Study guide
12.3 Derived Alternate Minimums: 1-Nav and 2-Nav Facility Rules
FAA Aircraft Dispatcher Certificate ADX
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1 Chapter 1: Regulations, Certification & Operational Control
1.1 Part 65 Subpart C: Certification, Eligibility & Currency 1.2 Operational Control & Dispatcher-PIC Joint Responsibility 1.3 Domestic, Flag & Supplemental Operations Framework 1.4 Dispatch Release Elements, Signatures & Recordkeeping 1.5 Regulations, Manuals, Crew Qualifications, and Source Control
2 Chapter 2: Atmospheric Physics, Pressure Systems & Fronts
2.1 Atmospheric Structure, Standard Atmosphere & Altimetry 2.2 Air Masses, Frontal Systems & Frontogenesis 2.3 Atmospheric Stability, Lapse Rates & Cloud Formation 2.4 Earth Motion, Regional Weather, Local Winds, and Water Phase Changes
3 Chapter 3: Aviation Weather Hazards & Severe Weather Avoidance
3.1 Structural and Induction Icing: Types, Accretion & Operations 3.2 Thunderstorms, Squall Lines, Microbursts & Low-Level Windshear 3.3 Clear Air Turbulence (CAT), Mountain Waves & Jet Streams 3.4 Low Ceilings, Fog Types & Visibility Restrictions 3.5 Hurricanes, Typhoons, Tornadoes, and Tropical Dispatch Hazards
4 Chapter 4: Weather Observations, Forecasts & Advisories Decoding
4.1 METAR and SPECI Surface Observations: Decoding & Operations 4.2 Terminal Aerodrome Forecasts (TAF): Groups & Timing 4.3 Pilot Weather Reports (PIREP): Coding, Severity & Urgent Alerts 4.4 In-Flight Advisories: AIRMETs, SIGMETs & Convective SIGMETs 4.5 Weather Collection, NWS Facilities, GFA, and EWINS
5 Chapter 5: Aviation Weather Charts & Upper-Air Analysis
5.1 Surface Analysis & Radar Summary Charts 5.2 Constant Pressure Analysis & Winds Aloft (FB) 5.3 Significant Weather Prognostic Charts (SIGWX) 5.4 Satellite Imagery, Moisture-Stability Analysis, and Forecast Integration
6 Chapter 6: Air Navigation, Airspace Architecture & Route Structure
6.1 National Airspace System: Classes, Rules & Special Use 6.2 Ground-Based Navigation Aids & Standard Service Volumes 6.3 Area Navigation (RNAV), GNSS/GPS, PBN & RNP Fundamentals 6.4 En Route Airways, SIDs, STARs & Chart Interpretation 6.5 Earth Coordinates, UTC, Chart Projections, and Flight Instruments 6.6 North Atlantic, Pacific, RVSM, and Global Navigation Differences
7 Chapter 7: Instrument Approaches, Minimums & Airport Operations
7.1 Precision Approach Systems: ILS Categories & Tolerances 7.2 Non-Precision Approaches, RNAV & Decision Minimums (MDA/DA) 7.3 Takeoff Minimums, Lower-Than-Standard & Takeoff Alternates 7.4 Holding Procedures: Patterns, Speed Limits & Timing
8 Chapter 8: Subsonic Aerodynamics & Jet Aircraft Performance
8.1 Subsonic Jet Aerodynamics: Lift, Drag & Flight Regimes 8.2 High-Altitude Flight: Mach Number, Buffet & Coffin Corner 8.3 Transport Turbine Engines: Thrust, Specific Fuel & Bleed Air
9 Chapter 9: Transport Category Takeoff & Landing Runway Analysis
9.1 Takeoff Performance Speeds & Balanced Field Concept 9.2 Takeoff Climb Segments & Obstacle Clearance Gradients 9.3 Landing Performance: Field Lengths, 60% Rule & Contamination
10 Chapter 10: En Route Performance, Driftdown Profiles & ETOPS Operations
10.1 One/Two-Engine-Inoperative En Route Net Profiles & Driftdown 10.2 Extended-Range Operations (ETOPS): Philosophy & Regulations 10.3 ETOPS En Route Alternates: Adequate vs. Suitable & Critical Fuel
11 Chapter 11: Aircraft Weight & Balance, Center of Gravity, and Loading
11.1 Transport Aircraft Structural Weight Limits: Definitions & Rules 11.2 Center of Gravity Computations: Datum, Arm, Moment & Index 11.3 Mean Aerodynamic Chord (%MAC) Computations & Stability 11.4 Payload-Range Analysis, Zero Fuel Weight Tradeoffs & Loading
12 Chapter 12: Destination Alternate Airport Selection & Weather Rules
12.1 Destination Alternate Selection: 91.169 & Part 121 1-2-3 Rule 12.2 Flag & Supplemental Operations Destination Alternate Rules 12.3 Derived Alternate Minimums: 1-Nav and 2-Nav Facility Rules
13 Chapter 13: Part 121 Fuel Planning, Reserves & En Route Re-Release
13.1 Part 121 Domestic Fuel Requirements: Reserves & Calculations 13.2 Part 121 Flag Fuel Requirements: 10% Reserve & Island Rules 13.3 Supplemental Fuel Requirements & Holding Fuel Management 13.4 Redispatch and En Route Re-release Procedures
