2.2 Dispatch Weather Minimums & Departure Rules

2.2 Dispatch Weather Minimums & Departure Rules

Quick Summary: Federal regulations draw a sharp legal line between takeoff weather minimums (14 CFR § 121.651, Part 97, and the carrier's OpSpecs) and destination dispatch minimums (14 CFR § 121.613). Standard takeoff minimums under 14 CFR § 91.175 are 1 statute mile (RVR 5000) for twin-engine aircraft and 1/2 statute mile (RVR 2400) for 3- or 4-engine aircraft. However, Part 121 carriers routinely operate down to RVR 600 or RVR 500 under OpSpec C078, provided the runway is equipped with High Intensity Runway Lights (HIRL), Runway Centerline Lights (CL), and multiple operative transmissometers. For destination dispatch under 14 CFR § 121.615, weather reports and forecasts must show conditions at or above authorized landing minimums at the estimated time of arrival (ETA). Under FAA Exemption 3585 or Exemption 17347, flights may be dispatched when conditional remarks (TEMPO, PROB) show weather below landing minimums at ETA, subject to strict 50% minimum thresholds and a mandatory, fully legal second alternate.

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1. Takeoff Weather Minimums (14 CFR § 121.651 & § 91.175)

Get the citations right before anything else — this is one of the most commonly mis-cited pairs in dispatcher study material:

Under 14 CFR § 121.651(a), no pilot may take off an airplane under IFR when the weather conditions reported by the U.S. National Weather Service, a source approved by that Service, or a source approved by the Administrator are below the takeoff weather minimums prescribed for that airport in:

1. 14 CFR Part 97 (the standard instrument departure / takeoff minimums published for that specific runway); or
2. The certificate holder's FAA-approved Operations Specifications (OpSpecs).

Standard Takeoff Minimums (14 CFR § 91.175(f))

When an air carrier does not have OpSpecs authority for lower-than-standard minimums, the statutory fallback minimums established in 14 CFR § 91.175(f) apply:

• Aircraft having two engines or less: 1 statute mile (1 SM) prevailing visibility, or Runway Visual Range (RVR) 5,000 feet.
• Aircraft having three or more engines: 1/2 statute mile (1/2 SM) prevailing visibility, or RVR 2,400 feet.

Why the difference? Multi-engine transport aircraft (3 or 4 engines) retain a significantly higher climb gradient and greater performance margin following an engine failure at $V_1$ than a twin-engine aircraft, which loses 50% of its total available thrust.

Controlling Visibility: Prevailing vs. Runway Visual Range (RVR)

In commercial airline dispatch, RVR is always controlling for the runway to which it applies. If the surface METAR reports a prevailing ground visibility of 1/4 SM, but the touchdown RVR for the departure runway reports RVR 5000, the flight is legally cleared for takeoff under standard twin-engine minimums because RVR represents the precise visual distance measured down that specific runway.

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2. Lower-Than-Standard Takeoff Minimums (OpSpecs C056 & C078)

Virtually all Part 121 scheduled air carriers hold FAA authorization in OpSpecs Part C to operate with lower-than-standard takeoff minimums. This authorization depends on specific runway lighting systems, surface markings, and transmissometers.

+-------------------------------------------------------------------------------------------------------------------------+
|                                 OPSPEC C078 LOWER-THAN-STANDARD TAKEOFF MINIMUMS                                         |
+-----------------------+-------------------------------------------------------------+-----------------------------------+
| Minimum Authorized    | Required Visual Aids / Runway Infrastructure                | RVR Transmissometer Rules         |
+-----------------------+-------------------------------------------------------------+-----------------------------------+
| RVR 1600 (1/4 SM)     | High Intensity Runway Lights (HIRL), OR                     | Touchdown (TDZ) RVR is            |
|                       | Runway Centerline Lights (CL), OR                           | controlling if installed.         |
|                       | Runway Centerline Markings (RCLM)                           |                                   |
+-----------------------+-------------------------------------------------------------+-----------------------------------+
| RVR 1200 / 1200 /     | Day: HIRL, CL, or RCLM                                      | Minimum 2 operative RVRs.         |
| 1000                  | Night: HIRL or CL required                                  | TDZ & Rollout (RO) controlling;   |
|                       |                                                             | Midpoint (MID) is advisory.       |
+-----------------------+-------------------------------------------------------------+-----------------------------------+
| RVR 1000 / 1000 /     | HIRL and Runway Centerline Lights (CL) required             | Minimum 2 operative RVRs.         |
| 1000                  |                                                             | All reporting RVRs controlling.   |
+-----------------------+-------------------------------------------------------------+-----------------------------------+
| RVR 600 / 600 / 600   | HIRL and Runway Centerline Lights (CL) required             | Minimum 2 operative RVRs.         |
|                       |                                                             | All available RVRs (TDZ, MID, RO) |
|                       |                                                             | are controlling.                  |
+-----------------------+-------------------------------------------------------------+-----------------------------------+
| RVR 500 / 500 / 500   | HIRL, CL, and Head-Up Display (HUD) with takeoff guidance   | TDZ, MID, and RO all operative    |
|                       | or approved flight guidance system                          | and controlling.                  |
+-----------------------+-------------------------------------------------------------+-----------------------------------+

