Introduction

In-flight icing is a serious hazard to aircraft because it can:

• increasethestallspeedoftheaircraftbyincreasingitsweightandchangingtheaerodynamicsofthewingandtail

• makeitalmostimpossibletooperatecontrolsurfacesandlandinggear

• destroythesmoothflowofairovertheaircraft

• increasedraganddecreaselift(testshaveshownthaticingnothickerorrougherthanapieceofcoarsesandpapercanreduceliftby30%andincreasedragby40%)

• causeenginefailure

• causepropellervibrations

• damagecompressorbladesofjetengines(chunksoficecaninjectintotheengine)

• produceerrorsininstrumentreadingsofairspeed,altitudeandverticalspeed

• interferewithcommunicationssystems

• reducevisibility

Aircraftflyingthroughcloudinsub-freezingtemperaturesarelikelytoexperiencesomedegreeoficing.Apilotcanreducethechanceoficingbecomingaseriousproblembyselectingappropriateflightroutes,remainingalerttothepossibilityoficeformationandknowinghowandwhentooperatede-icingandanti-icingequipment.

Theriskofanicingencountercanbeevaluatedbyconsideringtheairtemperature,andthenumberandsizeofwaterdroplets.Themoreliquidwaterthereis,thehigheristhepotentialforsevereicing.MoreicingincidentsinAustraliaoccurduringthewintermonthsthanatothertimesoftheyear.

Thethreetypesofaircrafticingarestructural(airframe),inductionandinstrumenticing.Aviationweatherforecastsareonlyconcernedwithstructuralicing,whichisiceaccumulationontheexteriorpartsoftheairframe.Itoccurswhenwaterdroplets(cloudorliquidprecipitation)attemperaturesbelowthefreezingpoint(i.e.theyaresupercooled)freezeonimpactwithaircraftsurfaces.

HAZARDOUS WEATHER PHENOMENA

Airframe IcingBureau of Meteorology › Weather Services › Aviation

Accumulation of ice on aircraft is one of the most significant hazards to the safe and efficient operation of aircraft as it can reduce aircraft performance in a number of ways. Aircraft flying through cloud in sub-freezing temperatures are likely to experience some degree of icing.

Whilethisbrochuredealswiththemeteorologicalaspectsofstructuralicing,itisimportanttonotethatmanynon-meteorologicalfactorswillinfluencetherateoficeaccretion.Forexample,thelengthoftimespentwithinicingconditions,theaircraftshape,thespeedoftheaircraftandthetype(ifany)ofde-icing/anti-icingequipmentonboardtheaircraft.Thereforeitcanbedifficulttocorrectlyidentifythethreatthaticingconditionswillposeonanyparticularaircraft.

Icing Severity ClassificationIcingseverityisgenerallyclassifiedastrace,light,moderateorsevere.Trace is used whentherateofaccumulationisslightlygreaterthanrateofsublimation(theprocessoficechangingdirectlytovapour,bypassingtheliquidphase)resultingintheicebecomingperceptible.Itisnothazardousandde-icing/anti-icingequipmentisnotutilizedunlessencounteredforanextendedperiodoftime(i.e.overonehour).Light meanstherateofaccumulationmaycreateaproblemifflightisprolongedintheenvironment(i.e.morethanonehour).Occasionaluseofde-icing/anti-icingequipmentremovesorpreventsaccumulation.Itdoesnotpresentaproblemifthede-icing/anti-icingequipmentisused.Moderatemeanstherateofaccumulationissuchthatevenshortencountersbecomepotentiallyhazardousanduseofde-icing/anti-icingequipmentordiversionisnecessary.Severemeanstherateofaccumulationissuchthatde-icing/anti-icingequipmentfailstoreduceorcontrolthehazard,andthusanimmediatediversionisnecessary.

Icing TypesIcingconditionsareonlypresentintemperaturesbetween0ºCand-40ºC,withthehighestriskoccurringbetween0ºCand-15ºC.Thesizeandtemperatureofthesupercooledwaterdropletsthatimpactontheaircraftdeterminethecategoryoficethatisformed.

