IFR alternate requirements: when to file one
The “1-2-3 rule” is the Part 91 alternate-filing test for aircraft other than helicopters; this guide focuses on airplane IFR flights. It answers whether an alternate airport must be listed on the flight plan under 14 CFR 91.169(b). It does not prove a flight is safe, decide whether a particular alternate is usable, or replace the rest of the weather and fuel plan.
The Part 91 airplane 1-2-3 test
Under 14 CFR 91.169(b), the destination must first have a standard instrument approach procedure prescribed in Part 97 or a special procedure issued to the operator. The weather exception then applies when reports or forecasts show both of the following for at least one hour before through one hour after ETA:
- Ceiling at least 2,000 feet above the airport elevation; and
- Visibility at least 3 statute miles.
That is the source of “1 hour, 2,000 feet, 3 miles.” The ceiling is relative to airport elevation (AGL), not 2,000 feet MSL. Both conditions must be met throughout the stated window. If either misses its threshold during the window, the exception is not met and a qualifying alternate must be listed for a flight plan subject to this rule.
A 2,300-foot ceiling and 4-mile visibility
Assume a standard Part 97 instrument approach is prescribed for the destination. Its forecast stays at a 2,300-foot ceiling above the airport and 4 statute miles visibility from one hour before until one hour after ETA. Both thresholds are met, so the 91.169(b) exception applies to the alternate-filing requirement for this Part 91 airplane example. If the ceiling drops to 1,900 feet for part of that window—even with 4 miles visibility—the ceiling condition fails, so an alternate is required. This test is about filing; a pilot may still choose to plan an alternate for sound operational reasons.
Then check whether the alternate itself qualifies
When an alternate is required, “an airport nearby” is not enough. Review the current instrument approach chart, forecast at the alternate’s ETA, airport and approach availability, NOTAMs, aircraft equipment, navigation capability, and any published alternate notes. Some procedures publish nonstandard alternate minima or restrictions. An “NA” restriction or unavailable approach/equipment can make a candidate unsuitable even when its weather looks good.
For aircraft other than helicopters, 91.169(c) sets standard alternate weather minima when the published procedure does not specify alternate minima: 600-foot ceiling and 2 statute miles visibility for a precision approach, or 800 feet and 2 statute miles for a nonprecision approach. If there is no instrument approach procedure, the regulation calls for forecast conditions that allow descent from the MEA, approach, and landing under basic VFR. Use charted/published minima and current FAA procedure information for the actual airport; do not substitute these standard values where published restrictions control.
Filing an alternate is not the same as having enough fuel
For a Part 91 airplane IFR flight, 14 CFR 91.167 separately addresses fuel: destination, then alternate when required, then 45 minutes at normal cruise. Other rules and operating specifications may impose different requirements. Weather, route changes, holding, approach delays, and aircraft-specific unusable fuel also affect a sound plan.
How to use the rule responsibly
- Use the applicable current forecast and determine the destination ceiling above airport elevation and visibility across the entire 1-hour-before to 1-hour-after window.
- If the exception is not met, select an alternate and verify charted minima, equipment, airport status, and forecast at its ETA.
- Plan the fuel and route under the rules that apply to the operation; account for realistic delays and weather changes.
- Recheck current charts, NOTAMs, and weather before departure. A forecast-based filing decision can change when the forecast changes.
The 1-2-3 explanation here is specifically for the Part 91 airplane rule. Helicopter rules and Part 121, 135, military, or other operations can differ; use the controlling regulation, OpSpecs, and procedures for that operation.
Primary sources
- eCFR 14 CFR 91.169 — IFR flight plan and alternate airport
- eCFR 14 CFR 91.167 — IFR fuel requirements
- FAA Instrument Procedures Handbook, Chapter 1
PilotDesk Sources & Methods explains source selection and tool limitations.