form of decreased thrust, decreased lift increased stall speed, trim changes, and altered stall characteristics and handling qualities;
• Various procedures available for aircraft ground deicing and anti-icing, including the use and effectiveness of FPD fluids;
• Capabilities and limitations of these procedures in various weather conditions;
• Critical areas of aircraft such as the wings and tail; and
• Recognition that final ensurance of a safe takeoff rests in confirmation of a clean aircraft.
FROZEN CONTAMINANTS Frozen contaminants in the form of ice, snow, or frost can accumulate on exterior surfaces of an aircraft on the ground. The type of accumulation on the aircraft surface is a key factor in determining the type of deicing/anti-icing procedure that should be used.
Ice, snow, and frost should be removed before takeoff. Dry, powdery snow can be removed by blowing cold air or nitrogen gas across the aircraft surface. Heavy, wet snow or ice can be removed by using solutions of heated FPD fluids and water or by mechanical means such as brooms and squeegees.
Frozen contaminants can also be removed from the surface of an aircraft by using FPD fluids. There are a number of FPD's available for use on commercial large transport category aircraft. The FPD's used most often are glycol-based fluids produced by a number of North American, European, and Russian chemical manufacturers.
DEICING AND ANTI-ICING FLUIDS Common practice, developed by the North American and European aviation communities over many years of experience, is to deice and anti-ice an aircraft before takeoff. Various techniques of ground deicing and anti-icing have been developed. The most common of these techniques is to use FPD fluids in the ground deicing process and to anti-ice with a protective film of FPD fluid to delay the reforming of ice, snow, or frost. Commercially available FPD fluids used for aircraft deicing are ethylene glycol or propylene glycol based. Today's FPD fluids have characteristics that are best defined by a phase diagram or treeze chart as shown in figure 1. The general characteristics of these fluids are described in table 1.
Note: Generally, the freeze characteristics of commercially available FPD fluids are based on the "neat" (undiluted premix) solution as furnished by the fluid manufacturer.
The basic philosophy of using FPD fluids for aircraft deicing is to decrease the freezing point of water in either the liquid or crystal (ice) phase. FPD fluids are highly soluble in water; however, ice is slow to absorb FPD or to melt when in contact with it. If frost, ice, or snow is adhering to an aircraft surface, the formation may be melted by repeated application of proper quantities of FPD fluid.
This process can be significantly accelerated by thermal energy from 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 applications of FPD become necessary until the fluid penetrates to the aircraft surface. When all ice has melted, the remaining liquid residue is a mixture of water and FPD. The resulting film could freeze (begin to crystallize) with only a slight temperature decrease.
Figure 1. Phase Diagram of Aqueous Glycol Solutions Caution: This Diagram Is Not Representative of Anv Commercially Available Aircraft Ground Deicing Type II
• Diethylene Glycol - Temperature (°F) - Ethylene Glycol. Temperature (°C
- Propylene Glyco 60 80 20 100
- 100% Water 100% FPD - (Pure, Undiluted) Pro Mixture win water, % FrO Dy weignt FPD Mixture with Water, percent FPD by Weight 12
Table 1. General Characteristics of Commercially Available FPD's Common Primary Active Viscosity Primary Notes (see AC 20-117 for Name Ingredients Use more complete information) North Ethylene glycol LOW Deicing Includes AMS** 1425, AMS American propylene glycol 1427, and Mil-A-8243 fluids.
May not meet SAE nor ISO Type I specs.
SAE Propylene/diethylene Low Deicing Propylene glycol based fluids Type I ethylene glycol not to be used undiluted at OAT < 14°F (-10°C).
ISO Aircraft performance changes Type I may result. AMS 1424 included. SAE, ISO specs similar.
SAE Propylene/diethylene High Deicing For use on aircraft with Vp > Type Il glycol with to and anti- 85 knots; lower viscosity than polymer thickener Low icing AEA*** Type Il produced ISO before 1988. AMS 1428 Type II included. SAE, ISO specs similar.
