When an air carrier dispatcher files a flight release, one of the most consequential weather-related decisions involves selecting a suitable alternate airport. The regulations governing that decision—found in 14 CFR Part 121, Sections 619 through 625—set precise standards for what ceiling and visibility values must be forecast at the alternate before it qualifies as a legal filing. Understanding how these minima are derived, why they exist, and where the traps lie is essential knowledge for every dispatcher candidate and every working dispatcher who signs a release under Part 121.
Unlike general aviation alternate rules, Part 121 alternate minima are not simply a matter of adding buffers to instrument approach minimums. They are tied to the category of approach available, the number of approaches available, and specific regulatory thresholds that the dispatcher must apply to forecast weather—not current weather. Getting this calculation wrong can result in an aircraft departing with no legal fuel alternate when actual conditions deteriorate, a scenario with obvious safety consequences.
The Regulatory Framework: Sections 619 Through 625
The chain of sections in 14 CFR Part 121 builds the alternate requirement from the ground up. Section 121.619 establishes the general requirement for domestic air carrier operations: a dispatcher must include at least one alternate airport in the dispatch release unless specific weather criteria at the destination airport make an alternate unnecessary. Section 121.621 addresses flag operations with similar logic. Section 121.623 covers supplemental operations. Section 121.625 is the section that actually prescribes how to determine what the weather at the alternate must be—it sets the minima against which the forecast is measured.
Critically, the weather used to evaluate an alternate must come from an appropriate forecast—typically the terminal aerodrome forecast (TAF) for the estimated time of arrival at the alternate, which is the estimated time of arrival at the destination plus one hour of additional flight to the alternate. The dispatcher is evaluating a future condition, not a current one, which means the TAF is the operative document.
How Alternate Minima Are Derived Under Section 121.625
Section 121.625 provides a tiered structure based on the type of instrument approach procedure available at the proposed alternate airport. The rules differ depending on whether the airport has a precision approach, a non-precision approach, or only visual approach capability.
Precision Approach Alternate Minima
If the alternate airport has an approved precision instrument approach procedure (such as an ILS, GLS, or PAR), the minimum weather required in the forecast is a ceiling of 600 feet and visibility of 2 statute miles. These values are the regulatory floor—they do not add to the ILS minimums, they replace the concept of approach minimums for alternate-filing purposes. A dispatcher must find that the TAF predicts at least 600-foot ceilings and 2 SM visibility at the alternate during the window of potential use.
Non-Precision Approach Alternate Minima
If the alternate airport has only a non-precision instrument approach procedure (such as a VOR, RNAV/GPS approach without vertical guidance, NDB, or LOC), the required forecast weather rises to a ceiling of 800 feet and visibility of 2 statute miles. The higher ceiling requirement reflects the lack of precision vertical guidance: without a glideslope, a missed approach from a non-precision procedure begins at a higher decision point and requires greater margin for safety.
No Instrument Approach Available
If the alternate airport has no approved instrument approach procedure, the forecast ceiling and visibility must allow descent from the minimum en route altitude (MEA), approach, and landing under basic visual flight rules (VFR). In practice, this standard is rarely met under IFR conditions, which effectively limits the use of non-approach airports as alternates to VFR-forecast weather situations.
The Two-Approach Rule
Section 121.625 also addresses the situation where an airport has two or more instrument approach procedures of different types. In that case, the ceiling and visibility minima are based on the lowest minima of the available approaches, as long as the forecast weather supports operating to that approach. In practical terms, if an airport has both an ILS and a VOR approach, the precision approach standard of 600-2 governs, because the ILS is the precision procedure. This is favorable for the dispatcher: a better-equipped alternate requires lower forecast weather to qualify.
Why These Minima Exist: The Safety Rationale
The purpose of alternate minima is to provide a reliable diversion option if the destination airport becomes unusable at the time of arrival. The one-hour buffer built into the timing calculation (ETA at destination plus one hour equals the planning window for the alternate) acknowledges that weather can change between dispatch and arrival. The ceiling and visibility thresholds above the actual approach minimums provide an additional buffer so that even if the forecast is slightly optimistic, the crew will likely have enough weather to fly a successful approach at the alternate.
A dispatcher who files a marginal alternate—one that barely meets the 600-2 or 800-2 standard—is relying on forecast accuracy that, for planning horizons of several hours, may be limited. Good dispatch practice involves not only meeting the legal minimum but also evaluating trend information, convective activity, and whether the forecast confidence is high. The dispatcher's authority and responsibility is co-equal to the pilot in command's under Part 121: if either believes the flight cannot be safely completed, it does not depart.
Key Numbers and Rules
- Precision approach alternate minimum: Ceiling 600 feet, Visibility 2 statute miles (14 CFR 121.625)
- Non-precision approach alternate minimum: Ceiling 800 feet, Visibility 2 statute miles (14 CFR 121.625)
- No instrument approach: Forecast must allow VFR descent and landing from MEA
- Two approaches available: Use the lower (more favorable) minima based on the best available approach type
- Weather source: Must be a valid aerodrome forecast (TAF) covering the estimated arrival time at the alternate
- Timing window: ETA at destination plus approximately one additional hour of flight time to alternate drives the forecast hour evaluated
- Alternate not required (121.619): If, for at least one hour before and one hour after the ETA at the destination, the forecast ceiling exceeds 2,000 feet and visibility exceeds 3 SM, no alternate need be filed—but this is a destination weather exemption, not an alternate minima question
Memory Aid
A reliable mnemonic for the alternate minima under Part 121 is "Precision is 6 and 2, Non-Precision is 8 and 2." Associate the lower numbers with the more precise approach (ILS gives you more help getting down, so the regulatory buffer starts lower) and the higher ceiling with the non-precision approach (you need more visual room when there is no electronic glidepath). Both share the same 2-statute-mile visibility standard, so visibility never changes—only the ceiling shifts.
Common Test Traps
- Confusing Part 91 and Part 121 alternate minima: Under 14 CFR 91.169, general aviation alternate minima are 600-2 for precision and 800-2 for non-precision—identical numbers to Part 121.625, and under Part 91 those values are used directly as the forecast ceiling/visibility criteria for alternate selection, not added to the published approach minimums. Under Part 121.625, the numbers ARE the alternate minima, not additions. Do not mix the two regulatory frameworks.
- Using current weather instead of forecast weather: The alternate qualification is based on the TAF (forecast) for the estimated time of arrival window, not on the current METAR. A station reporting 700 and 3 right now is irrelevant if the TAF shows 500 overcast at ETA.
- Forgetting the two-approach rule: If an airport has an ILS and a VOR approach, the precision minima (600-2) apply. Exam questions sometimes try to get candidates to apply the non-precision standard to an airport that also has a precision approach.
- Applying approach plate minimums to alternate filing: The ILS minimums on the approach chart (often 200-½ or 300-¾) are irrelevant to determining whether the airport qualifies as an alternate. The 600-2 or 800-2 values replace that analysis for alternate filing purposes under Part 121.
- Confusing the alternate-not-required rule with the alternate minima: The 2,000-3 ceiling/visibility window that eliminates the alternate requirement (from 121.619) applies at the destination, not the alternate. Many students misapply these numbers to the alternate airport standard.
Mastery of 14 CFR 121.625 requires keeping three items in sharp focus: the type of approach available, the applicable ceiling and visibility threshold, and the fact that a valid forecast—not a current observation—is the measuring stick. A dispatcher who internalizes these distinctions will not only pass the written examination but will make safer dispatch decisions throughout a career.