You're planning an IFR flight, and the weather at your destination is marginal. Do you need to file an alternate? And if so, what are the weather minimums for your alternate airport?

We'll cover both topics here, and we'll show you some quick tips for alternate planning if you're using ForeFlight.

First Off, When Do You Need to File An Alternate?

According to FAR 91.169, you always need to file an alternate airport unless...

• The airport has an instrument approach, AND...
• Weather reports and forecasts indicate that from 1 hour before to 1 hour 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 1-2-3 rule, and it's the easiest way to remember the regulation.

To recap, if the weather at your destination from 1 hour before to 1 hour after your ETA isn't at least 2000' AGL ceilings and 3 SM of visibility, you need to file an alternate.

Planning Your IFR Alternate

So if you need an alternate airport, how good does the weather at your alternate need to be?

For your alternate airport to meet the requirements of FAR 91.169 (c), the weather conditions at the ETA must meet or exceed:

• If a precision approach is available: 600' ceilings AND 2 SM of visibility.
• If only a non-precision approach is available: 800' AND 2 SM of visibility.
• If no instrument approaches are available: Descent from the MEA and landing must be conducted under basic VFR.

Planning For The Actual Alternate Minimums

In the real world, not all airports follow the standard minimums. If that's the case, the airport has nonstandard alternate minimums. So how do you find those alternate minimums?

If you're using FAA charts, finding alternate minimums is a 2-step process.

First, if the airport has nonstandard alternate minimums, you'll see this "A" in a triangle symbol on an approach chart for the airport:

Next, to find out what the alternate minimums actually are, you need to turn to the chart supplement.

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 Isn't A TAF At Your Destination Or Alternate?

Using a TAF to determine the weather conditions at your destination and alternate is the most common way pilots plan an IFR flight.

But what happens if your destination or alternate doesn't have a TAF?

The FAA's Graphical Area Forecast (GFA) is a good place to start.

Use the "Clouds" and "Ceiling/Visibility" dropdowns, along with the Zulu time slide bar, to determine the weather conditions at your destination or alternate.

Fuel Requirements

Next up, you need to make sure you have enough fuel for your destination and alternate airport.

Day or night, you need 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. There are other factors that you should include during your planning as well:

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

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. Filing an alternate on your IFR flight plan is for planning purposes only, and once you're airborne, you can change your alternate if you need to.

That being said, 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? Tell us about it in the comments below.

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