Skip to content

Destination alternate requirements and the 1-2-3 rule

When a domestic IFR flight needs an alternate listed on the release, how the ceiling and visibility test is applied, and how alternate minimums differ from the test itself.

Dispatch procedures16 min read

Video scheduled

The lesson video for this topic is in production on The Flight Dispatch Room.

Written lesson

The lesson

Two separate questions get confused constantly. First: does this flight need an alternate at all? Second: if it needs one, does the candidate airport qualify? They are governed by different regulations and different numbers.

Question one: is an alternate required?

14 CFR 121.619 states that no person may dispatch a flight under IFR in domestic operations unless at least one alternate airport is listed in the dispatch release for each destination airport. There is one exception, and it is the exception the exam tests.

An alternate is not required if, for at least one hour before and one hour after the estimated time of arrival at the destination, appropriate weather reports or forecasts, or any combination of them, indicate the ceiling will be at least 2,000 feet above the airport elevation and the visibility will be at least 3 statute miles.

If either value falls below the threshold at any point in that two-hour window, an alternate must be listed. The test is a floor across a period, not a snapshot.

Question two: does the alternate qualify?

Once an alternate is required, 14 CFR 121.625 governs which airports may be listed. An airport may be listed as an alternate only if the appropriate weather reports or forecasts, or any combination of them, indicate that the weather conditions will be at or above the alternate airport weather minimums specified in the certificate holder's operations specifications for that airport at the estimated time of arrival at the alternate.

  • The applicable time is the ETA at the alternate, not the ETA at the destination.
  • The applicable minimums come from the operations specifications, not from the 2,000 and 3 numbers used in the 121.619 test.
  • Forecasts, reports, or a combination of the two may be used.

Why the two rules feel similar but are not

121.619 asks whether the destination is confidently good enough to skip planning for a diversion. 121.625 asks whether a diversion airport is realistically usable when you would actually arrive there. The first protects against needing an alternate you never listed. The second protects against listing an alternate you could not use.

Examination focus

Key exam concepts

  • 121.619: a domestic IFR flight requires at least one destination alternate listed on the dispatch release, unless the 1-2-3 exception is met.
  • 1-2-3: from one hour before to one hour after ETA, ceiling at least 2,000 ft above airport elevation and visibility at least 3 statute miles.
  • The 1-2-3 condition must hold throughout the window, not just at ETA.
  • 121.625: an airport qualifies as an alternate only if forecast to be at or above the operations specifications alternate minimums at the ETA at the alternate.
  • The 2,000/3 values are the exemption test only. They are not alternate minimums.

Method

Worked examples

Does this flight need an alternate?

Given

  • Domestic Part 121 IFR flight, ETA at destination 1900Z.
  • TAF at destination: 1700Z to 1830Z, BKN025, visibility 5SM.
  • TAF at destination: 1830Z to 2100Z, BKN018, visibility 4SM.
  • Destination airport elevation 600 ft MSL.

Steps

  1. 1Identify the window: one hour before to one hour after 1900Z, so 1800Z to 2000Z.
  2. 2Convert the ceiling requirement: 2,000 ft above airport elevation. The TAF ceiling is already in feet AGL, so compare directly against 2,000 ft AGL.
  3. 31800Z to 1830Z: ceiling 2,500 ft AGL and visibility 5SM. Both above the threshold.
  4. 41830Z to 2000Z: ceiling 1,800 ft AGL. This is below 2,000 ft AGL.
  5. 5The condition fails for part of the required window.

Answer

An alternate is required. The 1-2-3 exception is only available when both values hold for the entire window from 1800Z to 2000Z, and the ceiling drops to 1,800 ft AGL at 1830Z.

Watch out

Common exam traps

  • Applying 2,000 ft as an MSL value. It is 2,000 ft above airport elevation.
  • Checking the weather only at ETA instead of across the full two-hour window.
  • Using 2,000 and 3 as the alternate airport minimums. Those come from the operations specifications under 121.625.
  • Evaluating the alternate at the destination ETA rather than at the ETA at the alternate.
  • Applying domestic rules to a flag or supplemental question without checking which operation is described.

Test yourself

Practice questions

Choose an answer to reveal the discussion. Every option is explained, including why the wrong ones are attractive.

  1. Q01

    A domestic Part 121 IFR flight has an ETA of 1400Z. Forecasts show ceiling 2,500 ft AGL and visibility 4 SM from 1300Z to 1430Z, then ceiling 2,500 ft AGL and visibility 2 SM from 1430Z to 1600Z. Is a destination alternate required?

  2. Q02

    Under 14 CFR 121.625, the weather at an airport listed as an alternate must be forecast to be at or above the alternate minimums:

  3. Q03

    The 2,000 ft and 3 statute mile values in 14 CFR 121.619 are best described as:

Verify

Official FAA references

  • 14 CFR 121.619Alternate airport for destination: Domestic operations.
  • 14 CFR 121.621Alternate airport for destination: Flag operations.
  • 14 CFR 121.625Alternate airport weather minimums.
  • 14 CFR 121.617Alternate airport for departure.
  • FAA-H-8083-25Pilot's Handbook of Aeronautical Knowledge.

Current regulation text is published on eCFR. Always confirm against the current version before relying on any citation.

Verify before use · AVIA Flow provides educational content only. Information on this website does not replace current regulations, approved manuals, official aviation information, company procedures or professional operational judgement. Always verify information using authoritative sources.