Wx Watch: LIFR Signs

Looking for lower than 500-and-one? Here's how to find it

February 1, 2000

Some instrument-rated pilots like to think that just because they have the rating in their wallets, they can take on the lowest ceilings and visibilities. But as any pilot with a fair amount of actual instrument time can tell you, low IFR conditions (ceilings less than 500 feet and/or visibilities less than one statute mile) pose some serious operational considerations. Taking off or landing in low IFR weather is very, very serious business, to say the least. It can be done safely, but only if you stack the deck in your favor. First, there's the skill and experience factor, which also assumes that you're completely familiar with the particular airplane that you're flying. When taking off into the clag, you'll go on instruments shortly after liftoff, and you may have to follow a complicated departure procedure before going on your way. This means there's little time to acclimate to solid instrument flying. You'll be at a low altitude, and obstructions or terrain may be factors. This is no time to be rusty on your instrument skills. You're visual for less than a minute after brake release, then it's presto! Gauges time. To minimize risk, make sure that you've given yourself a good preflight talking-to about what you'll be doing. Saying it out loud helps to drum it into your head, and saying it to another pilot front-seater keeps him or her in the loop—which may come in handy should confusion crop up. Study the instrument approach plate, making special note of your post-takeoff heading and altitude, plus the locations of the first fixes that you'll be flying to. Also, have a good idea of the minimum safe altitudes around the departure airport, and where the highest obstacle or terrain is located—so you know where not to turn should you momentarily drift off course. While warming up or waiting for your clearance, set up your communications and navigation radios for the departure procedures. Then check them to be absolutely sure you've plugged in what the procedure calls for. It just won't do to launch into the soup on the wrong frequencies or courses.

Here's something else that you should think very hard about before taking off into low IFR weather: What will you do if there's an engine or instrument malfunction right after takeoff, or another emergency of some sort' A return to the departure airport may be out of the question, especially if the departure airport doesn't have an ILS approach. You'll be out there in the soup, at low altitude, struggling with a problem, and have no viable or legal way of making it back home.

That's why there's a rule of thumb that you should never violate: If the weather is below your departure airport's approach minimums, don't take off. FAR Part 91 says that you can take off in these conditions, but don't do it. You want to be able to shoot an approach if difficulties arise. You may not make it to the nearest airport—the one you just took off from—if the field is below minimums. At the very least, however, you could keep in mind a rule from charter and airline operations and name what's called a takeoff alternate. That would be a relatively close airport with weather above minimums—a place where you could go if things start falling apart after taking off into low ceilings and/or visibilities. FAR Part 121 states that a takeoff alternate for twins must be no more than one hour away (at normal cruise speed with one engine inoperative) from the departure airport.

As for instrument approaches in low IFR weather, there are still more concerns. It's imperative that your procedural and tracking skills be top-notch, because you'll need all the accuracy you can muster if you're going to catch sight of the runway. This is especially true when flying an instrument landing system (ILS) approach. The ILS's localizer and glideslope become more sensitive the closer you fly to their antennas, so any course or glide path deviations must be recognized quickly and corrected just as quickly, or those deviations will become more and more radical. That's something you definitely don't want as you descend closer to terra firma. And since 200 feet agl is the typical ILS decision height, there's not much room for error during the final moments of an ILS approach.

In Category II and III ILS approaches, the needles are exquisitely sensitive, because they're designed to take you closer to the runway—or, in the case of Cat III, very, very close to touchdown.

Bear in mind that a standard one-dot localizer needle deflection equals roughly a 300-foot off-course situation at the outer marker or final approach fix (about five miles from the approach end of the runway). At the middle marker (about one-half mile from the runway), a one-dot deflection means that you're 100 feet off of the desired lateral course.

The glideslope is even more sensitive. At the outer marker, a one-dot deflection means that you're about 50 feet high or low of the glide path. At the middle marker, one dot equals an approximate eight-foot deviation. The exact values of localizer and glideslope deviations depend on a particular ILS's electronic beam width and any other considerations imposed on the approach procedure by obstacles, runway dimensions, and airport layout.

