When a domestic air carrier dispatches a flight under 14 CFR Part 121, the dispatcher and pilot in command share joint responsibility for ensuring the flight can be completed safely. A critical piece of that planning puzzle is the destination alternate airport. Even if the forecast at the destination looks fine, regulations may require listing an alternate, and if an alternate is required, the weather forecast at that alternate must meet specific minimums at the estimated time of arrival. These rules live in 14 CFR 121.619 through 121.625 and represent one of the highest-frequency topic areas on the Aircraft Dispatcher Knowledge Test.
Understanding these rules is not merely about passing a test — dispatchers must apply them operationally every single day. Getting them wrong can strand passengers, create legal exposure for the certificate holder, or worse, place a crew over an airport with no legal place to go. This article walks through the full framework: when an alternate is required, what weather qualifies an airport as a legal alternate, and the edge cases that trip up both students and working dispatchers.
When Is a Destination Alternate Required?
The starting point is whether you even need an alternate. Under 14 CFR 121.619(a), a person may not dispatch a flight under IFR unless the dispatch release lists at least one alternate airport for each destination airport, unless the weather conditions at the destination meet the so-called "one-two-three" rule. Specifically, no alternate is required if, for at least one hour before and one hour after the estimated time of arrival (ETA), the forecast ceiling is at least 2,000 feet and the forecast visibility is at least 3 statute miles. This window is centered on the ETA — so dispatchers are looking at a two-hour bracket of forecast weather.
If those forecast conditions cannot be confirmed for the entire two-hour window, an alternate must be listed on the dispatch release. This is not discretionary. The dispatcher cannot simply decide the flight will probably make it; the regulatory requirement is clear and absolute. Note that this rule applies to IFR operations; a separate consideration exists for VFR dispatching, though domestic Part 121 operations are predominantly conducted under IFR.
What Weather Qualifies an Airport as a Legal Alternate?
Simply listing an airport as an alternate is not sufficient. The weather forecast at the proposed alternate must meet the alternate airport weather minima specified in 14 CFR 121.625. These minima are tied to the type of instrument approach procedure available at the alternate airport and apply at the estimated time of arrival (ETA) at the alternate, which is typically calculated as the destination ETA plus holding/diversion time.
Precision Approach Alternates
If the alternate airport has an instrument landing system (ILS) or other precision approach procedure, the forecast weather at the alternate must be at least 600 feet ceiling and 2 statute miles visibility. The ILS approach, with its vertical guidance, allows tighter minima because the approach itself is more precise and reliable in low visibility conditions. This 600/2 standard is one of the most commonly tested numbers on the dispatcher exam.
Non-Precision Approach Alternates
If the alternate airport has only a non-precision approach (VOR, NDB, RNAV, LOC-only, etc.), the forecast weather must be at least 800 feet ceiling and 2 statute miles visibility. The higher ceiling requirement for non-precision approaches reflects the absence of electronic vertical guidance; pilots descending on a non-precision approach need more vertical clearance to safely execute the missed approach procedure if the runway environment is not in sight at the MDA.
No Instrument Approach
If the proposed alternate airport has no instrument approach procedure at all, the regulations allow it to be used as an alternate only if the ceiling and visibility at the ETA will allow descent from the MEA, approach, and landing under basic VFR conditions. As a practical matter, this means the forecast must support flight entirely in VFR conditions. Dispatchers rarely use airports without instrument approaches as alternates for domestic operations, but the option exists under the regulatory framework.
The Role of the Dispatcher and the Dispatch Release
Under the two-pilot, two-signature system that governs Part 121 domestic operations, both the dispatcher and the PIC must release the flight. The dispatch release must include the alternate airport and must confirm that the weather at that alternate is forecast to meet the applicable minima at the ETA. This is not a suggestion or a best practice — it is a legal requirement enforceable under the certificate holder's operations specifications.
