The IFR 0-1-2-3 rule is the memory aid instrument pilots use to decide whether an alternate airport must be listed on an IFR flight plan. If the forecast at your destination shows a ceiling below 2,000 feet or visibility below 3 statute miles at any point from one hour before to one hour after your estimated time of arrival, you must file an alternate. If the forecast stays at or above those numbers for that entire window, you don’t. The rule comes from 14 CFR 91.169.1eCFR. 14 CFR 91.169 – IFR Flight Plan: Information Required
What Each Number Stands For
Read the mnemonic left to right and it becomes a checklist.
The 0 is the outcome you’re checking for: zero alternates required if the forecast clears the bar. The 1 is the window, one hour on either side of your ETA. The 2 is the ceiling threshold, 2,000 feet above airport elevation. The 3 is the visibility threshold, 3 statute miles.
Both ceiling and visibility must hold for the full two-hour window. If either one drops below its threshold at any point in the forecast, the rule triggers and you file an alternate. The regulation refers to “appropriate weather reports or weather forecasts, or a combination of them,” which in practice means the TAF for airports that publish one.
One detail catches pilots out: the check runs against the forecast, not conditions at the moment you depart. Clear skies at the destination right now don’t matter if the TAF shows the ceiling dropping to 1,500 feet inside your arrival window.
When You Must File an Alternate No Matter What
The 0-1-2-3 exemption only applies when your destination has a published instrument approach procedure in Part 97. If it doesn’t, you must file an alternate no matter how good the forecast looks.1eCFR. 14 CFR 91.169 – IFR Flight Plan: Information Required This trips up pilots flying into smaller fields that rely entirely on visual approaches. Clear skies and unlimited visibility don’t change the requirement.
Weather Minimums the Alternate Itself Must Meet
Picking an alternate isn’t just finding the nearest paved runway. The forecast at the alternate, at the time you’d arrive there, must meet a separate set of minimums that depend on what kind of approach the alternate offers.
- Precision approach (ILS, PAR, or GLS): forecast ceiling at least 600 feet and visibility at least 2 statute miles.1eCFR. 14 CFR 91.169 – IFR Flight Plan: Information Required
- Non-precision approach (VOR, NDB, LOC, LDA, and similar): forecast ceiling at least 800 feet and visibility at least 2 statute miles.2Federal Aviation Administration. IFR Alternate Minimums
- No instrument approach at all: you can still list the airport, but the forecast must allow descent from the minimum IFR altitude to the airport under basic VFR at your expected arrival time.
Those are the standard minimums. Many airports publish non-standard alternate minimums that override them, marked on the approach chart by a triangle containing an “A” and detailed in the Chart Supplement. Non-standard minimums are almost always higher than standard, reflecting terrain, obstacles, or approach limitations. Some airports carry an “NA” designation for alternate use, meaning you can’t list them at all. That usually comes down to the lack of on-airport weather reporting, since without a forecast there’s nothing to evaluate the airport against.
RNAV and GPS Approaches
Alternate planning treats RNAV approaches in a way that surprises pilots used to flying LPV minimums that rival an ILS. For alternate planning, the FAA groups RNAV (GPS) and RNAV (RNP) approaches with non-precision approaches, which means the 800-2 minimum applies rather than 600-2.2Federal Aviation Administration. IFR Alternate Minimums The procedure flies like a precision approach, but the alternate minimums don’t treat it as one.
How Filing an Alternate Changes Your Fuel
Filing an alternate directly changes the fuel you must carry. Under 14 CFR 91.167, the calculation has three pieces: fuel to the intended destination, then fuel from the destination to the alternate if the 0-1-2-3 rule requires one, then a reserve of 45 minutes at normal cruising speed (30 minutes for helicopters).3eCFR. 14 CFR 91.167 – Fuel Requirements for Flight in IFR Conditions
When no alternate is required, the math collapses to destination fuel plus the 45-minute reserve. When one is required, the reserve stacks on top of the alternate leg, not the destination leg, so you land at the alternate with 45 minutes still on board. The regulation also tells you to factor in “weather reports and forecasts and weather conditions,” which in practice means adjusting for the headwinds you’d actually see on the alternate leg rather than planning it in calm air.3eCFR. 14 CFR 91.167 – Fuel Requirements for Flight in IFR Conditions
What the Filed Alternate Binds You To
Filing an alternate is a preflight planning requirement, not a commitment. Once you’re airborne, you can divert to any airport that works: your filed alternate, a completely different field, or the one you passed two minutes ago. The rule exists so you planned ahead and have the fuel to reach a backup. It doesn’t restrict your decisions in flight.
It cuts the other way too. Even when the 0-1-2-3 rule didn’t require an alternate on the flight plan, nothing stops you from diverting to one when conditions change at altitude.
ATC generally has no visibility into which alternate you filed. Controllers see your destination and route, and if you need to divert you tell them where you’re going and get a new clearance. There’s no approval process for changing your alternate in flight because no one on the ground is managing it.