14 Chapter 14: Transport Systems, Pressurization & MEL/CDL Procedures
14.1 Transport Jet Auxiliary Power, Electrical, Hydraulic & Pneumatic Systems 14.2 Cabin Pressurization, Rapid Decompression & Oxygen Requirements 14.3 Master Minimum Equipment List (MMEL), MEL & CDL Procedures 14.4 Flight Controls, Autoflight, Fuel, Oil, Brakes, and Fire Protection
15 Chapter 15: Hazardous Materials, Duty Regulations & Emergency Procedures
15.1 Dangerous Goods Carriage by Air (49 CFR Part 175) & NOTOC 15.2 Dispatcher Duty Limitations, Rest Requirements & Fatigue 15.3 Emergency Authority, Declaration & Diversion Decision-Making 15.4 NTSB Part 830 Accident Reporting & Wreckage Preservation 15.5 Security, Overdue Aircraft, and FAA Emergency Services
16 Chapter 16: Communications, ATC, and Flight Plan Procedures
16.1 Voice, Data Link, Position Reports, and Lost Communications 16.2 ATC Facilities, Separation, ICAO Flight Plans, and Traffic Management
17 Chapter 17: Human Factors and Practical Dispatch Applications
17.1 Dispatch Resource Management, Human Error, and Risk Decisions 17.2 Manual Flight Planning, PIC Briefing, Flight Watch, and Postflight Closeout
12.3 Derived Alternate Minimums: 1-Nav and 2-Nav Facility Rules
Key Takeaways Explain This Topic AI
Use the actual issued C055 table and provisions; published landing minima supply inputs rather than automatic alternate planning weather.
The common one-facility row adds 400 ft to the applicable rounded-up approach height and 1 SM to authorized landing visibility.
The common two-facility row adds 200 ft to the higher rounded-up height and ½ SM to the higher visibility; 250-foot height yields a 500-foot ceiling.
Opposite ends of the same pavement can be different suitable runways; shared GNSS or a single localizer/VOR does not create two facilities.
A NA restrictions, weather sources, winds including gusts, aircraft capability, and facility availability require the actual authority and current data.
Last updated: October 2026
Derived Alternate Weather Minimums
Alternate planning and an actual approach at landing are different decisions. Under § 121.625, the alternate forecast must meet the certificate holder's alternate weather minimums at the estimated arrival time. The carrier's issued C055 , including selectable provisions and aircraft restrictions, determines the calculation. Published landing minima are inputs to that calculation, not automatic alternate-planning values.
Read the Issued Table and Its Conditions
The common C055 table distinguishes the number of operational facilities and the qualifying procedures. Its basic one-facility row can cover a straight-in nonprecision approach, CAT I precision approach, or an applicable circling maneuver. GPS-based procedures need the authorization's specified provisions. Additional CAT II/III credit, if issued, requires its own conditions; published low minima alone do not establish that credit.
Common supplied table configuration Ceiling calculation Visibility calculation
One qualifying operational facility/procedure Add 400 ft to the applicable DA(H) or MDA(H) height Add 1 SM or 1,600 m to authorized landing visibility
Two qualifying operational facilities, each with a straight-in procedure to different suitable runways Add 200 ft to the higher DA(H) or MDA(H) height of the two Add ½ SM or 800 m to the higher authorized landing visibility
Apply the actual table's height rounding : the height value is rounded up to the next 100 feet when it is not already a multiple of 100, then the additive applies. Since the additives are multiples of 100, rounding the sum upward gives the same result. Use height , not the DA or MDA in feet MSL, as the ceiling basis. A DA of 1,250 feet MSL at a field whose relevant published height is 250 feet does not yield a 1,450-foot ceiling under the two-facility row.
Visibility uses the stated statute-mile or metric basis. Do not add RVR readings as though 2,400 RVR + 5,000 RVR were an authorized equivalent to adding 1 SM. Use any conversion only when the applicable authority expressly permits the relevant planning treatment. Reported runway visibility and forecast prevailing visibility have different functions.
