Winter Operations

De-icing/Anti-icing Review

Overview

• Clean Aircraft– To de-ice, anti-ice or

both

• Fluids• Methods– Contamination Removal

(De-ice)– Contamination

Prevention (Anti-ice)

• Holdover tablesCold weather operations require a vigilant eye to ensure a clean aircraft. When necessary, de-icing and anti-icing procedures must be properly executed.

Achieving a Clean Aircraft

The “clean aircraft concept” requires the critical surfaces be clear of all contaminates such as ice and frost prior to takeoff. That means you need to remove any accumulated contaminates and also ensure that they will not accumulate prior to takeoff.

Applying anti-ice fluid to the aircraft forms a protective shield against contamination for varying amounts of time.

Remove Protect

Fluid Types

Deicing formulas are categorized from Type I through Type IV, depending on fluid composition and application procedure.

Type I Type II Type III Type IV

The four fluid types can be identified by color

Type I Fluids

Low-viscosity Type I deicing fluids flow quickly from aircraft surfaces, making them ideal for light applications and composite surfaces.

Primarily a De-icer

Un-thickened/Low viscosity – Usually 50% water and either ethylene glycol or

propylene glycol– Can penetrate cracks/crevices to dissolve

contaminates

Heated before spraying (usually 600C/1400F)

Limited anti-ice protection

Type II III & IV Fluids

While the same fluids used for deicing are also used for anti-icing, SAE types II, III, IV fluids are more typically used for anti-icing because they are thickened to stay on the airplane and thus provide longer HOT protection. Type II and Type IV fluids are often used during heavy snowfall. Type III, a compromise between Types I deicing and Type II anti-icing fluids, is used for smaller, metal-skinned aircraft.

Primarily Anti-icersMost effective when applied unheated and undiluted to a clean airplane surface

Viscosity

Viscosity can be thought of as the thickness of the fluid. Honey has a much higher viscosity than water for example. Thickening agents can be added to anti-icing fluids to make them more viscous. The more the viscosity of the fluid, the longer the holdover time.

Viscosity and Holdover Time

The anti-icing fluid is only useful on the ground to prevent ice buildup. During the takeoff roll, it should bleed off the aircraft surfaces to allow clean airfoils. The more viscous the fluid, the higher the necessary rotation speed to bleed off the fluid.

TYPE VISCOSITY MIN TAKEOFF ROTATION SPEED

I Low None

III Medium 60

II High 100-110

IV High 100-110

Increasing holdover

time

De-icing Methods

• Fluids– Water (Heated or Cold)– Heated mixture of water and Type I fluids– Heated mixture of water and Type II or Type IV fluids

• Mechanical or manual– Brush or Broom

• Put aircraft in a heated hangar– Dry the aircraft surfaces after the ice melts

• Infrared Heat– Gas Heaters in a “drive-under” structure– Does not heat the air, but penetrates the frost/ice to melt it

De-icing removes accumulated frost, ice, or snow from an airplane, typically through the application of hot water or a hot mixture of water and de-icing fluid. Although there are other approved methods for de-icing, such as infrared heat or hot air, the primary method is the use of fluids.

JFK infrared station

Anti-icing Methods

• Mixtures of water and Type I fluids

• Type II or Type IV Fluids applied as unheated concentrate

• Mixture of water and Type II or Type IV Fluids

Once the critical surfaces are clear, it is necessary to anti-ice the aircraft if icing conditions still can cause surface contamination prior to takeoff. Anti-icing prevents ice from refreezing or reforming on the critical surfaces for a specified amount of time.

Fluid De-icing/Anti-icing Methods

• One application to both de-ice & anti-ice– Typically a heated mixture of

thickened fluid and water is applied

– The heated fluid is used to de-ice the aircraft surfaces

• Provides limited anti-ice capability

• Step one: de-ice – This process involves deicing with

heated Type I fluid, a heated mixture of Type i fluid and water, or a heated mixture of water and thickened (Type II, III, or IV) fluid

• Step two: anti-ice– application of thickened fluid for

anti-icing protection.

• Provides maximum anti-icing capability

Depending on the situation, fluid application is done in one or two steps. When anti-icing capability is needed, a two step process is always recommended. Let’s have a look at a few reasons.

One Step Method Two Step Method

The Case for Two Step Method

A one step method is quicker and often sufficient. However, a two step method is necessary at times for maximum holdover time and to ensure that first any fluid residue is removed before anti-icing again.

• The repeated application of Type II and IV fluids, without the subsequent application of type i fluid or hot water may cause a residue to collect in aerodynamically quiet areas. This residue may rehydrate and freeze under certain temperature, high humidity and/or rain conditions. This residue may block or impede critical flight control systems and may require removal

• Landing on runways where deicing fluid has been applied can leave a residual on the aircraft surfaces. You need to perform the two step process to remove this residual fluid as a means of protection for your next flight

• You need maximum holdover time

One Step Considerations

Use a heated mixture of water and either type I, II, or IV fluids when performing the one step method. Consider the weather conditions, holdover protection time necessary and condition of the airplane to determine the quantity of water necessary.

• You need sufficient fluid temperature and flow rate to flush the ice and snow from the airplane surfaces– More water increases the flow rate for better removal but makes the

fluid less viscous, lowering holdover time

• If there is only snow on the airplane and the temperature is 340 F or higher, stable and on the increase, then water may be used to remove the snow

Note: FAA POI’s strongly encourage use of type II or type IV anti-icing fluid, during light freezing rain or freezing drizzle.

Guidelines

The winter operations manual contains guidelines for fluid and mixture selections. Note mixing of type II or type IV fluids from the same or different manufacturers is not allowed.