I'm not mentioning nonprecision approaches (those without vertical guidance, such as VOR, NDB, localizer, LDA [localizer-type directional aid], SDF [simplified directional facility], or GPS approaches) for one good reason: You may not be able to legally descend low enough to see the runway environment if low IFR weather prevails. Most nonprecision approaches have minimum descent altitudes around 500 feet agl'right where low IFR's highest cloud bases may be.

When planning for a flight that begins or ends in low IFR weather, remember the fuel and alternate-airport variables. (These are spelled out in FAR Parts 91.167 and 91.169, respectively.) IFR regulations require that you name an alternate airport if the weather forecast for one hour before to one hour after your estimated time of arrival at your first intended destination includes anything less than a 2,000-foot ceiling and three miles— worth of visibility. The alternate's forecast weather for the ETA there must be at least 600 feet and two miles for an alternate with a precision approach, and 800 feet and two miles if the airport has only nonprecision approaches. If your named alternate doesn't have an instrument approach, then you must be able to descend from the minimum en route altitude, make your approach, and land under basic VFR conditions.

Another rule states that you must have enough fuel aboard to fly to the first intended destination airport, miss the approach, fly to your named alternate airport, and still have enough fuel for 45 minutes of flying at normal cruise speed.

Do these rules sound reasonable to you? Consider that 45-minute fuel reserve, then ponder that 800-and-two, 600-and-two alternate weather requirement. Let's say you descend to the missed approach point at your first destination airport, see nothing, perform a missed approach, fly to that airport you named as your alternate on your flight plan, then find out that instead of the 600-and-two that was advertised during your preflight briefing a few hours ago, the weather is actually at ILS minimums—which is usually around a 200-foot ceiling and/or a one-half-mile visibility. Will you be able to continue flying to an airport with better weather—in less than 45 minutes?

Here's where plenty of fuel can save your life. Why? Because if the weather is as low as 600-and-two, then it's likely that the same rotten conditions are fairly widespread. Depending on any number of variables, conditions may deteriorate even further, anywhere within a 100-nm radius or so of your first alternate. Risk another instrument approach that far away with less than an hour's worth of fuel aboard? Doesn't sound like a good idea to me—especially if icing or thunderstorms are in the picture.

Now you know why it's nice to have ice protection and thunderstorm detection equipment, along with long-range fuel tanks and enough gas to let you fly out of the worst weather and into conditions that take the pressure off any subsequent approaches—if any—that you may try. The last thing your nerves need after two consecutive missed approaches at two different airports is the thought of shooting a third low approach with minimal fuel.

Which brings us to the question: What combination of factors is most likely to create widespread low IFR weather? If you know which weather setups are most conducive to large-scale low weather, then you'll be that much more prepared to take it on, if you choose to—or wait it out, if your better judgment prevails.

Here are a few prescriptions for really lousy, vast areas of low IFR weather. If an Internet provider, a flight service station briefer, DUATS, or The Weather Channel mentions any of the following, think seriously about postponing your trip. Ask yourself: Do I really have to get there? Do I have the skill, experience, equipment, and fuel to be comfortable facing one or more low approaches to IFR minimums?

