What is the 1-2-3 rule in IFR flight planning?

The 1-2-3 rule, found in 14 CFR 91.169, states that an alternate airport is required if, from 1 hour before to 1 hour after your estimated time of arrival at the destination, the forecast ceiling will be less than 2,000 feet above the airport elevation or the visibility less than 3 statute miles. If the destination's forecast weather meets or exceeds those thresholds during that 2-hour window, you are not required to file an alternate. This rule applies to IFR flight plans filed under 14 CFR Part 91 and is a foundational concept on the FAA Instrument Rating Airplane Knowledge Test.

What weather minimums must an alternate airport meet for IFR filing purposes?

Under 14 CFR 91.169, if the alternate airport has a precision instrument approach (such as an ILS), it must have forecast weather of at least a 600-foot ceiling and 2 statute miles visibility at your estimated time of arrival. If it has only a non-precision approach, the alternate minimums are at least an 800-foot ceiling and 2 statute miles visibility; if the airport has no instrument approach, the alternate must allow a descent from the MEA under basic VFR minimums as defined in 91.155. Some airports publish non-standard alternate minimums in the U.S. Terminal Procedures Publication, indicated by an 'A' inside a triangle on the approach chart.

Why is filing an alternate airport important even if the destination looks good at departure?

Weather can deteriorate rapidly and unpredictably, so the FAA requires an alternate to ensure you have a legal and safe option if the destination becomes unusable upon arrival. The alternate requirement forces pilots to verify that a suitable airport with acceptable forecast conditions and an appropriate instrument approach exists within reach, which is a core element of IFR preflight planning as described in the Instrument Flying Handbook. Failing to file a required alternate or selecting one that does not meet the regulatory minimums is a violation of 14 CFR 91.169 and reflects inadequate aeronautical decision-making that is evaluated on the FAA Instrument Rating Airplane Practical Test.

Alternate Airport Weather Minimums (1-2-3 Rule)

Before any IFR flight departs, the pilot-in-command must answer two tightly related questions: Do I need to file an alternate airport? And if so, what weather does that alternate need? The FAA's answer to both questions is elegantly compact. Regulations found in 14 CFR Part 91 and Part 121/135 set out precise numerical thresholds that pilots call the 1-2-3 rule for alternate requirements and the alternate minimums for the weather that must be forecast at the chosen airport. Understanding both halves — when you need an alternate and what counts as a legal one — is essential for the instrument rating knowledge test and, more importantly, for safe IFR operations in the real world.

This article focuses on the rules that apply to Part 91 general aviation operations, which govern the vast majority of student pilots and private/instrument-rated pilots flying their own aircraft. Where Part 121 (airlines) differs, that distinction is noted for context.

When You Must File an Alternate: The 1-2-3 Rule

Under 14 CFR §91.169, a pilot filing an IFR flight plan is not required to list an alternate airport if the weather forecast at the destination meets a specific standard for a specific time window around the planned arrival. If the destination forecast falls short of that standard, listing an alternate is mandatory — and that alternate must itself meet its own minimum weather criteria.

The threshold is straightforward to memorize:

1 hour before to 1 hour after the estimated time of arrival (ETA), the destination airport must be forecast to have:
2,000 feet ceiling above the airport elevation or higher, AND
3 statute miles visibility or greater.

That three-number sequence — 1, 2, 3 — gives the rule its nickname. If the destination's terminal area forecast (TAF) shows those conditions throughout that two-hour window centered on your ETA, you are legally free to file without an alternate. The moment the forecast dips below 2,000 feet ceiling above airport elevation or below 3 statute miles visibility at any point during that window, you must file an alternate.

Reading the Rule in Practice

Suppose your ETA at the destination is 1800Z. You need the TAF to show at least a 2,000-foot ceiling above airport elevation and 3 statute miles visibility continuously from 1700Z through 1900Z. If the TAF shows a temporary condition (TEMPO) or a becoming condition (BECMG) that drops visibility to 2 statute miles at 1730Z, that window is broken and you must list an alternate — even if the main body of the forecast looks fine. Forecasters code uncertainty into TAFs using precisely these qualifiers, and the regulation treats any forecast condition within that window as representative of what you may encounter.

It is equally important to note what the 1-2-3 rule does not do: it does not tell you whether you can legally continue to the destination or land there. It is purely a preflight filing decision. Once airborne, actual weather and approach minimums govern what you may or may not attempt.

What Weather Must the Alternate Have?

Filing an alternate only satisfies the regulation if the alternate airport itself meets forecast weather minimums. The FAA divides alternate weather minimums into two categories based on whether the alternate has an instrument approach procedure.

Alternate Has a Precision Approach (ILS)

If the alternate airport is served by at least one precision approach — most commonly an Instrument Landing System (ILS) — the forecast weather at the alternate must be at or above:

600-foot ceiling
2 statute miles visibility

These values must be forecast at the estimated time of arrival at the alternate, not at your destination ETA. This is a common source of confusion: you use the destination ETA for the 1-2-3 filing decision, but you use the alternate's ETA for checking whether the alternate qualifies.

