Aircraft De-Icing/Anti-icing Fluids Basic Characteristics & Fluid Degradation Considerations

SofemaOnline (SOL) www.sofemaonline.com looks at Aircraft De-Icing /Anti-Icing Basic Criteria

Introduction

De-icing Fluids are typically ethylene glycol, or propylene glycol-based fluids containing water, corrosion inhibitors, wetting agents, and dye.

Anti-icing Fluids are similar in composition except that they also contain polymeric thickeners. They are formulated to prevent the formation of unabsorbed frozen contamination for a longer period of time than de-icing fluids; however, the protection is still for a limited period of time. (refer to “Hold Over Time” for details related to useful time availability.)

De-icing fluids are typically composed of either:

»  Ethylene Glycol (EG) 
»  Propylene Glycol (PG) - Also less toxic
»  Additionally, other constituents include

o   Thickening agents

o   Surfactants (wetting agents),

o   Corrosion inhibitors,

o   Colourants

o   UV-sensitive dyes

Both Ethylene Glycol (EG) and Propylene Glycol (PG) are known as freezing point depressants (FPD),

Types of De-Icing /Anti-icing Fluids

Type I Fluids

These have a low viscosity and are considered "unthickened".

» Type 1 fluids provide only short-term protection because they quickly flow off surfaces after use.
» They are typically sprayed on hot (130-180 °F, 55-80 °C) at high pressure to remove snow, ice, and frost.
» Оften they are dyed to permit identification during application.

Type II Fluids

Type II Fluids are described as pseudoplastic, which means they contain a polymeric thickening agent to prevent their immediate flow off aircraft surfaces.

»  Type II prevents snow, ice, or frost contamination from adhering to the aircraft from the apron to take-off.
»  Typically a Type II Fluid Film will remain in place until the aircraft attains approximately 100 knots at which point the viscosity breaks down due to shear stress.
»  The high speeds required for viscosity breakdown mean that this type of fluid is useful only for larger aircraft.

Note Type II fluid is often replaced with Type IV due to its increased holdover times Type II Fluids are generally Clear & Type IV are Typically Green in colour.

Type III Fluids

Type III fluids are intended for use on slower aircraft, with a rotation speed of fewer than 100 knots. Type III fluids are generally bright yellow in colour.

Type IV has the same purpose and meets the same AMS standards as Type II fluids, but they provide a longer holdover time. They are typically dyed green to aid in the application of a consistent layer of fluid.

Considering Anti-Icing Fluids

De-icing fluids Types II, III, and IV contain thickeners and are also known as anti-icing fluids because they are used primarily to prevent icing from re-occurring after an initial de-icing with either Hot Water, a Mixture of Hot Water & Type 1 Fluid or Heated Type 1 Fluid.

Typical De-icing /Anti-icing Additives

»  Benzotriazole and methyl-substituted benzotriazole used as corrosion inhibitors.
»  Alkylphenol and alkylphenol ethoxylates, non-ionic surfactants used to reduce surface tension.
»  Triethanolamine used as a pH buffer.
»  Dyes are used to aiding in identification.

Application of De-icing /Anti-icing Fluid

A Combination of the heat contained by the Type l (deice) fluid together with and hydraulic forces (high-pressure spray equipment) is used as the first step to removing the frozen contaminants. (De-icing)

Storage Considerations De-icing /Anti-icing Fluids

»  The storage temperature limits shall comply with the manufacturer’s guidelines.
»  The stored fluid shall be checked routinely to ensure that no degradation/contamination has occurred.

Pilot Responsibility related to Acceptance of the Aircraft Following De-Icing / Anti-Icing

Take-off must not be attempted until the Pilot in Command (PIC) has determined that all critical surfaces of the aircraft are free of frost, ice or snow contamination.

»  The operator is ultimately responsible for ensuring that only qualified fluids are used.
»  If the colour of the fluid being applied to the aircraft is NOT the colour anticipated, the procedure should be stopped and the situation investigated.

Lowest Operational Use Temperature (LOUT)

The LOUT is the lowest temperature at which a de/anti-icing fluid will adequately flow off aircraft critical surfaces and maintain the required anti-icing freezing point buffer which is

»  7 °C (13 °F) below OAT for SAE Type II, Type III, and Type IV fluids and
»  10 °C (18 °F) below (OAT) for SAE Type I fluids

Note: The anti-icing and aerodynamic properties of thickened fluids may be seriously

degraded by, for example, inappropriate storage, treatment, application, application

equipment and age.

Concerning Residue Build Up

Aeroplane de-icing and anti-icing fluids can leave residue in critical areas in the wings and stabilizers. This residue can rehydrate and expand into a gel-like material that can freeze during flight and cause restrictions in the flight control systems.

Therefore, attention to this residue should be part of a regularly scheduled inspection and cleaning process.

Note - Using a two-step de-icing/anti-icing process helps to reduce the amount of fluid residue that forms in the wings and stabilizers. (the use of heated Type I fluid/water high-pressure washing may alleviate the occurrence of fluid dry out)

Single Step - Dual Application Process

In Europe, it is common for a one-step de-icing / anti-icing process to be employed

This process involves the application of de-icing/anti-icing fluids in a single application for example

»  Using a heated mixture of Type II fluid and water, usually in a ratio of 75/25.

In North America, a two-step process is commonly used. This process involves de-icing with heated Type I fluid, or a heated mixture of Type I fluid and water, which is followed by an application of Type IV anti-icing fluid.

Note 1  - Experience and testing have shown that de-icing with heated Type I fluid will help clear away residue from previous anti-icing fluid treatments.

Note 2  - Early research also indicates that the interaction between aeroplane de-icing/anti-icing fluids and runway de-icing fluids may contribute to the formation of residue gels. (When these fluids combine, the separation of the anti-icing fluid thickeners may be enhanced, leading to a more rapid formation of the residue.)

Next Steps

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