One of the most critical planning tasks for any IFR flight is determining whether your destination airport requires an alternate airport on your flight plan — and if so, which airports actually qualify as legal alternates. Get this wrong and you may file an illegal flight plan, or worse, find yourself low on fuel with no legal place to land in deteriorating weather. The FAA's rules on alternate airports are found in 14 CFR 91.169 and are a staple of the Instrument Rating knowledge test. Understanding the logic behind the rules, not just the numbers, will help you apply them correctly in the real world.
This article walks through when an alternate is required, the famous "1-2-3 rule," what weather minimums an alternate airport must meet, how to read alternate minimums from the Terminal Procedures Publication (TPP), and the special cases that trip up even experienced pilots.
When Is an Alternate Airport Required?
Under 14 CFR 91.169, you must list an alternate airport on your IFR flight plan unless the weather at your destination airport is forecast to be at or above a specific ceiling and visibility window centered around your estimated time of arrival (ETA). That window and those thresholds are defined by the famous 1-2-3 rule.
The 1-2-3 Rule Explained
The 1-2-3 rule states: From 1 hour before to 1 hour after your ETA (a 2-hour window centered on your arrival), the destination forecast must show a ceiling of at least 2,000 feet and visibility of at least 3 statute miles. If the official forecast — typically the Terminal Aerodrome Forecast (TAF) — shows those conditions throughout that entire 2-hour window, you are not required to file an alternate.
Think of it as a safety buffer: weather forecasts are never perfect, so the FAA builds in an hour of margin on each side of your planned arrival. If forecast conditions are solid — 2,000-foot ceiling and 3 SM visibility or better — during that entire period, the regulations assume you have enough cushion to complete a visual approach or a well-executed instrument approach without needing a backup. If the forecast dips below either threshold at any point in that window, you must designate an alternate.
A few important clarifications: the 1-2-3 rule uses forecast weather, not actual weather. It is entirely possible that actual weather at your destination is perfect VFR while you are still legally required to file an alternate because the TAF predicts deterioration near your ETA. Conversely, if the forecast is fine, you need no alternate even if current METAR conditions are poor — though in practice a prudent pilot would reconsider the flight. Also note that the 1-2-3 rule applies to operations under 14 CFR Part 91; Part 121 and Part 135 operators face stricter requirements.
What Makes an Airport a Legal Alternate?
Once you have determined that an alternate is required, you must choose one that qualifies under 14 CFR 91.169(c). Not every airport with an instrument approach automatically qualifies. The alternate must meet weather minimums at the time you expect to arrive there — and those minimums depend on the type of approach available.
Standard Alternate Minimums
The standard alternate minimums specified in 14 CFR 91.169(c) are:
- Precision approach (e.g., ILS): Forecast ceiling of at least 600 feet and visibility of at least 2 statute miles at your ETA at the alternate.
- Non-precision approach (e.g., VOR, RNAV/GPS, NDB, LOC): Forecast ceiling of at least 800 feet and visibility of at least 2 statute miles at your ETA at the alternate.
- No instrument approach: The alternate must have weather that allows descent from the MEA, approach, and landing under basic VFR conditions. In practice this means the airport must have VFR conditions forecast at your ETA.
Notice that precision approaches allow a lower ceiling (600 feet versus 800 feet) because the ILS provides more precise vertical guidance all the way to decision altitude. Non-precision approaches, which generally have higher minimums by nature, require that extra 200-foot buffer in the alternate minimums to account for the reduced precision.
Non-Standard Alternate Minimums
Here is where many students get caught: the FAA publishes non-standard alternate minimums for specific airports in the front pages of each volume of the Terminal Procedures Publication (formerly called the NOS approach plate booklets or Jepp approach charts). Airports with non-standard alternate minimums are flagged with an inverted black triangle containing a white letter A (often called the "NA triangle" or "alternate minimums symbol") printed in the notes section of the instrument approach procedure chart.
This symbol can mean one of two things: either the airport has higher-than-standard alternate minimums (for example, requiring a 900-foot ceiling or 3 SM visibility because of terrain, obstructions, or limited approach types), or the symbol indicates that the airport is NA — meaning it is not authorized for use as an alternate at all. Common reasons for NA designation include airports with no weather reporting capability, airports whose only available approach is a circling approach with high minimums, or airports lacking the necessary ground equipment during certain hours.
