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{{Short description|System to limit ice on aircraft surfaces}}
[[File:Icing on a plane.jpg|thumb|Supercooled large droplet (SLD) ice on a NASA [[Twin Otter]] research aircraft]]

In [[aeronautics]], '''ice protection systems''' keep atmospheric [[moisture]] from accumulating on [[aircraft]] surfaces, such as wings, [[Propeller (aeronautics)|propellers]], [[Helicopter rotor|rotor blades]], [[Intake|engine intakes]], and [[Environmental control system|environmental control]] intakes.<ref>{{cite book |title=A Dictionary of Aviation |first=David W. |last=Wragg |isbn=9780850451634 |edition=first |publisher=Osprey |year=1973 |page=106}}</ref> Ice buildup can change the shape of [[airfoil]]s and [[flight control surface]]s, degrading control and handling characteristics as well as performance. An anti-icing, [[de-icing]], or ice protection system either prevents [[Atmospheric icing|formation of ice]], or enables the aircraft to shed the ice before it becomes dangerous.

==Effects of icing==
{{main|Icing (aviation)#Effect}}
[[File:Icing on a rotor.jpg|thumb|Ice accumulation on a rotor blade in a [[wind tunnel]]]]

Aircraft icing increases weight and drag, decreases lift, and can decrease thrust. Ice reduces engine power by blocking air intakes. When ice builds up by freezing upon impact or freezing as runoff, it changes the aerodynamics of the surface by modifying the shape and the smoothness of the surface which increases drag, and decreases wing lift or propeller thrust. Both a decrease in lift on the wing due to an altered airfoil shape, and the increase in weight from the ice load will usually result having to fly at a greater [[angle of attack]] to compensate for lost lift to maintain altitude. This increases fuel consumption and further reduces speed, making a [[Stall (fluid dynamics)|stall]] more likely to occur, causing the aircraft to lose altitude.

Ice accumulates on helicopter rotor blades and [[aircraft propeller]]s causing weight and aerodynamic imbalances that are amplified due to their rotation.

Anti-ice systems installed on [[jet engine]]s or [[turboprop]]s help prevent airflow problems and avert the risk of serious internal engine damage from ingested ice. These concerns are most acute with turboprops, which more often have sharp turns in the intake path where ice tends to accumulate.{{sfn|''Federal Aviation Administration''|2015|p=16–17}}

==System types==
=== Pneumatic deicing boots ===
[[file:Deicing boots on the leading edge of the wing.png|thumb|When ice builds up on the leading edge, an engine-driven pneumatic pump inflates the rubber boots. <ref>{{Cite PHAK|year=2016|chapter=7|page=40}}</ref>]]
The [[deicing boot|pneumatic boot]] is usually made of layers of [[Synthetic rubber|rubber]] or other [[elastomer]]s, with one or more air chambers between the layers. If multiple chambers are used, they are typically shaped as stripes aligned with the long direction of the boot. It is typically placed on the leading edge of an aircraft's wings and stabilizers. The chambers are rapidly inflated and deflated, either simultaneously, or in a pattern of specific chambers only. The rapid change in shape of the boot is designed to break the adhesive force between the ice and the rubber, and allow the ice to be carried away by the air flowing past the wing. However, the ice must fall away cleanly from the trailing sections of the surface, or it could re-freeze behind the protected area. Re-freezing of ice in this manner was a contributing factor to the crash of [[American Eagle Flight 4184]].