Mil-A- Propylene glycol Medium Deicing Less toxic to animals. Not to 8243D be used undiluted. Not similar Type I to Mil-A-8243C Type I or II.
Mil-A- 3 parts ethylene Low Deicing Similar to Mil-A-8243C Type I 8243D glycol, and II. Not approved as SAE Type Il 1 part propylene or ISO Type II.
glycol Arktika Ethylene glycol High Deicing Not approved as SAE or ISO (Russia) with thickener and anti-Type II. Considered thickened icing Type I. Effects on aerodynamics unknown to date.
**AMS - Aerospace Materials Specification ***AEA - Association of European Airlines 13
TRADITIONAL NORTH AMERICAN FLUIDS As shown in table 1, there are various types of FPD's available.
These fluids are produced by chemical manufacturers in North America and Europe. The FPD's used to deice aircraft in North America are usually composed of ethylene or propylene glycol combined with water and other ingredients. Users can purchase this deicing fluid in a concentrated form (80 percent-90 percent glycol) or in a solution that is approximately 50 percent glycol with 50 percent water by volume.
ISO COMMERCIAL FLUIDS These fluids were originally known as AEA Type I and Type II.
Specifications for these two types of FPD's are provided in the ISO guidelines as ISO #11075, "Aircraft deicing/anti-icing Newtonian fluids ISO Type I" and ISO #11078, "Aircraft deicing/anti-icing non- Newtonian fluids ISO Type II.i SAE Commercial Fluids.
SAE Type I and Type Il fluids are very similar in all respects to ISO Type I and Type Il fluids. The minor differences will not be presented in this AC. These FPD's, specified by the SAE and ISO as Type I and Type Il, are distinguished by material requirement, freezing point, rheological properties (viscosity and plasticity), and anti-icing performance.
SAE and ISO Type I Fluids.
These fluids in the concentrated form contain a minimum of 80 percent glycols and are considered "unthickened" because of their relatively low viscosity. These fluids are used for deicing or anti-icing, but provide very limited anti-icing protection.
SAE and ISO Type II Fluids.
These fluids contain a minimum of 50 percent glycols and are considered "thickened" because of added thickening agents that enable the fluid to be deposited in a thicker film and to remain on the aircraft surfaces until the time of takeoff. These fluids are used for deicing and anti-icing, and provide greater protection than do Type I fluids against ice, frost, or snow formation in conditions conducive to aircraft icing on the ground.
SAE and ISO Type Il fluids are designed for use on aircraft with VR greater than 85 knots. As with any deicing or anti-icing fluid, SAE and ISO Type Il fluids should not be applied unless the aircraft manufacturer has approved their use regardless of rotation speed.
SAE and ISO Type Il fluids are effective anti-icers because of their high viscosity and pseudoplastic behavior. They are designed to remain on the wings of an aircraft during ground operations or short term storage, thereby providing some anti-icing protection, but to readily flow off the wings during takeoff. When these fluids are subjected to shear stress, such as that experienced during a takeoff run, their viscosity decreases drastically, allowing the fluids to flow off the wings and causing little adverse effect on the aircraft's aerodynamic performance.
The anti-icing effectiveness of SAE and ISO Type Il fluids is dependent upon the pseudoplastic behavior which can be altered by improper deicing/anti-icing equipment or handling. Some of the North American airlines have updated deicing and anti-icing equipment, fluid storage facilities, deicing and anti-icing procedures, quality control procedures, and training programs to accommodate the distinct characteristics of SAE and ISO Type Il fluids. Testing indicates that SAE and ISO Type Il fluids, if applied with improper equipment, may lose 20 percent to 60 percent of anti-icing performance.
SAE and ISO Type Il fluids have been in the process of introduction in North America since 1985. Widespread use of SAE and ISO Type Il fluids began to occur in 1990. Similar fluids, but with slight 15
differences in characteristics, have been developed, introduced, and used in Canada.
