Icing – Taking Adequate Precautions

It's that time of year again. Time to shift your operational concerns from thunderstorms to icing. But where does airframe icing come from? How does it form? Where? How can I recognize the danger of icing during my flight planning and what do I do about it? In this first part of a series, AVweb contributor R. Scott Puddy tackles these questions and more. Consider it a primer for what's to come.


The FAA’s Airport and Aircraft Safety Research and Development Division, Aircraft Safety Research and Development Branch, issued a fact sheet on September 6, 2000, which begins:

“Aircraft icing continues to be one of the major safety threats to aircraft operations during hazardous weather conditions and can result in catastrophic accidents unless adequate precautions are taken.”

I’m sure that news is as shocking to you as it was to me. I guess we’d better prepare to take “adequate precautions” for icing during the upcoming winter flying months.

Icing — What Is It?

Icing occurs when liquid moisture comes into contact with an object that is at a below-freezing temperature. The temperature of the liquid moisture doesn’t matter. Step outside on any bitter cold winter morning. Using your moist, above-freezing tongue, lick the below-freezing, metal street post. That’s icing. (Don’t try this alone or you could be “stuck” outside for awhile.)

You can’t make it through an article about icing conditions without reading about “supercooled” water droplets and how they are THE cause of aircraft icing. Although they are not THE cause, supercooled water droplets do play a major role. As discussed below, if not for supercooled water droplets you usually would not encounter liquid moisture at altitudes where the outer skin of your airplane is at a below-freezing temperature. Liquid moisture is one of the prerequisites for icing. However, icing occurs because of the temperature of the airframe. If the temperature of the airframe is above freezing (such as on the surface of a windshield anti-icing panel) ice will not form.

You will pick up ice anywhere liquid moisture impacts any part of the airframe that is below freezing. The temperature of the airframe is a function of the ambient air temperature and the warming that results from the friction of the air passing over the surface. The techies would point out that the air molecules immediately adjacent to the airframe are not moving at all and that the friction actually occurs within the boundary air layer — but that is a distinction without a difference. The friction within the boundary air layer heats the air that heats the airplane skin and elevates the surface temperature to some extent. Just how much the temperature is elevated depends on the speed of the airframe in general as well as the speed of airflow across the skin at specific locations (and, one for the techies, the thickness of the boundary air layer).

The faster the airplane, the higher the skin temperature. The Concorde boasts a skin temperature of between 91-127 degrees Centigrade (196-261 degrees Fahrenheit) in a Mach 2 cruise at altitudes where the ambient air temperature is well below freezing. No need for anti-ice on the Concorde, at least in cruise. Similarly, transport-category aircraft climbing out at over 200 knots could encounter no ice in conditions that would load up a GA aircraft climbing out at under 100 knots.

Where Is It Coming From?

At subfreezing temperatures water vapor sublimates to ice crystals, so where is all this liquid moisture coming from? If you have watched videos of airplanes being certified for flight in known icing conditions, you know that one possible source is a large tanker aircraft flying directly in front of you trailing a water nozzle. If you ever find yourself in that position, radio the pilot ahead and tell him you will catch up on your child support as soon as you get paid for completing this delivery of checks for the Federal Reserve.

In all other cases, the two possible sources of liquid moisture are the above-freezing air below you and the above-freezing air above you. Most discussions of aircraft icing classify ice as either “clear,” “rime,” or “mixed.” If the source of liquid moisture is the air below, rime ice will form. If it is liquid moisture falling as rain from above, clear ice will form. If it is attacking you from both direction (such as in a cumulus cloud) the icing will be mixed. “Rime” is shorthand for “bad”; “clear” is shorthand for “really bad.”

In order to be able to predict how “bad” the icing you may encounter might be, you need have a detailed understanding of the phenomena that cause liquid moisture to be present at temperatures below freezing in order to answer the real issues, which are: “How much moisture is there?” and “How much of it is likely still to be in liquid form?” In order to be able to predict the answers to those questions, you need to understand the basic steps of the icing cycle.

