How to Choose the Best Alternate Airport: A Guide for Instrument Pilots

Pilots should always be evaluating alternate airports.

One of the most important skills pilots must continue to hone is sound decision-making. Every flight, regardless of the complexity of the airplane or the distance of the trip, is comprised of a continuous series of decisions involving a host of variables that the pilot must make in order to safely get from point A to point B.

When flying a VFR cross-country, one of the early lessons is to always be identifying and evaluating alternate airports in case you’re not able to land at the intended destination. This is not only common sense, but it’s a legal requirement too:

Preflight Action (FAR 91.103) – Each pilot in command shall, before beginning a flight, become familiar with all available information concerning that flight. For a flight under IFR or a flight not in the vicinity of an airport, weather reports and forecasts, fuel requirements, alternatives available if the planned flight cannot be completed, and any known traffic delays of which the pilot in command has been advised by ATC.

Though it’s rare, your destination airport could become unusable due to a thunderstorm moving in, or a disabled airplane on the runway. To be clear, you are not required to declare an alternate airport in any official capacity flying VFR, but rather you should have a few airports in mind along the way and near your destination.

IFR Alternate Airport Planning

Legal requirements for selecting an alternate airport

Additional GPS Approach Considerations

Which weather forecast should you use?

Flight Planning vs. Reality

IFR Alternate Airport Planning

Contingency planning is even more important for IFR pilots on cross-country flights where a cloud ceiling shift of 100’ or visibility change of a ½ mile can mean the difference between seeing the runway at the end of an instrument approach or having to go around and find another place to land.

While every IFR flight requires you to file an IFR flight plan and receive an ATC clearance, not every flight is flown in IFR conditions. After 3,000 hours of flying on IFR flight plans, I’d say most of that time was actually spent in VFR conditions and with only a handful of the flights in any given year requiring an instrument approach to be flown at the destination.

Regardless of the weather, my preference is to always file an IFR flight plan on cross-country trips, primarily for the benefit of the ATC services provided along the way. The IFR flight plan form requires much of the same information as when flying VFR, except you have to treat the Alternate Airport field more seriously. FAR 91.169 describes two scenarios as it relates to IFR alternate airport requirements:

1. If the weather at the destination, for at least 1 hour before and 1 hour after the estimated time of arrival, is forecast to have at least a 2,000 foot ceiling (above the airport elevation) and visibility of at least 3 statute miles, you are not required to list an alternate airport in the IFR flight plan. 
2. If either the ceiling or visibility is forecast to be less than 2,000 feet or 3 statute miles during that arrival window, you are required to file an alternate. Also, the destination airport must have at least one instrument approach procedure or an alternate airport will always be required.

Many pilots refer to this as the 1-2-3 rule: Plus or minus 1 hour from arrival, ceilings at least 2,000 feet and visibility at least 3 statute miles.

The first scenario above only states that you don’t have to legally file an alternate on the IFR flight plan if the weather is forecast to be good at the destination, but as discussed earlier during the VFR flight scenario, FAR 91.103 still requires you to have an alternate in mind in the event that the runway at the destination airport becomes unusable for any other reason.

Now, let’s assume that the weather forecast at the destination is for low IFR conditions and an alternate airport is required. There are legal requirements that the filed alternate airport needs to meet, but similar to the weather briefing, start with a big-picture view of what’s causing the IFR conditions at the destination and a few practical aspects about potential alternate airports:

Is there a cold front moving through at the arrival time forecast to cause localized areas of low visibility? Look for a nearby airport to the east of the destination ahead of the front.

Is there widespread fog? You’ll need to search further away from the weather system over the destination and carry plenty of extra fuel.

It’s important to understand what’s causing weather impacts at your intended airports.

Is there another cloud layer above the fog and no wind? The fog may not clear out as fast as forecast due to inadequate solar heating.

What time of day is the flight? Low visibility conditions tend to improve several hours after sunrise into late morning, but then deteriorate at night as temperatures cool.

