Professional Pilot, March 2018

when the deicing systems do release some it can break away leaving an ice ridge where it has separated from the remaining ice sheet Mixed icing is a combination of rime and clear and is the most dangerous type of inflight icing This is common within and beneath cumulonimbus clouds where an aircraft may encounter SLD of various sizes The result is an opaque and heavy layer of ice that combines the worst qualities of both rime and clear ice Not only will it increase weight and drag but it will reduce lift and is difficult to remove Often the rime that accretes on the surface of the ice can form into a dual horn shape along leading edges that severely disrupts airflow Although many aircraft are certified for flight into known icing conditions the presence of ice accretion means you should search for warmer air immediately even if you are able to shed most of it with deicing systems Because the lower atmosphere is heated from the surface warmer air is nearly always found below the icing zone However if the icing is happening along a frontal zone especially a warm front warmer air may actually be above the altitude at which you are experiencing icing Any time you think your flight may take you into a region where icing is a possibility it is worth a look at the temperatures aloft to determine where you might find warmer air Icing on the ground For aircraft on or near the ground icing is frequently produced by supercooled rain falling into subfreezing air below As with inflight icing as soon as the droplet strikes the subfreezing aircraft skin it will freeze with the droplet size determining the rate Aircraft 80 PROFESSIONAL PILOT March 2018 Frost may deposit on aircraft skin after a clear subfreezing night Even though it may be very thin frost can dramatically reduce lift and as with all ice must be removed completely before flight on the ground are moving slowly or are stationary which allows more droplets to strike and accrete away from any anti ice protected areas Freezing rain tends to occur at temperatures not much below 0 º C which also slows the crystallization process resulting in often heavy coats of clear ice over all upward facing surfaces Icing doesnt necessarily require supercooled precipitation If an aircraft has been moved out of a warm hangar into falling snow that snow will melt as it lands on the aircraft only to freeze once the aircraft skin temperature has equalized with the air temperature Conversely an aircraft that has recently descended or landed from high altitude cruise may accrete clear ice on its wings due to a phenomenon known as cold soaking an effect of wing fuel tanks The fuel in the tanks can remain liquid to temperatures of 40 C to 50 C and will take some time to warm So even though the aircraft may be in above freezing temperatures as high as 15 C the wing surfaces may still be below freezing and any rain hitting the wings will quickly freeze just as if the water was supercooled Pilots descending from cruise into an area of cloud or rain should expect that cold soak icing may occur Pilots need to take appropriate countermeasures A last icing danger is that of frost When air is near or at saturation and the temperature is below freezing additional cooling can produce deposition or the transition of water vapor directly to ice When this happens usually under clear night skies in relatively humid air a thin layer of ice will crystalize on the upper surfaces of the aircraft A relative to frost freezing dew can also coat an aircraft with ice But while frost will normally create a rough whitish coating freezing dew which occurs when dew settles while the air is above freezing and then freezes as the air cools below 0 C will create a clear slightly bumpy ice layer Regardless of how ice may form on an aircraft on the ground pilots must take steps to ensure that all ice is removed before takeoff In most cases a visual and tactile inspection of the wings will reveal whether ice or snow is present If it is deicing is critical But even if ice is not perceptible you should deice whenever conditions favor the formation of ice Your aircraft may appear ice free now you may even have just towed it from the hangar but you still need to taxi and takeoff in conditions that may lay down a layer of ice on your wings Of course one additional ice hazard of note is that icing doesnt only coat aircraft Freezing rain and subfreezing temperatures after a rainfall can leave slick spots on runways taxiways and tarmacs Although airports will do their best to melt this black ice many aircraft flown by unsuspecting pilots have slid off runways or taxiways and many pilots have fallen on ice just walking to the FBO If you notice icing either in flight or on the ground or even icy patches on runways or taxiways notify controllers and file a pirep Karsten Shein is a climatologist with NOAA in Asheville NC He formerly served as an assistant professor at Shippensburg University Shein holds a commercial license with instrument rating Freezing rain may coat all upward facing surfaces with a glaze that may be several centimeters thick Just 1 cm of ice on a 100 sq m wing surface equals around 2000 lbs