What is the 1-2-3 rule for IFR alternate airport filing?

The 1-2-3 rule, found in 14 CFR 91.169, requires you to file an alternate airport on your IFR flight plan if, from one hour before to one hour after your estimated time of arrival, the destination airport's forecast ceiling is less than 2,000 feet or visibility is less than 3 statute miles. If those conditions are expected to be met, no alternate is legally required, though carrying one is always good airmanship. If an alternate is required, it must meet its own weather minimums at your estimated time of arrival there.

What's the difference between standard and non-standard IFR alternate minimums?

Standard alternate minimums under 14 CFR 91.169 allow an airport to be filed as an alternate if it has a precision approach and forecast weather of at least 600-foot ceiling and 2 statute miles visibility, or a non-precision approach with at least 800-foot ceiling and 2 statute miles visibility. When a triangle with an 'A' inside it appears next to an airport's name on the IFR Alternate Minimums chart, that airport has non-standard alternate minimums published in the chart booklet that must be used instead. Some airports marked with a 'NA' notation may not be used as an alternate at all under IFR.

How do you read the IFR Takeoff Minimums chart and why does it matter?

The IFR Takeoff Minimums chart, published in the Terminal Procedures Publication, uses an inverted triangle with a 'T' to indicate that an airport has non-standard takeoff minimums or departure procedures that deviate from the standard 1-statute-mile visibility (or 1/2 mile for some turbine aircraft) established in 14 CFR 91.175. Pilots must look up the specific minimums or obstacle departure procedures listed in the chart booklet for that airport before departing under IFR. Ignoring these non-standard minimums can expose you to terrain or obstacle conflicts during climb-out, particularly at airports surrounded by rising terrain or obstructions.

IFR Alternate Minimums and Take-Off Minimums Chart Interpretation

When you file an IFR flight plan, two sets of minimums govern your safety margins at the bookends of the trip: takeoff minimums at the departure airport and alternate minimums at the airport you list as an alternate destination. Both sets of minimums appear on a dedicated chart page — the IFR Alternate Minimums and Takeoff Minimums and Obstacle Departure Procedures section published in the FAA Terminal Procedures Publications (TPPs, commonly called the approach plate booklets). Understanding how to read these charts is not just an exam requirement; it is a fundamental skill that keeps you legal and safe every time you fly IFR.

This article walks through the regulatory foundation, how to locate and decode the chart symbols, the standard versus non-standard values you will encounter, and the practical decisions you must make before every IFR departure and flight plan filing.

Regulatory Foundation

The rules for both takeoff minimums and alternate minimums come from 14 CFR Part 91 and the more detailed 14 CFR Part 97, which publishes Standard Instrument Approach Procedures (SIAPs). Part 91 sets the baseline obligation: no pilot operating under Part 91 is required by regulation to meet any specific takeoff minimum (91.175(f) notes that Part 91 pilots may take off in zero-zero conditions at their own risk), but Part 91.169 absolutely governs when and how you must file an alternate airport and what weather minimums apply to that alternate. Commercial operators under Parts 121 and 135 face stricter obligations, but even a private pilot filing IFR must understand these rules because the alternate requirements under 91.169 apply to everyone filing an IFR flight plan.

The 1-2-3 Rule for Filing an Alternate

Before you can worry about what the alternate minimums are, you need to know when you must file an alternate. The FAA's so-called 1-2-3 rule (found in 14 CFR 91.169) states that you must file an alternate airport unless, for the period beginning one hour before and ending one hour after your estimated time of arrival (ETA) at the destination, the forecast weather at the destination is at least a 2,000-foot ceiling and 3 miles visibility. If the destination weather forecast meets or exceeds 2,000 and 3 during that two-hour window, you are not required to list an alternate. If it falls below those values at any point in that window, an alternate is required.

A critical nuance: you use the Terminal Aerodrome Forecast (TAF) to make this determination, not a METAR or a pilot report. The TAF represents a forecast for the specific airport and is the correct source for the 1-2-3 rule evaluation. If no TAF exists for the destination, you should treat the destination as requiring an alternate.

Standard Alternate Minimums

Once you determine an alternate is needed, the next question is: does the alternate airport meet the required weather minimums? The standard alternate minimums under 14 CFR 91.169 depend on what type of instrument approach is available at the alternate:

Precision approach (ILS or equivalent): Forecast ceiling at least 600 feet and visibility at least 2 statute miles at ETA.
Non-precision approach (VOR, NDB, RNAV/GPS without vertical guidance, etc.): Forecast ceiling at least 800 feet and visibility at least 2 statute miles at ETA.
No instrument approach procedure: The airport cannot be used as an alternate unless the flight can descend from the Minimum En Route Altitude (MEA), reach the airport, and land under VFR. Weather must allow that VFR descent and landing.

These standard values apply unless the approach procedure chart or the alternate minimums chart specifies non-standard alternate minimums — which is where chart-reading skill becomes essential.

Reading the Alternate Minimums Chart

In every TPP booklet, the first few pages contain the Takeoff Minimums and (Obstacle) Departure Procedures section, followed immediately by the IFR Alternate Minimums section. These pages list airports alphabetically by city and state. If an airport appears in the alternate minimums section, it has non-standard alternate minimums or restrictions that override the standard values.