Is there rising terrain nearby or large variations in airport elevations within 50 NM of the destination? A nearby airport at a lower altitude may provide additional clearance under a cloud layer (with the caveat that it could be prone to fog if near a body of water).

What services are available at the alternate airport? As we’ll discuss later in this article, the alternate airport needs to have reliable weather reporting, and you’ll likely need fuel and/or FBO services when you get there. Think about choosing a towered airport with an ATIS broadcast and a 24-hour FBO.

If you are planning with ForeFlight or Garmin Pilot, you’ll find some helpful Alternate Airport planning tools to help you decide. In ForeFlight, enter your basic flight plan information, and then tap the Alternate field. This will display a list of suggested alternate airports and key information: airport name, forecast weather for the arrival time, longest runway length and the time/fuel to divert there after executing a missed approach at the planned destination.

Legal requirements for selecting an alternate airport

After selecting the best option based on the criteria above, you’ll next need to verify that it meets the legal requirements to use the selected airport as an alternate. Once again, this is based on the weather reports or forecasts. The ability to include “reports and forecasts” in this decision is important. On a relatively short flight, reports may be more meaningful than a forecast. You can also determine trends by examining a series of reports versus relying solely on a forecast.

The same regulation that outlines whether or not an alternate is required, FAR 91.169, also specifies the weather conditions that a filed alternate airport must meet. These rules are designed to build some extra ceiling and visibility margins above the approach minimums to make it as much of a sure thing as possible if you actually have to divert. The weather reports or weather forecasts, or a combination of them, indicate that, at the estimated time of arrival at the alternate airport, the ceiling and visibility at that airport will be at or above the following weather minima:

1. A ceiling of at least 600 feet and a visibility of two statute miles if the airport has a precision approach (ILS Approach)
2. A ceiling of at least 800 feet and a visibility of two statute miles if the airport has a non-precision approach (RNAV/GPS or VOR)

The key point here is that those are the standard alternate minimums, but there are exceptions and many airports have nonstandard alternate minimums that may require a forecast of higher ceilings and/or higher visibility. To determine if an airport has nonstandard alternate minimums or another exception, reference the chart for the planned instrument approach and look for a black triangle with the letter A:

This symbol indicates that nonstandard alternate minimums apply and you need to refer to the IFR Alternate Airport Minimums reference. If you are using paper charts, these can be found in the front of the approach chart book. If you are a ForeFlight user, navigate to the Airports screen, select the Procedures tab, select Arrival and then Alternate Minimums.

The first thing you’ll notice is that the majority of the airports include the line “NA when local weather not available”, which simply means you can’t plan to fly that approach at the alternate airport if the AWOS or ASOS is out of service (or in very rare cases the ATIS).

Next, you’ll see instrument approaches with notes indicating that higher alternate minimums are needed if you are planning to fly that approach. For example, at Pellston, MI (PLN), if your only option is to fly the RNAV Rwy 5 approach based on strong winds out of the northeast, the forecast would need to be for 900 foot ceilings and 2 miles visibility for Category A and B speeds for you to be able to file PLN as a legal alternate airport.

The last thing to pay attention to is that just because one instrument approach has nonstandard alternate minimums listed, it doesn’t mean that it applies to all the approaches at the airport. Take a look at Jackon, MI (JXN), and you’ll see that the ILS Rwy 25 approach chart shows the black A symbol with the notes that you cannot plan to use Jackson as an alternate airport and fly the ILS when the local weather is not available and/or the tower is closed:

There are actually five instrument approaches at JXN and if the RNAV 32 looks like a viable option, you can use the standard alternate minimums (forecast of 800 foot ceilings and 2 SM visibility) since there is no black A symbol on the chart. This means you can legally file JXN as an alternate even if your planned arrival time is after hours when the control tower is closed.

Additional GPS approach considerations

There is one last legal consideration when planning for the alternate and it relates to the avionics in your panel. The first generation of IFR-approach GPS receivers were approved to fly LNAV-only approaches and require the pilot to verify receiver autonomous integrity monitory (RAIM) will be available during the approach (e.g. Bendix/King KLN94, the original Garmin 430/530/G1000). 

