12.3 Derived Alternate Minimums: 1-Nav and 2-Nav Facility Rules
Key Takeaways
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.
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.
Under the supplied standard C055 one-facility table, how are alternate planning minima calculated?
Add 200 ft and 1/2 SM in every case
Use the DA in MSL as a reported ceiling
Add 400 ft to the applicable approach height and 1 SM to the authorized landing visibility
Use a universal 600-2 regardless of authorization
Under a supplied common C055 table requiring height rounded up to the next 100 feet, two suitable independent ILS procedures have DA(H) 200 ft/½ SM and 250 ft/¾ SM. What are the derived two-facility minima?
400 feet ceiling and 1 statute mile visibility
650 feet ceiling and 1 3/4 statute miles visibility
400 feet ceiling and 1 1/4 statute miles visibility
500 feet ceiling and 1¼ statute miles visibility
Which of the following approach configurations satisfies the OpSpec C055 requirement for 'two operational navigational facilities, each providing a straight-in approach procedure to different suitable runways'?
An ILS procedure to Runway 04 and a completely independent ILS procedure with separate localizer, glide slope, and frequency to Runway 22
An ILS approach to Runway 36R and a Localizer-only approach to Runway 36R sharing the same localizer antenna
Two VOR straight-in approaches to different runways, both based on radials from the same on-field VOR transmitter
An ILS approach to Runway 09 and an RNAV approach to Runway 27 when forecasted crosswinds on Runway 27 exceed the aircraft's maximum limit
An approach shows A NA. What should a Part 121 dispatcher establish before selecting that airport as an alternate?
That every other runway is automatically unusable
The restriction’s basis and whether an authorized, available approach and weather source permit alternate use under the issued OpSpecs
That the symbol may always be ignored
That any landing weather authorizes alternate use
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