12.1 Destination Alternate Selection: 91.169 & Part 121 1-2-3 Rule

Destination Alternate Selection: 91.169 & Part 121 1-2-3 Rule

Quick Summary: In Part 121 domestic operations, selecting a destination alternate airport is not merely a flight planning recommendation—it is a binding regulatory mandate governed by 14 CFR 121.619. Under the classic "1-2-3 Rule," no alternate is required if, from 1 hour before to 1 hour after estimated time of arrival (ETA), the ceiling is forecast to be at least 2,000 feet above airport elevation and visibility is at least 3 statute miles. If either metric falls below these thresholds—or if conditional groups like TEMPO forecast adverse conditions—an alternate must be listed on the dispatch release. Furthermore, when conditions at both the destination and first alternate are marginal, federal regulations mandate a second alternate airport.

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1. Regulatory Framework: Part 91 vs. Part 121 Domestic

Every instrument flight rules (IFR) operation requires an evaluation of destination weather to determine whether an alternate airport must be designated. However, the regulatory threshold and subsequent operational requirements diverge significantly between General Aviation (Part 91) and Scheduled Commercial Air Carriers (Part 121 Domestic):

• 14 CFR 91.169 (General Aviation): An alternate is required unless, for 1 hour before to 1 hour after ETA, the ceiling is at least 2,000 feet above airport elevation and visibility is at least 3 statute miles. If an alternate is required, standard filing minimums under Part 91 are 600-foot ceiling and 2 statute miles visibility for precision approaches, and 800-foot ceiling and 2 statute miles visibility for non-precision approaches, unless non-standard alternate minimums are published.
• 14 CFR 121.619 (Part 121 Domestic): Governs scheduled domestic air carriers. The dispatcher and Pilot in Command (PIC) share joint operational control and must evaluate whether an alternate is required. Unlike Part 91, Part 121 carriers do not use standard 600-2 / 800-2 filing minimums to qualify alternate airports; instead, they must derive custom alternate minimums under FAA Operations Specification C055.

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2. Domestic No-Alternate Conditions

Under § 121.619(a), an IFR or over-the-top dispatch normally lists at least one alternate for each destination. No alternate is required when the appropriate reports or forecasts, or a combination, indicate ceiling at least 2,000 feet above airport elevation and visibility at least 3 miles for at least one hour before and one hour after ETA. Both tests must hold across the window. This domestic exception has no six-hour flight-duration condition.

The Part 91 no-alternate rule additionally requires a prescribed instrument approach at the destination under § 91.169(b)(1). Do not forget that condition when comparing Part 91 with domestic Part 121. If an alternate is required, qualify it under the applicable regime: Part 91 published/standard alternate minima and exceptions differ from the Part 121 issued OpSpecs.

For an ETA of 1900Z, evaluate 1800Z–2000Z. A BKN019 forecast within that window fails the ceiling test even if visibility is P6SM. A ceiling of 2,000 feet and visibility exactly 3 SM meet these two thresholds, subject to the applicable approved weather interpretation and other operational requirements. A passing no-alternate test does not waive destination landing-weather or performance requirements.

3. Analysis of Terminal Aerodrome Forecasts (TAF) Groups

When evaluating destination suitability, dispatchers must scrutinize all sections of the Terminal Aerodrome Forecast (TAF) covering the ETA ± 1 hour window, including prevailing conditions and conditional modifiers.

Main Body vs. Conditional Forecast Groups

• Main Body / FM (From) Groups: Represent rapid, permanent prevailing changes. If an FM group valid over ETA indicates a ceiling < 2,000 feet or visibility < 3 SM, an alternate is mandatory.
• TEMPO (Temporary) Groups: Indicate fluctuations expected to last less than one hour in each instance and cover less than half the total forecast period. Under standard FAA Part 121 dispatch rules, conditional forecast groups carry full legal weight. If a TEMPO group during the arrival window forecasts conditions below 2,000 feet or 3 SM, an alternate airport must be designated.
• PROB (Probability) Groups: A PROB30 or PROB40 group indicates a 30% or 40% probability of occurrence. Under standard Part 121 domestic rules, any forecast group indicating adverse weather below 1-2-3 limits mandates an alternate.

