Aircraft Icing

As the winds cool and chill with impending snow and frozen weather, aircraft all over the world haul pilots, commuters, and cargo all over the world. As an aircraft is exposed to dangerously cold weather, it is put immediately at risk for aircraft icing, an extremely hazardous and potentially fatal safety concern. The phenomenon of aircraft icing only occurs in colder climates when the outside air temperature approaches the freezing mark. When the air around the craft chills to that extent, the temperature of outer metal skin of the aircraft also falls. While the metal sheets are built to withstand these lowered temperatures, the airplane assumes risk in flying through a cloud of liquid also cold.

As the aircraft enters the liquid, the water freezes when impacted with the frozen metal exterior of the plane, resulting in problems with wings, tail, propellers, and engine.

Aircraft Icing

As the winds cool and chill with impending snow and frozen weather, aircraft all over the world haul pilots, commuters, and cargo all over the world. While the passengers on board defend against the elements with woolen scarves, hats, and blankets, the skin of the aircraft remains vulnerable to the elements. The most dangerous of these is found not in the snowflakes the pilot brushes from his face as he boards the plane, but instead in the cold liquid and freezing rain the aircraft encounters as it drops to freezing. As the plane is exposed to the winter weather, it is put immediately at risk for aircraft icing, an extremely hazardous and potentially fatal safety concern.

The phenomenon of aircraft icing is most common on smaller and propeller aircraft. Physically, icing only occurs in colder climates when the outside air temperature approaches the freezing mark. When the air around the craft chills to that extent, the temperature of outer metal skin of the aircraft also falls. While the metal sheets are built to withstand these lowered temperatures, the airplane assumes risk in flying through a cloud of liquid also cold. When the craft flies through the cloud, the accumulating rainfall can either be of a low temperature near freezing or below that point, “supercooled.”

As the aircraft enters the liquid, the water freezes when impacted with the frozen metal exterior of the plane. When the rainfall is supercooled, the disturbance causes immediate freezing.

The now frozen aircraft is not always put into a hazardous situation as the ice accumulates on its skin, and aeronautical engineers take into account the weather hazard when building the craft. However, despite technological prowess and infrastructural advances, any ice that freezes on lifting surfaces such as the wings and tail results in an extremely dangerous reduction in performance and safety. While other ice accretion patterns are also hazardous, none are as risky as that affecting the lift.

When frozen liquid collects on wings or tail, the maximum lift and angle of attack are both decreased; a the same time, there is an increase in drag.

As the angle of attack is reduced, safety becomes more nebulous.

Typically, the ice collection that lessens the angle of attack is most problematic as the angle of attack increases, namely when the speed is reduced as the aircraft approaches land. Because the angle of attack is crucial to a safe landing, the impediment resulting from the aircraft icing becomes extremely hazardous. When the aircraft lands, what was once a proper angle of attack for landing becomes a dangerous one, and the impact of the icing forces the craft to lose lift as the wing stalls and the aircraft can nose downward suddenly. The nature of landing demands that the pilot steers the plane towards the ground quickly; a nosedive at such a close proximity is unwelcome in any situation, but with the affects of airplane icing, is most likely to result in a fatal conclusion.

Should the icing not occur on the wings but instead on the tail, the results are frequently as disastrous but the result of a different aeronautical affect. With tailplane icing, the accumulation of ice on the tail produces negative lift. The downward pull of the tail balances the craft’s pitching, then forcing the plane to nose down. Unfortunately, unlike icing on the wings, tailplane icing causes an extremely violent dive down for the craft, since the pull on tail is not in balance with the natural flow for which the plane was designed. This type of icing is just as dangerous as that which occurs on the wing, but far more elusive; it is difficult to diagnose in-flight and can be missed by needy pilots.

Aircraft icing is not limited in location only to control and lifting surfaces, where it is most common, but can happen anywhere. Hollywood has made famous the accretion of ice on propeller blades, but it can also gather on critical engine components. Should it accumulate in the engine or the blades, it forces a reduction in the craft’s thrust and, accordingly, increases drag. On these parts of the plane, when ice gathers, it is also likely to break off suddenly. This effect, called ice shedding, is as damaging as the hazardous initial icing. As chunks of ice break off of the moving aircraft, they can be thrust into the engine, resulting in significant and potentially fatal damage.

