3.2 Alternate Airport Weather Minimums (OpSpecs Table C055)

3.2 Alternate Airport Weather Minimums (OpSpecs Table C055)

When an aircraft dispatcher designates an airport as a destination alternate on a Part 121 dispatch release, what weather must be forecast at that alternate? A common misconception among student dispatchers is that an alternate is legal if its forecast weather is at or above the published landing minimums shown on the instrument approach procedure (IAP) chart (for example, 200-foot ceiling and 1/2 statute mile visibility). Under federal regulations, this assumption is false and legally invalid.

Filing an alternate airport requires a pre-flight safety buffer. Because an en-route diversion may occur hours after initial dispatch—under conditions of deteriorating regional weather, deteriorating fuel state, air traffic congestion, and potential facility degradation—the Federal Aviation Administration mandates that Part 121 certificate holders derive elevated alternate weather minimums using Operations Specifications (OpSpecs) Table C055.

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The Dual Standard: Filing Minimums vs. Approach Minimums

To master alternate planning, dispatchers must clearly separate the two operational environments:

1. Pre-Flight Dispatch / Filing Minimums: The weather conditions required by regulation at the alternate's ETA to legally list that airport on the dispatch release. These are derived higher minimums computed via OpSpecs Table C055.
2. In-Flight / Approach Landing Minimums: The actual published minimums on the instrument approach chart (Decision Altitude [DA] or Minimum Descent Altitude [MDA], and visibility). If the flight diverts and arrives overhead the alternate airport, the flight crew executes the approach down to the published chart minimums, NOT the derived C055 filing minimums.

Pre-Flight Planning at Desk                In-Flight Diversion Overhead Alternate
───────────────────────────────            ───────────────────────────────────────
Must meet Derived C055 Minimums           Flies to Published Chart Landing Minimums
(e.g., Ceiling 600 ft, Vis 1.5 SM)         (e.g., DA 200 ft, Vis 0.5 SM / RVR 2400)

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Part 91 Standard vs. Part 121 OpSpecs C055

Under general aviation rules (14 CFR § 91.169), standard alternate minimums are fixed: 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 "A" minimums are published).

Part 121 scheduled air carriers do not use Part 91 standard alternate minimums. Instead, each air carrier is issued OpSpecs paragraph C055 (Alternate Airport IFR Weather Minimums), which authorizes the carrier to derive operational alternate minimums tailored to the specific navigational facilities and runways available at the alternate airport. OpSpecs C055 establishes two primary methods: the One Nav-Aid Rule and the Two Nav-Aid Rule.

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The One Nav-Aid Rule (OpSpecs Table C055)

Applicability

The One Nav-Aid rule applies to any airport with at least one operational navigational facility providing a straight-in non-precision approach procedure, straight-in precision approach procedure, or circling maneuver from an instrument approach procedure.

The Mathematical Formula

To derive the required weather minimums at the alternate's ETA:

• Ceiling: Add 400 feet to the authorized Category I Decision Altitude (DA) or Minimum Descent Altitude (MDA).
• Visibility: Add 1 statute mile (1,600 meters) to the authorized landing minimum visibility.

$\text{Derived Ceiling} = \text{Authorized DA / MDA (HAT or HAA)} + 400\text{ ft}$ $\text{Derived Visibility} = \text{Authorized Visibility Minimum} + 1\text{ SM}$

Step-by-Step One Nav-Aid Calculation

Airport: Des Moines International (KDSM)

• Available Approach: ILS or LOC RWY 31
• Published Straight-in ILS Cat I Minimums: DA 200 feet HAT, Visibility 1/2 SM (RVR 2400)

Calculation:

1. Ceiling: 200 ft HAT + 400 ft = 600 feet ceiling.
2. Visibility: 1/2 SM + 1 SM = 1 1/2 statute miles visibility.
3. Result: At KDSM's estimated time of arrival, the forecast must indicate at least a 600-foot ceiling and 1 1/2 statute miles visibility.

If only a non-precision approach is available (for example, a VOR RWY 13 with an MDA of 480 feet HAT and 1 SM visibility):

1. Ceiling: 480 ft + 400 ft = 880 ft -> rounded up to the next reportable ceiling: 900 feet ceiling.
2. Visibility: 1 SM + 1 SM = 2 statute miles visibility.

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The Two Nav-Aid Rule (OpSpecs Table C055)

Applicability

The Two Nav-Aid rule applies to airports equipped with at least two operational navigational facilities, each providing a straight-in approach procedure to different suitable runways.

Because two independent facilities and two different suitable runways provide operational redundancy—meaning a failure of one transmitter or a blockage on one runway still leaves an open runway and functioning approach—the FAA allows lower derived filing minimums.

The Mathematical Formula

To derive the required weather minimums at the alternate's ETA:

• Ceiling: Add 200 feet to the higher authorized Category I DA or MDA of the two approaches used.
• Visibility: Add 1/2 statute mile (800 meters) to the higher authorized landing minimum visibility of the two approaches used.

$\text{Derived Ceiling} = \max(\text{DA}_1, \text{DA}_2) + 200\text{ ft}$ $\text{Derived Visibility} = \max(\text{Vis}_1, \text{Vis}_2) + 0.5\text{ SM}$

Step-by-Step Two Nav-Aid Calculation: Precision + Precision

Airport: Indianapolis International (KIND)

• Approach 1: ILS RWY 05R — DA 200 feet HAT, Visibility 1/2 SM (RVR 2400)
• Approach 2: ILS RWY 23R — DA 200 feet HAT, Visibility 1/2 SM (RVR 2400)
• Both runways are suitable and wind is calm.

