Appendix 3
PHASE DIAGRAM OF AQUEOUS GLYCOL SOLUTIONS 40 30 CAUTION: THIS DIAGRAM IS NOT
REPRESENTATIVE OF ANY
COMMERCIALLY AVAILABLE 20 AIRCRAFT GROUND DEICING
OR ANTI-ICING FLUIDS -10 10 -20 TEMPERATURE, oF •DIETHYLENE GLYCOL TEMPERATURE. °C
ETHYLENE GLYCOL
- 30
• PROPYLENE GLYCOL -40 T EUTECTIC -50 POINT -70 -
- 60 10 20 30 40 50 60 70 80 90 100 100% WATER 100% FPD FPD MIXTURE WITH WATER, % BY WEIGHT (FURE UNDILUTED) PIGURE 3-1 Par 5
Appendix 3 freeze point to be not greater than -25°F. However, it is very important for the user of the fluid to know the specific characteristics of the fluid being used.
6. CAPABILITIES AND LIMITATIONS OF FREEZING POINT DEPRESSANT (FPD) AIRCRAFT GROUND DEICING FLUIDS.
a. General. The basic philosophy of using FPD fluids for aircraft deicing is to decrease the freezing point of water either in the liquid or crystal (ice) phase. FPD fluids are very soluble in water. However, the rate at which ice will absorb FPD or melt is very slow. If frost, ice, or snow is adhering to a wing surface, that ice formation may be melted by repeated application of proper quantities of PPD fluid. This process can be significantly accelerated by thermal energy of heated fluids. As the ice melts, the FPD mixes with the water thereby diluting the FPD. As dilution occurs, the resulting mixture may begin to run off. If all the ice is not melted, additional application of FPD becomes necessary until the fluid reaches the ice/aircraft surface interface. When all ice has melted, the liquid residue that remains is a mixture of water and FPD.
The remaining film could freeze (begin to crystallize) with only a slight temperature decrease. Slight temperature decreases could be induced by many factors such as cold soaked fuel in wing tanks, reduction of solar radiation by cloud obscuration or the sun, ambient temperature cooling, wind effects, temperature depression during development of wing lift and other factors. If the freeze point of the remaining film is found to be insufficient or is unknown, a fresh coat of FPD fluid should be added before the aircraft is released for flight. Practice developed and accepted by the air carrier industry is to assure that the remaining film has a freezing point less than 20°F below ambient temperature. This entire process can be accomplished by any means available; i.e., pouring, mopping, brushing, or spraying. Freeze point of the remaining FPD film can be measured at specific locations on a surface using refractive index techniques.
b. Nonprecipitation Conditions.
(1) The above description is intended to describe the capabilities and limitations of FPD for deicing (getting the aircraft clean), in a very general way. As mentioned earlier there are many other methods of removing ice, frost, or snow deposits. Large quantities of FPD would be necessary to try to melt or timely, and snow and the use of this method alone may not be cost effective This is especially true with regard to snow deposits. The deicing process can possibly be accomplished more cost effectively by combining manual methods to first remove soft snow and then using the FPD fluid to remove any residual ice. In some instances, where the snow is not adhering to surfaces, all snow may be removed by sweeping, brushing, or blowing with cool air (either shop air or other pressurized air sources). Hot air should not be used to attempt to remove dry snow. In clear (nonprecipitation) weather, once proper inspection has been perfomed, no further action may be necessary.
(2) Where modern ground support equipment (GSE) is available, common practice is to use water heated to approximately 180°F and glycol heated to approximately 150°F. Some GSE manufacturers recommend that hot water alone be used to melt and remove snow, frost, or ice formations. This is possible Par 5 9
Appendix 3 because of the heat applied, as well as the mechanical force of the stream tending, to melt, loosen, and flush ice formations from the aircraft.
(3) Some manufacturers recommend that pure hot water can be used to deice the aircraft at ambient temperatures above +26°F. Below that temperature or if ice formations occur, sufficient FPD mixed with the water is recommended to prevent freezing during the deicing process. In the latter case, as discussed before, the freeze point of the remaining film may be unknown. Once the aircraft is clean (deiced) a final coat of FPD should be added to prevent freezing due to temperature depression by any of the factors mentioned earlier.
If hot water alone is used for deicing, all surfaces should be coated with a solution of FPD to assure that the freezing point of remaining films will be no greater than 20°F below ambient or surface temperature whichever is lower.