At best, the weight of the ice wears down the craft and causes a serious decrease in the overall performance of the plane. Because the other repercussions are far greater and more deadly, the aircraft engineering and architectural industries have been spurred to find an infrastructural solution to the roll ice can play in an aircraft’s performance. Most commonly used are fluid solutions that are sprayed over aircrafts prior to departure; both military and commercial airlines are subject to this wash with great frequency in cold weather. If a potential icing condition presents itself, the aircraft is sprayed with de-icing fluids prior to takeoff. The fluid is meant to melt and remove preexisting ice and prevent further ice from forming during flight.

While anti-icing and de-icing fluids are the most popular solution to the problem of aircraft icing, other technologies also exist. Electrothermal devices act to remove ice with heat and prevent any other ice from forming; this is a common tool used to prevent ice on the blades of the propeller. Commuter aircrafts also use a de-icing boot, a quickly inflating and deflating rubber instrument, to break ice off the wings. Jet aircrafts also route hot bleed air through the wings to prevent the initial accretion of ice.

“The economics of airline operations and the requirements of round-the-clock vigil defensive systems dictate the need for all-weather operational capabilities for civil aircraft.”

Aircraft icing is such a dangerous risk for flight that it garners international attention and is even the focus of government and academic grant and focus in the United States. The University of Illinois is home to the Smart Icing System, which measures atmospheric and aircraft limitations to determine if the craft is at risk for icing to warn the pilot and launch preventative measures to reduce the fatal risk. The NASA Glenn Research Center is located conveniently at the Cleveland-Hopkins International Airport in Ohio, where winter icing is made even more risky with lake-effect weather and harsh Canadian winter winds. There, NASA has developed a team of anti-icing researchers who run the Icing Research Tunnel (IRT), which examines both the actual risk of icing as well as developing potential solutions to the problem.

Most of these solutions are very effective. Very few planes incur fatal landings as a result to aircraft icing any longer, but the historical problem remains. The weather through which an airplane flies can put even the best-built craft at the mercy of the elements, and of these, icing is one of the most fearsome. Increasing drag, weakening the integral angle of attack, and putting the craft in harm’s way, icing presents fatal possibilities to all involved. Increased attention and ever-improving technology hold the key to the safety concerns presented by aircraft icing.

Bibliography

Coares, Andrew. “Space Weather and Aircraft – A Threat to Aircrew and Passengers? The Physics of the Universe.” New Statesman. May 20, 2002.

Fyall, A.A. “Practical Aspects of Rain Erosion on Aircraft and Missiles.” Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences. Vol 260, No. 1110. A Discussion on Deformation of Solids by the Impact of Liquids, and its Relation to Rain Damage in Aircraft and Missiles, to Blade Erosion in Steam Turbines, and to Cavitation Erosion. Jul. 28, 1966.

Kyle and Sand, Thomas G. And Wayne R. “Water Content in Convective Storm Clouds.” Science. New Series, Vol. 180, No. 4902. Jun., 1973.

MacFarlan, Ross A. Human Factors in Air Transport Design. New York: McGraw-Hill Book Co., 1946.

Lankford, Terry T. Aircraft Icing: A Pilot’s Guide. New York: McGraw-Hill Professional, 1999.

MacFarlan, Ross A. Human Factors in Air Transport Design. New York: McGraw-Hill Book Co., 1946. p. 243.

Kyle and Sand, Thomas G. And Wayne R. “Water Content in Convective Storm Clouds.” Science. New Series, Vol. 180, No. 4902. Jun., 1973. p. 1275.

MacFarlan, 245.

Coares, Andrew. “Space Weather and Aircraft – A Threat to Aircrew and Passengers? The Physics of the Universe.” New Statesman. May 20, 2002. p. 5.

Fyall, A.A. “Practical Aspects of Rain Erosion on Aircraft and Missiles.” Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences. Vol 260, No. 1110. A Discussion on Deformation of Solids by the Impact of Liquids, and its Relation to Rain Damage in Aircraft and Missiles, to Blade Erosion in Steam Turbines, and to Cavitation Erosion. Jul. 28, 1966. p. 161.