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When Do You Need To File An Alternate Under IFR?

Do you remember the criteria for filing an alternate in your flight plan when weather conditions are poor?

When You Don't Need An Alternate

Simply put, you always need to file an alternate airport unless...

  • The airport has an instrument approach, AND...
  • Weather reports and forecasts indicate that 1 hour before and after your ETA (at the listed destination) the ceiling will be at least 2,000' AGL and the visibility will be at least 3 SM.

It's called the 3-2-1 rule, and it's the easiest way to remember the regulation.

Standard Alternate Minimums

During flight planning, you must determine your time enroute between your destination and your filed alternate to find an alternate airport's ETA. For the alternate airport to meet the requirements of FAR 91.169, the weather conditions at the ETA must meet or exceed:

  • Usable Precision Approach Is Available: 600' ceilings AND 2 SM of visibility.
  • Only Non-Precision Approaches Are Available/Usable: 800' AND 2 SM of visibility.
  • No Instrument Approaches Available: Descent from the MEA and landing must be conducted under basic VFR.

Nonstandard Alternate Minimums

If an airport has nonstandard alternate minimums, you'll see this symbol on approach plates:

If you have a chart supplement, turn to the "A" pages for alternate minimums. If you're using ForeFlight, you can click on the "alternate minimums" tab under "arrival procedures." Sorted by city name, you'll find the new weather minimums you must use to determine if an airport is eligible to file as an alternate.

Many airports will have nonstandard alternate requirements due to terrain considerations, and they may vary by aircraft approach category. In rare cases, an airport might not be available as an alternate, often because there's no on-airport weather reporting.

What If There's No TAF?

Using a terminal aerodrome forecast (TAF) to determine weather conditions at your destinations is easy. They're found at most large airports around the country. If you have trouble decoding TAFs, try out our Aviation Weather training course.

Hundreds of smaller airports don't have TAF reporting capability, so what should you use to determine if you'll need an alternate? In the past, you had to use the Area Forecast (FA) from NOAA's Aviation Weather Center. Now that the FA has been officially discontinued for the contiguous United States, you'll need to use the Graphical Area Forecast (GFA). Use the "Ceiling/Visibility" tab and the Zulu time sliding bar to determine weather conditions around your arrival.

Aviation Weather Center

Analyze Weather Trends And Include Personal Minimums

When deciding on an alternate, always analyze the overall trends of nearby weather. Don't simply pick the closest airport to your destination. Are conditions improving? If weather is predicted to get worse, consider planning for an alternate that has a better forecast.

Legal alternate minimums are just that, legal requirements. You should always factor in personal minimums. If you're not comfortable flying into low conditions, plan a better alternative. Making a list of alternate requirements that fits your comfort level is a great strategy.

Fuel Requirements

Day or night, you must have enough fuel to (FAR 91.167)...

  • Fly to your intended destination.
  • Fly from the destination to the alternate (if required).
  • Fly for an additional 45 minutes at normal cruise speed (minimum).

It's Not Just About Weather

Alternate planning shouldn't be based on weather alone. Here are other factors that you should include during your planning:

  • ATC Services Available
  • Transportation
  • Lodging
  • Nearby Restaurants
  • FBO Services
  • Passenger Requirements
  • Maintenance
  • Runways and Conditions
Boldmethod

You Have To Divert. Now What?

Whether you're going missed off an approach at your destination or just diverting early, you're not required to fly to your filed alternate. You can pick any airport that meets legal requirements for you to begin an approach to landing.

Just because you don't have to fly to an alternate doesn't mean you shouldn't put a lot of consideration into your choice. Having a reasonable "Plan B" in mind is important when you actually need to divert. Instead of fumbling around the cockpit searching for nearby diversion options, you'll have an easy go-to strategy.


Have you ever diverted to your alternate? Why did you divert? Tell us in the comments below.

Swayne Martin

Swayne is an editor at Boldmethod, certified flight instructor, and an Embraer 145 First Officer for a regional airline. He graduated as an aviation major from the University of North Dakota in 2018, holds a PIC Type Rating for Cessna Citation Jets (CE-525), and is a former pilot for Mokulele Airlines. He's the author of articles, quizzes and lists on Boldmethod every week. You can reach Swayne at swayne@boldmethod.com, and follow his flying adventures on his YouTube Channel.

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