Step One: Evaporation

All moisture in the atmosphere comes from evaporation, the process through which water changes from a liquid to a vapor (a gas). Oceans (e.g., the Pacific Ocean and the Atlantic Ocean) and other large bodies of water (e.g., the Gulf of Mexico and the Great Lakes) are the most prominent sources of the water vapor that starts the icing process.

Step Two: Condensation

Air has a limited capacity to hold water vapor and that capacity diminishes as the temperature drops. The most usual cause of condensation is a lifting of the airmass. As the ambient pressure decreases at higher altitudes, the airmass expands and cools. When the air cools, its capacity to hold water vapor decreases, the airmass becomes supersaturated, and the vapor condenses on condensation nuclei (or existing cloud droplets) and forms cloud droplets. Not to be confused with rain drops, “cloud droplets” are tiny little suckers that range in size from a few micrometers to a few tens of micrometers. A single raindrop (one millimeter in radius) would consist of approximately 1 million cloud droplets with a 10-micrometer radius. A typical cloud contains 100 to 1,000 cloud droplets per cubic centimeter.

Step Three: Coalescence

In warm (liquid) clouds, drops of water form from water droplets through the process of coalescence. Water droplets of different sizes travel at different speeds and hence tend to run into one another. If the collision is strong enough to overcome the surface tension of the droplets, the two droplets combine (coalesce) as one. After as many as a million such unions, the drop of water eventually becomes too heavy for the cloud to support and it falls from the cloud as rain or drizzle. The size of the drop depends on the type and strength of the cloud. There is not a lot of circulation in a stratus cloud and the coalescence process proceeds slowly. Stratus clouds also lack the vertical circulation necessary to support larger drops of water and tend to produce drizzle (drops 0.50 millimeters in diameter or smaller). In convective clouds, in contrast, the complex circulation patterns afford ample opportunity for collisions and coalescence and the vertical circulation supports much larger drops (up to more than 6 millimeters in diameter).

Step Four: Supercooling

(Option 1: Supercooled Water Droplets)

The wing of a NASA Twin Otter after landing. This looks to be clear icing or perhaps mixed. Notice the runback well past the leading edge and on the underside of the wing.

Click image for larger version. Photo copyright NASA-Lewis.

Minute supercooled water droplets could exist at below-melting temperatures if a stratus cloud is lifted to cooler, higher altitudes. We all know from common experience that smaller bodies of water freeze more quickly. A large lake may remain unfrozen throughout the coldest of winters. A smaller lake may freeze over after a few weeks of subfreezing weather. The mud puddle next to your driveway will freeze in a couple hours. Absent antifreeze, the spray from your automobile windshield washer will freeze in the time it takes the wiper blades to complete a cycle. Anything as small as 1/1,000,000th the size of a rain drop must therefore freeze instantly if the temperature dips to 31 degrees.

Wrong. That’s why scientists have to go to graduate school. The temperature we commonly refer to as “the freezing point” should actually be called “the melting point.” Frozen water always melts at temperatures above 32 degrees, but the freezing process is much more complex. Freezing requires the formation of ice crystals which in turn requires the presence of “freezing nuclei” or “ice nuclei.” In contrast to the mud puddle in your front yard, the immaculate little water droplets were formed through sort of a scientific Virgin-birth process and consist of water of a purity that the Culligan Man could only pray for. No freezing nuclei here. Hence, no crystallization and no immediate freezing when the temperature drops below “melting.” For the pure water that exists in the atmosphere, there is no set “freezing point.”

Sponsored by those who would like to harvest rice in the desert, Ph.D. types have performed countless scientific studies relating to the spontaneous freezing point of water droplets of various sizes and the most efficient means of causing those water droplets to coalesce or crystallize and fall to Mother Earth as measurable rainfall. The “spontaneous” freezing temperature for undisturbed water droplets has been observed to be as low as -40 degrees Fahrenheit. Supercooled water droplets are liquid but unstable. If they encounter “freezing nuclei,” “ice nuclei,” or “your airplane,” they will immediately freeze.

