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On Instruments: Prepping for Plan B

Your alternate is below minimums. Now what?

Every instrument-rated pilot knows that an alternate must be named if the destination airport meets certain weather limits. You need to name an alternate airport on your IFR flight plan if, for the period of time one hour before and one hour after your estimated time of arrival, the weather is forecast to include a ceiling of 2,000 feet agl and/or visibilities less than three statute miles (the 1-2-3 rule).
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Illustration by Sarah Hanson

Assuming you take off for a destination requiring an alternate, you must be able to shoot an approach at the destination, miss that approach, and then fly to your alternate—and land with 45 minutes’ worth of fuel reserves. This is the letter of the law for operations under FAR Part 91, but most corporate flight departments use more conservative rules. Ones that require enough fuel to hold for five minutes at 5,000 feet after missing an approach, then fly to an alternate 100 or 200 nautical miles away at long-range cruise power, and landing at that alternate in visual meteorological conditions with 45 minutes of fuel remaining in the tanks. These are the rules recommended by the National Business Aviation Association, so they’re known as NBAA IFR reserves.

Most of us fly using the basic 1-2-3 rule, and plan for a landing at the alternate with 45-minute reserves. But there’s a whole lot more to it than memorizing these minimal rules. That’s because the weather plays a huge factor. And dealing with weather means carrying fuel reserves more generous than a mere 45 minutes.

Let’s say you miss the approach and fly to your alternate. This alternate must be forecasting weather with ceilings at or above 600 feet and two-mile visibility (600-2) if it’s served by a precision approach. If it only has nonprecision approaches, then 800-foot ceilings and two-mile visibilities (800-2) are required. Once committed to flying to your alternate, the published minimums apply.

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The Aviation Weather Center’s Graphical Forecasts for Aviation (GFA) tool can serve as a valuable source of ceiling and visibility information at candidate destination and alternate airports not served by TAFs. The GFA tool replaces the earlier, textual area forecasts.

None of this may matter if low IFR (ceilings below 500 feet, visibilities below one statute mile) conditions prevail over a wide area. Warm fronts are notorious for generating widespread low ceilings and visibilities, and they tend to move slowly. Same thing with overnight cooling of rain-soaked terrain. Will you have enough fuel to make it to your alternate and complete an approach successfully? Is there a chance you’ll miss the approach at the alternate? Then what? Hint: You can always call ATC and ask for a different alternate in flight; use ATC to provide weather updates as well.

What if the weather involves thunderstorms or fast-moving cold fronts? Will you have enough fuel to divert around adverse conditions and meet the regulations? And what if the weather is low and you make multiple approaches at your destination, hoping that the next approach will let you see the airport environment? With every approach you’re eating into your fuel reserves—enough that you may not be able to reach your named alternate.

You need to think about all this during your preflight planning. A check for nonstandard alternate minimums is also in order. Are they above 600-2 or 800-2? They could be if obstacles are nearby, or if a tower is closed. Are certain runways closed? Are all navaids fully operational? Is the airport itself closed? Does your alternate have any instrument approaches? If not, then you must be able to descend from the minimum en route altitude and land in VMC. Oh, and don’t forget to do a receiver autonomous integrity monitoring (RAIM) check of your GPS navigator before takeoff.

Ideally, your destination and alternate airports will provide terminal forecasts to help your preflight planning. METARs can help you understand initial weather conditions, and in flight they can provide forecast confirmation or blown forecasts. No TAFs? Then you’ll have to rely on more general predictions, such as the Aviation Weather Center’s Graphical Forecasts for Aviation tool for ceiling and visibility. Go online ( to familiarize yourself with this replacement for the now-obsolete, textual area forecasts of yore.

Thinking more carefully about alternate airports and low weather should convince you to carry plenty of fuel when flying in hard IFR. Perhaps now you can see how having those NBAA fuel reserves can make a lot of sense.

But perhaps most important is your comfort level with the thought of flying in low IMC. Don’t feel like having the pressure of worrying about missing an approach and flying into still more bad weather? Weather below minimums for your takeoff? Then maybe it’s time to wait it out until skies clear up a bit, and ceilings and visibilities rise.

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Thomas A. Horne

Thomas A. Horne

AOPA Pilot Editor at Large
AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.

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