Transmissometer Rules (TDZ, MID, RO)

• Touchdown (TDZ) RVR: When installed and reporting, the TDZ RVR is always controlling for the takeoff run.
• Midpoint (MID) RVR: Evaluates the middle section of the runway; becomes controlling at RVR 1000 and RVR 600 tiers.
• Rollout (RO) RVR: Evaluates the deceleration/abort zone of the runway; controlling at RVR 1200, 1000, and 600.
• Transmissometer Failure Rules: If a runway has three transmissometers (TDZ, MID, RO) and the MID fails, the carrier can still take off down to RVR 600 provided TDZ and RO both report at or above 600 ft.

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3. Destination Dispatch Weather Mandate (14 CFR § 121.613)

The departure weather rules get the airplane into the sky, but the dispatcher cannot issue a dispatch release unless the destination airport is legal under 14 CFR § 121.613:

"Except as provided in § 121.615, no person may dispatch or release an aircraft for operations under IFR or over-the-top, unless appropriate weather reports or forecasts, or any combination thereof, indicate that the weather conditions will be at or above the authorized minimums at the estimated time of arrival at the airport or airports to which dispatched or released."

Key Principles of § 121.613

1. Forecast Weather at ETA Governs: The current weather at the destination at the moment the release is signed is legally irrelevant. If a blizzard is currently producing 1/16 SM visibility at Chicago O'Hare at 0800Z, but the flight's ETA is 1400Z and the TAF forecasts 2 SM visibility at 1400Z, the flight is legal to dispatch.
2. Visibility is Primary: For straight-in instrument approach procedures (CAT I, II, III), visibility (reported in statute miles or RVR) is the governing regulatory minimum under standard OpSpecs. Ceiling is advisory unless the published instrument approach procedure specifically states that ceiling is a required minimum (e.g., circling approaches or non-standard procedures).
3. En Route Degradation (§ 121.651): Section 121.613 is strictly a dispatch rule. If the destination weather deteriorates below landing minimums after the aircraft is airborne, the flight is not required to divert immediately. The PIC and dispatcher may evaluate trends, hold, or continue toward the destination. However, under 14 CFR § 121.651, the pilot cannot begin the final approach segment (past the final approach fix) unless reported visibility is at or above landing minimums.

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4. Conditional Forecast Language & FAA Exemption 17347

Under standard Part 121 rules, a dispatcher reading a Terminal Aerodrome Forecast (TAF) must consider both the main body and all conditional remarks:

• Main Body: Prevailing conditions, "FM" (From) groups, and "BECMG" (Becoming) groups.
• Conditional Remarks: "TEMPO" (Temporary) and "PROB" (Probability, e.g., PROB30 or PROB40) groups.

Without an exemption, if the main body of a TAF forecasts 3 SM visibility at ETA, but a TEMPO 1/4SM FG is valid during the ETA window, the flight cannot be dispatched. The conditional remark alone invalidates the dispatch.

The Mechanics of FAA Exemptions 3585 and 17347

To prevent mass flight cancellations caused by transient low-visibility remarks, the FAA grants exemptions from §§ 121.613, 121.619(a), and 121.625 to industry petitioners. The two most often cited are Exemption 3585 (the long-standing airline-association grant) and Exemption 17347 (granted to Regional Airline Association members and renewed through lettered amendments such as 17347D). They are parallel grants to different petitioners, not a renumbering of one another — a carrier may operate only under the exemption it actually holds, and the authorization and its expiration date appear in that carrier's OpSpecs. Either exemption allows an air carrier to dispatch a flight when conditional remarks in the destination or first alternate forecast indicate weather below authorized minimums, provided specific strict criteria are met.

