Wx Watch: Ice Surprises

Icing's sucker punches

October 1, 2006

We all do the best we can when it comes to preparing for winter flying weather, right? We check with flight service, check Internet sources of weather information, and double-check any mention of icing airmets or sigmets.

(Forecasts or reports of "run of the mill" clear, rime, or mixed icing conditions will generate an airmet. It's the widespread severe icing — such as freezing rain — that brings on the sigmets.)

We even go the extra step and check for any late-breaking pireps that may confirm the existence of icing along our anticipated route. At the airport, just before departure time, we also make a final check for icing by making still another call to flight service for any updates, or by talking with pilots who have recently arrived.

But as anyone with a few hours flying in the soup will tell you, the weather can be full of surprises. You can prepare all you want, but nature always has a sucker punch waiting for even the best-prepared pilots. You know how it goes: One minute you're simply logging actual instrument time; the next, you're radioing air traffic control for a way out of a bad situation, or clearance to land at the nearest airport.

In this forum, we don't have enough space to mull over all the ways that icing can ruin your day — or night. So let's take a look at a few select "sucker punches" you should bear in mind this winter. Here are some of the traps that we may not readily anticipate:

Clear ice. Clear ice is made up of the larger supercooled (i.e., liquid, but still at a freezing temperature) water droplets. You'll find these sorts of droplets in cumulus clouds, with the majority of them concentrated near the cloud tops. Here's the trap: Forecasts may have advertised above-freezing temperatures at your planned altitude, but clear icing can be highly localized. Just because your outside air temperature (OAT) gauge has been steadily reading +5 degrees Celsius doesn't mean it will stay there. A drop of a few degrees may bring you into the clear icing range of +1 to -10 degrees C. Your "ice-free" cruise in cumulus clouds can suddenly change. Now you're faced with a climb or descent to escape — all the while carrying a load of ice. The best escape strategy is to descend into warmer conditions, turn around, or both. Before takeoff, you did make certain that there are warmer temperatures below your planned cruising altitude, didn't you? And you did make certain that there would be a safe terrain- and obstacle-clearance margin between you and your escape altitude, yes?

Large-droplet icing. Supercooled large droplet icing (SLD) is a subset of clear icing. In SLD conditions — which are most common near the Great Lakes, the Pacific Northwest, and North Atlantic coastal regions — droplets are many times larger than standard-issue clear-icing droplets. They typically occur in a very small, two- to three-degree band hovering around the 0-degree-C mark. This means that they often don't immediately freeze on impact. Their large size means that they splatter when they hit the airplane, then run back along airfoils and other airframe projections before freezing. Clear icing is bad enough, but SLD's runback can drastically change airfoil shape. In a matter of a few minutes, the airplane may become uncontrollable. It's this type of icing that felled an ATR-72 in Indiana back in 1994, an airline accident that prompted the research that led to the discovery of this phenomenon. SLD icing, like clear icing, also can be highly localized. For example, airplanes flying near that ATR-72 had no difficulties. Again, having warmer air, high cloud bases, and adequate terrain clearance is your best ace in the hole should you inadvertently have a run-in with SLD icing.

Freezing rain in clear air. Yes, you read correctly. In warm frontal conditions, warm rain falling into subfreezing air can cause big, big problems even if you're flying in the clear. That's because those falling droplets become supercooled as they make their way down. You can be in cloud-free weather and still receive a coating of clear ice if that overcast above you is producing rain. Talk about a shock! There are few good alternatives to this predicament. A descent to warmer conditions may not be possible because the cold air can be widespread, and extend to the surface. Climbing is out of the question — you'll only encounter more icing. Your airplane will be flying at a higher angle of attack in the climb, runback ice will ruin the wing's performance in seconds, and cloud tops may be thousands of feet above you. Even if you're flying a high-performance airplane with turbocharging and known-ice approval, getting to clear, on-top conditions will be doubtful at best. Come to think of it, the most powerful, best-equipped airplanes flown by the most experienced pilots would have a difficult time in all the conditions mentioned so far. Your best "out" may be a landing at the nearest suitable location. Hopefully, it's an airport with long, wide runways, and a tower you can call for assistance while gently maneuvering for a high-airspeed landing. Small bank angles and high-airspeed, no-flap approaches and landings are the rule when flying iced-up. That's because stall speeds increase with ice accretions, and flap deflections can reduce elevator effectiveness and cause tailplane stalls.

Windshield ice. OK, so you ran into some ice, and have descended out of the cloud bases. But the OAT has remained above freezing and your windshield is still iced over. Great. If your load of ice dictates an immediate landing (i.e., you can't maintain a safe airspeed or altitude) then you'll have to do it with a makeshift method of seeing the runway in order to judge when to flare and where to steer the airplane at low altitude. You may be able to squeeze in a view of the runway environment by slipping the airplane on final and craning your head, but with the higher stall speeds imposed by icing that may be questionable advice. Here's where the small storm window — or an openable side window — on many airplanes can come in handy. Pop it open and see if the view is any better, knowing all the while that you'll be relying mainly on peripheral cues. You hear stories about pilots sticking an arm out the storm window and scraping ice off a small patch of windshield for a better view ahead, but my guess is that this strategy will only work with a few airplane types. The sucker punch here is that the airplane may be flying reasonably well, but you risk a crash on the runway because of the iced-up windshield. It's something you don't often think of, but it certainly has happened. Having windshield heat is an antidote, but most of us don't fly with this type of ice-protection gear — let alone fly airplanes certified for flight into known icing (FIKI).