  • High pressure, high humidity, and low nighttime temperatures. High pressure isn't always nice, especially around four or five a.m., when surface temperatures are lowest. The temperature can drop to the dew point, creating large areas of dense fog. This can happen anywhere, but the Central Plains, the Central Valley of California, and Central Florida are places where huge areas of radiation fog can form fairly often.
  • On-shore flows. When low-level air flows from oceans or bays toward land, its high moisture content can cause large zones with low ceilings. Often, this is a problem in the southeastern states, when southeasterly flows ahead of an approaching cold front encounter rising terrain. Clouds thicken and ceilings lower as the onshore air is lifted by the upsloping Piedmont.
  • Slow-moving or stationary fronts. The longer a front hangs around, the more time there is for it to gather moisture and hug the ground. This is particularly true in the Great Lakes region and the northeastern United States, where large bodies of water are nearby.
  • Occluded fronts, or lows with multiple fronts radiating from them. An occluded front—or occlusion—is another slow-moving phenomenon. Occlusions happen when cold fronts catch up with the warm fronts ahead of them, and usually stretch to the north or northeast of a parent surface low. When they catch up they lift, or are lifted by, the fronts that precede them. That makes for two fronts'one at the surface and one aloft. It may be a few days before instrument weather clears out, and in the meantime there can be a sloppy mess of stratus clouds, rain, and/or snow. The Pacific Northwest, Great Lakes, and the northeastern states are places where occlusions like to ground airplanes.
  • The northeast "corner" of a surface low. The northeastern quadrant ahead of a fall or winter low has plenty of lift and plenty of instrument weather. The low and its attendant fronts are in close proximity, and low clouds and precipitation often wrap around the low, causing snow, freezing precipitation, and icing conditions as the air makes the counterclockwise turn to the low's northwest.
  • Gulf lows in winter. Keep an eye on them, because most major winter snowstorms that affect the eastern seaboard begin as lows in the Gulf of Mexico. Then they can track to the northeast, shutting down airports in several states at a time.
  • Winter warm fronts. East of the Rockies, you'll often find huge swaths of low IFR weather along and ahead of warm front leading edges. This happens when warmer rain falls through the narrow wedge of the cold air mass ahead of the warm front. Fog and low stratus form because the colder temperatures drop to the dew point, and moisture condenses. Freezing rain is another possibility.
  • Inversions. Whenever a warm, moist air mass overrides a shallow layer of colder air, expect instrument weather. This is a big cause of the coastal fogs of Northern California.
  • Cold air damming. This phenomenon occurs east of the Appalachians when a mass of cold, high-pressure air over New England or Canada spreads down the East Coast ahead of an approaching, slow-moving northeast-southwest cold front. The advancing cold air sets up a snaky-looking cold front of its own as it invades the warm sector ahead of the cold front to the west. It bulges south until it runs into the bases of the Appalachians, where it "dams" and its movement stops. This tongue of cold air is shallow—maybe just 1,000 to 2,000 feet deep—but it lowers surface temperatures in the former warm sector to the dew point and badabing, low IFR from Pennsylvania to Georgia. This is especially true if the ground has been soaked by rainfall prior to the cold air's arrival.
  • Cold, wet ground, and an overcast. Here's a guaranteed instrument-weather combo. When a slow-moving, precipitation-loaded front moves through, fog and low IFR can persist for days—if a tenacious overcast prevents the sun from breaking through. Fog burns off from the ground up, and that can't happen if the heat of the sun doesn't make it to the surface. A coating of wet snow is just as bad—maybe worse—than rain-saturated ground. The whiteness of the snow reflects any incoming solar heat better than rain-soaked ground, and keeps temperatures near the dew point. And we all know that a close temperature-dew point spread (less than 5 degrees Fahrenheit) means that fog is likely to form.

What do all these weather phenomena have in common, Cooling of moist air, to be brief. And why do I bring the subject of low IFR up at this time of year, Because ever since late autumn, nighttime temperatures have been steadily falling and fronts have been moving through at a faster rate. That, and the days are shorter. The upshot is more fog and low clouds in the fall and winter. If you haven't been flying for a while, you may be surprised by a sudden, unforecast deterioration in the weather. Remember the rotten-weather rules of thumb listed above, stay current on instruments if you intend to fly IFR in the fall and winter months, and carry as much fuel as you safely can. And remember to call home when you land.


Links to additional information about flying IFR may be found on AOPA Online ( www.aopa.org/pilot/links/links0002.shtml). E-mail the author at tom.horne@aopa.org.

Thomas A. Horne

Thomas A. Horne | AOPA Pilot Editor at Large, AOPA

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.