Appendix 3 treated first with FPD fluid can have totally different characteristics than areas treated last. In cases where hot water alone is being used for deicing, areas previously oversprayed with FPD fluids can be diluted by water splashed or flowing from other areas being deiced. All of these factors are variables that can influence the effectiveness.
g. Ambient Temperature. A specific mixture of FPD fluid will freeze (begin to crystallize) at a specific ambient temperature. For that specific mixture, the freeze point is accurate and repeatable. Some cockpit installed outside air temperature gages are known to have inaccuracies as great as 15°C. In addition, the ambient temperature at the ramp, gate, tiedown area, taxiway or runway, may vary significantly from the temperature in the airport tower or from the thermometer on the line shack wall. Ambient temperature may be steady, rising, or falling. Each of these variables, usually unknown to the pilot, can either independently or in combination influence the time of effectiveness of FPD fluids.
h. Aircraft Surface Temperature.
(1) Aircraft surface (skin) temperature also influences FPD fluid effectiveness time. Surface temperatures can be drastically different from ambient temperatures for many reasons: solar radiation (cloudy day, sunny day, parked in the shade, etc.); temperatures of fuel in fuel cells; the type and location of fuel cells; fuel cold soaked during previous high altitude flight;
quantity of fuel in fuel cells; the source of fuel (above or below ground storage); time since refueling; engine operation; APU operation; operation of subsystems or components that generate heat; thickness of the skin, type of skins, and skin materials. These are only a few of the many variables that can influence aircraft surface temperature.
(2) Common practice accepted in the aviation community is that if it can be determined that a small quantity of dry snow is not adhering to aircraft surfaces, it will blow off during the takeoff roll and thereby not affect aircraft flight characteristics. This practice is well proven if one can ascertain that the snow is not adhering to critical surfaces. Dry snow at cold ambient temperatures will not adhere to dry aircraft surfaces if surface temperatures are also very cold and will remain cold during start, taxi, and takeoff. However, operation of engines, or other equipment, refueling, solar radiation and many other factors may warm surface temperatures, causing melting or partial melting of snow and possibly cause freezing prior to takeoff or during the takeoff roll. Simply checking the consistency of snow during pre-flight inspection is not considered adequate. Pretakeoff inspections should also be performed. In addition, during preflight inspection and pretakeoff inspection, caution should be exercised when making this judgment to not rely upon spot checking, especially on areas of hollow structures, such as ailerons, spoilers, etc., since these devices are not normally exposed to certain heat Under certain conditions dry snow could adequately blow off of a dry cold spoiler but not from large fuel tankage areas.
i. Solar Radiation. Solar radiation can significantly influence aircraft surface and FPD fluid temperatures. In conditions of heavy precipitation, solar radiation is usually slight. Generally, solar radiation will warm aircraft surfaces, but, in shadow areas, surface and FPD fluid temperatures will be colder.
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Appendix 3 J. Wind. Ambient wind blowing over an aircaft surface tends to remove heat from the surface at a more rapid rate than under calm conditions.
Evaporation of liquids can cause surface temperatures to actually be lower than ambient. The rate of reduction of suriace temperatures and the temperature depression is a function of many variables that make it impractical to estimate, but this factor can influence FPD fluid effectiveness.
K. FPD Fluid Temperature. Elevated FPD fluid temperatures (hot fluid) are commonly used during the aircraft ground deicing process to assist in melting and reducing bond strength of ice formations. Fluid viscosity, however, is reduced with increases of fluid temperature. This can reduce the residual Also, if hot fluids are applied during precipitation conditions (snow or other ice crystal), during the time that elevated fluid temperatures remain, the heat provides a source of latent heat of fusion to melt impacting snow/ice crystals and to mix with and dilute the mixture. This process is favorable in that formation of water crystals in the upper surfaces of the fluid film will be delayed. However, in the presence of wind and other factors, the utilized, ice crystals (precipitation) impacting the fluid will melt and mix at a slower rate and the latent heat of fusion demand will depress the surface film temperature. Use of hot or cold FPD fluids is considered acceptable common practice, however, unheated fluids will leave greater amounts of residual film prolonging effectiveness while heated fluids are more effective in the deicing
1. Precipitation Rate and Type. Subfreezing precipitation occurs in many forms: supercooled rain (freezing rain or drizzle), snow, sleet, and combinations thereof. Each of these conditions can produce an infinite variety of conditions and resulting characteristics on the surface of an aircraft. Snow has been categorized into 22 varieties, each having different crystalline structure and each containing various amounts of frozen water (density). Each type of precipitation contains droplets or ice crystals having various temperatures which may be equal to or below ambient temperature. These combined effects introduce another set of variables that cannot be quantitatively assessed to determine their influence upon effectiveness of FPD fluids.
However, under these conditions surface roughness can vary significantly and in conjunction with other variables, time-to-initiation of freezing (surface roughness) can vary.
m. Operation on Snow, Slush, or Wet Surfaces. Such operations can cause snow or slush to be splashed or blown onto various aircraft surfaces. Jet or rotor wash, and taxi and takeoff operations can, through recirculation, blowing, or splashing, cause snow, ice particles or water to impact various aircraft surfaces and freeze or adhere to these surfaces or further dilute FPD deicing fluids. These occurrences are somewhat predictable and should be expected during ground operations. The effect of these occurrences is unpredictable. If ice formations are anticipated, the clean aircraft concept must be implemented. These occurrences can also have an influence upon FPD fluid effectiveness.
Par 7 21 #U.S. Government Printing Office: 1988 -516-018/80302