9.3 Dispatch Minimums & Alternate Airport Selection

Dispatch Minimums & Alternate Airport Selection

Core Airline Transport Principle: Before a transport category aircraft can be dispatched under Part 121, both the destination and all filed alternate airports must be legally qualified based on meteorological forecasts at their Estimated Times of Arrival (ETA). Dispatchers and captains must apply the statutory "1-1-2 Rule", evaluate marginal weather conditions requiring secondary alternates, and derive higher alternate weather minimums under FAA Operations Specification C055.

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1. Destination Alternate Requirement: The 1-1-2 Rule (14 CFR 121.619)

Under 14 CFR 121.619, every Part 121 domestic IFR flight must list at least one destination alternate airport on the dispatch release unless the weather at the destination meets strict exemption criteria:

+-----------------------------------------------------------------------------+
|                 14 CFR 121.619 DESTINATION ALTERNATE "1-1-2 RULE"           |
|                                                                             |
|   NO ALTERNATE IS REQUIRED IF:                                              |
|   For at least ONE (1) HOUR BEFORE to ONE (1) HOUR AFTER the Estimated      |
|   Time of Arrival (ETA) at the destination airport, appropriate weather     |
|   reports and forecasts indicate:                                           |
|                                                                             |
|   1. CEILING: At least 2,000 FEET above airport elevation                   |
|      - Must be evaluated against airport field elevation, NOT MSL.          |
|      - Example: Airport elevation 1,200 ft MSL -> Cloud base must be at     |
|        least 3,200 ft MSL (Broken or Overcast).                             |
|                                                                             |
|   2. VISIBILITY: At least 3 STATUTE MILES (SM)                              |
|                                                                             |
|   * IF EITHER CEILING (< 2,000 FT) OR VISIBILITY (< 3 SM) IS FORECAST AT   |
|     ANY POINT DURING THE +/- 1 HOUR WINDOW, AN ALTERNATE IS MANDATORY.      |
+-----------------------------------------------------------------------------+

Flag Operations Alternate Rule (14 CFR 121.621)

In Part 121 Flag operations, the rule is more stringent:

• An alternate airport is MANDATORY for all flag flights, regardless of weather, UNLESS:
  1. The flight is scheduled for 6 hours or less; AND
  2. For at least 1 hour before to 1 hour after ETA, the ceiling is at least 1,500 feet above the lowest circling MDA (or 2,000 feet above airport elevation, whichever is higher), and visibility is at least 3 statute miles (or 2 statute miles more than lowest usable approach minimums).

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2. When Two Destination Alternates Are Required: Marginal Weather & Exemption 3585

Federal regulations mandate the addition of a second destination alternate under two primary circumstances:

+-----------------------------------------------------------------------------+
|              TWO DESTINATION ALTERNATES REQUIREMENT (14 CFR 121.619)        |
|                                                                             |
|   CONDITION 1: MARGINAL WEATHER AT DESTINATION AND FIRST ALTERNATE          |
|   - If weather at the DESTINATION is "marginal" at ETA,                     |
|     AND                                                                     |
|   - Weather at the FIRST ALTERNATE is "marginal" at ETA,                    |
|   - THEN a SECOND ALTERNATE must be listed on the dispatch release.         |
|                                                                             |
|   FAA Industry Standard Definition of "Marginal":                          |
|   - Destination: Ceiling within 100 ft OR Visibility within 1/2 SM of       |
|     the lowest authorized CAT I landing minimums (e.g., 500-1 1/2).        |
|   - Alternate: Ceiling within 100 ft OR Visibility within 1/2 SM of the     |
|     OpSpec C055 derived alternate minimums (e.g., 600-1 1/2).              |
+-----------------------------------------------------------------------------+

Alternate Decision Matrix (Marginal Weather Determination)

FAA Exemption 3585 (Conditional Forecast Relief)

Standard Part 121 regulations prohibit dispatching if the TAF indicates weather below minimums at ETA. However, most major US airlines operate under FAA Exemption 3585, which allows dispatch when conditional language (TEMPO, PROB30, PROB40, or BECOMING) indicates weather below minimums:

+-----------------------------------------------------------------------------+
|                     FAA EXEMPTION 3585 RULES SUMMARY                        |
|                                                                             |
|   1. DESTINATION TAF:                                                       |
|      - Main Body: Must be at or above authorized landing minimums at ETA.   |
|      - Conditional (TEMPO/PROB): Can be down to 1/2 of lowest landing       |
|        visibility minimum (ceiling is not controlling for CAT I).           |
|                                                                             |
|   2. FIRST ALTERNATE TAF:                                                   |
|      - Main Body: Must be at or above OpSpec C055 derived alternate minimums|
|      - Conditional (TEMPO/PROB): Can be down to 1/2 of derived alternate    |
|        ceiling AND visibility minimums.                                     |
|                                                                             |
|   3. SECOND ALTERNATE TAF (Mandatory under Exemption 3585):                 |
|      - Main Body AND Conditional: MUST BE FULLY AT OR ABOVE OpSpec C055     |
|        derived alternate minimums (no 1/2 reduction allowed).               |
|                                                                             |
|   * Dispatch Release must contain explicit remark: "Dispatched under 3585". |
+-----------------------------------------------------------------------------+

