Rime and Clear and Mixed  

Rime and Clear and Mixed

by Phyllis-Anne Duncan

The pilot was only going to fly his newly purchased Cessna 140 three miles to the airport where he was going to store it in a hangar, safe from winter's wild weather. The ceiling was 500 feet overcast, five  a precautionary landing which caused substantial damage to the airplane. miles visibility, and fog and freezing rain were present.

The windshield iced up, and the pilot opted for Accident investigators determined the PIC as the probable cause for disregarding weather information and for flight into know adverse conditions. Even after receiving three complete weather briefings indicating icing and including PIREP's on icing, a Bonanza pilot departed on an IFR flight into known icing conditions. Less than an hour later, the pilot reported leading edge wing ice one inch thick. Controllers lost communication with the pilot while issuing vectors to a nearby airport.


The aircraft crashed in a farm field, and the wreckage was spotted by a passing motorist. Accident investigators cited the 4,000-hour pilot as the probable cause--poor preflight planning/preparation, poor inflight planning/decisions, poor weather evaluation, for attempting flight into known adverse conditions, and for poor judgment. The pilot of another Bonanza told investigators he had not received any PIREP's for icing, but a weather briefer indicated providing one PIREP and one SIGMET for icing along the pilot's proposed route. While en route, the pilot had to divert because of low ceilings and encountered heavy rain. The pilot then noticed an ice build-up. As he began an instrument approach, the build-up had grown to one inch, and the pilot had to apply full power to maintain speed. When over the approach lights, the pilot slowly reduced power and lowered the gear. The airspeed dropped, the left wing stalled, and the aircraft hit a snow bank 40 feet short of the runway. Post-accident examination revealed a one-inch thick layer of mixed rime and clear ice on the leading edges of the wings and tail. Accident investigators cited the PIC as probable cause for improper planning/decisions and for flight into known adverse weather.

Although the pilot had been issued PIREP's for icing during the preflight weather briefing, the pilot took off in a PA-32, Cherokee Six, not equipped for icing conditions. Soon after takeoff, the pilot reported instrument problems then an ice accumulation of one-half inch. The pilot then reported losing then regaining aircraft control before making a forced landing. Accident investigators cited as probable cause the weather conditions as well as the PIC, for flight into known adverse weather and for an inadequate weather evaluation.

There are about 50 general aviation accidents each year involving airframe icing. In a random look at 10 accidents one year, we found that the PIC was cited as the probable cause in all 10, usually with the addendum "poor preflight planning/decisions" and "flight into known adverse weather." With the wealth of weather forecasts and reports available for preflight planning, these accidents are mostly avoidable.

The psychological reasons why a perfectly rational pilot, who avails him- or herself of all this information, flies into icing conditions are varied and not the subject of this article. Maybe what these PIC's lacked was a little icing knowledge. When it comes to aircraft structural icing, a little knowledge should go a long way because it doesn't take much to know that icing is scary and that the only good use for it is as ice cubes in your drink.

The three-part article, "The Clean Aircraft Concept," deals with ground icing, but here we'd like to talk about structural icing that accumulates in flight.

AC 00-6A, "Aviation Weather," states quite simply that, "Aircraft icing is one of the major weather hazards to aviation." That is really somewhat of an understatement since icing is insidious, cumulative, and sometimes invisible. It can decrease thrust, reduce lift, and increase drag. It can slow the aircraft down, force it downward, or make it go out of control. Engine performance can suffer, and icing can also contribute to false indications on instruments and loss of radio communication. It may freeze a landing gear to the point where gear can' t extend or retract fully, and it can stop brakes from working properly.

In the face of all these negatives, why place oneself in so much danger by flying at all in the winter? Some pilots do remove themselves from winter flying's challenges by "grounding" themselves for the duration. But with careful preparation and planning, you don't have to miss out on winter flying's advantages: great aircraft performance, generally more stable air, and beautiful snow-covered scenery.

Structural icing and induction icing occur in flight, but we will deal here with structural icing, a primary concern to instrument pilots in the winter.

You only need two ingredients for structural icing--the presence of visible moisture and temperatures at or below freezing. Cooling that occurs when lift is produced can reduce aircraft skin temperatures to below freezing even if the ambient air temperature is above freezing. Supercooled water is water still in a liquid state even though its temperature is below freezing. When a cold aircraft strikes a supercooled drop of water, part of the drop freezes instantly, adhering to the aircraft's skin. Friction warms the remaining portion of the supercooled water, but aerodynamic cooling can cause it to refreeze. How that remaining liquid freezes determines what kind of ice forms on the aircraft, either clear, rime, or mixed.