(4) under conditions of precipitation or frost to reduce the workload and time required to prepare an aircraft for flight. Experience has shown that if ice formations are not allowed to adhere to an aircraft surface they are easier to remove. If ice formations can be prevented from forming, only final inspections need to be performed under some conditions. Under other conditions, however, further treatment may be required to assure that the residual film has a freezing point sufficiently low to prevent ice crystals from forming during taxi and takeoff.
c. Conditions of Freezing Precipitation.
(1) General.
and limitations of FPD fluids in clear (nonprecipitation) weather conditions.
This paragraph addresses the capabilities and limitations of FPD fluids under veather conditions where ice, frost, or snow may form. FPD fluids have the capability of preventing the formation of frost, ice, or snow for a finite period of time. The time of protection depends upon many factors and is therefore unpredictable. The primary factors that influence this time of protection are ambient temperature, surface temperature, FPD fluid film thickness, FPD mixture strength, and the rate of addition of moisture. These factors are influenced by many other variables such as solar radiation, relative humidity, surface contour, surface inclination angle, surface roughness, wind effects, fluid application procedures, residual moisture on the surface prior to deicing, etc. But to bring the physical process into perspective one must visualize a simple case where only a few of these variables are examined.
As discussed earlier, once the deicing process is completed, a thin film of deicing fluid remains on the surface and this film contains a mixture of FPD and water. The ratio of water to FPD is unknown and therefore the freezing point is unknown. Assume that a very rich mixture of fluid is uniformly added to the surface in a manner that most moisture is flushed away while realizing that some water will always remain in the solution. Also assume that the fluid film that remains is 80% ethylene glycol. Under these assumptions, the question then becomes "How much water must be added to this remaining film to cause the freeze point of the fluid to reach that of ambient temperature? To answer this question, test results have shown that when 10 Par 6
Appendix 3 glycol and 84 percent water will begin to crystalize.
(2) Snow Conditions.
(i) Precipitation rates in snow conditions have been found to contain water in amounts up to 0.7 in/hr. A general rule of thumb, commonly used by meteorologists, is that the depth of snow is about 10 times the depth of melted snow. This rule of thumb, although useful for some purposes, could be misleading, since snow densities vary widely. Very dry snow can have a density of 30:1 (30 inches of snow contains 1 inch of water) and very wet snow can have a density of 5:1. In snow conditions, the primary influence upon FPD life is the rate of addition of water and subsequent dilution of the FPD fluid. If snow is falling at a rate of 1/2 inch per hour and if the water content of this snow is 0.1, then the rate of addition of water to the FPD will be 0.05 in/hr.
(ii) If we assume that as the falling snow contacts the surface film it melts and the resulting water mixes thoroughly with the fluid contained in the film, the strength (glycol to water ratio) of 16 percent will be reached in a specific time. This time can be calculated using rationale and equations given in appendix 2.
(iii) From this assessment we can see that at a snowfall rate of 1/2 inch per hour onto a wing having been coated with a very rich mixture, 100 percent ethylene glycol, at an ambient temperature of 20°F, crystallization may begin in approximately 25 minutes. Figure 3-2 presents the estimated life of deicing fluids as a function of ambient temperature for several snowfall rates.
Extreme caution is emphasized that these estimates should not be used to attempt to estimate the time available from deicing or anti-icing to takeoff, as noted clearly on the chart. The reason for this caution is that these estimated times are influenced by many variables, and figure 3-2 takes into account only a few.
These variables are emphasized and their effects discussed in paragraph 7 of this appendix.
Par 6 11
Appendix 3 40 CAUTION: THIS REPRESENTS ONE EXAMPLE OF THE USE OF AN FPD FLUID.
DO NOT USE THIS CHART FOR 30 ESTIMATING TIME AVAILABLE TIME TO CRYSTALLIZATION (MIN) ASSUMPTIONS: FROM DEICING TO TAKEOFF.
6p = 0.05 mm 20 M, = 60% ETHYLENE GLYCOL Rw = 0.1 Rs
SNOW FALL RATE 10 1/4 in/hr (6.35 mm/hr) 1/2 in/hr (12.7 mm/hr) 1 in/hr (25.4 mm/hr) 10 15 20 25 30 TEMPERATURE °F FIGURE 3-2
(iv) It is emphasized that the above rationalization regarding the finite life of FPD fluids is simplified for understanding purposes. Other physical processes occur within an FPD fluid film in the dynamic condition of precipitation or addition of moisture by any means. It is imperative that this process be understood by all associated with winter operations of aircraft.