Rime iceisformedbysmallwaterdropletsfreezingquicklyuponimpact,trappingairinsidetheice,givingitawhite,opaqueandlumpyappearance.Becausethisicefreezesquickly,itisusuallyconfinedtotheleadingedgeofthewing.Itmaybeencounteredanywherebetween0ºCand-40ºC,butmostfrequentlyformsinthetemperaturerangeof-10ºCto-20ºCinstratiformclouds,althoughcumuliformcloudsmayalsoproducerimeattemperaturesbelow-10ºC.Muchofitcanberemovedbyde-icingandpreventedbyanti-icing.

Clear (or glaze) iceisformedbylargerwaterdropletsthatfreezemoreslowlyuponimpact,enablingtheicetoextendfurtheralongthesurface,initiallyproducingasmoothsheetoftransparentice,butafterfurtheraccumulationitcanformridges.Cleariceformsmostreadilyintemperaturesbetween0ºCand-10ºCbutcanoccur,withreducedintensity,atlowertemperatures.Itismostcommonlyexperiencedincumuliformcloudwherelargesupercooledwaterdropletsareoftenencountered,andbelowcloudinfreezingprecipitationThelattercanresultinthemostsevereformsofclearicingwhenanaircraftbelowthecloudbaseencountersthesupercooledrainordrizzle,whichmayresultinanaircraftbecomingenshroudedinaclearicelayerinamatterofseconds.Suchconditionscanoccuraheadofawarmfrontorsometimesbehindacoldfrontwhenwarmmoistairaloftoverrunssub-zeroairatlowerlevels.Rainfallingfromorthroughthewarmairintosub-zeroaircanbecomesupercooled.Onimpactwithanaircraftthesupercooledraindropsflowoverexposedsurfacesandfreezeasclearice.

Mixed ice,acombinationofcleariceandrimeice,isthemostfrequentformoficingbecausedifferentsizedwaterdropletscommonlyoccurincloud.Itismostlikelytoforminthetemperaturerangeof-10ºCto-15ºC.Itisformedwhenthewaterdropletsvaryinsizeoraremixedwithsnow,icepelletsorsmallhail.

Hoar frost,anothertypeoficing,isnotspecifictoaparticulartemperaturerange.Itisawhite,featherycrystallinedepositwhichcanoccurinclearairwhenanaircraftpassesquicklyfromsub-zerodryairtowarmmoistair,withthewatervapourinthe

SIGMET will be issued for severe icing. Area Forecasts will include any expectation of moderate or severe icing.

Anti-icing prior to flight

warmerairchangingdirectlytoice.Itcan,forexample,occuronafrosty,calmmorningwhenanaircraft,whichhascooledtobelowfreezing,takesoffintowarmer,moisteraircausingthewatervapourtodepositasiceonthewindscreenandcanopyTheicewillquicklydisappeariftheaircraftremainsinthewarmerenvironment,howeveritcancausesignificantproblemsintheinterimperiod,withthebuild-uponthegreatlyreducingvisibility.Itcandisruptthesmoothairflowoverthewing,inducingearlyseparationoftheairflowovertheuppersurface.Hoarfrostcanalsooccuronparkedaircraftwhenthetemperatureissub-zeroandthehumidityisclosetosaturationpoint.

The Icing EnvironmentThedistributionofsupercooledwaterdropletsandicewithinacloudvarieswithtemperature.Ingeneralthelargestsupercooleddropletsarefoundattemperaturesjustbelow0ºC,i.e.ataltitudesjustabovethefreezinglevel(FZL).Thesizeofthesupercooleddropletstendstodecreasewithdecreasingtemperaturesand/orincreasingaltitude.

Therateoficeaccumulationisdirectlyproportionaltotheamountofsupercooledliquidwaterpresent.Inclouds,theworst-casescenarioismostlikelytooccurintoweringcumulusandcumulonimbusbecauseoftheirverticalextent,theabundantsupplyofmoistureandthelargedropletsizefoundthem.

Dropletsizeandconcentrationarecharacteristicofdifferentcloudtypes.Thebroadrelationshipbetweencloudtypeandicingare:

Anvil of a cumulonimbus cloud

Towering cumulus cloud

Nimbostratus cloud

Altocumulus cloud

Stratocumlus cloud

Structural icing in high-level cloud such as cirrus is unlikely to be a problem as these are usually composed of ice crystals, which will not freeze on to the airframe.