Clear Ice

If after initial impact the remaining liquid from the supercooled drop flows out over the aircraft's surface and gradually freezes, it will form a smooth sheet of ice. Large drops, as found in rain or in cumuliform clouds, create clear ice. Clear ice (sometimes translucent) is hard and glossy, heavy and tenacious. Clear ice, as the accumulation increases, can build up in a horn-like shape on leading edges, either a single horn or a double horn;

consequently, aircraft inflight deicing equipment may be incapable of removing such large build-ups of clear ice. This horn-like accumulation produces a very large increase in drag and a correspondingly inverse decrease in lift.

Rime Ice

When the supercooled drops are small, such as in a light drizzle or moisture in stratified clouds, the liquid remaining after initial impact with the aircraft freezes quickly before the liquid has time to spread out over the surface. The small, frozen drops trap air between them, giving rime ice a rough, milky, opaque appearance--the look of your freezer (if not a frost-free) before you defrost it.

Although lighter than clear ice, rime ice has an irregular shape and a surface roughness, both of which reduce aerodynamic efficiency, i.e., reduce lift and increase drag. Because it is brittle, rime ice is more easily removed with aircraft inflight deicing equipment.

Mixed Clear and Rime Icing

When supercooled drops vary in size or are mixed with snow or ice particles, a combination of clear and rime ice can form and form rapidly. Ice or snow particles can actually imbed in the clear ice and create highly irregular shapes on airfoil leading edges.

Regardless of the form icing takes, the amount of the accumulation is directly proportional to the amount of liquid water in clouds. The worst case scenario involves large water droplets, temperatures close to freezing, and clouds with a high liquid-water content.

Effects of Icing

On an ice-free wing, lift increases with an increase in the angle of attack until the wing's critical angle of attack is reached. Accumulations of ice reduce lift, and maximum lift will occur at a lower angle of attack. The coefficient of drag increases tremendously. Thrust is also decreased because of ice buildup on propeller blades. All of these effects are cumulative, and soon you may need full power and a high angle of attack to maintain altitude. However, this attitude gives you a new problem: Ice can now form on the underside of the wing, adding more weight and drag. The need to get out of the icing conditions at this point is crucial.

Reduction of lift is the most important effect of ice on the wings and tail. An aircraft can lose 30% of its lift with just a small accumulation, and the stall speed can increase 15%. The next one is a real kicker--as if the loss of lift isn't depressing enough: When ice conforms to the shape of the wing, drag can increase 200%! Horn-shaped ice can mean a drag increase of 300% to 500%!!

Icing Intensities

When do you have too much ice? Some may say any ice is too much, especially when you look at the figures for lift reduction and drag increase in the above paragraph. Is a trace okay? Is light icing more or less than a trace? A trace of ice may seem pretty severe if it's your first encounter. FAA has defined some descriptions of icing intensity, used primarily when pilots report icing conditions. These descriptions will show up in reports and forecasts, and so they can be useful in preflight planning as well.

Trace: You can just barely see the ice. Trace ice is not usually hazardous, and aircraft inflight deicing/anti-icing equipment is probably not necessary unless the aircraft is exposed for an hour or more. Light: Accumulations of this intensity require occasional use of inflight deicing/anti-icing equipment. Light accumulations may create a problem for exposure longer than hour and if deicing/anti-icing equipment is not used.

Moderate: Moderate ice accumulates at a rate that short encounters are potentially hazardous and use of deicing/anti-icing equipment is necessary.

Severe: This is so much ice that deicing/anti-icing cannot reduce or control the accumulation. The only thing to do in this case is get out of the icing.

Clouds and Ice Formation

As we've stated there must be visible moisture and temperatures at or below freezing for ice to form. Clouds are the greatest source of that visible moisture, but does it always mean that clouds and low temperatures mean ice? Not always. To form, icing likes a temperature range from 0_ to about -10_C. At temperatures colder than -20_C, ice is rare, but don't automatically assume you'll be ice-free. Temperature can vary in clouds, and you could go from an ice-free zone into an icing one.

Stratus clouds have a very thin layer where icing occurs, usually only a 1,000 feet or so thick. A small change in altitude in stratus clouds could get you out of this narrow ribbon of ice. However, stratus clouds cover great distances. Low-level stratus clouds produce rime icing, but thick, extensive stratified clouds with continuous rain (altostratus and nimbostratus) have a lot of liquid water. In winter these cloud systems can cover thousands of square miles and present conditions for protracted exposure to icing. In thick stratified clouds, there is more liquid water at warmer temperatures, and heavy icing in these clouds can occur at or slightly above the freezing level.

Severe icing conditions can be found in stratus clouds over or downwind of large bodies of water, and when the clouds are lifted by convection over water or mountains, they become stratocumulous. That can mean thick layers of mixed clear and rime icing.