Worked One-Facility Examples
Assume a hypothetical issued table has the common one-facility row above. An authorized ILS has published DA(H) 200 feet and landing visibility ½ SM. The derived planning ceiling is 200 + 400 = 600 feet , and visibility is ½ + 1 = 1½ SM . These are exercise values, not the verified current minima of a named airport.
For a nonprecision procedure with published height 485 feet and visibility 1 SM, round the height to 500 feet and add 400: 900-foot ceiling . Visibility becomes 2 SM . A forecast 885-foot ceiling would not satisfy the rounded requirement. Also verify the required weather source, procedure availability, aircraft capability, MEL restrictions, and applicable forecast elements.
Worked Two-Facility Example
Suppose two qualifying facilities serve suitable runways. Approach A has DA(H) 200 feet and ½ SM visibility. Approach B has DA(H) 250 feet and ¾ SM visibility. Choose the higher height and higher visibility separately: 250 feet and ¾ SM. Round 250 to 300, then add 200 for a 500-foot ceiling . Add ½ SM to ¾ SM for 1¼ SM visibility . The unrounded 450-foot result is not the applicable table result.
The higher ceiling input and higher visibility input need not come from the same procedure. If approach A has the higher height but approach B the higher visibility, each governs its respective column. Do not select the lower of the two simply because the airplane can use that particular procedure at landing.
Two-facility treatment is not always the most favorable option. For an ILS at 200 feet/½ SM and a second approach at 700 feet/1 SM, the two-facility result is 900 feet/1½ SM, while considering the ILS under the one-facility row gives 600 feet/1½ SM. Select a qualifying method permitted by the issued authority; facility count alone does not require using a worse row in every circumstance.
Facilities, Runway Ends, and GNSS Dependence
Different suitable runways can be opposite ends of the same physical paved surface . Independent qualifying ILS systems to Runways 04 and 22 can meet the facility/runway configuration when both ends are suitable. Sharing one localizer for an ILS and localizer-only approach does not create two facilities. A single VOR serving procedures to multiple ends likewise remains one facility.
Example configuration Basic interpretation, subject to issued conditions
Separate qualifying ILS systems to suitable 04 and 22 ends Different ends can qualify even though the pavement is shared.
ILS and LOC using the same localizer One facility does not become two by naming two procedures.
Two procedures relying on one VOR One VOR remains one facility.
Two RNAV (GPS) procedures Shared GNSS dependence does not create two facilities.
Ground-based ILS plus permitted GNSS procedure Check actual GPS authorization and all qualifying conditions.
Dual WAAS receivers do not convert two GPS procedures into two independent facilities for the common C055 table. Where WAAS alternate use is authorized, the planning basis is the permitted LNAV or conventional/GPS procedure line specified by the approval; LPV or LNAV/VNAV service available on arrival can permit using that landing guidance then. Do not substitute a lower LPV planning minimum without issued authority. Required prediction, NOTAM, availability, and equipment conditions depend on the actual approval.
Suitability at the Expected Arrival Time
Two painted runway numbers are insufficient. Wind including gusts must be within the applicable operating limits, including contamination or low-visibility restrictions, and the authority's maximum-demonstrated-crosswind guidance must be considered. The demonstrated value is not universally the certified operating limit for every aircraft. Tailwind limits, landing performance, runway closures, approach outages, rescue services, and field conditions can remove suitability.
For a hypothetical north-south runway pair 18/36 and forecast wind 36028G42KT, Runway 18 has a strong tailwind while 36 has a headwind. Do not declare 18 suitable by overlooking direction. If only one of two previously qualifying runways remains suitable, reassess the table configuration. A more restrictive one-facility result can make the alternate fail even though its actual landing minima remain below the forecast.
Chart Restrictions and Continued Monitoring
The triangle A directs the user to nonstandard alternate minima; A NA marks an alternate-use restriction for the procedure. Read the relevant notes and issued authority. A prohibited IAP must not be used as the calculation basis. One restricted procedure does not automatically ban every other authorized alternate option at that airport. Lack of required weather reporting or a shared navigation outage can nevertheless remove the airport's practical qualification.
Qualification at release is not a guarantee of later availability. Continue monitoring weather, NOTAMs, runway conditions, fuel, and the actual diversion choices. If a facility fails, reassess the remaining method and communicate its effect. An amendment must meet the relevant operating rules; the fact that the airport qualified under two-facility minima earlier does not preserve that configuration after a required facility becomes unavailable.
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.
FAA-issued C055 example, including rounding and GPS provisions ; FAA 8900.1 C055 runway/facility guidance . Apply the carrier’s actual Part 121 issued authorization; the linked FAA guidance’s Part 129 applicability does not replace it.