Many would contend that icing is the most serious weather condition pilots can face as they fly through adverse weather. While not as monstrous as thunderstorms, icing is even more dangerous because of the insidious nature in which it can attack. A pilot can see a thunderstorm from miles away in most instances. In the rest of the cases, radar, ATC and other resources are available. In addition, a pilot knows when he is in a thunderstorm. The lightning, downdrafts, updrafts, microbursts, and even mesocyclonic activity make them hard to miss. But icing is different. It is a sneaky killer. One minute you are flying along seemingly with no problems and the next you a plummeting to Earth in an uncorrectable stall. Ice accretion on the aircraft wings wings has destroyed their ability to act as airfoils and provide lift. As we all know, lift may fail, gravity never does. So what is icing? Basically, it is the accretion of ice on aircraft surfaces. That accretion can cause a variety of woes ranging from instrument failures to engine power problems and finally a total loss of lift. Luckily, icing can only occur in a very narrow range near the freezing point. Now we could get into the formulas to determine the freezing level using lapse rates and such, but why bother when the work is basically done for us.
This is a chart showing the freezing level and any icing problems in Alaska. Why Alaska? Heck, it's summer and kind of hard to find U.S. freezing level graphics like this. Click on the chart to get a full view of the image. Then there are reports from other pilots _ probably the best information available. Here is a look at a graphic representation of PIREPs as they are known. 
The legend at the bottom identifies the three types of icing which can occur. The "C" stands for clear icing, which can be seen just north of the Colorado border; the "X" is mixed icing, shown here in northeast Colorado; and the "R" is for rime icing, seen here near the borders of California, Nevada, and California We will now look at how these various types of icing from and the effects they can have on an aircraft. Click on this image for the latest icing news. Let's take a look at the general parameters needed for icing, then we can look more closely at the three types of icing mentioned above. The following graphic developed by the National Weather Service gives the general basics of airframe icing. First, we need temperatures in the range of 14-32 degrees. Any warmer and nothing will freeze. Any colder, the air does not have the ability to hold the moisture needed to form supercooled droplets. These supercooled droplets are able to remain in a liquid state, though they may be as cold as 15-20 degrees. We will not get into the dynamics of how they exist in this tutorial, but accept that they do. So if you look at "1" we have the optimal temperature range, then "2" gives us a relative humidity allowing droplets to form form rising, warmer air. The fourth step shows that these droplets can be anywhere from 30 to 300 micrometers across. That size will become important later. Finally, we see the drops freeze on the airframe and disrupt the flow of air.
(National Weather Service Graphic) So why are there three different kinds of icing? Well it has to do with the tweaking of those parameters noted above. Clear Ice is formed when large supercooled droplets hit the airframe, freezing as they spread along the surface. This allows a solid sheet of smooth ice to form on the airframe. There is a good and the bad here. The good first: Since clear icing spreads as a smooth sheet on the airframe, there is little disruption of airflow. Unfortunately, this is outweighed, literally by the bad: Clear ice is heavy and hard. It is the heaviest of all types of icing and the toughest to remove. Add enough of it to the airframe and lift is overcome by gravity with serious effects. You can expect to find Clear ice in areas of rain and almost exclusively in cumulous types of clouds. Rime Ice is formed when smaller, fast moving, supercooled droplets hit the airframe and freeze instantly. They do not spread across the surface but freeze where they hit. As hundreds of these hit the airframe they trap air in pockets between frozen droplets. This gives Rime ice a milky appearance, compared to the "Clear" ice. Rime ice is much lighter due to the air trapped within. But the rough irregular surface can so significantly disrupt the airflow over the wings and other control surfaces that control is impossible. Rime icing is common in areas with drizzle and usually stratus types of clouds. Mixed Ice is just what it says, a mix of clear ice and rime ice. This is seen when droplets vary in size or when snow, various size droplets and ice pellets make up the mix hitting the plane. This is the most serious form of icing. It has the weight of clear ice and the airflow disruption of rime ice. A deadly combination. Before we look at how to handle icing, let's look at a couple of other icing problems not directly related to control surface icing. Induction icing forms when supercooled droplets are pulled into the engine. This can restrict the flow of air into the engine and even restrict the movement of engine components. Instrument icing occurs when pitot tubes and static vent ports are covered and clogged with ice. This can lead to the loss or malfunction of a host of instruments, many essential to safe flight in heavy weather. So what do we do? In most cases, this is not that big a deal. During pre-flight, check for any PIREPs along your path as well as freezing line and precipitation charts. If there is any chance you will enter icing conditions, well ahead of the adverse conditions, make sure pitot heat, engine anti-ice and structural anti-icing are all on. Do not wait until you are in icing conditions to turn these devices on. They all take a little time to come to full working status. So this takes care of the airliners with full anti-icing systems, what about planes with such systems? First of all, you should still have engine heat and pitot heat. Use them. Then try to avoid the icing conditions by climbing above the precipitation or descending below the freezing line. If this is not possible, land and sit out the passage of the conditions. (For Flight Simulation Use Only) |
