14 CFR 121.613: The 1-2-3 Rule, Alternates, and Fuel

Under 14 CFR 121.613, no airline operating under Part 121 may dispatch or release a flight under instrument flight rules unless the available weather reports or forecasts show that conditions at the destination will be at or above the authorized landing minimums at the estimated time of arrival.¹ If the forecast falls short, the flight cannot legally leave the gate on that release. A companion rule, 14 CFR 121.619, then decides whether the airline must also list an alternate airport before departure.²

What Section 121.613 Requires

Section 121.613 sets one broad gate for every IFR dispatch: forecast weather at the destination must be at or above authorized landing minimums at ETA.¹ The same rule applies to flights conducted “over the top,” meaning operations above a cloud layer rather than through it. The section also cross-references 14 CFR 121.615, which extends similar weather-planning requirements to extended overwater operations flown by flag and supplemental carriers.³

Section 121.613 does not, by itself, tell you when to file an alternate. It sets the minimum condition for launching at all. The alternate question lives in 121.619 for domestic operations and 121.621 for flag operations, and dispatchers apply both rules in tandem with 121.613 on every release.

When an Alternate Airport Must Be Filed: The 1-2-3 Rule

For domestic operations, 121.619 uses what dispatchers call the “1-2-3 rule.” An alternate is not required only when forecasts show, for the entire window from one hour before to one hour after ETA, that the ceiling will be at least 2,000 feet above the airport elevation and visibility will be at least 3 statute miles.² If either number dips below those thresholds at any point in that two-hour window, at least one alternate must appear on the dispatch release.

The forecast can come from a Terminal Aerodrome Forecast, current weather observations, or a combination of the two, whichever gives the most accurate picture of arrival conditions.

When a Second Alternate Is Required

Section 121.619 also handles the case where the backup plan is nearly as fragile as the primary. When the weather at both the destination and the first alternate is forecast to be marginal, the dispatcher must designate at least one additional alternate airport.² Many dispatchers file two alternates on stormy days even when the rule only forces one.

How Flag Operations Differ

International and long-haul flag operations follow 14 CFR 121.621, which uses different math. For flights scheduled at six hours or less, no alternate is required if the ceiling will be at least 1,500 feet above the lowest published instrument approach minimum (or 2,000 feet above the airport elevation, whichever is greater) and visibility will be at least 3 statute miles (or 2 miles more than the lowest applicable approach minimum, whichever is greater).⁴ For flights longer than six hours, an alternate is always required regardless of the weather forecast.

Weather Minimums at the Alternate Itself

Filing an alternate is only useful if that airport can actually take the airplane. Under 14 CFR 121.625, weather at the alternate must meet the minimums listed in the certificate holder’s operations specifications, which are tailored to each airline and each airport based on the instrument approaches available.⁵

The FAA’s standard OpSpecs template (C055) commonly sets alternate minimums along these lines:

With a precision approach available: a ceiling of at least 600 feet and visibility of at least 2 statute miles.
With only a non-precision approach: a ceiling of at least 800 feet and visibility of at least 2 statute miles.

These figures are lower than the 2,000-foot and 3-mile numbers used to decide whether an alternate is needed in the first place. The alternate does not need clean weather; it needs conditions good enough to land under instrument procedures. A carrier’s actual OpSpecs may impose higher minimums for airports with challenging terrain or limited approach infrastructure.

Conditional Forecasts and Exemption 3585

Forecasts do not always speak in absolutes. A TAF may include conditional language such as TEMPO (temporary fluctuation) or PROB (probability), indicating weather could briefly dip below minimums even when the main forecast is favorable. Read strictly, 121.619 could force an alternate filing every time a sub-threshold value appears in any conditional block.

FAA Exemption 3585 provides relief. It allows Part 121 carriers to dispatch to a destination or alternate even when conditional statements in the TAF suggest weather could drop below authorized minimums, so long as the main body of the forecast shows conditions at or above those minimums at ETA. The exemption was originally issued to People Express Airlines and later transferred to the Air Transport Association, making it available to any Part 121 operator that complies with 121.613, 121.619(a), and 121.625.⁶ It may only be used for forecasts prepared by the National Weather Service.

When a carrier invokes the exemption and both the destination and first alternate carry marginal conditional forecasts, a second alternate must be designated. The first alternate then has to meet at least half of the normal alternate weather minimums in the carrier’s OpSpecs, and the second alternate must show conditions solidly at or above full alternate minimums in both the main forecast body and any remarks.⁷

Fuel Consequences of Filing an Alternate

Listing an alternate is not a paperwork exercise. Under 14 CFR 121.639, a domestic flight must carry enough fuel to reach the destination, then continue to the most distant designated alternate, and still have 45 minutes of flying time at normal cruising fuel consumption on top of that.⁸

That extra fuel is weight, and weight comes out of payload. On longer flights where fuel margins are already tight, or on days when weather forces dispatchers to file distant alternates, the fuel penalty can be enough to bump passengers or freight from the release.

Amending the Release En Route

Weather does not stand still after takeoff. When conditions at the destination or alternate deteriorate in flight, 14 CFR 121.631 lets the dispatcher and pilot-in-command amend the dispatch release. The flight can be redirected to any alternate that falls within the airplane’s remaining fuel range under the applicable fuel regulations.⁹

Under Extended Operations (ETOPS) rules the amendment process is tighter. If weather at an ETOPS alternate drops below operating minimums, the dispatcher may add a replacement ETOPS alternate only if it falls within the maximum authorized diversion time and has weather at or above minimums.⁹ The pilot-in-command must update the flight plan through company communications before reaching the ETOPS entry point. Every amended release still has to satisfy the full fuel and weather requirements as if the flight were being planned from scratch.

1
eCFR. 14 CFR 121.613 – Dispatch or Flight Release Under IFR or Over the Top
2
eCFR. 14 CFR 121.619 – Alternate Airport for Destination: IFR or Over-the-Top: Domestic Operations
3
eCFR. 14 CFR 121.615 – Dispatch or Flight Release Over Water: Flag and Supplemental Operations
4
eCFR. 14 CFR 121.621 – Alternate Airport for Destination: Flag Operations
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eCFR. 14 CFR 121.625 – Alternate Airport Weather Minima
6
Federal Aviation Administration. Capital Cargo International Airlines Exemption 3585 Decision
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Regulations.gov. Denial of Exemption No. 18235 (Swift Air, LLC)
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eCFR. 14 CFR 121.639 – Fuel Supply: All Domestic Operations
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eCFR. 14 CFR 121.631 – Original Dispatch or Flight Release, Redispatch or Amendment of Dispatch or Flight Release