Ground icing operations - all about deice and anti-ice fluids
GROUND ICING CONDITIONS "No pilot may take off in an aircraft that has frost, ice or snow adhering to any rotor blade, propeller, windshield, wing or stabilizing or control surface, to a power-plant installation, or to an airspeed, altimeter, rate-of-climb or flight-attitude instrument system," according to the FAA basis for the “clean-aircraft concept.”
RELEVANT DISCUSSION: FAR 91.527, 91.1101, 121.629, 125.221, 135.227, 135.345, AIM 7-5-14, OPSPEC A023, AC 20-73, AC 20-117, AC 120-58, AC 120-60, AC 135-17, NASA There are four standard aircraft deicing and anti-icing fluid types: Type I, II, III and IV.
Type I fluids (red-orange in color) are the thinnest of the fluids. They are essentially deicing fluids but can also be used for anti-icing. Because they’re not thickened, they will shear or blow off the airplane at relatively low airspeeds (around 60 knots). However, their low viscosity results in the shortest set of holdover times (HOT). Type I is always applied heated and diluted; it flows off easily, and can fail suddenly. Type I is available throughout the world.
Type II fluids (clear or straw color) add thickening agents to increase viscosity and provide better protection against refreezing than Type I fluids, but require a minimum 100-knot rotation speed. The thickeners allow fluid to remain on the aircraft longer to absorb and melt the frost or freezing precipitation. They’re applied when a long elapse time is anticipated between deicing and takeoff. Type II is common throughout Europe, but has limited availability in North America.
Type III fluids (yellow-green) are relatively new and have a viscosity higher than Type I fluids but much lower than Type II or IV fluids. Type III was formulated for use on small commuter-type aircraft with takeoff rotation speeds of 60 knots or higher, but they can also be used on aircraft with much higher rotation speeds. Type III fluids, unlike Type II or IV fluids, can be used in a handheld sprayer because they do not require specialized low-shearing-application equipment. Type III became available in 2004 in Europe and North America.
Type IV fluids (emerald green) meet the same fluid specifications as Type II fluids (i.e., thickening agents), and have a significantly longer HOT. Therefore, Type IV should be used on aircraft with rotation speeds (Vr) above 100 knots when long elapse time is anticipated between deicing and takeoff. Type IV fluids are available throughout the world.
How the Fluids Work
As noted, Type I fluids are always applied heated and diluted. For deicing, it is the heat and hydraulic force that accomplish the task. For anti-icing, it is the heat imparted to the airframe that accomplishes the task.
Type I fluids have the shortest HOT. Therefore, when a Type I fluid fails, it fails suddenly.
Type II and IV fluids may be applied heated or cold, and diluted or full strength.
In North America, Type IV fluids typically are applied cold, and only for anti-icing. In the U.K., Type II or IV fluids usually are applied heated to accomplish deicing as well as anti-icing.
Cold-Temperature Limitations Using fluids in very cold conditions — around minus-20 degrees Celsius or below — raises some issues:
Aerodynamic acceptance — In general, the colder the fluid, the more viscous it becomes. At some point,
the fluid will become too viscous to adequately shear or flow off the aircraft. This creates an unacceptable aerodynamic situation.
Freezing point — If the freezing point of the fluid is within the freezingpoint buffer (10 degrees Celsius above the OAT for Type I, or 7 degrees Celsius above the OAT for Type II, III or IV), the fluid cannot be used.
Lowest operational use temperature, or LOUT — Aerodynamic acceptance and freezing point are both considered in the concept known as LOUT. The LOUT is the coldest air or aircraft skin temperature at which a deice or anti-icing fluid can be used.
1. Ground icing procedures are specifically detailed in the OpSpecs and operations manual of every certificate holder that chooses to operate during ground icing conditions.
Holdover time (HOT) is the estimated length of time that deicing or anti-icing fluid will prevent the accumulation of ice, snow or frost on the aircraft. “HOT tables” are used to determine the legal requirements for takeoff under various conditions.
Holdover time begins when the final application of deice or anti-ice fluid starts, and expires when the fluid loses its effectiveness.
Holdover time may be exceeded when at least one of the following conditions exists: a. A visual check within five minutes of takeoff determines that the aircraft is free of ice, snow or frost. b. An otherwise FAA-approved procedure (included in the operations manual) is used to determine that the aircraft is free of frost, ice or snow. c. The aircraft is re-deiced and a new holdover time is determined.
Newly developed technologies for ground deicing, such as liquid water equivalent (LWE), aka holdover-time determination systems (HOTDS), are highly automated systems that measure winter precipitation quantity and temperature to determine an LWE value. This LWE value is compared to a database of deice and anti-ice fluid properties to determine the precise moment in time when the deice or anti-ice fluid will no longer be effective. This information is transmitted to the cockpit via aircraft communications addressing and reporting system (ACARS) or other means so that the flight crew always knows how long the fluid applied to their airplane will remain effective.
NOTE: A visual check of the wings within five minutes prior to takeoff is required whether holdover times have been exceeded or not.