Alternate Has Only Non-Precision Approaches

If the alternate has instrument approaches but none of them are precision approaches, the required forecast weather is higher:

800-foot ceiling
2 statute miles visibility

The logic here reflects the fact that non-precision approaches (VOR, RNAV/LNAV, NDB, LOC) do not provide vertical electronic guidance down to the runway and therefore require better ceiling conditions to allow the pilot to descend from the Minimum Descent Altitude (MDA) and complete the landing.

Alternate Has No Instrument Approach

An airport with no published instrument approaches can still be used as an alternate under Part 91, but only if the forecast allows a descent from the Minimum En Route Altitude (MEA), an approach, and a landing under basic VFR minimums as defined in 14 CFR §91.155. In practical terms, this means the pilot must be able to visually identify and land at the airport without relying on any published instrument procedure. This scenario is rare in planning, but the regulation permits it — a useful fact to know for test purposes.

Airports with Published Non-Standard Alternate Minimums

Some airports have terrain, obstacles, or procedural constraints that make the standard alternate minimums unsafe. For these locations, the FAA publishes non-standard alternate minimums in the front matter of approach procedure charts (the booklets and digital equivalents pilots use for instrument approaches). When an airport has non-standard minimums, a black triangle with the letter A inside it appears on the approach chart. Pilots must look up the specific published values and use those instead of the standard 600-2 or 800-2 figures. Ignoring the A symbol and defaulting to standard values is a dangerous oversight and a favorite FAA test trap.

Conversely, some airports are designated NA for alternate purposes, meaning they cannot be used as an alternate at all. This designation also appears in the approach chart notes and must be respected.

Why This Matters: The Safety Logic

The alternate requirement exists because IFR flight planning must account for the possibility that your destination becomes unusable — equipment failures at the destination, unexpected weather deterioration below approach minimums, runway closures, or traffic conflicts can all force a diversion. The alternate airport is your pre-planned backup, and the weather minimums ensure that backup is actually viable at the time you would need it.

Arriving over a destination in IMC with no alternate, low fuel, and deteriorating weather throughout the region is one of the most dangerous situations a pilot can face. The 1-2-3 rule and alternate minimums are pre-flight safeguards designed to prevent exactly that scenario by forcing the decision — and the fuel planning — before the flight ever leaves the ground.

Fuel planning and alternate planning are inseparable. Under 14 CFR §91.167, IFR fuel requirements state that a pilot must carry enough fuel to fly to the destination, then to the alternate (if one is required), and then for 45 additional minutes at normal cruise. Choosing a distant alternate demands more fuel; the rule creates an economic and practical incentive to choose an alternate that is genuinely reachable.

Key Numbers and Rules

1-2-3 Rule: No alternate required if destination forecast shows at least 2,000 ft ceiling above airport elevation and 3 SM visibility from 1 hour before to 1 hour after ETA.
Precision approach alternate minimum: Forecast ceiling 600 ft and visibility 2 SM at alternate ETA.
Non-precision approach alternate minimum: Forecast ceiling 800 ft and visibility 2 SM at alternate ETA.
No approach alternate: Must allow descent from MEA and VFR landing.
Non-standard minimums: Look for the circled or filled triangle-A symbol on the approach chart; published values replace standard values.
NA designation: Airport cannot be filed as an alternate at all.
IFR fuel reserve: Destination + alternate (if required) + 45 minutes at normal cruise.
Governing regulation: 14 CFR §91.169 (alternate requirements) and §91.167 (fuel requirements).

Memory Aid

1-2-3: Think of it as counting up — 1 hour before and after, 2 thousand feet ceiling, 3 statute miles visibility. The numbers increase by one each step, making the sequence easy to recall under pressure. Once you know an alternate IS required, shift to 6-2 or 8-2: precision approaches need a 600-foot ceiling with 2 SM, non-precision need 800 feet with 2 SM. The visibility stays constant at 2 SM regardless; only the ceiling changes based on approach type.

Common Test Traps

Mixing up the two windows: The 1-2-3 rule uses the destination ETA to decide if an alternate is needed. The 600-2 / 800-2 minimums use the alternate's ETA. Confusing these two reference times is the single most common error on knowledge test questions.
Thinking 1-2-3 is the alternate minimum: Some students believe 2,000 and 3 SM are the weather required at the alternate. They are not — they are the destination forecast threshold for determining whether an alternate is needed at all.
Ignoring the triangle-A symbol: Questions frequently present an airport with published non-standard minimums. The correct answer always requires checking for and using those published values, not defaulting to 600-2 or 800-2.
Forgetting the NA designation: An airport marked NA in the alternate minimums section cannot be used as an alternate regardless of its forecast weather or approach type.
Confusing ceiling and visibility requirements by approach type: Both precision and non-precision alternate minimums require 2 SM visibility. Only the ceiling changes — 600 feet for precision, 800 feet for non-precision. Students who reverse these numbers or believe visibility changes between the two categories will miss these questions.