As mentioned above, ethylene and propylene glycol are components of deicing fluid, along with other things such as thickening agents, corrosion inhibitors and dyes. It should be noted that many deicing fluids are designed to be mixed, in varying concentrations, with water.
There are four types of deicing fluid (three in common use) specified by the Society of Automotive Engineers (SAE); Type I fluid (dyed orange) is basically raw, unthickened glycol - very similar to automotive engine coolant, apart from the additives, that is. As it is unthickened, Type I fluid does not really provide much inhibition to icing at temperatures above about -25 Celsius, and is often used for initial deicing only, or for anti-ice protection at very low temperatures. Type II
fluid (dyed yellow) is a thickened glycol mixture, and is designed to shear off the wings (due to aerodynamic forces) of an aircraft at speeds in excess of 100 knots. The thickness of Type II
fluid makes it stick to surfaces, thus allowing longer holdover times. Type II
is no longer in common use today, as it is extremely expensive and is quite toxic, from what I understand. Type III fluid (dyed yellow) is similar to Type II
, but it is designed to shear off at speeds of less than 100 knots. Mostly, this fluid is used for anti-ice protection for regional aircraft and business jets. Type IV
fluid (dyed green) is largely to the same specification as Type II
fluid, but it is somewhat less expensive (and not nearly as toxic), and provides slightly longer holdover times.
It should be noted that the anti-icing fluids (Types II
, III and IV
) are only used if there is precipitation falling, as there is obviously no need for anti-icing fluids if it is clear out. Also, the cost of the anti-icing fluids is astronomical. To give you an idea how much the difference is, I was told several years ago that the cost of deicing a Boeing 737 with Type I fluid would be about $800 or so. Now, to deice with Type I and then apply a layer of Type IV
to the same aircraft would cost about $8000. Needless to say, air operators use anti-icing fluids very sparingly.
|Quoting ANITIX87 (Thread starter):|
3) How long is a de-ice effective? Does it ever happen that an aircraft gets de-iced and then is delayed long enough to require another run? Or do they intentionally wait until they know it is going to leave?
All deicing fluids have holdover times, or HOTs. Basically, these times (found on tables published by the fluid manufacturer) indicate how long the fluid will protect against surface contamination, based on temperature, fluid concentration and the amount and type of precipitation falling. It should be noted that there is no holdover times exist for anything more than moderate freezing rain or heavy snow, even for Type IV
|Quoting ANITIX87 (Thread starter):|
6) What surfaces of a plane MUST be de-iced prior to flight? What surfaces are non-critical and can be overlooked if need be?
"Critical surfaces" of an aircraft include the wings, control surfaces, rotors, propellers, horizontal stabilizers, vertical stabilizers or any other stabilizing surface of an aircraft and, in the case of an aircraft that has rear-mounted engines, includes the upper surface of its fuselage. It should be noted that section of the fuselage through which the wings pass is considered to be part of the wing, and is therefore considered to be a critical surface.
Of course, airlines will often have even more strict rules than the bare minimum set forth by the regulator - some airlines require there to be no contamination whatsoever on any surface of the aircraft, and nobody that I know of would allow accumulations of snow or ice to remain on any surface - with one exception. It is permissible to allow an aircraft to takeoff with frost or ice on the undersides of the wings, so long as it is the result of cold-soaked fuel, and the accumulation remains within the limits published by the aircraft manufacturer.
|Quoting KingAir200 (Reply 1):|
Everything. If you see snow or ice on the airplane, it's gotta come off.
Snow or ice, yes. Frost - not necessarily.