In practice, dispatchers pull forecast data from area forecasts, terminal aerodrome forecasts (TAFs), and, where applicable, model output statistics (MOS) data. The dispatcher must ensure the TAF or equivalent forecast covers the ETA window. If a TAF does not extend to cover the ETA at the alternate, the dispatcher must use other approved meteorological sources or select a different alternate. The forecast used must be an official aviation weather product, not a general consumer weather application.
Special Considerations: RNP and RNAV Approaches
As Required Navigation Performance (RNP) approaches with vertical guidance (sometimes referred to as RNP AR APCH) and LPV approaches have proliferated at airports across the National Airspace System, a practical question arises: does an LPV approach with Decision Altitude (DA) qualify as a precision approach for alternate purposes? Under FAA policy and the regulatory structure, only approaches designated as Category I, II, or III ILS (or GLS in some contexts) are classified as precision approaches for 14 CFR 121.625 purposes. LPV and LNAV/VNAV approaches provide vertical guidance but are classified as APV (Approach with Vertical Guidance), not precision approaches, for this regulatory purpose. As a result, an airport served only by an LPV approach typically must meet the 800-foot ceiling and 2 SM visibility non-precision alternate standard, not the 600-foot precision standard. Dispatchers must verify the approach classification carefully.
Key Numbers and Rules
- No alternate required: Forecast ceiling ≥ 2,000 ft AND visibility ≥ 3 SM for 1 hour before and 1 hour after ETA (14 CFR 121.619).
- Alternate with precision approach (ILS): Forecast ceiling ≥ 600 ft AND visibility ≥ 2 SM at ETA (14 CFR 121.625).
- Alternate with non-precision approach: Forecast ceiling ≥ 800 ft AND visibility ≥ 2 SM at ETA (14 CFR 121.625).
- Alternate with no instrument approach: Weather must allow VFR descent and landing at ETA.
- Weather source: Must be an official aviation weather forecast product (e.g., TAF) covering the applicable time window.
- Joint release authority: Both the Aircraft Dispatcher and PIC must sign the dispatch release listing the alternate (14 CFR 121.595).
Memory Aid
For the no-alternate rule, use the phrase "one-two-three": 1 hour before and after ETA, 2,000 feet ceiling, 3 statute miles visibility. For alternate minima, think "6 for precision, 8 for non" — 600 feet for a precision approach alternate, 800 feet for a non-precision alternate, both with 2 SM visibility.
Why It Matters Operationally
From a safety perspective, the alternate airport rules exist because forecasts are imperfect. Even the best meteorological prediction carries error bars, and an aircraft that arrives at a destination below minimums with no legal alternate and insufficient fuel has no good options. The alternate requirement is a built-in safety buffer that ensures every dispatched flight has a pre-planned escape valve. Dispatchers who understand the weather minima rules can confidently select viable alternates — not just legally compliant ones, but practically usable ones given the actual forecast confidence and fuel reserves.
From a business perspective, selecting an alternate that is too close to minimums can create operational problems if weather deteriorates unexpectedly. Experienced dispatchers often look for alternates with a margin above the regulatory floor, particularly on routes where weather is volatile or the ETA window spans a frontal passage. The regulations set a floor, not a target.
Common Test Traps
- Forgetting the 1-hour window applies to BOTH sides of ETA. A student who checks weather only at ETA, or only after ETA, will miss the full two-hour requirement and may incorrectly conclude no alternate is needed.
- Confusing 3 SM (no alternate) with 2 SM (alternate minima visibility). The visibility threshold for the no-alternate exception is 3 SM; the visibility requirement at the alternate itself is 2 SM. These are different numbers for different purposes.
- Applying precision minima to LPV approaches. LPV and LNAV/VNAV are not precision approaches under Part 121 alternate rules. Applying 600/2 to an LPV-only airport is a regulatory error.
- Using the destination approach minima instead of the alternate minima. The alternate minima (600/2 or 800/2) are separate from and generally higher than the published approach minimums on the approach plate. Do not confuse the two.
- Ignoring the TAF time coverage requirement. If the TAF does not cover the ETA at the alternate, it cannot legally serve as the forecasting basis for that alternate selection. Students sometimes overlook this and assume any TAF is sufficient.