Operational Example: TAF Interpretation

TAF KMKE 111720Z 1118/1218 21014KT P6SM BKN035
     TEMPO 1118/1121 21022G35KT 2SM TSRA BKN015CB
     FM112200 28018KT P6SM OVC040

• Flight Profile: Dispatching to Milwaukee (KMKE) with an ETA of 1930Z. Airport elevation is 723 feet MSL.
• Evaluation Window: 1830Z to 2030Z (ETA ± 1 hour).
• Prevailing Conditions: Wind 210 at 14 kt, visibility > 6 SM, ceiling 3,500 ft AGL (BKN035). By itself, this satisfies the 1-2-3 rule.
• Conditional Group: Between 1800Z and 2100Z, TEMPO 2SM TSRA BKN015CB forecasts temporary thunderstorms with 2 SM visibility and a 1,500-foot ceiling.
• Dispatch Action: Because 1,500 feet < 2,000 feet and 2 SM < 3 SM during the arrival window, an alternate airport is legally mandatory on the dispatch release. Fuel must satisfy § 121.639: destination, most distant required alternate, and thereafter 45 minutes at normal cruising consumption, plus all applicable planning considerations.

Carrier-Specific Conditional-Weather Relief

Some carriers hold specific FAA relief addressing conditional forecasts. Historical exemption numbers do not establish present authorization. Verify the actual grant, holder, validity, airport and approach scope, minimum-weather conditions, number of alternates, and required fuel before applying it. Do not teach a universal half-minimum formula or assume one exemption series automatically replaced another.

Without a supplied applicable relief provision, evaluate all relevant forecast groups against the ordinary destination and alternate rules. A conditional group affecting the ETA window can defeat the no-alternate weather exception. The destination dispatch minimum and the 2,000-foot/3-mile no-alternate test are separate questions.

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4. When Two Alternate Airports Are Required (§ 121.619(a))

Under 14 CFR 121.619(a), a domestic air carrier is required to list a second alternate airport on the dispatch release under specific adverse conditions:

14 CFR 121.619(a): ...if the weather conditions at the destination and the alternate airport are marginal, at least one additional alternate airport must be designated.

Defining "Marginal" Weather

The federal regulation itself does not supply a quantitative number for the word "marginal." Instead, the precise definition is established within each air carrier's FAA-approved Operations Specifications (OpSpecs) and Flight Operations Manual (FOM). Across major US air carriers, the standard industry definition of marginal weather is:

$$\text{Marginal Weather} = \begin{cases} \text{Destination Weather} & \approx \text{Published Landing Minimums} \\ & \text{AND} \\ \text{First Alternate Weather} & \approx \text{Derived Alternate Minimums} \end{cases}$$

Common OpSpec Marginal Criteria

• Destination is marginal if: The forecasted ceiling is within 100 feet of the lowest straight-in approach decision altitude / minimum descent altitude, OR the forecasted visibility is within 1/2 statute mile of the lowest authorized landing visibility minimum at ETA.
• First Alternate is marginal if: The forecasted ceiling is within 100 feet of the derived alternate ceiling minimum, OR the forecasted visibility is within 1/2 statute mile of the derived alternate visibility minimum at ETA.
• The Two-Alternate Trigger: If BOTH the destination AND the first alternate meet their respective marginal criteria, a second alternate airport must be added to the release. If either airport has weather clearly above marginal thresholds, a second alternate is not legally mandated.

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5. Do Not Import an Isolated-Airport Exception

Section 121.619 does not contain a general isolated-airport/two-hour-reserve exception. C059 is not an island-reserve authorization. The specifically approved route-without-an-available-alternate provisions are in § 121.621(a)(2) for flag and § 121.623(b) for applicable supplemental operations, with their applicable propulsion-specific fuel rules. If a domestic scenario fails 1-2-3, carrying extra cruise fuel by itself does not eliminate the required alternate. A specific FAA relief document must actually authorize any different treatment.