The most important symbol to memorize is the inverted A with a triangle (▲ or the NA notation). On individual approach charts, look for a small symbol in the notes box or briefing strip:

Alternate Minimums symbol (an 'A' inside a triangle): This symbol on an approach chart means the airport has non-standard alternate minimums. You must turn to the alternate minimums section of the TPP to find the specific values before listing this airport as an alternate.
NA (Not Authorized): When the alternate minimums section lists an airport with the notation NA, it means that airport is not authorized as an alternate under any circumstances using IFR procedures. This is common for airports that lack weather reporting capability, do not have a usable instrument approach at certain times, or have unique operational restrictions.

When you look up an airport in the alternate minimums section and find specific values listed — for example, a VOR approach requiring 900-2 instead of the standard 800-2 — those higher values become your legal minimum. The airport may still be usable as an alternate, but only if the forecast meets the elevated criteria.

Non-standard minimums arise for many reasons: mountainous terrain requiring higher obstacle clearance, part-time control towers affecting approach availability, non-standard approach categories, or approaches that are only usable with certain equipment operating (such as DME or radar). Always read the notes carefully; they may restrict the alternate authorization to certain hours, certain runways, or require specific airborne equipment.

Takeoff Minimums and Obstacle Departure Procedures

The same TPP section also covers takeoff minimums. By default, Part 91 pilots have no legally mandated takeoff minimum — you could legally depart in zero-zero visibility (though this would be extremely dangerous and operationally inadvisable). However, when an airport has non-standard takeoff minimums or a published Obstacle Departure Procedure (ODP), this appears in the Takeoff Minimums section.

The symbol on the individual airport diagram or approach chart that alerts you to non-standard takeoff minimums is a bold T inside a triangle (▲T). When you see this symbol, you must look up the airport in the takeoff minimums section to find the specific values or procedures.

Standard takeoff minimums for Part 91 operators, while not regulatory requirements, are considered operationally standard by convention and are the baseline for commercial operators: 1 statute mile visibility for aircraft with 1 or 2 engines, and ½ statute mile visibility for aircraft with more than 2 engines. Non-standard values can be higher (e.g., 1,600 RVR, or a specific climb gradient like 300 feet per nautical mile) due to terrain, obstacles, or noise restrictions.

An Obstacle Departure Procedure (ODP) is a charted or textually described procedure designed to ensure obstacle clearance during climb after departure. ODPs may specify a heading to fly, a climb rate to maintain, or a fix to cross at a certain altitude. Unlike a Standard Instrument Departure (SID), an ODP does not require ATC clearance to fly and may simply appear as text in the takeoff minimums section describing a specific departure heading and minimum climb gradient.

Why It Matters in the Cockpit

Misreading or ignoring alternate and takeoff minimum charts creates cascading safety risks. Arriving at a destination in IMC with no legal alternate — or arriving at an alternate that is below its own published minimums — puts the entire flight in jeopardy. Equally, departing without reviewing the ODP in low-visibility conditions near mountainous terrain has been a factor in controlled-flight-into-terrain (CFIT) accidents. The chart symbols exist precisely to draw your attention to these exceptional situations before you ever start the engine.

Key Numbers and Rules

1-2-3 rule: File an alternate if destination forecast is below 2,000 feet ceiling or 3 SM visibility within ±1 hour of ETA.
Standard alternate — precision approach: 600-2 (600 ft ceiling, 2 SM visibility).
Standard alternate — non-precision approach: 800-2 (800 ft ceiling, 2 SM visibility).
NA symbol in alternate minimums: Airport is NOT authorized as an alternate — period.
Triangle-A symbol on approach chart: Non-standard alternate minimums exist — look them up.
Triangle-T symbol on approach chart: Non-standard takeoff minimums or ODP published — look them up.
Standard takeoff minimums (Part 91, for reference): 1 SM for 1-2 engine aircraft; ½ SM for 3+ engine aircraft.
Alternate minimums weather source: Use the TAF forecast for the alternate, not current METAR.

Memory Aid

For the 1-2-3 rule, many instructors teach: "One hour either side, Two-thousand and Three." That is: look at the one-hour window on each side of your ETA; the destination forecast must show at least 2,000 feet and 3 miles during that window or you must file an alternate. The numbers themselves — 1, 2, 3 — are the rule's name, making it self-reinforcing once you know what each digit represents.

Common Test Traps

Confusing destination minimums with alternate minimums. The 1-2-3 rule governs when you need an alternate; the 600-2 / 800-2 values govern what weather that alternate must have. These are two separate determinations.
Using current weather instead of a forecast. Both the 1-2-3 rule (for the destination) and the alternate minimums check require you to use forecast weather (TAF), not the current METAR.
Ignoring the NA notation. Students sometimes list an airport as an alternate without checking the alternate minimums section. An NA designation makes that airport entirely off-limits as an alternate regardless of actual weather.
Assuming Part 91 pilots are exempt from everything. Part 91 pilots are exempt from mandatory takeoff minimums but are NOT exempt from the alternate filing requirements under 91.169. The exam will test this distinction.
Forgetting that non-standard minimums can be higher OR more restrictive in other ways. Non-standard alternate minimums are not always just higher ceilings and visibility; they can also restrict usage to certain hours, certain approaches, or require specific equipment. Always read the full text of the listing.