real forecasts

Both TAFs below are the real product issued on 6 November 2026, unedited. Reading the groups correctly is most of the skill, and how to read a METAR covers the change groups in detail.

Example 1: the obvious fail, and the subtlety inside it

TAF KBOS 060538Z 0606/0712 20010KT P6SM FEW023 FEW040 OVC080
  PROB30 0606/0609 4SM -SHRA OVC012
  FM060900 20008KT 5SM -RA BR OVC010
  FM061100 20007KT 3SM -RA BR OVC003
  TEMPO 0702/0706 1SM RA BR
  FM070600 21005KT 3SM -RA BR OVC008
  FM070900 26005KT 4SM -RA BR OVC008

ETA 1200Z on the 6th. The window is 1100Z to 1300Z, and the FM0611 group governs all of it: 3 SM and OVC003. The visibility is exactly 3 SM, which satisfies "at least 3 statute miles" on its own. The ceiling is 300 feet above the field, nowhere near 2,000, so the exemption fails and an alternate is required. The test is an and: one figure passing changes nothing.

ETA 0730Z on the 6th. The window is 0630Z to 0830Z, and the base group gives OVC080 with P6SM, comfortably clear. But the PROB30 group runs 0600Z to 0900Z and forecasts OVC012 with 4 SM, so it covers the whole window. The regulation asks what the forecast indicates the ceiling "will be", and this forecast does not indicate 2,000 feet throughout, so plan the alternate. The regulation does not say how to weigh a 30 percent probability, which makes this the judgement call rather than the arithmetic. The price of the conservative answer is one fuel calculation.

Example 2: the pass, and how close it runs

TAF KPIT 060520Z 0606/0712 24003KT P6SM BKN250
  FM061400 25005KT P6SM BKN035
  FM061700 27008KT P6SM BKN025
  TEMPO 0618/0622 6SM -SHRA
  FM070000 30005KT P6SM SCT035

ETA 1800Z on the 6th. The window is 1700Z to 1900Z. FM0617 gives BKN025, which is 2,500 feet above the field, with P6SM. The TEMPO group begins at 1800Z and sits inside the window, so it has to be evaluated: it drops visibility to 6 SM and says nothing about the ceiling, so the worst case in the window is 2,500 feet and 6 SM. Both are above the line, and KPIT has published approaches, so no alternate is required and no alternate fuel is needed.

The margin is 500 feet. Move the ETA back to 1630Z and the window opens at 1530Z, where the earlier BKN035 group still applies, so the answer holds with more room. Move it the other way, or take one forecast revision that trades BKN025 for BKN018, and the result flips. That is the case for running the test against the current TAF rather than the one you read at breakfast.

What the fuel looks like when the answer is yes

Take the first example. Suppose the alternate is 55 nautical miles from KBOS, you make 110 knots on that leg and burn 9 gallons per hour. 91.167(a) wants the trip fuel, plus the alternate leg, which is 30 minutes and so 4.5 gallons, plus 45 minutes at normal cruising speed, which is another 6.75 gallons. That is 11.25 gallons unavailable to the trip. In a typical trainer with 50 usable gallons it is close to a quarter of the tanks, and on a long leg it is the difference between filing direct and planning a stop. The E6B fuel panel takes the reserve as a number of hours, so you can price the alternate leg before committing to it.

How this tool decides

It parses the live destination TAF, selects every forecast group that overlaps the window from 1 hour before to 1 hour after your ETA, including FM, BECMG, TEMPO and PROB groups, and applies the worst ceiling and visibility it finds against 2,000 feet and 3 statute miles. It then lists nearby airports that qualify structurally: a published approach you are equipped to fly, plus a reporting weather station, filtered to a ground based approach when you tell it you are without WAAS. The candidate list is a shortlist, not a clearance. The forecast at the alternate, the charted alternate minima and the current NOTAMs are yours to check. If the destination forecast is the part in doubt, the personal minimums check is the companion question: legal and sensible are different tests.

This is an advisory aid, not an authoritative source. You are the pilot in command. Verify against official sources before you file.

FAQ

What is the 1-2-3 rule?

It is the memory aid for 14 CFR 91.169(b) and 14 CFR 91.167(b). You may leave the alternate off the flight plan, and the alternate leg out of the fuel plan, only if the destination has a published instrument approach and the forecast says that for at least 1 hour before and 1 hour after your ETA the ceiling will be at least 2,000 feet above the airport elevation and the visibility at least 3 statute miles. Fail either part and an alternate is required.

Is the 1-2-3 rule 2,000 feet AGL or 2,000 feet above the airport?

The regulation says 2,000 feet above the airport elevation. Cloud heights in a TAF are already given above field elevation, so you can read a TAF group straight across: BKN018 is 1,800 feet above the field and fails, BKN025 is 2,500 feet and passes. The distinction matters when you are working from a source that gives a MSL figure, where the field elevation has to come off first.

When is an IFR alternate not required at all?

Only when both halves of 91.169(b) are met: part 97 prescribes a standard instrument approach procedure to the destination, or the Administrator has issued the operator a special one, and the forecast for the window from 1 hour before to 1 hour after ETA is at least a 2,000 foot ceiling above the airport and 3 statute miles. The exemption is written as an exception. The default in 91.169(a)(2) is that an alternate goes on the flight plan.

