Un­der­stand­ing air­craft ice pro­tec­tion


Flying - - CONTENTS - By Rob Mark

With win­ter nearly in full swing north of the equa­tor, it’s only a mat­ter of time be­fore in­stru­ment-rated pi­lots will need to make de­ci­sions about how to es­cape from ic­ing sit­u­a­tions, whether that be be­fore take­off or while en route. Ice adds weight and acts as a lift spoiler across wings and tail sur­faces. Ice can also af­fect en­gine op­er­a­tions for all air­craft. Ice pro­tec­tion is broadly cat­e­go­rized as ei­ther anti- or de­ic­ing, de­pend­ing on the equip­ment in­stalled and the cer­ti­fi­ca­tion of the air­craft.

When it comes to pro­tect­ing an air­craft from ic­ing, there are few ab­so­lutes, only guidelines about when to ex­pect ice. In vis­i­ble mois­ture when the out­side air tem­per­a­ture is be­low 37 de­grees, pi­lots should ex­pect to see the white stuff be­gin to stick. While ic­ing equip­ment is cer­tainly help­ful in rid­ding the air­frame or power plant of ice, the best strat­egy is to leave the alti­tude or weather sys­tem pro­duc­ing the ice as soon as pos­si­ble. High-per­for­mance air­craft hold the ad­van­tage here

when it comes to es­cape ma­neu­vers.

De­icer boots — Boots are rub­ber sleeves at­tached to the lead­ing edges of the wings and the hor­i­zon­tal sta­bi­lizer, pow­ered by pneu­matic air. Boots cycli­cally ex­pand and con­tract when ac­ti­vated by the pi­lot to break the grip of the ice at­tempt­ing to spoil lift. Boots ex­pand se­quen­tially in such a way that not all sec­tions of the air­foil are de­iced at the same time. The trick is not to wait too long to ac­ti­vate them lest they be un­able to keep up with a rapid ac­cu­mu­la­tion of ice.

Heated lead­ing edges — Most trans­port-cat­e­gory air­craft use lead­ing edges heated from the in­side by bleed air si­phoned from their tur­bine en­gines and then piped to the ap­pro­pri­ate lo­ca­tion. The air­foils are heated be­fore the air­craft en­coun­ters ice. One draw­back to bleed-air heat­ing of the lead­ing edges is the power it draws, which can limit air­craft per­for­mance, such as at take­off.

TKS — This sys­tem cov­ers the lift-pro­duc­ing lead­ing edges with a fine wire mesh that in­cludes holes al­most im­pos­si­ble to see with the hu­man eye. As a TKS-equipped air­craft ap­proaches ic­ing con­di­tions, the pi­lot ac­ti­vates the sys­tem, which then squirts an anti-ic­ing fluid out the holes along the lead­ing edges to pre­vent the buildup of ice.

Pi­tot tube heat — Ice ad­her­ing to pi­tot tubes ex­posed to the slip­stream and wa­ter-laden at­mos­phere are just as sus­cep­ti­ble to ic­ing as air­foils. Pi­tot tubes, stall vanes and out­side air tem­per­a­ture gauges can, in an era of glass-cock­pit air­craft car­ry­ing ex­tremely so­phis­ti­cated sen­sors, quickly be­come use­less or er­ror-prone should they be­come en­cased in ice. On so­phis­ti­cated air­craft, these tubes are usu­ally elec­tri­cally heated, of­ten au­to­mat­i­cally be­fore en­coun­ter­ing ice. On a Cessna 172 or a Piper Archer, for ex­am­ple, a heated pi­tot tube is pretty much stan­dard equip­ment, ex­cept the pi­lot must re­mem­ber to turn it on be­fore en­coun­ter­ing ic­ing con­di­tions.

En­gines — Pis­ton en­gines de­mand a free flow of air to op­er­ate, a flow that ic­ing can dis­turb. Pis­ton air­craft will nor­mally of­fer the pi­lot an al­ter­nate air­source op­tion to suck air from a lo­ca­tion out of the slip­stream in the event of an ic­ing en­counter. Tur­bine en­gines have no love for ice ei­ther, and are usu­ally pro­tected by elec­tri­cally heated in­lets that must be switched on be­fore the ice be­gins form­ing.

Pro­pellers — Be­cause pro­pellers are air­foils, they too must be pro­tected from ice ac­cu­mu­la­tion lest they lose their abil­ity to pro­duce thrust. Mod­ern air­craft use elec­tri­cal power to pre­vent or shed ice at­tempt­ing to ad­here to the blades. In ear­lier days, pro­pellers were of­ten pro­tected by a sys­tem that squirted al­co­hol on the blades to shed ice.

A Elec­tric prop de­icers must be ac­ti­vated be­fore en­ter­ing ic­ing con­di­tions to have enough time to heat the pro­pel­ler blades. B De­spite their ef­fi­ciency, there are lim­its to how much ice pneu­matic boots are ca­pa­ble of re­mov­ing from wing lead­ing edges.

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