Wx Watch: The Scoop on Airmets

I once asked a flight service station briefer to give me the first word that pops into pilots’ minds when he mentions the word airmet. It was an attempt at word association. "You’re crying ‘wolf’ again," the briefer answered. "Most pilots just think, ‘Well, here we go with the standard-issue warnings that have a way of never panning out,’ and they blow them off as some kind of verbal tic or another."

Maybe, maybe not. While it’s certainly true that airmets (abbreviator WA) can be false alarms, there are also good reasons why they should be respected. The reason for this ambivalence lies in some of the definitions and terminology used in airmets. Interpret them strictly and you’ll understand why forecasters have a lot of leeway when it comes to flight precautions.

But first, a quick review. Airmets are issued when forecasters at the National Weather Service’s Aviation Weather Center in Kansas City, Missouri, think that one or more of the specified conditions could happen in a region or regions within an area forecast. Airmet perimeters are defined in text using VORs and other identifiers, which can make visualizing their extent a chore. Fortunately, airmets are plotted graphically on many aviation weather Web sites, and this makes locating their coverage areas a snap.

Airmets come out four times a day, cover a forecast period of six hours, and give an outlook for the next six hours. If there’s some late-breaking nastiness before the next usual issue time, an amendment (look for the abbreviator AMD) to any previously issued airmets is appended to the text header. Any continuing airmets are updated in subsequent scheduled reports, and carry the UPDT abbreviator in the header.

Airmets are aimed at pilots of all types of aircraft, but concern weather phenomena that could be "potentially hazardous to aircraft having limited capability because of lack of equipment, instrumentation, or pilot qualifications," according to the Aeronautical Information Manual’s (AIM) pilot/controller glossary. That’s code for lightplanes flown by low-time or noninstrument-rated pilots. The definition sounds condescending, but airmets are published for three weather phenomena that can be truly aggravating for any pilot, regardless of the airplane being flown. At worst, the weather implied by some airmets can set the boldest of legs to shaking.

Airmets are issued for:

• Moderate icing. This is covered under the "Airmet Zulu" heading, so you know right up front that icing could be a factor. What’s moderate icing? Again, the AIM defines it by saying, "The rate of accumulation is such that even short encounters become potentially hazardous, and use of deicing/anti-icing equipment or flight diversion is necessary." Sound serious to you? It does to me, especially if I’m flying an airplane without a full complement of ice-protection equipment. Also, an airmet may mention MXD/CLR ICICIP (mixed/clear icing in clouds and precipitation). You can interpret this to mean the most dangerous sort of icing—of the supercooled large-droplet kind—that felled two commuter airliners a few short years ago. The icing research that followed those accidents identified this mixed/clear accretion as particularly dangerous because ice shapes can form aft of the wing leading edge and other areas protected by deice boots. If icing takes place in conditions where large droplets are splattering on the windshield, one report stated, then you can assume you’re flying in supercooled large-droplet conditions. In other words, you’re picking up ice in precipitation, which is very, very bad. You should immediately work to get out of any type of icing condition, but with icing in precipitation there’s an extreme urgency to do so.
• Moderate turbulence. This would appear under the "Airmet Tango" heading. Moderate turbulence involves uncommanded attitude, altitude, and airspeed changes that can cause you and your passengers to feel strains against seat belts or shoulder harnesses. Unsecured objects become "dislodged," according to the definition. Oh, and "food service and walking are difficult." Every pilot and passenger hates turbulence, so an Airmet Tango is likely to cause most pilots to sit up and take notice—even though turbulence intensity is a subjective thing. What an old vet may consider merely a bother, the neophyte may dread as much as a sudden venture into inverted flight. It’s these subjective evaluations that help color the tone of a turbulence pirep. Also, the degree of turbulence depends on the airplane—something else that you should consider when looking at pireps. One with a high wing loading rides the bumps better than one with a low wing loading. A few sharp bumps in a Beech Bonanza can translate into a quick 30-degree bank in a Cessna Skyhawk.
• Instrument meteorological conditions (IMC) covering more than 50 percent of an area at a time. This comes under the "Airmet Sierra" heading.
• Mountain obscuration (MTOS) by clouds.
• Surface winds greater than 30 knots.

I think we can all agree that these meteorological events are the types we’d rather avoid. Whether it’s an inadvertent foray into instrument weather, a head-bumping bout of turbulence, moderate icing, or a workout with 30-kt winds in a landing flare, airmets can be ugly indeed.

So why do pilots shrug them off? To be blunt, because forecasters have covered their posteriors. They do this by means of the modifiers they use in airmets.

Often, you’ll see the abbreviators OCNY or OCNLY (translation: occasionally) in an airmet. This means that there’s a greater than 50-percent probability of airmet weather for less than half of the forecast period. The forecast period is six hours, so if icing crops up or a gale smites your airport, then the authorities have forecasted correctly. Then again, if there’s no ice or wind for three of the six hours, forecasters are equally well covered. Heads they win, tails you lose.

This helps explain why some pilots run into honest-to-goodness forecast airmets and some don’t. Let’s say an Airmet Tango comes out at 1400 Zulu (which it does during daylight-savings time, along with 2000Z, 0200Z, and 0800Z) and that you’re flying in the Eastern time zone. You take off at 1400Z (10 a.m.) and fly for three hours—and experience a ride as stable as if you’d been sitting at home in your easy chair. Your buddy takes off about the time you land—after hearing of your pirep advertising smooth air—and cusses you out for the bumps on his head and the way he had to reach under the back seat for the chart that flew out of his lap. And the airmet was correct for each of you.

The same thing can happen with the other airmet phenomena, of course. It’s all a matter of timing, and the quixotic, sometimes localized nature of adverse weather.

While we’re on the subject of forecast caveats, here are a few more:

• ISOLD (isolated). Isolated events affect areas of less than 3,000 square miles or are widely separated in time. Talk about a loophole. Though not as big as Texas, 3,000 square miles is a big chunk of geography.
• WDLY SCT (widely scattered). This means an event that covers less than 25 percent of a forecast area. Let’s hope you’re not flying in that 24 percent and that the other odds (the 50-percent/three-hour odds carried by all airmets) are with you.
• SCT (scattered). This means that 25 to 50 percent of a forecast area may be hit with airmet weather.
• NMRS (numerous). More than 50 percent of a forecast area. Now we’re getting somewhere—more than half of an area’s coverage!

It’s easy to be cynical about airmets, and make fun of all the outs that forecasters have made for themselves. On the other hand, airmets can be valuable shortcuts in preflight weather briefings. Personally, I always check the graphic airmets and sigmets first, before every flight. That way I get an instant handle on what might be.

If a picture is worth a thousand words, then a look at graphically plotted airmets (or sigmets, which can coincide with airmets—but sigmets are a story for another day) can tell you volumes about what to expect about the day’s flying. Maybe that’s why airmets and sigmets are at the top of a flight service briefer’s checklist. When it comes to bad flying weather, it’s always best to get the bad news first.

No bad news in today’s briefing? Then you’ve got a forecaster’s tacit assurance that things will be pretty tame indeed. So tame that there are no caveats, no conditional terms, no 50/50 chances. Sure, a local shot of turbulence may still swat you, but generally speaking the heat’s off. In times like these, what a joy to see NO SGFNT TURBC EXPCTD or NO SGFNT ICING EXPCTD, instead of a jumble of coordinates and an OCNLY this or a NMRS that.

Links to additional information on airmets may be found on AOPA Online ( www.aopa.org/pilot/links/links0006.shtml). E-mail the author at [email protected].