If you are flying today with one of these systems, you can only plan to fly an RNAV approach based on LNAV minimums at either the destination OR alternate airport, but not at both. From a practical standpoint, this means that if your original destination only has RNAV approaches, your alternate airport must have an approved instrument approach procedure, other than GPS, that is anticipated to be operational and available at the estimated time of arrival, and which the aircraft is equipped to fly.

The current generation of GPS navigators which include WAAS capability (e.g. Garmin 430W/GTN650/G1000 NXi) provide additional flexibility when it comes to alternate planning with one exception. You can plan to fly RNAV approaches at both the destination and the alternate, but for flight planning purposes, you have to make the assumption that you will be flying to the LNAV minimums. The reasoning here is to build extra safety margins as the LNAV minima will be higher than LPV minimums and helps to provide additional assurance that you will be able to complete the approach when arriving at the alternate.

Keep in mind, this is just for planning purposes; if you actually divert to the planned alternate, the rules say it’s perfectly ok to still fly the LPV approach when you get there. And it is also perfectly fine to select a diversion airport other than what was filed if the circumstances warrant.

Which weather forecast should you use

We just spent a lot of time reviewing alternate airport selection criteria that is 100% dependent on the weather reports and forecast for the time of arrival. If you’re flying to a larger airport that publishes a TAF, that forecast type is your main product for determining the ceiling height and visibility for your destination and alternate airport. Pay close attention to TEMPO or PROB30 periods that apply to your arrival time as you’ll need to use the ceiling or visibility listed here for legal planning, even if it’s just speculation on the part of the forecaster.

If your destination or alternate doesn’t offer a TAF, you’ll need to use the Graphical Forecast for Aviation resource, available here. This provides a model-based forecast for the entire U.S., showing expected cloud bases and tops 18 hours into the future. You can enter your flight plan at the top right of the map, select the Clouds overlay, and then Bases from the top left of the map. Drag the time slider to your planned arrival time and zoom in on your destination to see the forecast ceilings. You can check the forecast visibility the same way, by selecting the CIG/VIS layer. The resolution of the color-shading isn’t great, but it is helpful to locate areas where the visibility or ceiling is forecast to be better (or worse) than the location of your destination airport.

Flight planning vs. reality

Everything we’ve discussed so far relates to flight planning only and the process to choose an alternate airport for the IFR flight plan form. While it may seem arduous, it is designed to make you do your homework and ensure that is always a viable alternate airport available for every flight.

For a  three hour cross-country flight, this is probably done at least an hour before takeoff, meaning at least four hours before you actually arrive at your destination. The reality is that the weather doesn’t know or care what was forecast in the TAF, and is often much different when established on the approach.

Keep an eye on the weather as you approach the destination, using both ADS-B datalink weather, and by tuning in the local ATIS/AWOS on the radio. If it looks like the weather will be near the minimums for the approach, it is time to start thinking about what the best alternate airport will be based on the current weather. If it still looks like the planned alternate is the best option, go for it. And remember that the 600-2 or 800-2 ceiling/visibility was just for planning; you’re just looking for the weather to be above the approach minimums for the airport at this point.

However, if the weather reports are showing that the planned alternate is no longer a viable option, it’s time to throw all that planning out the window and start looking for a new option. Again this is where flying with ADS-B datalink weather and an iPad is worth every penny as you can scan the visibility and ceiling reports on the map to help determine the best option.

Keep ATC informed of your plans too, even if you do decide it’s best to go to the filed alternate. They do not have access to all of the information you filed and cannot see the airport you listed as an alternate (and don’t care for that matter), so choose the best option based on all the information you have at that moment in time.

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Bret Koebbe

As Sporty’s lead aviation course and video producer, Bret is usually flying an interesting airplane, and often with a camera rolling. He oversees the development, production and distribution of over 30 aviation courses from Sporty's, including the Private Pilot Learn to Fly Course. He is a flight instructor and check airman at the Sporty's Academy flight school and a senior captain on a Cessna Citation.

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