Inoperative pitot heat. Sure, you turned the pitot heat on. But does it work? Just because the loadmeter or ammeter reacted doesn't mean the pitot heat is doing its job! You may simply be feeding a short circuit. During a preflight, when was the last time you tested the pitot heat by turning on the master switch and actually touching the pitot tube? (It can get very, very hot, so don't grab it forcefully or you risk a burn; touch it lightly a few times.) Don't laugh. I once heard of a long cross-country, instructional flight that involved crossing a mountain range covered in stratus clouds. Over the mountains, on the gauges, the crew noticed that the airspeed indicator had dropped to zero. Was the pitot heat on? Well, the switch was, but the pitot heat was inoperative. A tense descent through the clouds ended with a landing at the nearest airport, where repairs were made. It's worth remembering what can happen when your pitot tube is blocked by ice: It can act like a crude altimeter. The airspeed indicator can erroneously show an increase in a climb and a decrease in a descent. In airplanes with a pitot mast equipped with a water drain hole (e.g., Piper airplanes), if the pitot tube's ram inlet is blocked but the drain hole isn't, airspeed will drop to zero. If the static ports are frozen over, then the air trapped in the pitot-static system leaves the altimeter and vertical speed indicator reading the altitude and vertical speed at the time the blockage occurred. The proper remedial action is to open the alternate static air source or, in airplanes not equipped with an alternate source, break the glass facing of the vertical speed indicator to allow static air to enter the system.

Induction system icing. We do a pretty good job of recognizing carburetor icing — a drop in power, engine roughness — and in applying full carburetor heat when it occurs. But in both carbureted and fuel-injected engines induction system air filters can be blocked by ice accretions. Usually, a spring-loaded door in the induction system is automatically drawn open to bypass the blocked filter. If it doesn't do its job, remember that you can turn on the carburetor heat to bypass the blockage manually. In fuel-injected engines, select the alternate source for engine induction air. It's scary enough to get surprised by an icing encounter, let alone a rough-running engine. Selecting carb heat or alternate engine air helps keep the heart rate down.

Ignoring the OAT. This is a big problem with many pilots. If you don't keep an eye on the OAT, ice will surely catch you unaware. Beware the basic icing temperature ranges — +1 to -10 degrees C for clear ice and -10 to -20 degrees C for rime icing. If you're in the clouds and notice OATs heading for these zones, be ready to carry out your escape plans. And remember, small temperature changes can mean a lot. Skimming through cumulus tops one day, I noticed that the OAT would drop from -8 to -10 degrees C every time I entered a cloud. And with each cloud passage, another frosting of rime would dust the airplane.

Have you experienced an unexpected surprise when flying in or around icing conditions? If so, I'd like to hear from you.

Remember the cardinal rules for winter flying when icing conditions are around — warm temperatures below your altitude, high cloud bases, low cloud tops, and avoidance of large-scale fronts and low-pressure systems.

E-mail the author at tom.horne@aopa.org.

Links to additional information about icing avoidance may be found on AOPA Online.

Wx Briefs

Omega Blocks

Sometimes, high-pressure systems are so big and so firmly entrenched that they prevent any new weather systems from entering their sphere of influence. These are called blocking highs. One particular type of blocking high is called an "Omega Block." That's because this type of high makes the height contours on a 500-millibar chart (which roughly corresponds to the 18,000-foot msl level) look like the Greek letter Omega (i.e., Ω). When this happens, the high pressure within the Omega Block keeps surface temperatures relatively high, and winds relatively low. Meanwhile, the low-pressure areas on either side of the block are prevented from moving. Because the moisture created within the lows cannot penetrate the high, thunderstorm complexes can be shunted up the front and back sides of the blocked zone. When this happens, meteorologists sometimes refer to the storm patterns as a "ring of fire," because they travel around the perimeter of the blocking high. Next time you're checking the winds aloft, look at the 500-millibar chart for signs of an Omega Block. If you see the telltale shape of the contours, expect good flying weather within the block. But beware the periphery late in the day, when surface heating can cause convection.

NASA T-38s get datalink

The NASA Lyndon B. Johnson Space Flight Center in Houston has ordered new cockpit displays for its fleet of T-38N astronaut trainer/proficiency aircraft. Among other capabilities, the displays will feature WSI Corporation's InFlight AV300 "next generation" datalink weather systems, which feature near-real-time Nexrad radar imagery and a wide range of other weather reports and graphics. The display hardware is manufactured by L-3 Communications' Display Systems division. It includes high-resolution primary flight displays and electronic engine displays. Under the four-year contract, display deliveries should begin later this year.