Older pneumatic boots were thought to be subject to ice bridging. Slush could be pushed out of reach of the inflatable sections of the boot before hardening. This was resolved by speeding up the inflation/deflation cycle, and by alternating the timing of adjacent cells.<ref>{{cite web|url=https://www.nbaa.org/ops/safety/in-flight-safety/icing/deicing-boots-training-20090421.pdf|title=FAA Information for Operators 09005}}</ref> Testing and case studies performed in the 1990s have demonstrated that ice bridging is not a significant concern with modern boot designs.{{sfn|Federal Aviation Administration|2015|p=20}}

Pneumatic boots are appropriate for low and medium speed aircraft, without leading edge lift devices such as [[Slat (aircraft)|slats]], so this system is most commonly found on smaller [[turboprop]] aircraft such as the [[Saab 340]] and [[Embraer EMB 120 Brasilia]]. Pneumatic de-icing boots are sometimes found on other types, especially older aircraft. These are rarely used on modern jet aircraft. They were invented by [[Goodrich Corporation|B.F. Goodrich]] in 1923.

===Fluid deicing===
{{distinguish|ground deicing of aircraft}}
[[image:Propeller blade with TKS deice boot.JPG|thumb|Propeller blade with fluid deicing system – [[glycol]] is sprayed from hub outward to cover blades]]
Sometimes called a weeping wing,{{sfn|Szurovy|1999|p=31}} running wet, or evaporative system, these systems use a deicing fluid, typically based on [[ethylene glycol]] or [[isopropyl alcohol]], to prevent ice forming and to break up accumulated ice on critical surfaces of an aircraft.{{sfn|''Federal Aviation Administration''|2015|p=22}}
One or two electrically-driven pumps send the fluid to proportioning units that divide the flow between areas to be protected. A second pump is used for redundancy, especially for aircraft certified for flight into known [[icing conditions]], with additional mechanical pumps for the windshield. Fluid is forced through holes in panels on the leading edges of the wings, horizontal stabilizers, fairings, struts, engine inlets, and from a slinger-ring on the propeller and the windshield sprayer. These panels have {{convert|1/400|in|mm}} diameter holes drilled in them, with {{convert|800|/sqin|/cm2|adj=pre|holes}}. The system is self cleaning, and the fluid helps clean the aircraft, before it is blown away by the slipstream.<ref>{{cite web|last1=E. McMann|first1=Michael|title=TKS Ice Protection: Flying year-round becomes a possibility with the TKS Ice Protection system|url=http://www.planeandpilotmag.com/products/pilot-supplies/tks-ice-protection.html#.VEEsIvl4pec|website=Plane & Pilot Magazine|publisher=Werner Publishing Corporation|access-date=17 October 2014}}</ref><ref name="flight|1942-04-11">{{cite magazine |date=11 April 1946 |url=http://www.flightglobal.com/pdfarchive/view/1946/1946%20-%200710.html |archive-url=https://web.archive.org/web/20120315043955/http://www.flightglobal.com/pdfarchive/view/1946/1946%20-%200710.html |archive-date=2012-03-15 |title=De-Icing for To-day |magazine=Flight |access-date=2013-12-11}}</ref>  The system was initially used during [[World War II]] by the [[United Kingdom|British]], having been developed by [[TKS (company)|Tecalemit-Kilfrost-Sheepbridge Stokes (TKS)]].<ref name="flight|1942-04-11" />

Advantages of fluid systems are mechanical simplicity and minimal airflow disruption from the minuscule holes; this made the systems popular in older [[business jets]]. Disadvantages are greater maintenance requirements than pneumatic boots, the weight of potentially unneeded fluid aboard the aircraft, the finite supply of fluid when it is needed, and the unpredictable need to refill the fluid, which complicates en route stops.{{sfn|Szurovy|1999|pp=31-32}}

=== Bleed air ===
[[Bleed air]] systems are used by most large aircraft with jet engines or turboprops. Hot air is "bled" off one or more engines' [[Gas turbine engine compressors|compressor sections]] into tubes routed through wings, tail surfaces, and engine inlets. Spent air is exhausted through holes in the wings' undersides.

