Running low on fuel in instrument meteorological conditions (IMC) is one of the most dangerous situations a pilot can face. Unlike VFR flight, where a pilot can often spot a suitable field and land, an IFR pilot operating in clouds has almost no margin for fuel miscalculation. The FAA has therefore established clear, legally binding rules governing how much fuel you must carry and when you must file an alternate airport on an IFR flight plan. These rules live primarily in 14 CFR 91.167 and the associated alternate airport weather minimums found in the Aeronautical Information Manual (AIM) and the instrument approach procedure charts. Mastering them is required for the Instrument Rating knowledge test — and, far more importantly, for every IFR flight you will ever conduct.
This article walks through the fuel requirement rules, the alternate airport filing requirement (including the famous "1-2-3 rule"), the standard and non-standard alternate minimums, and the practical cockpit thinking that ties it all together.
IFR Fuel Requirements: The Legal Baseline
Under 14 CFR 91.167, no person may operate a civil aircraft in IFR conditions unless it carries enough fuel to complete the flight to the first airport of intended landing, then fly from that airport to the alternate airport (if one is required), and then fly for an additional 45 minutes at normal cruising speed. This 45-minute reserve applies to non-turbine (reciprocating engine) civil aircraft operating under Part 91; turbine-powered aircraft are also required to carry a 45-minute reserve under 91.167(a)(3)(ii), so the 45-minute figure is not unique to reciprocating-engine airplanes.
It is critical to understand that this is a minimum legal standard, not a recommended planning target. The FAA and experienced instructors routinely counsel pilots to carry more than the minimum, particularly when operating in winter, at night, or when weather forecasts carry any uncertainty. The regulations set a floor, not a ceiling.
The calculation proceeds in distinct steps:
- Trip fuel: The fuel required to fly from departure to the destination airport, accounting for winds, altitude, and aircraft performance data from the Pilot's Operating Handbook (POH).
- Alternate fuel: The additional fuel to fly from the destination to the alternate airport (if an alternate is required), again using forecast winds and your aircraft's normal cruising fuel burn.
- Reserve fuel: 45 minutes at normal cruising speed after completing both legs above.
If no alternate is required (more on the 1-2-3 rule below), the alternate fuel segment is simply omitted, and you need only trip fuel plus the 45-minute reserve.
The Alternate Airport Requirement: The 1-2-3 Rule
Not every IFR flight requires an alternate airport on the flight plan. The requirement hinges on the forecasted weather at your destination airport. Under 14 CFR 91.167(b), you do not need to list an alternate airport if, for at least one hour before and one hour after your estimated time of arrival (ETA), the forecast weather at the destination calls for a ceiling of at least 2,000 feet and visibility of at least 3 statute miles.
This is universally known as the "1-2-3 rule":
- 1 hour before to 1 hour after your ETA
- Ceiling at least 2,000 feet
- Visibility at least 3 statute miles
If the Terminal Aerodrome Forecast (TAF) — the official forecast used for this determination — calls for conditions at or above 2,000 feet ceiling and 3 SM visibility throughout that two-hour window centered on your ETA, no alternate is required and you drop the alternate fuel leg from your fuel calculation.
If the TAF shows conditions below either of those thresholds at any point during that two-hour window — or if the destination airport simply has no TAF — you must file an alternate airport and carry the fuel to reach it.
Alternate Airport Weather Minimums
Selecting an alternate airport is not as simple as picking a nearby field. The FAA imposes specific weather minimums that the alternate airport must be forecast to meet at the time you expect to arrive there. These are called alternate minimums, and they come in two flavors: standard and non-standard.
Standard Alternate Minimums
If an airport does not have a published non-standard alternate minimums note (a black triangle with an "A" on the approach chart), the following standard minimums apply when using that airport as an alternate:
- Precision approach (ILS): Forecast ceiling at least 600 feet and visibility at least 2 statute miles at your planned arrival time.
- Non-precision approach (VOR, RNAV, etc.): Forecast ceiling at least 800 feet and visibility at least 2 statute miles at your planned arrival time.
