One of the most consequential decisions an aircraft dispatcher makes is choosing an alternate airport for a Part 121 flight. An alternate is not merely a bureaucratic checkbox — it is the contingency plan that keeps passengers and crew safe when the destination becomes unusable. Federal regulations found in 14 CFR 121.619 through 121.625 define when an alternate is required, how its weather minimums are calculated, and what additional factors must be weighed before the alternate is legally and operationally acceptable.
This article walks through every layer of that decision: the regulatory triggers, the approach-capability-based weather minimums, how NOTAMs can quietly invalidate an otherwise perfect alternate, and the field condition factors that a dispatcher must evaluate before signing the release. Understanding these elements in concert — not in isolation — is what separates a proficient dispatcher from one who merely fills in blanks on a form.
When an Alternate Is Required
Under 14 CFR 121.619, a dispatcher must designate at least one alternate airport in the dispatch release for every domestic Part 121 flight unless a specific exception applies. Domestic operations under 121.619 use a different test than the Part 91 "1-2-3 rule": no alternate is required if, for the period beginning one hour before and ending one hour after the estimated time of arrival at the destination, the appropriate weather reports or forecasts indicate the ceiling will be at least 1,500 feet above the lowest circling MDA (or, if a circling approach is not authorized, 1,500 feet above the airport elevation) and visibility will be at least 3 statute miles, or 2 statute miles more than the lowest applicable visibility minimum, whichever is greater. If the destination is forecast to be below those values at any point in that two-hour window, an alternate must be listed.
It is important to recognize that this is a threshold test, not a minimums test. Even if actual conditions are excellent, a forecast dip below the required ceiling or visibility within that window triggers the alternate requirement. Dispatchers must pull terminal aerodrome forecasts (TAFs) and verify there is no applicable TEMPO, BECMG, or PROB30 condition that breaches those thresholds during the window.
Establishing Legal Alternate Minimums
Once an alternate is required, the dispatcher must confirm that the chosen airport actually qualifies. This is governed by 14 CFR 121.625, which establishes the weather minimums an alternate must be forecast to meet at the estimated time of arrival. The minimums are tied directly to the approach capability at the alternate — specifically, what instrument approaches are available and authorized for the operation, and must meet or exceed the airport's approved landing minima at ETA (plus any carrier OpSpec additives).
Precision Approach Alternate Minimums
If the alternate has an approved precision approach (such as an ILS, PAR, or GLS), standard alternate minimums require a forecast ceiling at the alternate of at least 600 feet and visibility at least 2 statute miles at the estimated time of arrival. These are the standard precision alternate minimums for Part 121 carriers unless the carrier's operations specifications (OpSpecs) authorize different values, or unless the airport's published approach minima require otherwise. The logic is that a precision approach provides vertical guidance, giving the crew a meaningful ability to execute a successful approach even in reduced conditions.
Non-Precision Approach Alternate Minimums
If only a non-precision approach (VOR, NDB, RNAV without vertical guidance, LOC without glide slope) is available at the alternate, the required forecast values are higher: at least 800 feet ceiling and 2 statute miles visibility. The extra 200 feet of ceiling buffer acknowledges that without electronic vertical guidance, the crew has less precision during the approach and needs more margin to identify the runway environment and execute a safe landing.
Airports with No Instrument Approach
Some airports have no published instrument approach at all. Under 14 CFR 121.625, such airports may still be used as alternates, but only if it is reasonable to expect that descent from the minimum en route altitude (MEA) and a landing can be made under visual conditions. In practice, this means the dispatcher must have solid justification — typically a forecast well above VFR minimums — to use a non-instrument airport as an alternate. Carriers routinely restrict this option further through their own OpSpecs.
The Role of NOTAMs in Alternate Selection
Weather minimums are only the first filter. A legally compliant alternate must also be operationally usable, and NOTAMs are the primary real-time data source that can disqualify an otherwise acceptable airport. Dispatchers must review NOTAMs for the alternate as carefully as for the destination.
Navigation and approach NOTAMs are the most common disqualifiers. If the ILS that justifies the 600/2 precision alternate minimums is NOTAM'd out of service, the airport may fall back to non-precision status, requiring 800/2 instead — or may no longer qualify at all if no other instrument approach meets the forecast. Dispatchers must cross-check which approaches are available at the time of the estimated arrival, not just at the time of dispatch.
Lighting NOTAMs can also matter significantly. An approach light system (ALS) or runway end identifier lights (REIL) NOTAM'd inoperative may not disqualify the alternate legally, but it effectively raises the practical ceiling needed for a successful approach, especially at night. Dispatchers should factor this into the safety margin evaluation.