A disadvantage of these systems is that supplying an adequate amount of bleed air can negatively affect engine performance. Higher-than-normal power settings are often required during cruise or descent, particularly with one or more inoperative engines. More significantly, use of bleed air affects engine temperature limits and often necessitates reduced power settings during climb, which may cause a substantial loss of climb performance with particularly critical consequences if an engine were to fail. This latter concern has resulted in bleed air systems being uncommon in small turbine aircraft, although they have been successfully implemented on some small aircraft such as the [[Cessna CitationJet]].{{sfn|''Federal Aviation Administration''|2015|p=21}}{{sfn|Szurovy|1999|p=58}}

=== Electro-thermal ===
[[File:Propeller with electric deice detail.JPG|thumb|Detail of propeller with electro-thermal deicing system]]
Electro-thermal systems use heating coils (much like a low output stove element) buried in the airframe structure to generate heat when a current is applied. The heat can be generated continuously, or intermittently.<ref>{{cite web|url=http://www.compositesworld.com/articles/787-integrates-new-composite-wing-deicing-system|title=787 integrates new composite wing deicing system|first=Jeff|last=Sloan|website=www.compositesworld.com|date=30 December 2008 }}</ref>

The [[Boeing 787 Dreamliner]] uses electro-thermal ice protection. In this case the heating coils are embedded within the composite wing structure. Boeing claims the system uses half the energy of engine fed [[bleed-air]] systems, and reduces drag and noise.<ref>{{cite web|url=http://www.boeing.com/commercial/aeromagazine/articles/qtr_4_07/article_02_4.html|title=AERO – 787 No-Bleed Systems|website=www.boeing.com}}</ref>

Etched foil heating coils can be bonded to the inside of metal aircraft skins to lower power use compared to embedded circuits as they operate at higher power densities.<ref>http://papers.sae.org/2009-01-3165/ | Capitalizing on the Increased Flexibility that Comes from High Power Density Electrothermal Deicing</ref> For [[general aviation]], [[Thermawing|ThermaWing]] uses a flexible, electrically conductive, graphite foil attached to a wing's leading edge. Electric heaters heat the foil which melts ice.

Small wires or other conductive materials can be embedded in the windscreen to heat the windscreen. Pilots can turn on the electric heater to provide sufficient heat to prevent the formation of ice on the windscreen. However, windscreen electric heaters may only be used in flight, as they can overheat the windscreen. They can also cause [[Heading indicator|compass]] deviation errors by as much as 40°.<ref>{{Cite PHAK|year=2016|chapter=7|page=41}}</ref>

One proposal used [[carbon nanotube]]s formed into thin filaments which are spun into a 10 micron-thick film. The film is a poor electrical conductor, due to gaps between the nanotubes. Instead, current causes a rapid rise in temperature, heating up twice as fast as [[nichrome]], the [[heating element]] of choice for in-flight de-icing, while using half the energy at one ten-thousandth the weight. Sufficient material to cover the wings of a [[Boeing 747|747]] weighs {{cvt|80|g}} and costs roughly 1% of nichrome. [[Aerogel]] heaters have also been suggested, which could be left on continuously at low power.<ref>{{cite news|url=https://www.economist.com/blogs/babbage/2013/07/de-icing-aeroplanes |title=De-icing aeroplanes: Sooty skies |publisher=The Economist |date=2013-07-26 |access-date=2013-12-11}}</ref>

=== Electro-mechanical ===
Electro-mechanical expulsion deicing systems (EMEDS) use a percussive force initiated by [[actuators]] inside the structure which induce a shock wave in the surface to be cleared.<ref>{{cite web|title=How They Work: Ice Protection Systems|publisher=Aviation Week|year=2010|url=http://www.aviationweek.com/aw/generic/story_channel.jsp?channel=bca&id=news/coldops1010p08.xml&headline=null&next=20}}{{Dead link|date=July 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref><ref>{{cite web|title=Electro- mechanical Deicing|publisher=Air & Space Magazine|year=2004|url=http://www.airspacemag.com/how-things-work/deicing.html}}</ref>
Hybrid systems have also been developed that combine the EMEDS with heating elements, where a heater prevents ice accumulation on the leading edge of the airfoil and the EMED system removes accumulations aft of the heated portion of the airfoil.<ref>{{cite web|title=Deicing and Anti-Icing Unite|publisher=NASA STI|year=2002|url=http://www.sti.nasa.gov/tto/spinoff2002/ps_1.html|archive-url=https://web.archive.org/web/20030405054438/http://www.sti.nasa.gov/tto/spinoff2002/ps_1.html|url-status=dead|archive-date=2003-04-05}}</ref>