- No instrument approach: The airport cannot be used as an alternate unless you can descend from the MEA, fly to the airport, and land under VFR. In practice this means you need a forecast that clearly allows VFR arrival.
Notice that precision approach minimums for alternates are lower than non-precision minimums. This makes logical sense: an ILS gives you more guidance and allows safe approaches to lower ceilings, so the FAA permits its use as an alternate down to a lower weather threshold.
Non-Standard Alternate Minimums
Some airports have unique terrain, limited NAVAID coverage, or operational restrictions that make the standard minimums unsafe or impractical. These airports publish non-standard alternate minimums in the front section of the applicable instrument approach procedure booklet (the TERPS chart package). If you see the black triangle with an "A" on an approach plate, you must look up and apply the published non-standard minimums when considering that airport as an alternate. Ignoring the triangle is a common and dangerous error.
Additionally, some airports are marked with a "NA" (Not Authorized) notation, meaning the airport may not be used as an alternate at all. This might be because there is no weather observation service or because the approaches are not suitable for use as alternate minimums.
Practical Cockpit Application
Before every IFR flight, your fuel planning should flow through a clear mental checklist. First, retrieve the current TAF for your destination and check the entire two-hour window around your ETA. If conditions are forecast below 2,000 feet or 3 SM at any point in that window, you need an alternate. Select an alternate airport whose forecast weather meets the alternate minimums for its best available approach. Then add up trip fuel, alternate fuel, and the 45-minute reserve. Cross-check the total against your usable fuel as listed in the aircraft's POH, not the fuel gauges.
A point that catches many students: the 1-2-3 rule is based on forecast conditions, not actual conditions. Even if the destination is reporting clear skies when you depart, if the TAF shows a low ceiling moving in around your ETA, you are legally required to file an alternate and carry the fuel for it. The regulation is forward-looking by design.
Key Numbers and Rules
- 45 minutes — Required fuel reserve after reaching destination (and alternate, if required), at normal cruising speed, for Part 91 IFR flight.
- 1-2-3 rule — No alternate needed if TAF shows ceiling ≥2,000 ft and visibility ≥3 SM from 1 hour before to 1 hour after ETA.
- ILS alternate minimums — 600 feet ceiling / 2 SM visibility (standard).
- Non-precision alternate minimums — 800 feet ceiling / 2 SM visibility (standard).
- Triangle-A symbol — Non-standard alternate minimums published; always look them up before filing that airport as an alternate.
- NA notation — Airport not authorized for use as alternate.
- TAF is the source — Alternate filing decisions are based on Terminal Aerodrome Forecasts, not METARs or pilot reports.
Memory Aid
For the 1-2-3 rule, many instructors use the phrase "One hour, Two thousand, Three miles" recited in order: one hour either side of your ETA, two thousand feet ceiling, three statute miles visibility. If all three numbers are met in the TAF, no alternate is needed. If any one of the three fails, file an alternate and add the fuel.
Common Test Traps
- Using METAR instead of TAF: The alternate filing requirement is based on the forecast (TAF), not the current observed weather. Students who check only the METAR will get this wrong.
- Forgetting the full two-hour window: The TAF must show acceptable conditions from one hour before to one hour after ETA — not just at ETA. A forecast that dips below minimums even briefly in that window triggers the alternate requirement.
- Confusing destination minimums with alternate minimums: The 1-2-3 rule governs whether you need to file an alternate. The 600/2 and 800/2 minimums govern how good the weather must be at the alternate airport itself. These are two separate checks.
- Ignoring the Triangle-A: Standard alternate minimums do not apply if the approach chart shows the non-standard alternate minimums symbol. Always check the front of the chart booklet for the published figures.
- Omitting alternate fuel when required: If an alternate is required but the student calculates only trip fuel plus 45-minute reserve, the total fuel is insufficient and the answer on the knowledge test will be wrong. Always include the alternate leg in the fuel total when applicable.
Understanding IFR fuel and alternate rules is not merely a matter of passing a written test — it reflects the discipline that separates safe instrument pilots from those who push luck into dangerous territory. Internalizing the 1-2-3 rule, knowing the alternate minimums cold, and building conservative fuel planning habits will serve you for an entire flying career.