Cumulous clouds because their moisture is being lifted can produce the severest icing. The water droplets in cumulous clouds are larger than stratus, and when they strike the aircraft, they form a film of water that can freeze into clear ice. The layer of icing can be thousands of feet thick in cumulous clouds. Cumuliform clouds are vertical, so prolonged exposure to icing conditions occurs only in a broad zone of storms or heavy showers.

Cumuliform clouds in winter are best avoided altogether, and you will find that PIREP's on icing conditions in them are rare. If you encounter ice in cumuliform clouds, divert or descend into warmer air immediately.

Snow and Freezing Rain

Snow doesn't usually stick to an aircraft unless it is wet snow. If snow starts to stick to your aircraft, treat it as you would any structural icing: climb, descend, divert, whatever, to get out of it.

Freezing rain is created when warm, moist air aloft overruns cold air (sub-freezing) at the surface. Rain falls through the warmer air into the colder air and becomes clear ice. However, encountering freezing rain gives you a clue as to your way out. The air above you is warm--climb for it. Freezing rain usually accumulates at the moderate to severe intensity, and it is better avoided, even if the aircraft is certificated for flight into known


Where to Look

The leading edges of your airfoils or any objects that protrude into the air flow (antennas, OAT probe, etc.) are likely going to be the first place where ice will form. As it travels through the air, the leading edge of a wing, for example, disturbs the pressure field around it. This creates a pressure wave ahead of the wing. Air moves smoothly around the curve of the wing, and droplets of water try to make the turn as well, but they are too heavy.

Instead, they collide with the leading edge and, under the right conditions, form ice.

If you are in a low-wing airplane, spotting this leading edge accumulation is easier than in a high-wing plane. In a high-wing plane, you may see only what looks like trace amounts of ice, while a big horn of ice could be just out of sight. Just like scanning for traffic, move your head a little to scan for ice accumulations.

Also, if you have an accumulation on the leading edges of your wings, expect the tail--which is out of sight--to have it, too, perhaps in a heavier concentration.

Once you start to accumulate ice, don't stay in it. The length of time of your exposure is crucial. If you can get quickly out of it, you'll experience tremendous joy at watching it disappear. If you don't, you could end up carrying that accumulation all the way to the ground--one way or another.

Other Factors


Freezing rain below a frontal surface is an ice maker. Rain at the frontal surface is warmer than freezing, but when it falls through air that is below freezing, it becomes supercooled. Supercooled droplets freeze on contact with objects--like airplanes. This can happen with a warm front overrunning a cold front (Figure 2), or when a cold front pushes a warm front up and over (Figure 3). [NOTE: examples of fig 2 and 3 in AC00-6A] Icing accumulations can be severe because of the large amount of supercooled liquid.


Air that blows upslope is cooled adiabatically, and if it is cooled below the freezing point, it becomes supercooled. When stable air blows up a gradual slope, the droplets are small; large drops fall out as rain. Ice accumulation is slow and allows ample time to get out of the icing. Unstable air moving upslope forms convective (cumulus) clouds, which we've already talked about.

Icing is more likely and more hazardous over mountainous terrain. The vertical cloud formations support large water drops. Frontal movement over mountainous terrain combined with normal upslope airflow create extremely hazardous ice zones-most dangerous above the crests and windward of the ridges. The zone can extend up to 5,000 feet above the mountain tops but can go much higher if cumuliform clouds are present.

'Tis the Season...

We associate winter with icing, but it can occur in any season. In the winter, freezing levels in clouds are closer to the ground, so light, general aviation aircraft are more vulnerable to icing in winter. Cloud systems containing supercooled water droplets are more extensive in winter, and flight planning around them may be difficult.

Flying in Icing Conditions

After all we've said here, how could we talk about flying in icing conditions?

Well, we don't want you to fly in them, but we also don't want you to feel you have to park the airplane between October and March. Careful, conscientious, attentive preflight planning is the key to enjoying winter flying. That, and accepting the idea that there might actually be days in winter that you should not exercise the privileges of your instrument rating in IMC.

Icing--its location, intensity, type, and duration--are virtually unpredictable.

The forecast may call for ice, then it never shows up. Or you may encounter ice where you weren't expecting it. Careful attention to weather reports and forecasts, PIREP's, or any other source of weather information is essential in winter time preflight planning--even if you're planning just a short hop to the next airport. (Remember the first accident cited at the beginning of the article?)

If your route of flight takes you into an area where icing may occur, you need to find out from the weather briefer were the moisture is (clouds and what type they are) and what the temperature is aloft. The secret--and it's not so secret--to dealing with ice is to always have an "out;" i.e., somewhere to go to get rid of the ice or get away from it. Depending upon the conditions you have determined from your preflight planning, that may be up or down, a 180 to get to where you just came from and knew there was no ice, a diversion from your planned route of flight into clear air or to a nearby airport short of your intended destination. The key is to stay flexible, don't lock yourself into only one option, and plan to update your weather information through the various methods as you proceed en route.