===Passive (icephobic coatings)===
{{See also|Icephobicity}}
Passive systems employ [[Icephobicity|icephobic]] surfaces. Icephobicity is analogous to hydrophobicity and describes a material property that is resistant to icing. The term is not well defined but generally includes three properties: low adhesion between ice and the surface, prevention of ice formation, and a repellent effect on supercooled droplets.<ref>{{Cite journal|last1=Hejazi|first1=Vahid|last2=Sobolev|first2=Konstantin|last3=Nosonovsky|first3=Michael|date=2013-07-12|title=From superhydrophobicity to icephobicity: forces and interaction analysis|journal=Scientific Reports|volume=3|issue=1|page=2194|doi=10.1038/srep02194|pmid=23846773|pmc=3709168 |bibcode=2013NatSR...3E2194H|issn=2045-2322|doi-access=free}}</ref> Icephobicity requires special material properties but is not identical to [[hydrophobic]]ity.<ref>{{Cite journal|last1=Jung|first1=Stefan|last2=Dorrestijn|first2=Marko|last3=Raps|first3=Dominik|last4=Das|first4=Arindam|last5=Megaridis|first5=Constantine M.|last6=Poulikakos|first6=Dimos|date=2011-02-14|title=Are Superhydrophobic Surfaces Best for Icephobicity?|journal=Langmuir|volume=27|issue=6|pages=3059–3066|doi=10.1021/la104762g|pmid=21319778|issn=0743-7463|doi-access=free|hdl=20.500.11850/32592|hdl-access=free}}</ref>

To minimize accretion, researchers are seeking [[icephobic]] materials. Candidates include carbon nanotubes and [[Liquid-impregnated surface|slippery liquid infused porous surfaces]] (SLIPS) which repel water when it forms into ice.<ref>{{cite journal|last1=Kim|first1=Philseok|last2=Wong|first2=Tak-Sing|last3=Alvarenga|first3=Jack|last4=Kreder|first4=Michael J.|last5=Adorno-Martinez|first5=Wilmer E.|last6=Aizenberg|first6=Joanna|date=28 August 2012|title=Liquid-Infused Nanostructured Surfaces with Extreme Anti-Ice and Anti-Frost Performance|journal=ACS Nano|volume=6|issue=8|pages=6569–6577|doi=10.1021/nn302310q|pmid=22680067|via=ACS Publications}}</ref>

== See also ==
* [[Atmospheric icing]]
* [[Icing conditions]]
* [[List of aircraft icing accidents and incidents]]

==References==
{{Reflist}}

==Bibliography==
* {{cite report |author=<!--none credited--> |date=8 October 2015 |title=Pilot Guide: Flight in Icing Conditions |url=https://www.faa.gov/documentlibrary/media/advisory_circular/ac_91-74b.pdf |work=[[Federal Aviation Administration]] |id=AC 91-74B |access-date=9 March 2021 |ref={{sfnref|''Federal Aviation Administration''|2015}}}}
* {{cite book |last=Szurovy |first=Geza |date=1999 |title=Cessna Citation Jets |location=Osceola, Wisconsin |publisher=MBI Publishing Company |isbn=0-7603-0785-7}}

==External links==
* [http://papers.sae.org/2009-01-3165/ SAE paper on Electro-Thermal Ice Protection] by Strehlow, R. and Moser, R.

{{Aircraft components}}

[[Category:Aircraft ice protection systems| ]]
[[Category:Ice in transportation]]