Wx Watch: Six for the Road
Go or no-go? A checklist to answer the weather question
November 1, 2007
With winter weather now gearing up to deal us its best punches, it's time to gear up our risk awareness for the season, and think hard about our strategies for safe flying. Obviously, it all starts with a thorough preflight weather briefing.
With winter weather now gearing up to deal us its best punches, it's time to gear up our risk awareness for the season, and think hard about our strategies for safe flying. Obviously, it all starts with a thorough preflight weather briefing. Briefings can vary in quality and quantity of information, so here are some checklist items you should complete to help ensure that the weather doesn't give you nasty surprises. Make sure you ask the briefer for the relevant items below, or call them up using an Internet aviation weather provider. Here are six must-do preflight checks to help your situational awareness, and maybe even prevent an accident.
- File IFR. Or at the very least, obtain VFR advisories (formerly known as "flight following," a term most of us still use) if you're not instrument-rated or are simply flying under VFR. You will be better informed as you fly (you can ask for weather conditions ahead, and hear of any adverse conditions from Center Weather Advisories when they are broadcast), and ATC can be a great help clearing you to airports having better weather conditions. Calling ATC in the blind, or to airfile, can waste time that may well be precious if you're icing up.
- Check freezing levels. Sure, you learned about freezing levels during your briefing, but by calling up the current icing potential (CIP) and forecast icing potential (FIP) from the Aviation Digital Data Service Web site you can examine each altitude for the extent, severity, and probability of icing conditions. Click on the "Icing" tab and start navigating.
- Know how high the cloud tops are. Educated guesses on cloud tops can be found in area forecasts and, if enough are posted along your route of flight, pireps can provide tops information. Just be sure to check the valid times for pireps; don't be fooled into believing a day-old report. The CIP product can also give an idea of cloud tops. Scroll up the altitudes, and notice when and where areas of icing disappear. Although cloud tops are generally lower in the winter months, "lower" is relative. Anyone flying a non-turbocharged piston single will find it impossible to clear 10,000- to 20,000-foot cloud tops. Especially if icing is encountered on the way up.
- Beware the warm front. The idea of a warm front may sound cozy in the winter, but "warm" is relative too. Warm fronts can cover vast areas, have multiple cloud layers, and harbor some of the worst icing. A warm front's stratus clouds can cause rime ice to accrete on your airplane's leading edges, and a path to clear air may be a long, long way away. Rain falling from above-freezing clouds into colder air below a warm frontal surface can cause freezing rain to envelop your airplane in a sheath of clear ice. Cold fronts are trouble, too. Their cumuliform clouds are usually made up of larger cloud droplets—the kind that run back from leading edges and freeze on all lifting surfaces. This clear ice is as dangerous as that encountered in freezing rain—but freezing rain does its deadly work in seconds. With cumulus clouds, escape is often easier.
Web site of the month
Ever want to research the frontal locations, precipitation patterns, and upper-air profiles for a certain day? Then check out the Daily Weather Maps Web site. This is an archive of weather charts for the years beginning in 2002. The site is useful when recreating the conditions that existed on a past flight, or studying accident reports. Visit the Web site, select a date using the drop-down menus, and up pop the charts. A drawback is that the surface analysis charts are valid only for 7 a.m. Eastern Standard Time. But it still makes for an educational visit.— TAH
- Cloud bases. Mention of escape raises the issue of cloud bases. Why? For most of us, climbing to flee icing conditions is a tempting strategy. But it's a bad idea. I know, I know, everyone's heard the hoary old advice to climb in order to exit freezing rain. This is based on the notion that warmer air is above you. Which is correct, but the real questions are "how far above you?" and "can I even expect to climb at all in icing conditions—let alone freezing rain?" The answer to the first question is that you don't often know; to the second, the answer is a resounding no. And for many of us, the same questions and answers apply to garden variety clear, rime, and mixed icing conditions. Many pilots have come to grief trying to top a cloud layer, tempted by the occasional sighting of patches of blue sky through breaks in the clouds (see " Safety Pilot Landmark Accidents: Ice Crisis," page 68). But some tops are deceptively higher than you think.
Instead, you'll have to lower the nose in order to maintain a safe airspeed when iced up. If you're in cloud, you won't be out of icing conditions until you either reach an above-freezing altitude—or descend below cloud bases. That's why you need to know that cloud base will be at an altitude high enough to allow safe clearance from terrain. Again, CIP can give you an idea of where the lowest levels of icing conditions are located, and TAFs and pireps can be other good sources of cloud-base and ceiling information. The problem with TAFs and pireps is that these reports are spread far and wide. It's not very data-dense, and in mountainous areas—or in the West—TAFs and pireps can be hundreds of miles apart.
The imperative rule for those flying aircraft without equipment certified for flight into known icing (FIKI) conditions: Make sure that cloud bases and ceilings are above minimum en route altitudes, and high enough above the ground to permit approaches and landings in visual meteorological conditions. Ideally, your flight conditions should include warmer temperatures as you descend. You want to lose ice on the way down—not pick more up!
- The deal-breakers. There are icing situations no airplane should take on—even large, powerful turbine-powered airplanes with thousands of pounds of thrust. Some icing conditions can overwhelm any ice protection system. These include supercooled large-droplet (SLD) conditions (which are identified on CIP imagery), freezing rain, and extended exposure to clear icing conditions. Any mention of these conditions should keep you on the ground—unless you have the range and speed to circumnavigate them altogether.
- Want some short and sweet advice to ensure ice-free flights this winter? It's very simple, assuming your trip plans and the proper weather coincide. You want:
- High cloud bases with above-freezing temperatures below them.
- Low cloud tops
- No large-scale frontal complexes
- Low terrain
- Day conditions and no mention of SLD, freezing rain, or low-IFR conditions (assuming you're flying on an instrument flight plan).
For the absolute worst setup, just make the above entries their opposites. Does all of this argue for maintaining visual separation during all winter flights when icing clouds are broken or overcast? If you answered yes, and you fly an airplane without known-icing certification, you bet it does. For those of you lucky enough to fly more powerful airplanes certified for flight into known icing, your options are considerably wider. Just remember to steer clear of the deal-breakers.
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