Wx Watch: The Trouble With Zero

Is your OAT showing you SAT?

December 1, 2005

From the earliest days of ground school we've all learned about the temperature ranges associated with icing conditions. Clear icing — that slick, tenacious, transparent coating associated with cumulus clouds — happens most often in the zero-to-minus-10-degree-Celsius range. (That's 32 to 14 degrees Fahrenheit for those of you who prefer what is rapidly becoming an obsolete method of temperature measurement, at least in technical circles. We'll stick to Celsius from now on — besides, it makes most icing rules of thumb easier to remember.) Rime icing favors stratus clouds and the minus-10-to-minus-20-degree range. Let the mercury drop below minus 20 degrees and the chances of icing are greatly minimized. Below minus 40 degrees, it's way too cold for icing's supercooled droplets (subfreezing droplets that are still liquid), so the threat of icing disappears. At that temperature, any moisture will be in the form of ice crystals.

But when it comes to the most dangerous icing temperatures for all aircraft, let there be no doubt. It's the narrow temperature band that straddles zero degrees. This includes temperatures as high as plus 5 degrees, and as low as minus 5 degrees. Of course, this is not to say that deep, deep trouble can't be found below minus 5 degrees. It certainly can, and many icing-related accidents have occurred in conditions where temperatures were at, or well below, minus 5 degrees. But recent research and accident studies have proven that zero degrees is an especially bad neighborhood to be flying in. And it doesn't much matter what kind of airplane you're flying, its power rating, its size, or whether it's certified for flight into known icing (or FIKI, in acronymspeak) conditions.

There are many good reasons to be wary of zero degrees. For one thing, supercooled droplets tend to be larger around this temperature. Warmer air (a relative term, to be sure) can hold more moisture than colder air, and this is especially true when an air mass is saturated — as it is in clouds. This translates into more liquid water content — the amount of liquid water in a given air parcel — and therefore larger water droplets. How large, you may ask, and why should you care?

Testing parameters for compliance with FIKI rules specify that candidate aircraft fly in cloud conditions with droplets as large as 40 to 50 microns in diameter, and these sizes are most likely to occur near the zero-degree mark. (One micron is one-thousandth of a millimeter.) While 40 to 50 microns may not sound very large, it's the size at which an oncoming droplet begins — after splattering on impact — to run back on a leading edge. Supercooled drizzle drops (identified as the main meteorological culprit in the landmark, October 31, 1994, crash of an American Eagle ATR-72 at Roselawn, Indiana) can have diameters as great as 400 microns, or 10 times as large as "conventional" icing droplets. And freezing rain's droplets can be a whopping 4,000 microns in diameter.

When large droplets like the ones just mentioned hit an airplane, they are more likely to run back. In other words, after hitting the leading edges of the aircraft the droplets still have enough mass and fluidity to flow past the impact zone — and past any ice-protection panels, such as inflatable deice boots — and coat an entire airfoil.

Eventually, this runback will freeze to the wing or other unprotected surface. And this brings up another danger of flying near zero degrees. Frozen runback can form ridges and other lift-destroying shapes just aft of leading edges. In aircraft with deice boots, these ridges and shapes can cancel out any benefits of the ice-protection system. The leading edges may be clear of ice, thanks to the boots' inflation, but the frozen runback can pile up just aft of them.

By now it should be obvious that having an ice-protection system in no way guarantees safe flight in all icing conditions. Yes, the leading edges may be protected, but it's the runback ice and the rest of the ice accumulated on unprotected surfaces — wing tips, fairings, antennas, strakes, and other components — that can cause airspeed and lift to plummet. That's particularly true in temperatures near zero degrees.

Other issues with flight near zero degrees center around your aircraft's outside air temperature (OAT) gauge. These gauges tend to be fairly crude in light airplanes, and usually involve a simple probe jutting into the relative wind. They can consist of a bimetallic spring — attached to the temperature pointer — that expands and contracts with temperature changes. Or sometimes OAT gauges use an electrically powered, or thermocouple-driven, temperature sensor that posts its results on a liquid-crystal display or other display. Either way, OAT gauges can have errors of several critical degrees. An OAT may read plus 3 degrees, but is it really that warm outside?

Other factors also can make OATs read on the high side. One is ram rise. While we usually think of ram rise as affecting the temperatures around fast-moving, high-flying turbofans and turboprops, it also can affect much slower-flying aircraft. Ram rise is the temperature increase caused by the frictional effects of air striking an aircraft — and its temperature probe. Studies have shown that aircraft flying as slow as 150 knots can post OAT readings up to 3 degrees higher than the actual air temperature. This temperature is called the "static air temperature," or SAT. SAT is the temperature that would be recorded if the aircraft had somehow been stopped in flight. There would be no frictionally induced errors in this hypothetical situation, and it's this static condition that the OAT gauge tries to duplicate. Some OAT probes have a holed, cylindrical glove around them to try to make the air striking the probe as "static" as can be.

Turbofan and other high-performance airplanes have air data computers that compensate for ram rise and produce a ram air temperature (RAT) reading, and use still other variables to produce a total air temperature (TAT) reading that's useful for calculating Mach numbers and other performance data. But TAT is usually higher — sometimes much higher — than SAT. Most of us don't have TAT readouts, and most of us don't need to know TAT. But the point remains: Your OAT probably doesn't yield an accurate SAT, and it's the SAT that's important for accurately identifying the critical icing temperature ranges.

Moral: Don't become complacent if you see a plus-3-, or even a plus-5-degree reading on your OAT. Your actual outside air temperature may be zero degrees. For this reason, many turbofan pilots make sure their anti-ice equipment is on when they see a plus-5-degree RAT reading. When you see a plus-5-degree OAT, you should too. This provides a conservative margin against the chance of any ice forming near the zero-degree point. Here's another big-iron tip that will keep you out of trouble when flying in and near any clouds or precipitation — turn on your pitot heat before taking off, and leave it on.

Here's one more tip to remember this winter: Keep an eye on the OAT gauge! So many pilots all but forget about the OAT, but it should be part of your normal instrument scan. One winter, I flew with a very skilled instrument pilot on an IFR cross-country, and he handled the airplane perfectly. But even after an extended period of time flying in the clouds, he never checked the OAT. Meanwhile, it was minus 5 degrees outside. I noticed the first signs of icing, and it came as a surprise to this high-timer. If he'd been scanning his OAT he would have had plenty of warning. Good thing the cloud bases were high that day, and there was above-freezing air below us.


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


Links to additional information on icing may be found on AOPA Online ( www.aopa.org/pilot/links.shtml).