The 1-2-3 rule and the standard alternate minimums, checked against your forecast numbers. Do you need an alternate, and does the one you picked qualify?
No approach means an alternate is required, full stop
Worst forecast from 1 hr before to 1 hr after your ETA
Standard minima apply only if the chart doesn't publish non-standard ones
Forecast for your ETA at the alternate
The 1-2-3 rule: from 1 hour before to 1 hour after your ETA, the destination forecast must show at least a 2,000 ft ceiling and 3 SM visibility, or you must file an alternate. The alternate itself has to qualify: standard minima are 600-2 with a precision approach and 800-2 without one, unless the chart publishes different numbers. A TAF showing 1,500 broken at your ETA means you're filing an alternate.
14 CFR 91.169 lets you skip the alternate only when both of these hold for the window one hour either side of your ETA:
Read the TAF for the whole window, including TEMPO and PROB groups; a two-hour TEMPO of 1,000 overcast inside your window fails the test even if the prevailing line looks fine. And if the destination has no published instrument approach at all, the forecast is irrelevant: an alternate is required.
An airport qualifies as your alternate only if, at your ETA there, the forecast meets the alternate minimums for its approaches:
| Best approach at the alternate | Required forecast at your ETA |
|---|---|
| Precision (ILS or equivalent) | 600 ft ceiling and 2 SM |
| Non-precision only | 800 ft ceiling and 2 SM |
| No instrument approach | Descent from the MEA, approach, and landing in basic VFR |
Two traps hide behind that table. First, many airports publish non-standard alternate minimums that replace 600-2 and 800-2; the approach chart carries the alternate-minimums symbol when they do, and some approaches are marked NA for alternate use entirely (often because the weather reporting or navaid monitoring goes away when the tower closes). Second, GPS: a non-WAAS installation may plan an RNAV (GPS) approach at the destination or the alternate, not both, and WAAS planning generally uses the LNAV line of minima. AIM 1-1-17 and 1-1-18 carry the details.
Filing an alternate changes your fuel math. 14 CFR 91.167 requires enough fuel to fly to the destination, then to the alternate, then 45 minutes at normal cruise. Run the trip through the fuel and endurance calculator with the alternate leg included; a legal alternate you can't reach with reserves intact isn't an alternate.
Canada doesn't have a 1-2-3 escape hatch. CAR 602.122 requires an alternate on an IFR flight plan regardless of the destination forecast, with only narrow exceptions. The alternate weather minima also work differently: the Canada Air Pilot's alternate-minima table keys on how many usable approaches the airport has, not on a precision/non-precision split. If you fly both sides of the border, don't let the FAA numbers drive a Canadian plan; our Canadian weather briefing guide covers the sources.
The 1-2-3 rule is a floor, not a strategy. A destination forecast of 2,100 and 3 passes the test and still leaves you one TEMPO group from an approach to minimums with no plan B. Plenty of instrument pilots file an alternate whenever the destination is below their personal VFR-ish numbers, and pick alternates with weather good enough to be boring. The personal minimums worksheet is where those numbers get written down.
Common QuestionsYes, if the forecast at your ETA lets you descend from the MEA, approach, and land under basic VFR. In practice the weather there needs to be solidly VFR from the MEA down.
Non-WAAS GPS: at the destination or the alternate, not both. WAAS: more flexibility, but plan on the LNAV or circling minima unless the chart says otherwise. AIM 1-1-17 and 1-1-18, plus the chart's alternate notes, settle it for your installation.
Treat them as if they do. The regulation says "weather reports or forecasts, or any combination of them" must indicate 2,000 and 3; a TEMPO group forecasting less inside your window is a forecast of less. File the alternate and let the argument stay academic.
CAR 602.122 requires an alternate on an IFR flight plan regardless of the destination forecast, with narrow exceptions, and the Canada Air Pilot's alternate-minima table replaces the 600-2 / 800-2 numbers.
Destination, alternate, then 45 minutes: check the whole 91.167 chain.
The TEMPO and PROB groups that quietly fail your 1-2-3 test.
If the approach doesn't work out, this is the next thing ATC will ask of you.
FlightDecide pulls the METARs and TAFs for your route and your specific flight window, flags the groups that fail your minimums, and scores the flight GO, CAUTION, or NO GO with the raw reports one tap away.
Get FlightDecide on the App StoreEducational tool for flight-planning practice. It applies the standard 14 CFR 91.169 tests and standard alternate minimums; published non-standard minimums, equipment-specific GPS rules, and operator requirements override it. Advisory only, and not a substitute for the regulations, current charts, an official briefing, or your own judgment as pilot in command (14 CFR 91.3). Sources: 14 CFR §§91.167, 91.169; AIM 1-1-17, 1-1-18; CAR 602.122. Last reviewed: August 26, 2026.