Do I need an alternate if the destination has no instrument approach?

Yes, always. The exemption in 91.169(b)(1) depends on a standard instrument approach procedure being prescribed in part 97 for the destination, or a special procedure having been issued to the operator. No procedure means no exemption, however good the forecast is.

What are the alternate weather minimums?

Under 91.169(c)(1)(i), for aircraft other than helicopters, you use the alternate airport minima specified in the approach procedure, and only if none are specified do you fall back on the standard minima: ceiling 600 feet and visibility 2 statute miles for a precision approach, ceiling 800 feet and visibility 2 statute miles for a nonprecision approach. Those figures have to be forecast at the estimated time of arrival at the alternate, not across a window around it.

Can an airport with no instrument approach at all be my alternate?

Yes, and this surprises people. 91.169(c)(2) says that where no procedure is published in part 97 and none has been issued to the operator, the ceiling and visibility minima are those allowing descent from the MEA, approach and landing under basic VFR. It is a high bar in practice, but the regulation permits it rather than forbidding it.

What are the minimums for a helicopter?

Both helicopter cases are written separately. For the 1-2-3 test, 91.169(b)(2)(ii) uses the estimated time of arrival and 1 hour after it, a ceiling of at least 1,000 feet above the airport elevation or at least 400 feet above the lowest applicable approach minima whichever is higher, and visibility of at least 2 statute miles. At the alternate, 91.169(c)(1)(ii) is a ceiling 200 feet above the minimum for the approach to be flown and visibility of at least 1 statute mile, never below the minimum visibility for that approach. The reserve in 91.167(a)(3) is 30 minutes for helicopters rather than 45.

Can I use an RNAV (GPS) approach at the alternate?

It depends on the equipment, and the common claim that you cannot without WAAS is too strong. AIM 1-1-17 b.5(c) lets a TSO-C129 or TSO-C196 user whose system has fault detection and exclusion, who performs a preflight RAIM prediction for the approach, and who holds any required training or approval, file on a GPS based approach at either the destination or the alternate airport, but not at both locations. AIM 1-1-18 c.9(a) lets a WAAS user plan any procedure their avionics are approved for at the alternate, but the planning must be based on the LNAV or circling line, using the part 91 nonprecision weather requirement. Vertical guidance may still be used on arrival if the receiver reports the service is available.

What does NA mean on the approach plate?

It is the symbol the FAA calls Alternate Minimums Not Authorized, and it means the procedure may not be used to qualify that airport as an alternate. AIM 1-1-18 c.9(a) notes that the FAA has begun removing it from selected RNAV (GPS) and GPS procedures so approach approved WAAS receivers can use them at an alternate, and that some procedures keep it for other reasons, such as no weather reporting. A procedure can also carry nonstandard alternate minimums, which send you to the alternate minimums pages at the front of the Terminal Procedures Publication rather than to the 600-2 and 800-2 defaults.

What if my destination has no TAF?

The regulation asks for appropriate weather reports or weather forecasts, or a combination of them, so it does not name the TAF specifically. In practice a destination with no terminal forecast leaves you nothing that shows the window from 1 hour before to 1 hour after ETA is above 2,000 and 3, so the usual answer is to file the alternate. A nearby TAF plus an area forecast can sometimes support the case, and that is a judgement call you should be ready to explain.

Does a TEMPO or PROB30 group count against the 1-2-3 test?

A TEMPO group forecasting below a 2,000 foot ceiling or below 3 SM inside your window means the forecast does not say the condition will be at or above those figures, so plan the alternate. PROB30 forecasts a probability rather than a condition, and the regulation does not say how to treat one. It is the only PROB group in a US TAF, though an ICAO TAF abroad can carry PROB40. The conservative reading, and the one this tool takes, is that a low condition forecast anywhere in your window costs you the exemption. The price of being wrong that way is one extra fuel calculation.

What is the 0-1-2-3 rule?

The same rule with a zero added as a memory aid: 0 for zero instrument approaches at the destination, which means an alternate is required regardless of the forecast. Then 1 hour either side of ETA, a 2,000 foot ceiling and 3 statute miles.

How much fuel does an alternate add?

Under 91.167(a) you carry enough fuel to complete the flight to the first airport of intended landing, then to fly from that airport to the alternate, then to fly after that for 45 minutes at normal cruising speed, or 30 minutes for a helicopter. The 1-2-3 test in 91.167(b) is word for word the test in 91.169(b), so the same forecast that puts an alternate on the flight plan adds the alternate leg to the fuel. If that leg eats your reserve, the alternate decision has become a fuel stop decision.

Sources: 14 CFR 91.167 and 14 CFR 91.169, current text; AIM paragraphs 1-1-17 (global positioning system), 1-1-18 (wide area augmentation system) and 1-2-3 (use of suitable area navigation systems on conventional procedures and routes), AIM Basic with Changes 1, 2 and 3; TAFs issued 6 November 2026. Educational material, not a substitute for the regulations, the approach plate or an official briefing.