3.3 Marginal Weather Rule & Second Alternate Selection

3.3 Marginal Weather Rule & Second Alternate Selection

In domestic airline dispatch planning, selecting a single destination alternate is often sufficient to satisfy legal release criteria under 14 CFR § 121.619(a). However, weather systems rarely affect only a single aerodrome. When a widespread regional low-pressure system, warm front, or maritime fog bank envelopes an entire geographical corridor, both the primary destination and the planned diversion alternate can simultaneously hover near legal operating limits. If conditions deteriorate even slightly below forecast, an airborne airliner could find both airports below landing minimums, creating an emergency fuel exhaustion scenario.

To safeguard against this threat, the Federal Aviation Administration established the Marginal Weather Rule under 14 CFR § 121.619(a)(2), requiring the designation of a second alternate airport under specific meteorological conditions.

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The Regulatory Foundation: 14 CFR § 121.619(a)(2)

The federal code provides:

"If the weather conditions at the destination and the alternate airport are marginal, at least one additional alternate must be designated." (14 CFR § 121.619(a)(2))

While the regulatory text is concise, it introduces a critical operational question that every aircraft dispatcher must master: What constitutes "marginal" weather?

The Regulatory Silence on "Marginal"

Remarkably, nowhere in Title 14 of the Code of Federal Regulations is the term "marginal" mathematically or quantitatively defined! The FAA intentionally omitted a universal numerical definition from the CFR, leaving the definition to each air carrier's FAA-approved General Operations Manual (GOM / FOM) and Operations Specifications (OpSpecs).

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The Standard Airline Industry Definition of "Marginal"

Across major US Part 121 air carriers, the standard, FAA-accepted definition of marginal weather applied to § 121.619(a)(2) is structured as follows:

Standard Definition of Marginal Weather
├── Destination Airport is Marginal IF: Ceiling OR Visibility is AT published landing minimums.
└── First Alternate Airport is Marginal IF: Ceiling OR Visibility is AT derived alternate minimums (C055).

The Joint Condition Requirement

The single most tested principle on the FAA ADX exam is that BOTH airports must be marginal to trigger the statutory mandate of § 121.619(a)(2):

$\text{Second Alternate Mandatory} \iff \text{Destination is Marginal } \mathbf{AND} \text{ First Alternate is Marginal}$

If either airport is forecast to be comfortably above its respective threshold, the statutory marginal rule is not triggered, and a second alternate is not legally required under this specific regulation.

Operator Variations (The Additive Buffer Definition)

Some conservative airline OpSpecs establish a numerical buffer definition for marginal weather (for example, destination ceiling within 100 feet or visibility within 1/2 statute mile of landing minimums; and alternate ceiling within 100 feet or visibility within 1/2 statute mile of derived alternate minimums). Dispatchers must strictly follow their airline's specific GOM definition, but on the FAA ADX knowledge examination, the strict "at minimums" standard is tested.

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FAA Exemptions 3585 and 17347 and the Second Alternate Mandate

While § 121.619(a)(2) governs standard marginal conditions, dispatchers frequently encounter a separate regulatory scenario that mandates a second alternate: FAA Exemption 3585 and the parallel Exemption 17347.

The Operational Problem Solved by Exemption 3585

Under basic Part 121 dispatch rules, if any forecast remark (even a temporary fluctuation like TEMPO 1618 1/2SM FG) predicts weather below landing minimums at the destination ETA, the flight cannot be legally dispatched. Exemption 3585 provides legal relief, allowing the flight to depart provided specific conditional standards are met.

The Three-Tier Weather Standards under Exemption 3585

1. Destination Airport:
   • Main Body (Forecast): Must be at or above authorized Category I landing minimums (ceiling and visibility) at the ETA.
   • Conditional Remarks (TEMPO, PROB): May be down to 1/2 of the lowest authorized Category I landing visibility (ceiling is not restricting for straight-in approaches unless specified).
2. First Alternate Airport:
   • Main Body (Forecast): Must be at or above derived alternate minimums (OpSpecs C055) at the ETA.
   • Conditional Remarks (TEMPO, PROB): May be down to 1/2 of the derived alternate ceiling and visibility minimums.
3. Second Alternate Airport: ALWAYS MANDATORY UNDER EXEMPTION 3585!
   • The release must designate a second alternate.
   • Weather Standard: The second alternate's forecast—BOTH the main body AND all conditional remarks (TEMPO, PROB)—must be at or above 100% of the full derived alternate minimums under OpSpecs C055! No 1/2 relief is ever permitted for the second alternate.