Ground equipment and services NOTAMs are equally important. If the only available fuel source at the alternate is NOTAM'd out of service, or if ground deicing equipment is unavailable in icing conditions, the alternate may be legally compliant but operationally unusable. Part 121 regulations and carrier OpSpecs often impose additional requirements ensuring that adequate servicing capability exists at the alternate.
Runway and taxiway NOTAMs — including closures, displaced thresholds, construction activity, and surface condition reports — must be assessed in combination with the aircraft's performance data. A runway closure that reduces usable landing distance below what the aircraft requires in the forecast conditions renders that alternate unusable regardless of the weather minimums.
Field Condition Factors
Beyond NOTAMs, dispatchers must evaluate actual and forecast field conditions at the alternate. This analysis mirrors the performance-based approach used for destination planning but may require even more conservatism because by the time the aircraft arrives at the alternate, fuel may be near minimums and a go-around followed by diversion to a third airport may not be viable.
Runway condition reports (RCRs) and Runway Condition Codes (RwyCC) provide critical friction data in winter operations. Under 14 CFR 121.195, dispatchers must ensure that the aircraft can stop within the available landing distance for the prevailing conditions. If a forecast of freezing precipitation at the alternate suggests deteriorating RwyCC values by the estimated time of arrival, the dispatcher should either select a different alternate or build in a conservative performance buffer.
Crosswind components must also be evaluated. If the only open runway at the alternate has a forecast crosswind that exceeds the aircraft's demonstrated crosswind limit (or the carrier's more restrictive OpSpec limit), the airport effectively becomes unusable. Dispatchers should verify forecast wind direction and speed relative to available runway orientations.
Ceiling and visibility trends at the alternate deserve special attention. Choosing an alternate whose forecast shows improving conditions is preferable to one that shows marginal and deteriorating weather. A dispatcher who selects an alternate sitting exactly at the legal minimum with a worsening trend has satisfied the regulation but may have created a trap.
Putting It All Together: The Dispatch Decision
Legal compliance and operational safety are both necessary but neither alone is sufficient. A dispatcher's process should follow this logical sequence: (1) Determine whether an alternate is required using the 121.619 forecast test. (2) Identify candidate alternates with approach capability that matches forecast conditions — applying 600/2 for precision or 800/2 for non-precision. (3) Scrub NOTAMs to confirm that the approaches, lighting, navigation aids, and ground services supporting those minimums will actually be available at the estimated time of arrival. (4) Evaluate runway condition, landing performance, and crosswind for the forecast conditions. (5) Consider trend — is the alternate improving, holding steady, or deteriorating?
Carriers frequently impose more restrictive alternate minimums than the regulatory floor through their OpSpecs and General Operations Manual. Dispatchers must know their company's specific requirements and apply whichever standard is more restrictive. The FAA sets the floor; the carrier sets the policy; the dispatcher integrates both with real-world conditions on every flight release.
Key Numbers and Rules
- 121.619 alternate-required trigger: Forecast ceiling below 1,500 ft above the lowest circling MDA (or airport elevation if no circling approach) OR visibility below 3 SM (or 2 SM above the lowest applicable minimum, whichever is greater) during the period 1 hour before to 1 hour after ETA at destination. (Note: the Part 91 "1-2-3 rule" under 91.169 is a different test and does not apply to Part 121 dispatch.)
- Precision approach alternate minimums: Forecast ceiling at least 600 ft and visibility at least 2 SM at ETA.
- Non-precision approach alternate minimums: Forecast ceiling at least 800 ft and visibility at least 2 SM at ETA.
- Regulatory authority: 14 CFR 121.619 (alternate requirement), 14 CFR 121.625 (alternate weather minimums).
- Landing distance requirements: 14 CFR 121.195 governs wet and dry runway performance and must be satisfied at the alternate.
- NOTAMs are evaluated for the ETA window — not just at time of release.
Common Test Traps
- Confusing the alternate-required window: The two-hour window is centered on the ETA (1 hour before, 1 hour after). Candidates sometimes apply it as a one-hour window, apply the Part 91 1-2-3 rule to a Part 121 flight, or treat it as beginning at departure time.
- Applying minimums based on planned approach, not available approach: Alternate minimums are based on the approach capability at the alternate airport at ETA, not what the crew prefers to fly. A NOTAM'd-out ILS changes the calculation from 600/2 to 800/2.
- Ignoring NOTAM timing: A NOTAM with a future start time that falls within the ETA window must be considered; dispatchers who only check active-at-release NOTAMs will miss this.
- Treating the regulatory minimum as operationally acceptable: An alternate forecast exactly at 600/2 with worsening trends is legally compliant but operationally questionable. Exam scenarios may test whether the candidate applies professional judgment beyond the minimum.
- Overlooking ground services: A fuel or deicing NOTAM does not automatically appear in weather or approach NOTAM categories. Dispatchers must check all NOTAM categories for the alternate, not just D-NOTAMs related to approaches.