Holdover Table Overview

• Current Weather Conditions– Outside temperature– Type and intensity of

precipitation• Fluid

– Type of fluid– Dilution of fluid– Time started

To use the holdover time tables you will need to know several aspects of the current weather condition, as well as the fluid application. Be sure and read the cautions at the bottom of the holdover time table.

767 Composite Surfaces

The holdover tables have charts for predominantly aluminum and predominately composite aircraft surfaces. Even though there are composite surfaces, the 767 obviously has predominately aluminum surfaces.

Establishing HOT Range

• Snowfall rate– The relationship between visibility and snowfall intensity is analyzed

and contained in the Snowfall Intensity Chart

• Precipitation rates for other than snow– Due to the wide range of possible precipitation rates, the only

acceptable decision criteria time for all other precipitation is the shortest time within the applicable HOT cell. The shortest time in the HOT cell correspond to the highest precipitation rate

• With a one step process, the holdover time begins at the start of the operation. With a two step process, the holdover time begins at the start of the second stepHoldover time guidelines in the tables give an indication as the time frame of protection that could reasonable be expected under conditions

of precipitation. However, due to the many variable that can influence holdover, these times should not be considered as minimums or maximums as the actual time of protection may be extended or reduced, depending upon the particular condition existing at the time.

Snowfall Intensity vs. Visibility Chart

The snowfall intensity vs. visibility chart can give you a snowfall intensity, if you don’t know it. It can also refine the snowfall intensity reported in the METAR to give you a more accurate holdover time. In fact, the holdover tables now require you to use it. Let’s look at an example.

METAR Snowfall Intensity

You are planning on de-icing using Cryotech Polar Guard II type II fluid diluted at 75/25 at Churchill Airport. These are your current conditions. The METAR automatically displays light snow if it is snowing and the prevailing visibility is greater than ½ statue mile. Let’s use the chart to get a more accurate snowfall rate.

Determine Snowfall Intensity

It is daytime, visibility 10 miles with a temperature of 00 Celsius. That gives us very light snowfall instead of the light snowfall reported in the METAR. Note, we didn’t even need the precipitation from the METAR to get this. So if it is snowing, you only need the visibility and temperature to get the intensity.

Notice:Anytime the visibility is greater than 2 ½ miles, the intensity is considered very light

Holdover Tables

Be sure and select the correct holdover table according to the fluid used.

Holdover Time Calculation

If we are using the Cryotech Polar Guard II type II fluid, then our holdover time range is 2:25 to 2:55.

Holdover Time Use

If the existing weather conditions prevail for the next three hours, then we do not need an additional check if we takeoff before 2 hours twenty five minutes. Takeoff’s between 2 hours 25 minutes and 2 hours 55 minutes require a check. If we wait longer than that, we need to get another treatment.

Less than 2:25 no contamination check required

Between 2:25 and 2:55 consider contamination check

More than 2:55 contamination check required re-apply as necessary

Recall that the holdover time started at the BEGINNING of the final application, not when it was finished

Holdover Time Factors

The heavier the precipitation, the shorter

the HOT.

High winds or jet blast that cause the fluid to flow off, decreasing the protection afforded by the fluid layer.

Wet snow, which causes fluids to dilute and fail more quickly than dry

snow.

The use of incorrect equipment to apply fluids.

An airplane skin temperature lower than outside air temperature.

Direct sunlight followed by precipitation.

Heavy precipitation rates or high moisture content, high wind velocity or jet blast may reduce holdover time below the lowest time stated in the range and holdover times may also be reduced when aircraft skin temperature is lower than OAT.

Different Table Type

Note if we would have used this fluid, we do not have the light and moderate snow categories. In this case, the lower number is the holdover time for moderate snow and the upper number for light snow. So our holdover time with no check is 1 hour 10 minutes, after which we need a check. Heavy snow always requires a contamination check.

Generic Holdover Table

SAE Type is the generic table you use if you can’t find a specific table. It is the most conservative table for all brands.

METAR Snowfall Intensity

Finally let’s look at another example where the snow is reported as light, but the visibility is different. Here in St. Louis we have ½ sm visibility with light snow reported.

Determine Snowfall Intensity

With the same light reported snowfall as the previous example, our snowfall intensity is now Heavy. So we will need contamination check.

Notice:Anytime the visibility is less than ¾ miles, the snowfall intensity is Moderate or Heavy

De-icing Review Questions

It’s time for a few questions.

Question

Please select the best choice.

Select the best statement regarding viscosity and holdover time:

a) Type I fluid is the most viscous, with the least holdover time

b) Type II and IV are the most viscous, giving the longest holdover time

c) Type III is more viscous than type II and has a longer holdover time

d) Type I fluid is the most viscous giving it the longest holdover time

Question

Please select the best choice.

a) You need a Pre-takeoff check and a Contamination check

b) You need more information to determine holdover

c) There is no holdover time

d) Statements a & c are correct

The visibility is ¼ mile, select the best statement(s) ?

Question

Select the correct answer.

The METAR reports visibility at ¾ mile. Which chart would you use to determine the snowfall rate?

Question

Please select the best choice.

Which of these factors decreases the holdover time?

a) Dry snow, which causes fluids to dilute and fail more quickly than wet snow

b) An airplane skin higher than outside air temperature

c) Direct sunlight followed by precipitation

d) No wind

Question

a) It is not necessary to remove contaminates from control surfaces before anti-icing.

b) For maximum effectiveness and minimum consumption, the de-icing solution should be only lukewarm.

c) Apply only enough fluid to remove the ice and snow or to leave a light coating of fluid on the aircraft.

d) The greater the precipitation the longer the holdover time.

Select the correct statement:

Module Complete

You have come to the end of this module.