U.S. MILITARY AIRCRAFT
DEICING FLUIDS The U.S. Department of Defense has issued military specifications, "Anti-Icing and Deicing-Defrosting Fluids." These documents specify the following types of FPD's:
• MIL-A-4823C Type I-standard
• MIL-A-4823C Type II -standard with inhibitor
• MIL-A-4823D Type I (propylene glycol base)
• MIL-A-4823D Type II (ethylene and propylene glycol mix) Military Types I and Il fluids are essentially the same, except that Military Type Il fluids contain a fire inhibitor. Military Types / and I/ fluids are unrelated to SAE and ISO Types I and Il fluids (see table 1).
CHARACTERISTICS OF FPD FLUIDS Chemical Composition of FPD Fluids Commercially available FPD fluids are of the ethylene glycol or propylene glycol family. The exact formulas of various manufacturers' fluids are proprietary. It is important to understand that some commercially available FPD fluids contain either ethylene glycol or derivatives of ethylene glycol, such as diethylene glycol, with small quantities of additives and water. Various FPD manufacturers, upon request, will premix aqueous solutions of FPD for specific customer reasons. Before using a solution of FPD, it is imperative that the ingredients be checked by close examination of the stock number and by a quality control examination to ascertain that the fluid supply conforms to the customer need. FPD fluid 16
manufacturers can supply methodology and suggest equipment needed for quality control examinations. It is desirable that the pilot understand the criticality of effective quality control.
Freezing Characteristics of FPD Fluids Before a fluid is used on an aircraft, it is crucial that the user knows and understands its freezing characteristics. These characteristics can be determined through understanding of the fluid procurement specifications and tolerances and through quality control inspections.
FPD fluids are either premixed (diluted with water) by the manufacturer or mixed by the user from bulk supplies. To ensure known freezing characteristics, samples of the final mixture should be analyzed before use.
FPD Fluid Strength When Applied Fluid strength or the ratio of FPD ingredients, such as glycol, to water should be known if proper precautions, such as those outlined above, are taken before application. It is crucial to realize that fluid strength is a significant factor in deicing properties, as is the time that the FPD fluid may remain effective (holdover time).
Tables 1 and 2 in appendix 1 present guidelines for holdover times achieved by SAE and ISO Type Il and SAE and ISO Type I fluids respectively as a function of weather conditions and OAT.
Do not use pure (100 percent) ethylene glycol or pure propylene glycol fluids in nonprecipitation conditions. The reasons for this caution are explained below.
The freezing point of pure ethylene glycol is much higher than that diluted with water. Slight temperature decreases can be induced by factors such as cold-soaked fuel in wing tanks, reduction of solar radiation by clouds obscuring the sun, ambient temperature cooling, wind effects, and lowered temperature during development of wing lift. If the freezing point of the remaining film is found to be insufficient, the 17
deicing/anti-icing procedure should be repeated before the aircraft is released for flight.
• Full strength (undiluted) propylene glycol, having a strength of about 88 percent glycol at temperatures less than
- 10°C (+ 14°F), is quite viscous. In this form, propylene glycol based fluids have been found to produce lift reductions of about 20 percent. Propylene glycol FPD fluids are not intended to be used in the undiluted state unless specifically recommended by the aircraft manufacturer.
TEMPERATURE BUFFER American Practice The practice developed and accepted by the North American air carrier industry using traditional North American fluids is to ensure that the remaining film has a freeze point of at least 20°F below ambient temperature.
European and Canadian Practice The practice developed by the European air carrier industry has been to ensure that the freezing point of residual SAE and ISO Type I fluids is at least 10°C (18°F) below ambient temperature. This is similar to the North American practice, except for metric conversion differences. For SAE and ISO Type Il fluids, the freeze temperature should be at least 7°C (13°F) below ambient temperature. This temperature difference between SAE and ISO Type I and SAE and ISO Type II FPD fluids is primarily to accommodate differences in fluid dilution rates which occur in freezing precipitation. Type Il fluids, which are thicker, will not dilute to the same extent in a given period of time.