Code Cloud Type Icing threat

CB Cumulonimbus Possible severe clear ice

TCU Towering cumulus Possible severe clear ice

NS Nimbostratus Moderate mixed icing in lower levels.

SC Stratocumulus Moderate rime when freezing level is low enough

AS Altostratus Light to moderate rime. Clear ice possible in lower levels.

AC Altocumulus Light to moderate rime

ST Stratus Nil to light rime

Airservices Australia is the official distributor of aviation forecasts, warnings and observations issued by the Bureau of Meteorology. Airservices’ flight briefing services are available at www.airservicesaustralia.com. Telephone contact details for elaborative briefings are contained in Airservices’ Aeronautical Information Publication Australia (AIP), which is available online through their website.

Other brochures produced by the Bureau of Meteorology’s aviation weather services program can be found at www.bom.gov.au/aviation/knowledge-centre.

© Commonwealth of Australia, 8 February 2013

Cloud-Producing SystemsCloudformationandsubsequenticingismostlikelytooccurinsystemswithenhancedupmotionsuchas:

Active frontswhichforceoneairmassupandoveranother.Althoughtheliftingoveramovingcoldairmasscanhaveabroadextent,themoreintenseliftingcausedbyacoldfronttendstobelimitedtonarrowbandsofcloudstensofkilometreswidenearthesurfacefrontallocation.Frontsingeneralcanbeareasofenhancedicingduetothepresenceofconvectionandamplemoisture.

Cyclonic circulationsgenerateconvergenceofairnearthecentresoflow-pressuresystems,producinglarge-scale(overhundredsoreventhousandsofkilometres)risingmotionandresultantcloudformation.Theextensivenature,bothverticallyandhorizontally,ofasynoptic-scalecyclonecanresultinlongexposuresofaircrafttoicingconditions.

Orographic liftingoverhillsormountainsislikelytoincreaseboththedepthofacloudlayerandtheliquidconcentrationwithinthecloud.Icingmaythereforeoccurmorerapidlyinelevatedareas.Theseveritywillbeincreasediffrontalsystemsarealsointhevicinity.

Warm Air Advection,oftenassociatedwithpolewardmovingair,resultsinlarge-scaleupmotion.Cloudresultingfromwarmairadvectionismostrecognisableintheformofanorthwestcloudband.Icingcanbeseveredueititswidespreadnatureandwithupmotionprovidingaconstantsupplyofliquidwater.

Detection & MonitoringNotoolscurrentlyexistforpilotstoreadilyidentifyandmonitoricingconditions,otherthanfirsthandexperience.However,currentandforecasticingriskmaybeinferredusingtheinformationprovidedinthisbrochureinconjunctionwiththeforecasts,warnings&observationsprovidedbytheBureauofMeteorology.Pilotsshouldkeepinmindthattheprerequisitesforairframeicingare:

• Aircraftflyingthroughvisiblesupercooledcloudorprecipitation.

• Airframeatzeroorsub-zerotemperature.

ApilotincommandofanaircraftmustadviseAirTrafficServicespromptlyofanyhazardousweatherencounteredorobserved.Wheneverpracticable,thoseobservationsshouldincludeasmuchdetailaspossible,includinglocationandseverity.Hazardousweatherincludesthunderstorms,severeturbulence,hail,icing,linesqualls,andvolcanicashcloud.Moreover,apilotincommandshouldmakeaspecialAIREPreportwhenrequestedorassoonaspracticableafterencounteringanySIGMETphenomenon,oranyothermeteorologicalconditionlikelytoaffectthesafetyormarkedlyaffecttheefficiencyofotheraircraft.

Forecasts & WarningsTheBureauofMeteorologyprovidesanumberofforecastsandwarningscontaininginformationonthetiming,severityandextentoficingconditions.Theseinclude:

• SIGWXCharts–bothmidandhighlevelchartscontainicingforecastsinagraphicalformat.

• AreaForecasts–providesanicingforecastbasedoncloudtype,temperatureandsynoptic-scaleinfluences.

• SIGMETs–provideswarningofareas(horizontalandverticalextent)expectedtocontainsevereicingconditions,eitherobservedorforecast.

Synoptic weather patterns govern the movement and overall location of icing environments.

Northwest cloud band