You've plotted, planned, talked to flight service, studied weather maps from DUATS, checked the Weather Channel, and you've made the decision to go. We're not going to go into a great deal of detail on preflighting here, because winter preflighting and ground operation are covered in a three-part series called,


If your aircraft is equipped with deicing boots, wipe them down with an approved anti-icing fluid to help the boots resist ice accumulation. Check your braking action after you leave your parking space and during taxi.

Before you takeoff, check the aircraft's critical surfaces for any accumulation of ice, snow, or frost. Don't takeoff unless the aircraft is clean!

If your aircraft is equipped icing protection systems, you can plan to climb through a layer of icing to on-top, but if your climb is delayed by ATC and you begin to pick up ice, let ATC know immediately. If you can't get a higher altitude right away, ask for vectors away from traffic in order to climb. Mentioning ice to a controller will usually get his or her attention and you a positive response. If not, you have your FAR ¼ 91.3 emergency authority; be ready to use it.

Quick action is the key to dealing with in-flight icing. You cannot wait until you're so loaded with ice that you may lose control or stall with any reduction of power or change of attitude. Main ice accumulations begin on the leading edges of airfoils, as we said above, but the OAT probe may pick up ice earlier, giving you an indication of what's to come. Such early detection means you can take evasion action with plenty of options still open.

In Flight When you just begin to see ice and it accumulates slowly, you have a "trace." Report it as just that to ATC, and make plans to get out of it, unless you have icing protection systems. "Light" icing, remember, requires occasional use of the icing protection system and immediate action to get out of the conditions.

Be alert for any odd vibrations and strange noises if you're picking up ice. Ice build-up on antennas, for example, can cause them to separate, usually noisily and adding to your tension. Don't panic; just accept vibrations and things that go bump in the ice as a sign it's time to get out of there. Remember, fly the aircraft first and foremost.

Hold onto your options: Climb, descend, divert, or land. Don't get into moderate or severe icing, where the only option is down.

Arrival Even if you successfully climb through a layer of icing conditions and cruise on top, what goes up must come down. If you now have to descend through a freezing level close to the surface, you face the prospect of arriving at your destination iced-up and unable to see out the windows. A majority of icing accidents actually occur during approach and landing because the pilot loses control. Even a trace of ice might warrant coming in hot on the approach (up to a 20% increase in normal approach speed), but plan carefully because a higher approach speed means a longer roll-out on the runway. Make sure you have enough room. After you extend the gear, make your flap extensions and power changes in small increments, to avoid loss of control or stall. You might find you have to open a window and look outside to land. In Closing AC 00-6A, Aviation Weather, closes its discussion of icing with this summary: "Icing is where you find it....Forecaster can identify regions in which icing is possible. However, they cannot define the precise small pockets in which it occurs. You should plan your flight to avoid those areas where icing probably will be heavier than your aircraft can handle. And you must be prepared to avoid or escape the hazard when encountered enroute." Perhaps we can sum it up simply this way: There is no such thing as a little ice, so PLAN AHEAD!

Some Points to Remember about Icing

Before you takeoff, check the weather for possible icing areas along your planned route. Check PIREP's, and, if possible, talk to other pilots who have flown along your route. If your aircraft is not equipped for flight into known icing, avoid areas of icing. Rule of Thumb: Moisture must be visible and outside air temperature must be near 0_C or colder.

Icing PIREP's

FAA air traffic facilities solicit pilot reports (PIREP's) when bad weather conditions are reported or forecast. They are voluntary on the part of pilots, but their information is extremely helpful to other pilots planning flights. You can give PIREP's to any air traffic facility with which you have contact. EFAS--Enroute Flight Advisory Service--is a collection and dissemination point for PIREP's. When giving a PIREP, use the following format:

Aircraft identification, Location, Time (UTC), Altitude, Type aircraft, Sky cover, Weather, Temperature (OAT), Wind Icing intensity (trace, light, moderate, or severe), Remarks


From 15 miles north of Beckley VOR to Charleston VOR; Time 1815 UTC; altitude 12,000 feet; type aircraft BE-99; in clouds; rain; temperature -8_C; wind 290 degrees true at 30 knots; light rime icing; encountered moderate mixed icing during climb northwest bound from Roanoke, VA between 8,000 and 10,000 feet at 1750 UTC.

Help a fellow pilot out: Offer ATC an icing PIREP before they ask. Also remember that just because one aircraft flew through an are doesn't mean you can. It might have deicing/anti-icing equipment which you don't.
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