What weather minima does Exemption 3585 allow for filing an alternate airport under Part 121?

Exemption 3585 allows FAA-approved Part 121 domestic carriers to file an alternate airport using the lowest published instrument approach minima at that airport, rather than the standard 600-2 (precision) or 800-2 (non-precision) buffer required by 14 CFR 121.621. This means the alternate only needs to be forecast to meet its own published approach minimums at the estimated time of arrival. The carrier must hold explicit OpSpec authorization for this to apply.

How is Exemption 3585 similar to ETOPS rules for domestic alternate planning?

Both Exemption 3585 and ETOPS use a philosophy of replacing standard conservative weather buffers with reduced minima justified by rigorous, documented maintenance reliability and dispatch system quality. Just as ETOPS approves reduced diversion-airport weather thresholds for airlines with proven engine and systems reliability programs, Exemption 3585 grants reduced domestic alternate minima to carriers that demonstrate equivalent operational rigor. Neither framework removes safety — both substitute demonstrated reliability data for statistical conservatism.

Does Exemption 3585 change the 1-2-3 rule that determines when a Part 121 flight must file an alternate?

No. Exemption 3585 only affects the weather standard the alternate airport itself must meet, not the rule that triggers the need to file an alternate in the first place. Under 14 CFR 121.619, a domestic Part 121 flight must still file an alternate whenever the destination is forecast below 2,000 feet ceiling or 3 statute miles visibility during the window from one hour before to one hour after ETA. That threshold is unchanged by the exemption.

Exemption 3585 and Eligible-Alternate (ETOPS-Equivalent) Minima

When a domestic airline dispatcher plans a flight, selecting a suitable alternate airport is one of the most consequential weather-planning decisions of the day. Standard alternate minima — generally a 600-foot ceiling and 2 statute miles visibility for precision approaches, or 800 and 2 for non-precision, subject to any different alternate minima published on the approach chart for that airport — are familiar to most aviators. But for certain carriers operating under Part 121, a regulatory mechanism known as Exemption 3585 unlocks a different, more refined framework for evaluating alternates. Sometimes called eligible-alternate minima or, colloquially, ETOPS-equivalent minima because of the analytical rigor borrowed from Extended Operations (ETOPS) programs, this exemption allows approved operators to use reduced weather thresholds when the operational and maintenance infrastructure justifies the safety equivalence.

Understanding Exemption 3585 requires grounding in the underlying alternate-airport regulations found in 14 CFR 121.619 through 121.625, which govern when alternates must be filed, what weather is required, and what exceptions exist. Dispatchers preparing for the Aircraft Dispatcher certificate exam — and those already holding it — must understand not just the raw numbers, but the logic, the approval pathway, and the practical application of this framework.

The Regulatory Foundation: 14 CFR 121.619–.625

Part 121 imposes explicit alternate-airport requirements that differ depending on the type of operation (domestic, flag, or supplemental) and the destination airport's forecast weather. The relevant sections are:

14 CFR 121.619 — Domestic operations alternate airport requirements. Requires a certified dispatcher and pilot in command to jointly agree that the destination forecast (from one hour before to one hour after ETA) meets certain thresholds; if the destination is forecast to be below those thresholds, an alternate must be listed on the dispatch release.
14 CFR 121.621 — Alternate airport weather minima for domestic operations. Establishes the standard precision/non-precision approach minima an alternate must forecast to meet at ETA.
14 CFR 121.623 — Alternate airport requirements for flag operations.
14 CFR 121.625 — Alternate airport weather minima for flag operations. Mirrors the domestic framework but applies to international flag routes.

Under the standard rule, an alternate airport qualifies only if its forecast weather at the estimated time of arrival meets the prescribed minima: for a precision instrument approach (ILS), a ceiling of at least 600 feet and visibility of at least 2 statute miles; for a non-precision approach, a ceiling of at least 800 feet and visibility of at least 2 statute miles; where two or more approaches exist, the lowest applicable standard applies. These standard values apply unless the approach chart for that airport publishes different alternate minima, which take precedence. These are commonly tested values on the dispatcher written exam.

What Exemption 3585 Is and How It Works

Exemption 3585 is a grant of exemption issued by the FAA to qualifying Part 121 domestic carriers, relieving them — under precisely defined conditions — from the standard alternate weather minima of 14 CFR 121.621. In place of those standard values, the exemption allows the use of eligible-alternate minima, which are tied to the actual published approach minima for the alternate airport, rather than to a fixed ceiling/visibility buffer above those minima.

In practical terms, this means an approved carrier can file an alternate airport as long as the forecast weather at that airport meets the ceiling and visibility of the lowest published instrument approach procedure available there — essentially the same minima a crew would use to actually land. The logic mirrors what ETOPS rules do for oceanic diversion airports: because ETOPS requires rigorous analysis of aircraft reliability, maintenance tracking, and dispatch systems, the FAA accepts that comparable rigor in a domestic program justifies tighter weather thresholds.