+-----------------------------------------------------------------------------------------------------------------+
|                                 FAA EXEMPTION 17347 DISPATCH WEATHER MATRIX                                      |
+-----------------------+------------------------------------------------+----------------------------------------+
| Airport               | Main Body Forecast at ETA                      | Conditional Remarks (TEMPO/PROB)       |
+-----------------------+------------------------------------------------+----------------------------------------+
| Destination           | MUST be >= Authorized Landing Minimums         | Cannot be less than 50% of the lowest  |
|                       | (Ceiling & Visibility)                         | authorized landing visibility minimum. |
+-----------------------+------------------------------------------------+----------------------------------------+
| First Alternate       | MUST be >= Derived Alternate Minimums          | Cannot be less than 50% of the derived |
|                       | (OpSpec C055 Ceiling & Visibility)             | alternate ceiling and visibility mins. |
+-----------------------+------------------------------------------------+----------------------------------------+
| Second Alternate      | MUST be >= Derived Alternate Minimums          | MUST be >= Derived Alternate Minimums  |
| (MANDATORY)           | (100% of OpSpec C055 Ceiling & Visibility)     | (100% of C055 - NO RELIEF PERMITTED).  |
+-----------------------+------------------------------------------------+----------------------------------------+

Key Exemption 17347 Operational Requirements

1. Main Body is Inviolable: The main body of the forecast at both destination and first alternate must always be 100% legal. Exemption 17347 never provides relief for the main body.
2. The 50% Rule: Conditional remarks at the destination cannot be less than 1/2 of the authorized landing visibility (e.g., if CAT I minimum is 1/2 SM, the conditional visibility cannot be lower than 1/4 SM). At the first alternate, conditional remarks cannot be less than 1/2 of the derived alternate ceiling AND 1/2 of the derived alternate visibility.
3. Mandatory Second Alternate: The dispatcher must list a second alternate airport on the release. The forecast for the second alternate (both main body and all conditional remarks) must be 100% legal under OpSpec C055 alternate minimums. No relief applies to the second alternate.
4. Release Annotation: The dispatch release must explicitly contain the remark: "Dispatched under Exemption 17347" (or Exemption 3585).
5. Fuel Requirements: The aircraft must carry enough fuel to fly to the destination, fly to the most distant alternate (the second alternate), and fly for 45 minutes at normal cruising fuel consumption.

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5. Step-by-Step Dispatch Weather Scenarios & Exam Traps

Worked Example: Applying Exemption 17347

• Flight: ORD to MSP. Scheduled ETA: 1530Z.
• MSP Landing Minimums: ILS Rwy 12R: 200 ft ceiling, 1/2 SM (RVR 2400).
• TAF KMSP: 1512/1612 24010KT 3SM -SN BKN015 TEMPO 1514/1517 1/4SM FZFG OVC002
• First Alternate (KMSN): Derived alternate minimums: 400-1. TAF KMSN: 1512/1612 26012KT 4SM OVC010 TEMPO 1515/1518 1/2SM -SN OVC002
• Second Alternate (KGRB): Derived alternate minimums: 600-2. TAF KGRB: 1512/1612 27014KT P6SM BKN025 TEMPO 1515/1518 5SM -SN BKN020

Analysis:

1. Destination (KMSP):
   • Main body at 1530Z: 3 SM visibility, 1500 ft ceiling. Minimum is 1/2 SM. Main body is legal.
   • Conditional (TEMPO): 1/4 SM. Half of 1/2 SM minimum is 1/4 SM. Conditional visibility is at or above 50% of landing minimum. Legal under Exemption 17347!
2. First Alternate (KMSN):
   • Main body: 4 SM, 1000 ft ceiling. Derived minimum is 400-1. Main body is legal.
   • Conditional (TEMPO): 1/2 SM visibility, 200 ft ceiling. Derived minimums are 400-1. Half of 400 ft is 200 ft; half of 1 SM is 1/2 SM. Conditional is at exactly 50%. Legal under Exemption 17347!
3. Second Alternate (KGRB):
   • Main body: P6SM, 2500 ft ceiling. Minimum is 600-2. Legal.
   • Conditional (TEMPO): 5 SM, 2000 ft ceiling. Both are well above 600-2. Fully legal (100%).
4. Conclusion: Flight is legal to dispatch under Exemption 17347 with both KMSN and KGRB listed as alternates.

Critical Exam Traps

• Trap 1: Applying 50% to the Main Body. If the main body of the destination TAF reads 1/4SM, the flight cannot be dispatched under any circumstances, even with Exemption 17347.
• Trap 2: Forgetting the Second Alternate. An Exemption 17347 dispatch release with only one alternate is invalid and violates federal regulations.
• Trap 3: Applying Relief to the Second Alternate. The second alternate must be 100% clean in both main body and conditional language. A TEMPO below alternate minimums on the second alternate makes the entire release illegal.