CURRENT FAA RECOMMENDATIONS Generally the holdover time is increased with an expansion of the temperature buffer. Therefore, if the choice is available, use the maximum buffers. Greater buffers require the use of more glycol, which is more costly and which increases the burden for collection and processing of FPD spillage and runoff. FPD fluid mixtures and their attendant buffers should be determined after consideration of the following factors in the listed order of priority.
• Safety Environmental impact Cost For traditional North American and Type I SAE and ISO Fluids, the freeze point buffer of the anti-icing fluid should be as great as possible but not less than 10°C (18°F).
For SAE and ISO Type II Fluids, the freeze point buffer should not be less than those recommended by the SAE and ISO which is currently 7°C (13°F) at ambient temperatures below -7°C (19°F) and 3°C (5°F) at ambient temperatures above - 7°C (19°F).
AC 20-117 Recommendation The FAA's recommendation, published in AC 20-117 in December 1982 and reissued in 1988, is to ensure that the fluid freeze point is at least 20°F (11°C) below the colder of the ambient or aircraft surface (skin) temperature. The reasons for this differential are to delay refreezing of the anti-icing fluid and to take into consideration such factors as:
• Temperature reduction during climb or in the production of aerodynamic forces, and the possibility that residual fluids (on surfaces, in balance bays, etc.) will freeze at altitude;
Freezing potential in conditions conducive to icing. As freezing precipitation or moisture from any source contacts and is absorbed by the residual anti-icing fluid, the freeze point is 19
increased. A greater temperature buffer provides a longer holdover time due to this effect; and
• Quality control margin for error.
VARIABLES THAT CAN INFLUENCE HOLDOVER TIME This section provides a listing of some of the major variables that can influence the effectiveness of FPD fluids, especially when the fluids are being diluted by precipitation. The influence of these variables on the FPD fluids' time of effectiveness is described in detail in appendix 3 of AC 20-117. These major variables include:
• Aircraft component inclination angle, contour, and surface
• Are nut are on emperature:; FPD fluid application procedure;
• FPD fluid aqueous solution (strength);
FPD fluid film thickness;
FPD fluid temperature;
• FPD fluid type;
Operation in close proximity to other aircraft, equipment, and structures;
Operation on snow, slush, or wet ramps, taxiways, and runways;
• Precipitation type and rate;
• Presence of FPD fluid;
Radiational cooling;
Residual moisture on the aircraft surface;
• Relative humidity;
Solar radiation; and Wind velocity and direction.
HEALTH EFFECTS Pilots must be aware of the potential health effects of deicing and anti-icing fluids in order to ensure that proper precautions are taken during the deicing and anti-icing process and to better ensure the well-being of passengers and flightcrew. Passengers and crew should be shielded from all FPD fluid vapors by turning off all cabin air intakes during the deicing and anti-icing process. Exposure to vapors or aerosols of any FPD fluid may cause transitory irritation of the eyes. Exposure to ethylene glycol vapors in a poorly ventilated area may cause nose and throat irritations, headaches, nausea, vomiting, and dizziness.
All glycols cause some irritation upon contact with the eyes or the skin. Although the irritation is described as "negligible," chemical taken to den at as manufacturers recommend avoiding skin contact with FPD and wearing protective clothing when performing normal deicing operations.
Ethylene and diethylene glycol are moderately toxic for humans.
Swallowing small amounts of ethylene or diethylene glycol may cause abdominal discomfort and pain, dizziness, and effects on the central nervous system and kidneys. Because the glycol contained in FPD fluids is considerably diluted with water and other additives, it is highly unlikely that deicing personnel would ingest anything close to a lethal amount (3 to 4 ounces of pure glycol). Detailed information on health effects and proper safety precautions for any commercial FPD fluid is contained in the material safety data sheet for that fluid which is available from the fluid manufacturer and should be on file with the operator providing the deicing or anti-icing service.
DEICING AND ANTI-ICING PROCEDURES Depending on the type of accumulation on the surface of the aircraft and the type of aircraft, operational procedures employed in aircraft ground deicing and anti-icing vary. The general procedures used by