(v) To assist the understanding of this process, figure 3-3 depicts a surface containing a film of FPD fluid.
12 Par 6
AC 20-117 12/17/82 Appendix 3 H20 FROM SNOWFLAKE #2
ICE CRYSTAL H20 FROM MELTED SNOWFLAKE #1
FPD FLUID
AIRCRAFT SURFACE FIGURE 3-3
(vi) As snow contacts the upper surface of this film it melts and the water, begins to mix with the FPD fluid. It is known that glycols are very soluble in water but it is also known that to assure a homogeneous fluid, the mixture must be agitated or stirred. So when a snowflake or other ice crystal, supercooled rain drop, or water in any form contacts the fluid iilm surtace, a Einite time is required for the water to mix with the fluid. During this time, the water remains somewhat in pure form or a weak aqueous solution of FPD. If during this process, before the water from a snowflake mixes with the fluid, another snowflake impacts the surface, as depicted in figure 3-3, a portion of the snowflake may not melt immediately but will remain in ice-crystal form.
This process has been observed during simulated snow testing. As snow continues to impact the surface, ice crystals can be seen to form on localized areas of the surface of the FPD fluid. Whether or not these ice crystals will bond to the aircraft surface depends upon many factors, such as: fluid tilm thickness, PD fluid strength, surface temperature, ambient temperature and temperature or snow, rain, or ice crystals impacting the surface. Another factor is the latent heat of fusion; however, this effect is slight in comparison to heat transfer to ambient air or to massive aircraft structures. As a snowflake impacts the FPD surface and melts, heat is transferred from the fluid to the ice crystal. This reduces the surface temperature and, as precipitation continues, the rate at which snow will melt is reduced.
(vii) If the FPD film is sufficiently thick, these surface ice crystals may initially be localized on or near the surface or be floating on the surtace. But as the process continues, water being added will gradually mix with the FPD fluid and, in time, the fluid in the surface film will reach an aqueous solution that will freeze (become slush) at the prevailing temperature Par 6 13
Appendix 3 and with time become frozen or adhere to the aircraft surface. As the quantity of slush increases, the viscosity of the surface film increases, requiring greater forces to cause the surface slush to be blown off of aircraft surfaces.
Once the crystallization process begins, the chances of proper blow-off during takeoff roll are minimal. In addition, once the crystallization process begins, the surface may be expected to be a rough texture which can adversely impair aircraft performance and flight characteristics. It is also very important to understand that many varieties of snow occur in nature and each will result in differing characteristics (surface textures or roughness) during the process of dilution and crystallization of FPD fluid mixtures.
(3) Frost Conditions.
(i) Frost is formed from the sublimation of water vapor onto a surface where the temperature is below freezing. A method of retarding the formation of frost on aircraft surfaces is by applying a film of FPD fluid.
in previous examples, the maximum thickness of fluid that can be expected to be retained is 0.10 mm. A typical heavy frost condition will produce a rate of accumulation of liquid water (in the form of frost) of 0.0008 in/hr or 0.02 mm/hour. Again, we may, as an example, estimate how long the protective illm or FPD fluid may last and assess the condition of the fluid as a function of time using rationale and methods contained in appendix 2. The rationale used for snow applies as well for frost since the FPD fluid will absorb the water rather than allow formation of frost, i.e., until the FPD fluid mixture is diluted to its freeze point. Then frost can begin to accumulate. However, it is emphasized that, under frost conditions, moisture will be absorbed by FPD fluids at a faster rate than moisture would be deposited in the form of frost. This is
(ii) Assume that the ambient temperature is 20°F and the mixture that will begin to freeze at that temperature is 16 percent FPD fluid. In this case, we have assumed that a 50 percent solution of ethylene glycol and water was applied to a clean dry surface prior to the beginning of the dew or frost process. This example would be representative of a heavy frost condition, but it is emphasized that film thickness also is an extreme case. The estimated time of effectiveness for this example is 10.4 hrs. It is emphasized that hygroscopic effects and other variables can significantly reduce this time.