Exemption 3585 Alternate Summary:
├── Destination ───────> Main Body: 100% Landing Mins | Conditional: ≥ 1/2 Landing Vis
├── First Alternate ───> Main Body: 100% C055 Mins    | Conditional: ≥ 1/2 C055 Ceiling & Vis
└── Second Alternate ──> Main Body: 100% C055 Mins    | Conditional: 100% C055 Mins (NO RELIEF!)

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Operational Criteria for Selecting a Second Alternate

When the dispatcher is required (or elects) to list a second alternate, simply picking any airport from the route map is unacceptable. The second alternate is the flight's ultimate sanctuary. Dispatchers must evaluate five core operational criteria:

1. Meteorological Independence (Air Mass Separation)

The second alternate should reside in an entirely different weather regime or geographic basin from the destination and first alternate. If the destination (e.g., Philadelphia / KPHL) and first alternate (e.g., Newark / KEWR) are both socked in by a coastal warm front and advection fog, selecting John F. Kennedy (KJFK) or LaGuardia (KLGA) as a second alternate is poor dispatch practice—all four airports belong to the same microclimate. A superior second alternate would be located inland across the Appalachian ridge, such as Pittsburgh (KPIT) or Harrisburg (KMDT), where maritime stratus is physically blocked by terrain.

2. Fuel Planning and Payload Consequences (14 CFR § 121.639)

Under 14 CFR § 121.639, domestic turbine aircraft must carry fuel to:

1. Fly to and land at the destination airport;
2. Thereafter, fly to and land at the most distant alternate airport specified in the dispatch release; and
3. Thereafter, fly for 45 minutes at normal cruising fuel consumption.

When two alternates are listed, the regulatory alternate fuel burn is calculated from the destination to the most distant alternate: $\text{Required Minimum Fuel} = \text{Taxi} + \text{Burn (Origin } \to \text{ Dest)} + \text{Burn (Dest } \to \text{ Most Distant Alternate)} + 45\text{ min Reserve} + \text{Contingency}$

If Alternate 1 is 60 NM away and Alternate 2 is 250 NM away, the dispatch release must carry fuel to reach Alternate 2 from the destination. Tankering this additional fuel increases the aircraft's gross takeoff weight, potentially forcing the dispatcher to offload revenue cargo or bump passengers.

3. Runway Length & Factored Landing Distance (14 CFR § 121.195)

The second alternate must possess runways that accommodate the aircraft at its anticipated divert landing weight. Under § 121.195, turbine-powered transport category aircraft must be capable of landing within 60% of the effective runway length on dry runways, with an additional 15% added for wet or contaminated runways.

4. Airport Rescue and Firefighting (ARFF) and Services

The second alternate must meet the air carrier's authorized ARFF index for the aircraft type (for example, Index C for a Boeing 737-800 or Airbus A321). Dispatchers must also verify passenger handling capabilities, ramp gate space, ground power units (GPU), aircraft deicing availability, and passenger terminal customs/security clearances.

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Dispatch Decision Workflow: Second Alternate Determination

Step 1: Does destination meet 1-2-3 rule (Ceiling ≥ 2,000 ft, Vis ≥ 3 SM)?
        ├── YES ──> No alternate required (unless Exemption 3585 or company policy).
        └── NO  ──> First alternate required. Proceed to Step 2.

Step 2: Are you dispatching under Exemption 3585 / 17347?
        ├── YES ──> Second alternate is MANDATORY (must meet 100% C055 minimums).
        └── NO  ──> Evaluate marginal weather. Proceed to Step 3.

Step 3: Is destination forecast AT landing minimums (Marginal)?
        ├── NO  ──> Second alternate NOT required by § 121.619(a)(2).
        └── YES ──> Check First Alternate. Proceed to Step 4.

Step 4: Is first alternate forecast AT derived C055 minimums (Marginal)?
        ├── NO  ──> Second alternate NOT required by § 121.619(a)(2).
        └── YES ──> Second alternate is MANDATORY under 14 CFR § 121.619(a)(2)!

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Common ADX Exam Traps & Real-World Pitfalls

1. The "One Marginal" Trap: An exam question describes a destination at landing minimums (200 - 1/2) and a first alternate reporting 1,500 ft ceiling and 5 miles visibility (well above C055 derived minimums of 600 - 1 1/2). Candidates reflexively declare a second alternate required. It is NOT required. Both airports must be marginal under § 121.619(a)(2).
2. Exemption 3585 Second Alternate Relief: Candidates often believe that because Exemption 3585 allows conditional remarks at the destination and first alternate to be halved, the second alternate also gets 1/2 relief. This is completely false. The second alternate's forecast must meet full 100% derived minimums in every line of the TAF.
3. Fuel Calculation Error: Assuming fuel is calculated to the first alternate when two alternates are listed. The regulation (§ 121.639) requires alternate fuel to be calculated to the most distant alternate.