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3. Derived Alternate Weather Minimums: OpSpec C055

While standard civil alternate minimums printed on approach charts (800-2 for non-precision, 600-2 for precision) apply to Part 91, Part 121 air carriers derive customized alternate minimums using FAA Operations Specification C055. This table provides higher safety buffers to ensure that an aircraft diverting to an alternate will successfully land even if one approach aid fails.

+-----------------------------------------------------------------------------+
|                 OPSPEC C055 DERIVED ALTERNATE MINIMUMS TABLE                |
|                                                                             |
|   Approach Facility Configuration           Add to Authorized Minimums      |
|   -----------------------------------------------------------------------   |
|   1-NAV FACILITY RULE                       Ceiling: ADD 400 FT to MDA/DA   |
|   (For airports with at least one usable    Visibility: ADD 1 SM (1,600 m)  |
|   navigational facility providing a         to landing visibility minimum   |
|   straight-in approach)                                                     |
|                                                                             |
|   2-NAV FACILITY RULE                       Ceiling: ADD 200 FT to higher DA|
|   (For airports with at least two usable    Visibility: ADD 1/2 SM (800 m)  |
|   facilities providing straight-in          to higher landing visibility    |
|   approaches to DIFFERENT SUITABLE RUNWAYS)                                 |
+-----------------------------------------------------------------------------+

The "Two Different Suitable Runways" Requirement

To legally apply the lower 2-Nav Rule (+200 ft / +1/2 SM), the alternate airport must satisfy three strict technical criteria:

1. Two Separate Navigation Facilities: Must utilize independent ground navigation transmitters and frequencies (e.g., ILS RWY 04R on 110.3 MHz and ILS RWY 22L on 109.1 MHz, or an ILS and an independent RNAV/GPS system). If two approaches share the same localizer array or VOR facility, the 1-Nav rule must be used.
2. Different Suitable Runways: The runways must be physically suitable for the aircraft type (length, width, pavement classification number [PCN]) and authorized by company OpSpecs.
3. Wind and Environmental Limits: The runways must be usable considering forecast crosswind and tailwind components at ETA (including maximum aircraft crosswind limits with one engine inoperative).

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4. Worked Step-by-Step OpSpec C055 Calculations

+-----------------------------------------------------------------------------+
|                  OPSPEC C055 WORKED CALCULATION EXAMPLES                    |
|                                                                             |
|   EXAMPLE 1: Single Available Approach (1-Nav Rule)                         |
|   - Alternate Airport: Des Moines (KDSM)                                    |
|   - Approach: ILS RWY 31 (DA 200 ft AGL, Visibility 1/2 SM [2400 RVR])     |
|   - Calculation:                                                            |
|     * Derived Ceiling:    200 ft + 400 ft = 600 FEET AGL                    |
|     * Derived Visibility: 1/2 SM + 1 SM   = 1 1/2 STATUTE MILES             |
|   - REQUIRED ALTERNATE WEATHER AT ETA: 600 ft Ceiling & 1 1/2 SM Visibility |
|                                                                             |
|   EXAMPLE 2: Two Usable Approaches to Different Runways (2-Nav Rule)        |
|   - Alternate Airport: Kansas City (KMCI)                                   |
|   - Approach A: ILS RWY 01L (DA 200 ft AGL, Visibility 1/2 SM [2400 RVR])   |
|   - Approach B: RNAV (GPS) RWY 19R (DA 250 ft AGL, Visibility 3/4 SM)       |
|   - Selection: Higher DA is 250 ft; Higher Visibility is 3/4 SM             |
|   - Calculation:                                                            |
|     * Derived Ceiling:    250 ft + 200 ft = 450 FEET AGL                    |
|     * Derived Visibility: 3/4 SM + 1/2 SM = 1 1/4 STATUTE MILES             |
|   - REQUIRED ALTERNATE WEATHER AT ETA: 450 ft Ceiling & 1 1/4 SM Visibility |
+-----------------------------------------------------------------------------+

[!IMPORTANT] Dispatch Planning vs. Inflight Execution: Derived alternate minimums under OpSpec C055 are strictly preflight dispatch qualification criteria. If the aircraft subsequently diverts to the alternate in flight, the flight crew is NOT required to have 450-1 1/4 to land. The crew executes the approach down to the published chart minimums (e.g., 200 ft DA and 1/2 SM visibility) under 14 CFR 121.651.