Ground ic­ing op­er­a­tions - all about de­ice and anti-ice flu­ids

Flying - - CONTENTS - By Richie Len­gel

GROUND IC­ING CON­DI­TIONS "No pi­lot may take off in an air­craft that has frost, ice or snow ad­her­ing to any ro­tor blade, pro­pel­ler, wind­shield, wing or sta­bi­liz­ing or con­trol sur­face, to a power-plant in­stal­la­tion, or to an air­speed, al­time­ter, rate-of-climb or flight-at­ti­tude in­stru­ment sys­tem," ac­cord­ing to the FAA ba­sis for the “clean-air­craft con­cept.”

REL­E­VANT DIS­CUS­SION: FAR 91.527, 91.1101, 121.629, 125.221, 135.227, 135.345, AIM 7-5-14, OPSPEC A023, AC 20-73, AC 20-117, AC 120-58, AC 120-60, AC 135-17, NASA There are four stan­dard air­craft de­ic­ing and anti-ic­ing fluid types: Type I, II, III and IV.

Type I flu­ids (red-or­ange in color) are the thinnest of the flu­ids. They are es­sen­tially de­ic­ing flu­ids but can also be used for anti-ic­ing. Be­cause they’re not thick­ened, they will shear or blow off the air­plane at rel­a­tively low air­speeds (around 60 knots). How­ever, their low vis­cos­ity re­sults in the short­est set of holdover times (HOT). Type I is al­ways ap­plied heated and di­luted; it flows off eas­ily, and can fail sud­denly. Type I is avail­able through­out the world.

Type II flu­ids (clear or straw color) add thick­en­ing agents to in­crease vis­cos­ity and pro­vide bet­ter pro­tec­tion against re­freez­ing than Type I flu­ids, but re­quire a min­i­mum 100-knot ro­ta­tion speed. The thick­en­ers al­low fluid to re­main on the air­craft longer to ab­sorb and melt the frost or freez­ing pre­cip­i­ta­tion. They’re ap­plied when a long elapse time is an­tic­i­pated be­tween de­ic­ing and take­off. Type II is com­mon through­out Eu­rope, but has lim­ited avail­abil­ity in North Amer­ica.

Type III flu­ids (yel­low-green) are rel­a­tively new and have a vis­cos­ity higher than Type I flu­ids but much lower than Type II or IV flu­ids. Type III was for­mu­lated for use on small com­muter-type air­craft with take­off ro­ta­tion speeds of 60 knots or higher, but they can also be used on air­craft with much higher ro­ta­tion speeds. Type III flu­ids, un­like Type II or IV flu­ids, can be used in a hand­held sprayer be­cause they do not re­quire spe­cial­ized low-shear­ing-ap­pli­ca­tion equip­ment. Type III be­came avail­able in 2004 in Eu­rope and North Amer­ica.

Type IV flu­ids (emer­ald green) meet the same fluid spec­i­fi­ca­tions as Type II flu­ids (i.e., thick­en­ing agents), and have a sig­nif­i­cantly longer HOT. There­fore, Type IV should be used on air­craft with ro­ta­tion speeds (Vr) above 100 knots when long elapse time is an­tic­i­pated be­tween de­ic­ing and take­off. Type IV flu­ids are avail­able through­out the world.

How the Flu­ids Work

As noted, Type I flu­ids are al­ways ap­plied heated and di­luted. For de­ic­ing, it is the heat and hy­draulic force that ac­com­plish the task. For anti-ic­ing, it is the heat im­parted to the air­frame that ac­com­plishes the task.

Type I flu­ids have the short­est HOT. There­fore, when a Type I fluid fails, it fails sud­denly.

Type II and IV flu­ids may be ap­plied heated or cold, and di­luted or full strength.

In North Amer­ica, Type IV flu­ids typ­i­cally are ap­plied cold, and only for anti-ic­ing. In the U.K., Type II or IV flu­ids usu­ally are ap­plied heated to ac­com­plish de­ic­ing as well as anti-ic­ing.

Cold-Tem­per­a­ture Lim­i­ta­tions Us­ing flu­ids in very cold con­di­tions — around mi­nus-20 de­grees Cel­sius or be­low — raises some is­sues:

Aero­dy­namic ac­cep­tance — In gen­eral, the colder the fluid, the more vis­cous it be­comes. At some point,

the fluid will be­come too vis­cous to ad­e­quately shear or flow off the air­craft. This cre­ates an un­ac­cept­able aero­dy­namic sit­u­a­tion.

Freez­ing point — If the freez­ing point of the fluid is within the freez­ing­point buf­fer (10 de­grees Cel­sius above the OAT for Type I, or 7 de­grees Cel­sius above the OAT for Type II, III or IV), the fluid can­not be used.

Low­est op­er­a­tional use tem­per­a­ture, or LOUT — Aero­dy­namic ac­cep­tance and freez­ing point are both con­sid­ered in the con­cept known as LOUT. The LOUT is the cold­est air or air­craft skin tem­per­a­ture at which a de­ice or anti-ic­ing fluid can be used.

Holdover Times

1. Ground ic­ing pro­ce­dures are specif­i­cally de­tailed in the OpSpecs and op­er­a­tions man­ual of ev­ery cer­tifi­cate holder that chooses to op­er­ate dur­ing ground ic­ing con­di­tions.

Holdover time (HOT) is the es­ti­mated length of time that de­ic­ing or anti-ic­ing fluid will pre­vent the ac­cu­mu­la­tion of ice, snow or frost on the air­craft. “HOT ta­bles” are used to de­ter­mine the le­gal re­quire­ments for take­off un­der var­i­ous con­di­tions.

Holdover time be­gins when the fi­nal ap­pli­ca­tion of de­ice or anti-ice fluid starts, and ex­pires when the fluid loses its ef­fec­tive­ness.

Holdover time may be ex­ceeded when at least one of the fol­low­ing con­di­tions ex­ists: a. A vis­ual check within five min­utes of take­off de­ter­mines that the air­craft is free of ice, snow or frost. b. An oth­er­wise FAA-ap­proved pro­ce­dure (in­cluded in the op­er­a­tions man­ual) is used to de­ter­mine that the air­craft is free of frost, ice or snow. c. The air­craft is re-de­iced and a new holdover time is de­ter­mined.

Newly de­vel­oped tech­nolo­gies for ground de­ic­ing, such as liq­uid wa­ter equiv­a­lent (LWE), aka holdover-time de­ter­mi­na­tion sys­tems (HOTDS), are highly au­to­mated sys­tems that mea­sure win­ter pre­cip­i­ta­tion quan­tity and tem­per­a­ture to de­ter­mine an LWE value. This LWE value is com­pared to a data­base of de­ice and anti-ice fluid prop­er­ties to de­ter­mine the pre­cise mo­ment in time when the de­ice or anti-ice fluid will no longer be ef­fec­tive. This in­for­ma­tion is trans­mit­ted to the cock­pit via air­craft com­mu­ni­ca­tions ad­dress­ing and re­port­ing sys­tem (ACARS) or other means so that the flight crew al­ways knows how long the fluid ap­plied to their air­plane will re­main ef­fec­tive.

NOTE: A vis­ual check of the wings within five min­utes prior to take­off is re­quired whether holdover times have been ex­ceeded or not.

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