The exemption does not remove the obligation to file an alternate where one is required; it changes only the weather standard that alternate must meet. The requirement to file when the destination is forecast below minimums still applies in full.

Eligibility Requirements for Approved Operators

Not every Part 121 carrier can simply elect to use Exemption 3585. The FAA imposes a layered approval process, and operations specifications must explicitly authorize the use of eligible-alternate minima. Key eligibility pillars include:

Approved maintenance and reliability program: The carrier must demonstrate an aircraft maintenance tracking and reliability program comparable in rigor to what ETOPS demands. This includes continuous airworthiness monitoring, engine reliability data, and MEL discipline.
Dispatch system integrity: The airline's dispatch system must be capable of real-time weather monitoring and must have established procedures for updating the alternate selection if weather deteriorates after the dispatch release is issued.
Training and qualification: All dispatchers and relevant flight crew must receive specific training on the eligible-alternate concept, including how to identify qualified airports and how to apply the minima correctly.
Operations specifications (OpSpecs): The carrier's OpSpecs, issued by the FAA Certificate Management Office, must contain the specific authorization. Without the OpSpec paragraph, the exemption does not apply to that operator's flights regardless of fleet or route.
Aircraft and route suitability: Not all aircraft types or routes may be eligible. The carrier must demonstrate that the aircraft systems, particularly propulsion reliability, support the reduced conservatism in alternate planning.

Why It Matters Operationally

The practical benefit of Exemption 3585 is significant, especially in geographic regions where usable alternate airports are sparse or where standard 600-2/800-2 minima would force a carrier to fly with excess fuel reserves on every leg, day after day, adding cost and weight penalties. By permitting alternates to be filed against the actual published approach minima, dispatchers gain access to more airports as viable alternates during marginal weather, and they do so with a safety equivalence that the FAA has scrutinized and endorsed through the exemption process.

Consider a dispatcher routing a narrowbody aircraft to a mountain-valley airport in winter. The only realistic alternate within fuel range has an ILS approach with published minima of 300-1. Under the standard rule, the alternate would need a forecast of 600 and 2 — weather the region rarely sees in winter — making the alternate essentially unusable. Under Exemption 3585, that alternate qualifies if its forecast meets 300-1, a far more realistic threshold. This single change can be the difference between a commercially and operationally viable dispatch and a cancellation.

From a safety standpoint, the FAA's position is that the rigorous maintenance, dispatch, and training infrastructure required to hold the exemption produces safety equivalent to or better than the standard buffer, because the buffer's original conservatism was designed partly to compensate for less sophisticated operational oversight systems. The exemption recognizes that approved carriers have replaced statistical conservatism with demonstrable, documented reliability.

Key Numbers and Rules

Standard precision alternate minima (domestic, 14 CFR 121.621): Ceiling 600 feet, visibility 2 statute miles.
Standard non-precision alternate minima (domestic, 14 CFR 121.621): Ceiling 800 feet, visibility 2 statute miles.
Eligible-alternate minima (Exemption 3585, approved carriers): Forecast must meet the lowest published instrument approach minima at the alternate airport — no added buffer above the published procedure minimum.
No-alternate rule (1-2-3 rule): Under 14 CFR 121.619, no alternate is required if, from one hour before to one hour after ETA, the destination is forecast to have a ceiling of at least 2,000 feet and visibility of at least 3 statute miles. Exemption 3585 does not change this threshold.
OpSpec requirement: Authorization must appear in the carrier's operations specifications; the exemption is not self-executing.
Flag operations: 14 CFR 121.625 governs flag alternate minima; a parallel exemption framework may apply, but domestic Exemption 3585 is specifically tied to 14 CFR 121.621 domestic operations.

Common Test Traps

Confusing who holds the exemption: The exemption belongs to the carrier, not to the individual dispatcher or pilot. A dispatcher at an unapproved carrier cannot apply eligible-alternate minima even if they are personally familiar with the concept.
Thinking the 1-2-3 rule changes: Exemption 3585 modifies only the weather standard for the alternate airport, not the threshold that triggers the need to file an alternate in the first place. The 2,000-3 destination forecast rule under 14 CFR 121.619 remains unchanged.
Mixing up domestic vs. flag regulations: 14 CFR 121.621 is domestic; 14 CFR 121.625 is flag. Exam questions often test whether students know which section applies based on the operation type described in the scenario.
Assuming all ILS alternates automatically qualify: Even under Exemption 3585, the alternate must be forecast to meet the published minima at ETA. A beautiful ILS alternate that is forecast below its own published minima at the estimated time of arrival still cannot legally be filed.
Overlooking the OpSpec requirement: Students sometimes memorize the weather numbers without noting that an explicit OpSpec authorization is a prerequisite. On a scenario-based question, if the carrier lacks the OpSpec, standard minima apply regardless of the exemption's existence.