Considering that the average cloud has up to 1,000 unstable droplets in each of its cubic centimeters, you might imagine that there is some variation between them. Although they all come into life in a virginal state, some droplets bite of the forbidden fruit and freeze. Once some of the droplets have frozen, they constitute the nuclei that will cause other droplets to freeze. Given enough time and enough cold, the supercooled water droplets will depart their unstable liquid state and reach their stable frozen state.

(Option 2: Supercooled Drops of Water)

Close-up of a test wing section. A 16-inch portion of a wing being tested is lifted through a port in the Twin Otter’s fuselage and exposed to supercooled liquid water then pulled back inside and photographed.

Click image for larger version. Photo copyright NASA-Lewis.

The studies suggest that the much larger drops of water are not as resistant to freezing as the minute water droplets. Drops of water nevertheless can exist in liquid form at below melting temperatures, usually in convective clouds. The strong updrafts in convective clouds lift liquid drops of water from the warmer temperatures at lower altitudes to the below-melting temperatures above. Until the drops of water have an opportunity to freeze, they remain in liquid form. Strong thunderstorms can produce liquid moisture well into the flight levels in this manner.

(Option 3: Supercooled Raindrops)

Liquid raindrops, supercooled or not, can exist where rain falls from above-melting air into cooler below-melting air. If you’ve ever run out of ice at a party and refilled the ice cube trays in a futile effort to replenish supplies before your guests request a second round, you know that there is a time element to the freezing process. Where there is rainfall aloft and ice pellets below, there will be a layer of supercooled water (freezing rain) in between.

Hence there are at least three ways that liquid water could find its way to your below-melting flight altitude. The next question is, “Do you care how it got there?” Absolutely, positively, yes, you do.

Icing From Above

Icing from above (freezing rain) is a component of the natural selection process. Out west, those conditions occur perhaps a couple times a year. The pilots who fly in those conditions are eliminated from the aviator gene pool and you no longer have to avoid them while they are taking off from taxiways at uncontrolled airports.

Freezing rain requires a strong temperature inversion. It is usually associated with an advancing warm front but can also occur in conjunction with a cold front or an occluded front. In an advancing warm front, the warm air is elevated as it advances over the underlying cooler air and it expands and cools. Water vapor condenses into rain in the higher above-melting air, falls into the lower, below-melting air, locates your airplane and turns it into a flying popsicle. Supercooled raindrops are huge in comparison to the supercooled water droplets that you might encounter flying through a cloud and they generate a correspondingly greater volume of ice when your plane runs into them. Furthermore, the larger drops of liquid readily spread before they freeze (that is why the ice is “clear.”) That can create problems even for airplanes with deicing equipment if the clear ice spreads back on the airfoil to beyond the deice-protected regions. For most GA pilots, the appropriate precaution for freezing rain is to avoid it at all costs. On the other hand, if you and four of your drinking buddies just left the Raiders Monday night football game and departed from an unlighted taxiway in a C-172, go for it!

Icing From Below

Another close-up of the test wing section.

Click image for larger version. Photo copyright NASA-Lewis.

During the wet months, the Pacific Coast is a veritable ice-making machine and the conditions will require you to draw upon your knowledge of icing and to exercise your judgment as PIC (assuming you do not intend to mothball your plane until spring training). In the case of icing from above, the “Go/No-Go” decision could be made using a breathalyzer. In contrast, when conditions would support icing from below there usually is no forecast, pilot report, regulation, or other tool that will make the “Go/No-Go” decision for you. It all comes down to your exercise of judgment based on your knowledge of the prevailing conditions, your understanding of the causes of icing, your personal level of risk aversion, and your willingness and capability to tackle a tough flight on any given day.

The Pacific Ocean supplies ample water vapor to the air masses passing over it. When an airmass hits the Pacific Coast, the coastal mountain range provides a topographical mechanism that lifts the airmass to cooler altitudes aloft and promotes condensation and water droplet formation. If the airmass is unstable, the initial topographical lifting will trigger convective activity which will provide yet another lifting mechanism. A little more than a hundred miles inland, the airmass will encounter the inland mountain range which provides still another lifting mechanism. If the airmass is a front, the front itself will provide a lifting mechanism.