Calculation:

1. Higher DA: Both are 200 ft. Higher = 200 ft.
2. Higher Visibility: Both are 1/2 SM. Higher = 1/2 SM.
3. Derived Ceiling: 200 ft + 200 ft = 400 feet ceiling.
4. Derived Visibility: 1/2 SM + 1/2 SM = 1 statute mile visibility (RVR 5000).

Step-by-Step Two Nav-Aid Calculation: Precision + Non-Precision

Airport: Omaha Eppley Airfield (KOMA)

• Approach 1: ILS RWY 14R — DA 200 feet HAT, Visibility 1/2 SM
• Approach 2: RNAV (GPS) RWY 32L — MDA 380 feet HAT, Visibility 1 SM
• Both runways are suitable and within aircraft limits.

Calculation:

1. Higher DA/MDA: Compare 200 ft vs. 380 ft -> Higher is 380 feet.
2. Higher Visibility: Compare 1/2 SM vs. 1 SM -> Higher is 1 statute mile.
3. Derived Ceiling: 380 ft + 200 ft = 580 feet ceiling (rounded to 600 ft in practical reporting).
4. Derived Visibility: 1 SM + 1/2 SM = 1 1/2 statute miles visibility.

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Technical Constraints: Nav-Aid Independence & Runway Suitability

The Two Nav-Aid rule contains two critical legal conditions that dispatchers must verify before applying the lower 200/ 1/2 additive: Navigational Facility Independence and Runway Suitability.

1. Navigational Facility Independence

The two approaches must be based on separate, independent navigational facilities. What constitutes a separate facility?

• Two distinct ILS localizer frequencies and ground antennas on different runways = Two Facilities (Qualifies).
• An ILS on Runway 04 and a standalone VOR on the field providing a VOR approach to Runway 22 = Two Facilities (Qualifies).
• A single VOR station providing VOR RWY 09 and VOR RWY 27 = ONLY ONE FACILITY! Even though there are two approaches to different runways, if that single VOR transmitter trips offline, both approaches fail simultaneously. The One Nav-Aid rule must be used.
• Collocated Localizers / Shared Frequencies: Backcourse approaches or paired localizers sharing equipment do not qualify as independent facilities.
• RNAV (GPS) and RNP Approaches: Under current OpSpecs C055, operators without approved Satellite-Based Augmentation Systems (WAAS / SBAS) must perform a Receiver Autonomous Integrity Monitoring (RAIM) prediction. An operator with un-augmented GPS cannot plan for GPS-based approaches at both the destination and the alternate airport.

2. Different Suitable Runways and Environmental Wind Limitations

Can opposite ends of the same strip of asphalt (for example, Runway 09 and Runway 27) count as "different suitable runways"?

• FAA Interpretation: Yes, opposite ends of the same runway pavement qualify as two different runways under OpSpecs C055, PROVIDED that the prevailing and forecast surface wind at the ETA permits a safe landing within aircraft operating limitations on at least one of those runways.
• The Tailwind / Crosswind Trap: Every transport category aircraft has certified Maximum Crosswind and Maximum Tailwind limitations in its Aircraft Flight Manual (AFM) (typically 30–35 knots crosswind on dry runways and 10–15 knots maximum tailwind).

Operational Scenario:

• Airport has ILS RWY 09 (DA 200, 1/2 SM) and ILS RWY 27 (DA 200, 1/2 SM).
• Surface wind forecast at ETA: 27035KT (270 degrees at 35 knots).
• Aircraft maximum tailwind limit: 10 knots.
• Landing on Runway 27: Direct headwind of 35 knots (Suitable).
• Landing on Runway 09: Direct tailwind of 35 knots (UNSUITABLE—violates 10-knot limitation).

Because Runway 09 is unusable due to excessive tailwind, the airport has only one suitable runway. The dispatcher cannot use the Two Nav-Aid rule! The airport collapses to the One Nav-Aid rule based on Runway 27 alone, requiring derived minimums of 600 feet ceiling and 1 1/2 SM visibility.

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Summary Table: Deriving Alternate Weather Minimums

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Common ADX Exam Traps in Table C055 Calculations

1. Adding to the Lower Minimum: When applying the Two Nav-Aid rule to an ILS (DA 200 - 1/2) and a VOR (MDA 500 - 1), candidates mistakenly add 200/ 1/2 to the ILS. You must add 200 ft to the higher MDA (500 + 200 = 700 ft) and 1/2 SM to the higher visibility (1 + 1/2 = 1 1/2 SM).
2. Ignoring Inoperative Components (NOTAMs): If an airport has two ILS runways, but a NOTAM states "RWY 22L ILS GLIDESLOPE OUT OF SERVICE", the ILS RWY 22L degrades to a Localizer (non-precision) approach. The MDA will increase from 200 ft to ~450 ft HAT and visibility will increase to 3/4 or 1 SM. The derived Two Nav-Aid minimums must use the higher localizer values!
3. RVR Conversion Errors: Flight releases report visibility in RVR when available. Remember the standard conversion table: RVR 2400 = 1/2 SM, RVR 4000 = 3/4 SM, RVR 5000 = 1 SM. When adding 1/2 SM to RVR 2400 (1/2 SM), the result is 1 SM (RVR 5000).