12.2 Flag & Supplemental Operations Destination Alternate Rules

Flag and Supplemental Destination Alternates

A dispatcher must identify the applicable operating category before selecting the no-alternate test. Flag and supplemental IFR or over-the-top operations do not simply borrow the domestic 1-2-3 rule. The controlling destination-alternate requirements are §§ 121.621 and 121.623. Weather qualification of an alternate comes from the applicable issued OpSpecs, not a generic 600-2 or 800-2 mnemonic.

Flag default and weather exception

Section 121.621(a) normally requires at least one listed alternate for each destination. Its weather-based exception in paragraph (a)(1) requires a scheduled flight time of not more than six hours and appropriate reports/forecasts showing the required conditions for at least one hour before and one hour after destination ETA. A flight scheduled for exactly six hours can satisfy this duration condition; six hours and one minute cannot.

For a required and authorized circling approach, paragraph (a)(1)(i) requires the ceiling at least 1,500 feet above the lowest circling MDA. For the other branch in paragraph (a)(1)(ii), the ceiling must be at least 1,500 feet above the lowest published instrument approach minimum or 2,000 feet above airport elevation, whichever is greater. Do not take a maximum of all three terms indiscriminately: the circling branch and the other branch have different applicability.

In either case, the visibility must be at least 3 miles or 2 miles more than the lowest applicable visibility minimum, whichever is greater, for the procedures to be used. Evaluate the full time window, not merely the forecast at one instant. Also confirm that the planned procedure and aircraft/crew capability support the destination authorization.

Put ceiling calculations on the same datum

A ceiling is reported as a height above the airport, while an MDA or DA can be an MSL altitude. Subtract airport elevation from the chosen approach altitude before converting a result to a ceiling. Do not add a buffer to a DA in MSL and compare that directly with a reported ceiling in AGL.

For a hypothetical straight-in-only example, airport elevation is 200 feet MSL and the applicable lowest approach MDA is 1,200 feet MSL. The approach height above the airport is 1,000 feet. Add 1,500 to obtain 2,500 feet, then compare with the 2,000-foot airport-height baseline: required ceiling is 2,500 feet above airport elevation. If approach visibility is 1.5 SM, compare 1.5 + 2 with 3: required visibility is 3.5 SM. The example's four-hour schedule meets the duration criterion, but the weather still has to meet the calculated values throughout ETA ± one hour.

For a circling example, airport elevation is 5,000 feet MSL and lowest circling MDA is 6,800 feet MSL. The required circling-based ceiling is 1,800 + 1,500 = 3,300 feet above airport elevation. Determine visibility using the applicable approach value plus the separate visibility comparison. Do not mix a runway's touchdown-zone height with airport elevation when doing these calculations.

Flag routes without an available alternate

Paragraph 121.621(a)(2) supplies a separate exception for a route approved without an available alternate airport for the particular destination, when the airplane carries the required fuel under § 121.641(b) or § 121.645(c). This is not granted merely because a dispatcher cannot find a convenient alternate. The route approval and propulsion-specific fuel requirement both matter.

Therefore a flag flight over six hours does not qualify for the weather-based exception, but the separately approved no-available-alternate provision still exists. A quiz saying “every flag flight over six hours must always list an alternate regardless of any other authorization” is too broad. State whether the special route approval is present.

For applicable turbine jets under § 121.645(c), the no-available-alternate route requires destination fuel plus at least two hours at normal cruising fuel consumption. For nonturbine or turbopropeller flag airplanes, § 121.641(b) uses three hours. This difference reinforces why “island reserve equals two hours for every airplane” is incorrect.

Supplemental requirements

Section 121.623(a) normally requires at least one listed alternate for each IFR or over-the-top destination. It has no domestic-style weather waiver, so excellent destination weather does not by itself waive the alternate. Paragraph (b) recognizes the applicable operations outside the 48 contiguous states and DC over routes without an available alternate, with fuel satisfying §§ 121.643 and 121.645 as applicable.

For a charter jet wholly within the contiguous states, the ordinary supplemental alternate requirement applies. For an approved outside-contiguous route lacking an available alternate, evaluate the exact exception and its fuel rule. The release document is a flight release, and the operational-control arrangement is the applicable supplemental system. A certificated dispatcher may participate under carrier procedures, but do not automatically impose the domestic/flag signature model.

Second alternates and marginal weather

The explicit “destination and first alternate marginal” statutory second-alternate requirement is in § 121.619(a) for domestic operations. § 121.621(b) concerns flag alternate weather in the OpSpecs; it does not state that same second-alternate mandate. Section 121.623 similarly does not reproduce it. Carrier procedures, an exemption, or issued authorizations may require additional alternates, and prudent planning may select them, but cite the actual applicable source.

For a practical-test decision, separate three questions: Is an alternate required? Does the chosen alternate meet its planning minima and operational conditions? Does any additional applicable authority require a second alternate? Verify runway and approach availability, forecast winds and contamination performance, and fuel to the required most distant alternate before signing. Explain each answer from the appropriate rule rather than collapsing all operating categories into a single table of absolute statements.

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.