Dealing With It

The first step to tackling a flight in winter conditions anywhere that icing is commonly a problem is to check to see what the forecasters think might be in store. The forecasts and AIRMETs for icing conditions may or may not be any more accurate than they used to be, but they are much easier to interpret now that the NOAA has made graphs available. Check out the following sites for graphic representation of the areas covered by all AIRMETs and SIGMETs:

Don’t worry if you lose the URLs. They are available in AVweb’s weather section. As long as you’re online, check out the rest of the weather as well at :

Out west there usually is a forecast or an AIRMET for occasional light to moderate rime and mixed icing in clouds and precipitation and it usually covers an area of thousands of square miles. Whether used in a forecast or an AIRMET, “occasional” is defined to mean a MORE than 50/50 chance of occurrence during LESS than half of the forecast period. A forecast of “occasional” icing is therefore a prediction both that icing will probably exist and that icing will probably not exist at any given time and place in the coverage area. You are expected to understand that icing is frequently a localized and transitory phenomenon. A forecast of icing in an area consisting of thousands of square miles is not intended to suggest that icing is likely to exist throughout that area at any given time. Rather, it is a “heads up” that you could encounter icing. A PIREP is likewise just another clue. It tells you where icing was encountered a little while ago.

Armed with the knowledge that there is a risk of icing, your next step is to determine the probable location and severity of the condition along your route of flight. The essential element to icing is that your airframe be at a below freezing temperature. The winds aloft forecast will give you the freezing levels which you can compare to the MEAs along your route.

You will want to determine the active lifting mechanism(s) because the more lifting, the bigger the drops, the more severe the icing. As discussed above, lifting results from terrain, frontal activity and/or convective activity. The lifting effects are additive so the biggest supercooled water drops will occur at the highest altitudes inland from where the airmass is unstable and a front is crossing the coastal mountains. The surface analysis and prog charts will show you the current and forecast frontal locations. The synopsis in the Area Forecast will alert you to convective activity. Laying your VFR sectional chart alongside your IFR low-altitude chart will assist you in identifying areas where your planned route crosses areas where rising terrain will provide topographical lifting. If the synopsis would support developing of icing on shore (over the coastal mountains) and your route of flight is offshore, you might determine that icing is not probable along your route.

The final necessary component for icing is visible moisture. Check out the satellite photos on line. At times, although icing is forecast over a broad area, only a small portion of the area is actually covered by clouds. On a recent flight up to Seattle, icing was in the forecast because of predicted cumulous development. Sure enough, upon my arrival there were numerous towering cumulus clouds upwards of 10,000 feet tall. There were PIREPs of icing from several inbound pilots. At the same time, the basic conditions were VFR. As the controller told me, “Why don’t they just fly around those things?”

Once you have determined the specific areas where you might likely encounter icing conditions, you need to assess your available escape routes from those areas. If there is an easy escape, you can be more aggressive on your go/no-go decision making. If there is no easy escape … well … surviving nine flights out of ten just doesn’t cut it.

Once you’re en route, keep an eye out for the first indication of ice. Movement of air across the skin surface heats the skin and tends to sweep the liquid moisture away before it contacts the aircraft skin. The first indications of icing will therefore occur in areas of dead air, facing the airplane’s flight path, such as at the base of the glare screen, the area around the OAT probe and the leading edge of the wing. In those locations, the airflow is not across the skin surface because of obstructions to the airflow (such as around the OAT probe), because the air cannot flow smoothly along the surface (such as at the intersection of the cowling and the glare screen), or because the surface is perpendicular to the airflow (such as at the stagnation point along the wing’s leading edge). That keeps the skin surface relatively cool at those locations and prevents the airflow from sweeping the liquid moisture out of harm’s way. If you see it coming, get out of Dodge.

Based on all of the above, despite forecasts for icing, AIRMETs for icing, and PIREPs of icing, you might determine that icing presents a manageable risk for your flight and that it is appropriate to proceed. You have taken and can take “adequate precautions” to complete the flight safely, but is it legal? Keep a watch at AVweb — we’ll discuss that issue in a couple weeks.