(iii) Figure 3-4 provides the estimated life of FPD fluids for various frost conditions. Extreme caution is emphasized that these estimates should not be used to attempt to estimate time available from deicing or anti-icing to takeoff, as noted clearly on the chart. Many other factors must be considered, such as type of FPD fluid, surfaces to which the fluid is applied, surface temperature, and other variables mentioned earlier. In addition, it is important to realize that when the FPD fluid has been diluted to the freeze point, crystallization has begun.
temperature could result in further crystallization and dangerous surface roughness. After the frost protection process has been utilized, the aircraft surfaces affected should be thoroughly cleaned and inspected to assure that applies to conditions where ambient temperature has increased since the application of the initial or repeated anti-frost solutions, because surface 14 Par 6
Appendix 3 temperature depression on the upper surface of a wing producing lift can be significant. Temperature depression is a function of wing design, airspeed, and the amount of lift being produced. The aircraft manufacturer can provide specific values of temperature depression for their aircraft. Care should be taken to assure greater than 20°F below ambient temperature whichever is lower.
20 CAUTION: THIS REPRESENTS ONE EXAMPLE OF THE USE OF AN FPD FLUID.
DO NOT USE THIS CHART FOR
ESTIMATING TIME AVAILABLE
FROM DEICING OR ANTI-ICING TIME TO CRYSTALLIZATION (HOURS) 10 15 TO TAKEOFF.
FROST 5 0.01 mm/hr
0.02 mm/hr 0.03 mm/hr ASSUMPTIONS: óp = 0.05 mm M, = 60% ETHYLENE GLYCOL 5 10 15 20 25 30 TEMPERATURE °F FIGURE 3-4
(4) Freezing Rain and Drizzle Conditions.
(i) Precipitation rates in freezing rain and drizzle conditions have been found to vary from about 0.3 to 5.0 mm per hour or about 0.01 to 0.20 in/hr. These conditions can occur at ambient temperatures between 32°F and Par 6 15
Appendix 3 approximately 12°F. Freezing rain and drizzle water droplets are in the supercooled liquid state and freeze upon impact with other objects. Because of the large size of freezing rain droplets (400 to 1300 microns) the ice produced is usually glaze ice. When an aircraft surface is coated with a film of FPD fluid, freezing rain and drizzle droplets impacting the fluid film will mix readily with FPD fluid, causing rapid dilution and under certain conditions can wash away the FPD fluid. The effectiveness of FPD fluids to provide protection in freezing rain conditions can be estimated using the techniques contained in appendix 2. Under the most severe freezing rain conditions (5.0 mm/hour or about 0.2 in/hour) crystallization would be expected to begin in less than 11 minutes at 25°F if pure ethylene glycol were used and if FPD fluid film thickness is assumed to be 0.1 mm. Airline operational experience indicates that FPD fluids are short-lived in most freezing rain conditions. In many instances, one side of the aircraft already deiced will begin to refreeze while the other side is being deiced. Figure 3-5 provides estimated FPD fluid life as a function of ambient temperature for several freezing rain precipitation rates.
Extreme caution is emphasized that this chart should not be used to attempt to estimate time available from deicing to takeoff.
20 CAUTION:
THIS REPRESENTS ONE
EXAMPLE OF THE USE OF AN FPD FLUID. DO NOT
FREEZING RAIN RATE
USE THIS CHART FOR TIME TO CRYSTALLIZATION (MIN) 15 0.01 in/hr (0.25 mm/hr) ESTIMATING TIME AVAILABLE FROM
DEICING TO TAKEOFF.
10 0.02 in/hr (0.51 mm/hr) 5 0.04 in/hr (1.02 mm/hr)
0.10 in/hr (2.54 mm/hr) ASSUMPTIONS:
= 0.05 mm
0.20 in/hr (5.08 mm/hr) M1 = 60% ETHYLENE GLYCOL Л 5 10 15 20 25 30 TEMPERATURE °F FIGURE 3-5 16 Par 6
Appendix 3
(ii) It is emphasized that aircraft ice protection systems are designed basically to cope with the supercooled cloud environment (not freezing rain). Supercooled cloud water droplets have a median volumetric diameter (MVD) of 5 to 50 microns. Freezing rain MVD is as great as 1300 microns. Large droplets of freezing rain impact much larger areas of aircraft components and will, in time, exceed the capability of most ice protection equipment. Flight in freezing rain should be avoided where practical.
7. VARIABLES INFLUENCING THE EFFECTIVENESS OF AIRCRAFT GROUND DEICING FLUIDS.
Many variables can influence the effectiveness of FPD fluids for deicing and the life of these fluids, especially under conditions where moisture is being added to the fluid. This section is intended to provide a basic understanding of these variables and their influences. Each of these variables is discussed in the following subparagraphs.
a. Chemical Composition of FPD Fluids. FPD fluids intended for use in deicing or anti-icing aircraft are manufactured and distributed by a wide variety of companies either to manufacturer or customer specifications. Each may contain a combination of chemicals that have various freeze characteristics.
