Alternate Airports

Why select an alternate?

When filing an IFR flight plan, when are you required to select an alternate airport?  Practically speaking, if the weather at your destination is anywhere close to minimums, there is a possibility that the weather will deteriorate further by the time you arrive, and you will be unable to see the runway upon reaching approach minimums.  At this point, you are faced with a decision: Where do I go now?  You have only a certain amount of fuel remaining and you need to find someplace to land.  The prudent pilot has considered this possibility before takeoff and has another airport already in mind- one with better weather.

Now, not all pilots are prudent, and in an effort to promote good decision making, the FAA has mandated that if the weather forecast at your destination is below certain set values, the pilot is legally required to select an alternate in case the weather gets worse.  This document is designed to help you select an appropriate legal alternate that you can use on your flight plan form.  As we proceed, remember that we are fulfilling a legal requirement here, and the alternate that you select for your flight plan form may or may not be the best practical option.  Additionally, you are not required to actually proceed to your alternate in the case that you can’t see the runway at your destination.  The decision at that point is yours to make.  The legal requirement is there simply to make sure that you have considered the possibility of not being able to land at your destination.  None of the following guidelines dictate what you must actually do in the airplane.  They are simply weather and planning requirements that must be met for the flight to be LEGAL.

Do you legally need an Alternate?

How bad does the weather have to be before you must file an alternate airport?  Not that bad, actually.  CFR 14 91.169 states that if, from 1 hour before to 1 hour after your proposed time of arrival, the ceiling is forecast to be less than 2,000 ft AGL, OR the visibility is less than 3 statute miles, you must select an alternate.

Remember this as the 3-2-1 rule:

alternates_321

But how do we determine the forecast for the destination airport?  If the airport has a TAF, it should be used, and if not, then the Area Forecast should be used instead.

Checking weather using a TAF

We are planning a flight to Gainesville, FL, and our ETA is 13:00Z.  The current TAF reads:

KGNV 180911Z 1806/1906 VRB03KT 2SM BR SCT005 SCT010
 FM181300 076005KT P6SM SCT010 BKN020
 FM181800 08008KT P6SM VCSH SCT025 SCT035 
 FM190200 07004KT P6SM FEW030

At 13:00Z, the forecast calls for visibility greater than 6SM, and a ceiling of 2,000 feet.  However, look at the weather just prior to 13:00Z.  The visibility is forecast to be 2SM, and this weather is forecast from 06:00Z until 13:00Z, which will fall in the period of “1 hour before” our estimated time of arrival.  In this case, an alternate airport is required.

Checking weather using an Area Forecast

Next, consider a flight to Ocala, FL, with an ETA of 13:00Z.  There is no TAF for Ocala, and thus we must use the area forecast.

CNTRL PEN...

    SCT030 SCT-BKN060 TOP 120. ISOL -SHRA. 
     03Z SCT030 SCT CI. 
     09Z SCT010. VIS 3SM BR. 
     12Z BKN010 TOP 015. VIS 3SM BR.

    OTLK...IFR CIG BR 15Z VFR.

Beginning at 12:00Z, the area forecast calls for ceilings of 1,000 ft and 3SM visibility, and conditions are not forecast to improve until 15:00Z.  The visibility is enough, but the ceilings are forecast to be below 2,000ft, and again, an alternate airport is required.

Selecting an alternate

Now that we know an alternate airport is required, how do we select one?

It all depends on the weather forecast and the approaches available.

If the airport does not have any instrument approaches, the rule states that you must be able to descend all the way from the MEA to the airport under VFR conditions.  Common MEA altitudes in the flatlands range from 2,000 to 7,000 feet.  So as you can see the weather must be quite good, well above basic VFR requirements.

If the alternate airport has a precision approach (LPV, ILS) then ceilings must be at least 600 feet AGL, and visibility must be at least 2SM.  If the airport has a non-precision approach (LNAV, LOC, VOR) then ceilings must be at least 800 feet AGL, and visibility must be at least 2SM.

The memory aid is:

600-2 Precision
800-2 Non-Precision

These values are known as Standard Alternate Minimums.

Unfortunately, not all approaches are authorized for Standard Alternate Minimums.  Instead, Non-Standard Alternate Minimums apply to many approaches, especially at non-towered airports.  In fact, many approaches indicate that using it as an alternate is simply Not Authorized (NA).  It is therefore the pilot’s responsibility to browse the approaches for the intended alternate airport to see if it can be used.

When browsing the approaches, look for the “A” symbol inside a black triangle in the notes section (near the top).  If it is present, then Standard Alternate Minimums do not apply to this approach, and you must investigate further to see if the Standard Alternate Minimums have simply been modified, or if the approach is not authorized at all for use as an alternate.  If it is Not Authorized, the letters “NA” will appear right next to the “A” triangle, and your investigation can stop here.  You must find another approach and/or airport.