When you see that "A" symbol on an approach chart, you must look up the published non-standard alternate minimums in the TPP front matter to find out exactly what weather is required, or whether the airport is prohibited as an alternate entirely. You cannot assume standard minimums apply.
Weather at the Alternate: Forecast, Not Actual
Just as with the 1-2-3 rule, the determination of whether an airport qualifies as a legal alternate is based on forecast weather at your estimated time of arrival at that alternate. You will use the TAF for the alternate airport if one is available. If no TAF is issued for that location, you may use other forecast products (such as area forecasts), but in practice, airports without TAFs are often problematic choices for alternates because you cannot easily demonstrate the required forecast conditions.
Keep in mind: meeting alternate minimums legally qualifies an airport as your alternate for flight plan purposes, but sound aeronautical decision-making demands that your alternate have a comfortable margin above those minimums. Filing an alternate where the TAF shows exactly 800 feet and 2 SM — the bare legal minimum for a non-precision approach — is legal but leaves you almost no buffer if the forecast is off by even a small amount.
GPS and RNAV as Alternate Approaches
With the widespread use of GPS/RNAV approaches, an important regulatory nuance applies. For GPS-based approaches at an alternate, the GPS approach may only be used to qualify an alternate if the aircraft's avionics are capable of and approved for the approach, and RAIM/integrity must be available at the ETA at the alternate. Additionally, for planning purposes, LNAV minimums (the non-precision line) must be used when evaluating whether a GPS/RNAV alternate meets the 800/2 standard — you cannot plan to fly an LPV approach (which has vertical guidance similar to an ILS) to satisfy alternate requirements unless the approach is specifically authorized. Consult your GPS Airplane Flight Manual Supplement and current NOTAMs for RAIM availability to ensure your alternate is truly available.
Key Numbers and Rules
- 1-2-3 Rule: No alternate required if destination TAF shows ceiling ≥2,000 ft and visibility ≥3 SM from 1 hour before to 1 hour after ETA.
- Precision alternate minimum: 600 feet ceiling, 2 SM visibility (forecast at ETA).
- Non-precision alternate minimum: 800 feet ceiling, 2 SM visibility (forecast at ETA).
- No instrument approach: VFR conditions forecast at ETA.
- Non-standard minimums: Indicated by the "A" triangle on approach charts; consult TPP front matter.
- NA designation: Airport may not be used as an alternate at all — check the chart notes.
- Regulatory basis: 14 CFR 91.169 (Part 91 operations).
Memory Aid
For the 1-2-3 rule, simply remember the sequence of numbers: 1 hour before, 1 hour after, 2,000-foot ceiling, 3 SM visibility. Some students remember it as "one, one, two, three" — one hour before, one hour after, two thousand feet, three miles. The numbers climb in order: 1, 1, 2, 3. Once you have that pattern locked in, applying it to any scenario becomes automatic.
Common Test Traps
- Confusing forecast with actual: The 1-2-3 rule and alternate qualification are both based on forecast weather at ETA, not current conditions. A METAR showing 500 overcast right now does not automatically require an alternate if the TAF projects improvement before your arrival window.
- Forgetting the full 2-hour window: The forecast must be at or above 2,000/3 for the entire period from 1 hour before to 1 hour after ETA. If the TAF shows a TEMPO or PROB30 dip below minimums at any point in that window, an alternate is required.
- Assuming all airports with an approach qualify: The "A" triangle on the approach plate is a red flag. Many airports are designated NA for alternate use or carry higher-than-standard minimums. Always check the TPP front matter.
- Mixing up precision vs. non-precision minimums: ILS (precision) = 600/2; VOR, NDB, RNAV LNAV (non-precision) = 800/2. Students sometimes reverse these or apply a single number to both.
- GPS alternate planning: You cannot plan on flying an LPV approach to qualify an alternate in most cases. Use LNAV minimums for planning, and verify RAIM availability at your ETA at the alternate.