Although each FPD fluid is manufactured within tolerances contained in the specification, batches may differ and may have significant variations in characteristics.
b. Freezing Characteristics of FPD Fluids. These characteristics were discussed in detail in a previous paragraph of this appendix. It is emphasized here that before a fluid is used on an aircraft, the freeze characteristics should be known by the user. A method of determining this is by thorough understanding of fluid procurement specifications and tolerances and quality control inspections or sampling by either the manufacturer or the user. In addition, FPD fluids are either premixed (diluted with water) by the manufacturer or mixed by the user from bulk supplies. To assure known freezing characteristics, samples of the final mixture should be analyzed.
c. FPD Fluid Strength When Applied.
(i) Fluid strength or the ratio of FPD ingredients, such as ethylene glycol to water, may be known if proper precautions are taken prior to application such as those outlined above. It is important, however, to realize that fluid strength is a very significant factor in deicing properties as well as the time that the FPD fluid may remain effective (anti-icing).
(ii) Figure 3-6 presents estimates of ethylene glycol life as a function of ambient temperature for several commonly used fluid strengths. It can be seen that as fluid strength is increased, the time of effectiveness increases dramatically. inis figure is presented for reierence purposes only to illustrate the variation of FPD fluid life and should not be used for flight planning purposes. It is also emphasized that pure (100 percent) ethylene glycol fluids should not be used in nonprecipitation conditions, since the freeze point is much higher than fluids diluted with water, as illustrated in figure 3-1. Common practice of the aviation community is never to exceed the eutectic point when using ethylene glycol base fluids.
Par 6 17
Appendix 3 50 CAUTION: THIS REPRESENTS ONE EXAMPLE OF THE USE OF AN FPD FLUID.
40 - DO NOT USE THIS CHART FOR
ESTIMATING TIME AVAILABLE FROM DEICING TO TAKEOFF.
TIME TO CRYSTALLIZATION (MIN) 30 ASSUMPTIONS:
d = 0.05 mm (1.97 x 10-3 in) Rs = 1/2 in/hr RW = 0.1 Rs 20 10 M, = 100% M1 = 70% M, = 50% -M, = 40% • -M, = 30% 5 10 15 20 25 30 TEMPERATURE °F FIGURE 3-6
d. Residual Moisture on the Surface. If a known (quantified strength) FPD fluid is added to a clean dry surface, the residual strength can generally be relied upon to remain at that strength if precipitation is not in process.
However, if residual moisture (from any source; e.g., dew, melted ice, precipitation, etc.) is present on the surface, that moisture can be expected to 18 Par 7
Appendix 3 further dilute the FPD fluid being used. This will change the freeze characteristics and influence effectiveness time of the final film of FPD fluid.
e. Fluid Film Thickness. After FPD fluid has been added to an aircraft surface, a thin film of FPD fluid will remain on the surface. This fluid film thickness has been found to vary from 0.002 mm to about 0.100 mm and is influenced by many parameters, but primarily surface and fluid temperature, fluid viscosity, suriace contour, surface inclination angie, and suriace tension. Film thickness (on a finite area of the surface) influences the effectiveness of the fluid. The effectiveness of the FPD fluid will be different on various parts of the aircraft. Figure 3-7 presents an empirically derived relationship for estimating fluid film thickness as a function of several of these variables. It is emphasized that FPD fluid film thickness can vary significantly in the vicinity of surface discontinuities such as seams, cowling, inspection plates, rivets and screws, etc.
0.060
0.050 = FILM THICKNESS (mm) 0.040
0.030 dr = 0.0267 = 0.000 Ve sin € WHERE:
0.020 • = FLUID VISCOSITY (Ib/ft-hr) @ = FLUID DENSITY (lb/fr) 8 = INCLINATION ANGLE (degrees)
0.010
0.000
1.0 2.0 3.0 4.0 sin 8 FIGURE 3-7
f. FPD Fluid Application Techniques. Generally, FPD fluids, given time, will seek an equilibrium film thickness regardless of the method of application if the fluid is applied in a uniform manner using sufficient quantities of fluid. All FPD fluids in common use contain wetting agents to aid in this process. If sufficient fluid is not added to the surface, desired film thickness may not be achieved. If the FPD fluid is not uniformly applied, localized areas may be left with less than desired film thickness.
conditions of precipitation, if the rate of FPD application is slow, areas Par 7 19