Flight under instrument flight rules (IFR) is largely procedural. There’s little room or tolerance for zany spontaneity; if you love surprises, look elsewhere. But although we fly by the book, when the plot thickens, we do in fact have options (although they’re more like regulatory provisions) for choosing a different ending. Usually, the thickening agent affecting our best-laid plans is weather related.
Before we can exercise that freedom of choice however, IFR pilots must fulfill certain obligations. Some of these rules are similar to those for VFR flight, such as how much fuel we should have on board. Some however, go literally a step beyond, such as the requirement for specifying an alternate destination (as well as hopefully having some rough plan for flying there). It’s nothing other than simply having a Plan B. (What happens when there are low ceilings and visibility over a wide area? The answer in this case is, of necessity, humorless and unequivocal: if you find out that your first choice is out of the running, you’d better know where the nearest better weather is and have enough fuel to fly there.)
At first glance, the Federal regulations covering the choice and specification of alternative destinations can seem convoluted and hard to remember. Actually, it’s not exactly as easy as falling off a log, but it isn’t really that bad. Here is one way to remember the rules for IFR alternates and minimums: First, the first cornerstone or anchor memory aid is, literally, as simple as one-two-three. They represent three “ifs”: the “one” refers to hours of time; the “two” is for ceiling in thousands of feet; and the “three” represents visibility in statute miles. (Think of it like the sequence of factors for considering missed approaches: how long, how low, and how far.) To paraphrase this regulation, if between one hour before and one hour after the expected arrival time at your primary destination, the forecast ceiling is at least 2000 feet and the visibility is at least three statute miles—note that’s an “and” not an “or”—you don’t need to specify an alternate.
The second anchor point, one which follows along nicely, is the number 45. (1-2-3, and then 4-5…get it?) This is the rule that says that if you don’t need an alternate, you must have enough fuel to fly another 45 minutes past your destination, at normal cruise. It’s your job to calculate your likely en route time given forecast winds aloft, intended power settings and true airspeed (and I’d advise throwing in another half hour for unexpected vectors and other delays). The other part of this second part, if you will, says that if the weather does dictate filing an alternate, you must have enough gas to fly to the primary destination, then fly to your alternate, and then on top of that, fly 45 minutes more, again at normal cruise power.
I should note here that pilots can certainly fly under instrument flight rules to a grass strip with no instrument approach whatsoever. In most cases if your destination doesn’t have an instrument approach, the ceiling and visibility must be good enough for you to make a VFR approach from the so-called minimum en route altitude, or MEA. (In cases where you’re flying in a radar environment ATC may be able to descend you down to what’s called a minimum vectoring altitude, which can be lower. Usually only the controller will know what altitude that is, for whichever sector of airspace you’re flying through at that time.)
The 1-2-3 rule covers the primary destination, but the regulations go a little further than this: the next one covers your alternate, as far as something known as “alternate minimums”. This one can be remembered as the “602 or 802” rule. It says that as far as your alternate goes, if specific alternate minimums are not published for that airport (more on this in a minute), and if between an hour before and an hour after you expect to get there (same as the 1-2-3 rule), that airport is expected to have at least a 600 foot ceiling and two miles visibility, you can use it if and only if it has a precision approach procedure. If it has only a non-precision approach procedure, everything stays the same except the 600-foot minimum ceiling goes up to an 800-foot one. Those were the so-called standard alternate minimums. Some airports, because of terrain or obstructions, have higher ones. Jeppesen includes them right on the approach plates for each airport whether they’re standard or not (usually on the back or “airport layout” side of just the first approach plate when an airport has more than one), but you’ll have to look elsewhere for this information if you’re using the book of approach plates produced by the gummint.
The one other very important thing to note here is that these alternate minimums aren’t for actually landing at your alternate; they’re just for your advance planning option in case you need to divert there. If you do “go missed” at your chosen primary destination and you then fly towards your alternate, you simply pull out the approach plate for your alternate airport (or much better, you already have it out, ready to go, underneath your first ones) and fly according to the published minimums for that airport, just like you did with the first one.
So much for the FAA rules (See, that wasn’t so bad, was it?), but now we come to the realm of judgment and strategy. How do you know what alternate will work out and which ones might not? Of course as in-flight data trickles down to general aviation cockpits everywhere, it will eventually all be right in front of us. But if you aren’t an early adapter (or even a middle level one) and you don’t have up to the minute METAR reports on display in the cockpit, Flight Watch certainly will be able to provide them to you. It’ll just take a bit longer. Also, you can never be sure that those forecasts will come true, so never assume that the destination weather and winds will match those for which you have so carefully planned. But what else can you do about it?
As a general rule, when you’re fishing for a good alternate, pick something BIG. (After all, you’ll need to know what the forecast is so you can plan whether to use it or not, so why not remove as much uncertainty as possible?) It’s the bigger airports that have weather reporting capability, not to mention precision approaches, fantastically well-lit runways and approach light systems, as well as helpful ATC personnel right there when you may need them most. One caveat about large airports though is that they can be busy, so plan for enough “extra, extra fuel” to cover delays (or simply being vectored hither and yon) before you get to land there. As far as the weather surprises go, until there are National Weather Service metrics for unexpected drops in ceilings and visibility, compared to the current forecast, given right along with each updated METAR (and don’t hold your breath), the one thing you can do to get a jump on changes is compare them yourself as you go, listening for AWOS and ATIS broadcasts along the way. However, it doesn’t require any work at all to know that the odds of your needing that alternate will go way up when there are widespread low ceilings and visibilities; if several reporting points are updating their sequence reports with “specials” (or the terminal aerodrome forecasts are being amended), watch out! The other time-honored way that you can get updated information of course is, again, via Flight Watch.
Where I fly, in the mid-Atlantic region, the cost of 100LL has reached well over four dollars a gallon. (At this writing, it’s $4.50 at my home airport.) However when you’ve gone missed and then couldn’t get into your alternate because of worsening weather, the cost for having taken on extra fuel probably won’t even register on your radar screen of unwelcome annoyances. And on the subject of worsening weather and having to go even further away to get back to Earth, in populated areas such as mine, that 45 minutes I mentioned earlier is probably getting close to having enough extra fuel. But if you fly west of the Mississippi, or anywhere else where airports are few and far between, you might want to make that two hours of extra fuel.
There are two motivations for planning IFR flights in such exhausting detail like this. (The truth be told, IFR flight planning is actually easier than VFR planning in a way, because it’s so procedural, and because many of the airspace permissions are already taken care of, so to speak.) The first motivation is safety of course. These minimums and backup planning as I’ve described them exist to keep you and your passengers at as practicably great a distance as possible from having no options left. The second motivation is a three-letter acronym: CYA. If something ever does go amiss on a flight, you can be sure that the FAA will be looking to see how well you had planned for it and prepared accordingly.