Also distinguish an equal-time point from a point of safe return. Equal diversion times do not demonstrate that fuel is sufficient to return. A point-of-safe-return calculation considers remaining endurance, return winds, and required reserves. Geography alone does not establish a legal no-alternate operation.

6. Dispatcher Alternate Selection Workflow

A professional aircraft dispatcher executes a systematic, seven-step decision workflow when evaluating destination alternates for a Part 121 domestic flight:

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Step-by-Step Breakdown

1. Establish ETA Window: Identify scheduled landing time and compute the critical 2-hour window ($[\text{ETA}-1\text{h}, \text{ETA}+1\text{h}]$).
2. Check 1-2-3 Rule: Examine ceiling and visibility across prevailing and conditional forecast groups. If both ceiling $\ge 2,000\text{ ft}$ and visibility $\ge 3\text{ SM}$, no alternate is required.
3. Candidate Screening: If an alternate is required, screen candidate airfields for runway length, pavement classification number (PCN), approach lighting, aircraft servicing, passenger facilities, and curfew restrictions.
4. Apply OpSpec C055: Derive required ceiling and visibility for candidate airports using the One-Nav or Two-Nav facility rule.
5. Evaluate Weather Suitability: Verify that candidate airport forecast weather at the alternate ETA meets or exceeds derived alternate minimums.
6. Marginal Weather Analysis: Check if destination is marginal and first alternate is marginal. If both are marginal, select and qualify a second alternate.
7. Fuel Calculation: Calculate burn from destination to the designated alternate (or to the furthest alternate if two are listed), missed approach fuel, and 45-minute domestic reserve fuel at normal cruising consumption.

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7. Practical Traps & Operational Scenarios

Practical Trap: AGL vs. MSL Ceiling Altitudes

In METAR and TAF reports, cloud bases are reported in hundreds of feet Above Ground Level (AGL). However, sectional and instrument charts display terrain in Mean Sea Level (MSL). If an airport has an elevation of 5,431 feet MSL (such as Denver KDEN), a TAF reporting OVC025 represents an overcast layer at 2,500 feet AGL (7,931 feet MSL). A candidate who erroneously subtracts airport elevation from the reported AGL figure would mistakenly believe the ceiling is below 2,000 feet. Always remember: TAF cloud bases are already expressed in feet above ground level!

Practical Trap: Broken vs. Scattered Clouds

A forecast reports FEW010 SCT015 BKN025. The lowest clouds are at 1,000 feet AGL, but they cover only $1/8$ to $2/8$ of the sky (FEW). The layer at 1,500 feet covers $3/8$ to $4/8$ (SCT). The lowest ceiling layer is BKN025 at 2,500 feet AGL. Because the ceiling is 2,500 feet (which is $\ge 2,000$ feet) and assuming visibility $\ge 3$ SM, no alternate is required.

Operational Scenario: Marginal Weather Dispatch

A Boeing 737 is dispatched from Atlanta (KATL) to Chicago Midway (KMDW). Destination weather at ETA is forecast to be ceiling 200 feet overcast and visibility 1/2 SM in fog (precisely at Category I landing minimums). The primary alternate selected is South Bend (KSBN), where derived alternate minimums under OpSpec C055 are 400-1; South Bend is forecast to be ceiling 400 feet and visibility 1 SM. Because KMDW is at landing minimums and KSBN is at derived alternate minimums, both airports are marginal. The dispatcher must designate a second alternate (such as Fort Wayne KFWA or Indianapolis KIND) and fuel the aircraft to reach the furthest alternate.

Official References

Source check: 2026-10-12. Domestic alternates § 121.619; Flag alternates § 121.621; Supplemental alternates § 121.623; Alternate minima § 121.625. For operational use, apply current issued authorizations and the assigned aircraft’s approved data.