However, if you simply see the “A” triangle (without NA), you must dig further to see in what way the alternate minimums are non-standard.  There could be certain times of the day when it is not authorized (perhaps at night), certain equipment on the airport that must be working (perhaps AWOS), or the standard 600-2 / 800-2 weather minimums have been increased.

alternates_nonstandard

If you see the “A” by itself then you must continue to look up the specific restrictions that have been placed on this approach.  These are found in the front section of the Terminal Procedures Publication (Approach Plates Book).  Look for the pages with the black “A” triangle at the bottom.  Look up the airport in question, and you will find a list of approaches with the restrictions applicable to each.

alternates_nonstandard_details

Why are alternate minimums listed on approach plates and not for the whole airport? 

Because alternate airport selection depends on the approaches that you can fly.  Perhaps you are not GPS equipped, and thus the GPS approaches will all be unavailable to you.  The next best thing you can fly is a VOR approach.  In this case, for you, the airport would need to have 800 foot ceilings and 2 miles visibility, or whatever Non-Standard Alternate Minimums are specified for the VOR approach.

Perhaps you are GPS equipped, but can fly LNAV approaches only (your GPS is non-WAAS).  Despite the GPS approach being a precision approach, you can only fly it as non-precision, and thus again, non-precision minimums will apply to you.

Finally, consider the possibility that there are some approaches authorized for use as an alternate, but the ones you can fly are listed as “NA”.  This disqualifies the entire airport for your use as an alternate.

When selecting an alternate, head straight for the approach plates for that airport.

  • Look for approaches that you can fly. If you find some…
  • Determine if they are authorized for use as an alternate, and if they are…
  • Check to see if the weather meets the appropriate alternate minimums for that approach.

If not, move to the next approach, and then on to the next airport if needed.

Sample Flight – Solid IMC

There are times when it will be very difficult to find a suitable alternate.  Consider the possibility that the weather at your destination not only fails the 3-2-1 check, but ceilings over the entire area are less than 600 feet, or visibility is less than 2 miles.  In this case, none of the nearby airports’ weather will qualify them for use as an alternate.  You may have to search hundreds of miles away for better weather, and then for an airport that qualifies as an alternate.

alternates_example1

For example, a flight from KLAL to KPBI, factoring in the time required to fly an approach, takes about 1.5 hours in a Cessna 172.  An alternate airport is required because of the forecast 500 foot ceilings all across central and southern Florida.  However, even if they offer a precision approach that you can fly, none of the nearby airports qualify as a legal alternate because ceilings are lower than the standard 600 foot minimums (Non-Standard Alternate Minimums will never be lower than standard).  So you start looking for the nearest airports where ceilings are forecast to be higher than 600 feet, and the nearest turns out to be at St Augustine in northern Florida, a 2.25 hour trip.  Now the question becomes – do I have enough fuel to make it to my destination, alternate, and then have the required 45 minutes of fuel still remaining?  Assuming the average Cessna 172 has about 4.25 hours of fuel on board, you make the call.  Is this flight legal?

Sample Flight – Marginal VFR

It can be more difficult than you’d think to find an airport that qualifies as a legal alternate.  When most of the nearby airports are small and non-towered, you will find that most of them are not authorized for use as an alternate.  Consider a flight from KLAL to 2IS (Airglades Airport) in central southern Florida.  2IS does not have a TAF, and so we use the area forecast which calls for 1,500 overcast throughout the entire region.  This fails the 3-2-1 rule, and thus we must select an alternate.

As an aside, note that 2IS is in fact forecast to be VFR at our time of arrival!  Legally, we could fly (read, scud run) under VFR on this trip, but if we choose to file IFR, we must select an alternate.  The point is, seeing that an airport is VFR does not mean that you need not select an alternate.  Be sure to read the forecast carefully.

Looking at nearby airports, we immediately realize that all of them will pass the 600-2 or 800-2 standard alternate minimums, so for convenience, we pick the next closest airport to 2IS, KPHK only to discover that no approaches are authorized for use as an alternate.  Finally, after expanding our search area, we select KFMY, a towered field with standard alternate minimums.

alternates_example2

The FAA uses particular criteria to determine an airport’s suitability as an alternate.  For example, ILS approaches should be monitored to ensure they are working, the airport must have a weather reporting station on the field (to ensure accurate altimeter settings), and so on.  Non-towered and small airports often do not meet these criteria.  Thus, when selecting an alternate, the easiest thing is to jump straight to towered airports that offer a variety of approaches.

Special note for Non-WAAS GPS equipped aircraft

If your GPS unit does not offer WAAS, then you may only plan to use a GPS approach at either your destination OR your alternate airport – not both.  Thus if you are flying to an airport that only offers GPS approaches, you will need to find an alternate airport with VOR or ILS approaches to be legal.

Tips:

  • Towered airports are more likely to have standard (or close to standard) alternate minimums, as well as a variety of approaches. They typically make good choices for an alternate.
  • If the weather is worse than 800 feet or 2 miles visibility over a wide area, you may be looking a long distance away for your alternate.
  • Even if the weather is VFR at your destination, you may still need an alternate. In this case, the nearest towered field is usually a good option.

 

 

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