I'm very grateful that I'm in a job where I don't have to put a price on safety, because sometimes safety is incredibly expensive. Allow me to elaborate...
We flew west today for a whole long while, and on the way west we stopped in Omaha Nebraska to clear US Customs. The Customs people were friendly and quick and that part was great, but the part that wasn't too hot was the weather.
It was overcast and snowing today in Omaha, and we ended up picking up some ice. Not quite the "Can't see out the windshield" kind we had a couple of weeks ago (scroll down a couple of posts for that story), but any ice is too much ice on a plane, so we had to remove it before continuing our journey westbound.
When deicing in North America, there are generally 4 different types of fluid used, conveniently named Type I, II, III and IV.
Type I is a de-ice fluid, meaning it's used to remove ice that has already accumulated on the plane. It's a mixture of 90% glycol (alcohol) and 10% water which is heated to about 90 degrees Celsius before being sprayed on the plane. It's usually colored bright orange, like in the video I attached to this post. Type I is great for removing ice, but it doesn't provide a very long period of protection from re-icing after it's applied.
Type IV is the most commonly used anti-ice fluid and is normally colored green. It's a mixture of glycol and a thickener, usually vegetable oil. The thickener prevents the glycol from dripping off the wings after the fluid is sprayed on, which provides decent protection for a while if it's snowing outside etc. The stuff is supposed to stick like glue to the wings until you are doing about a hundred miles an hour down the runway, when it's supposed to drip off and leave no trace. Oh, and as a random observation, it tastes terrible.
Anyway, you are less likely to come across the other two types of fluid, but I'll include them here for the sake of completeness:
Type II is an anti-ice fluid, meaning it's put on to prevent ice from forming on the wings it's snowing outside, etc. It's being phased out in favor of Type IV, which is more effective.
Type III is an anti-ice fluid but it's hardly used any more, if at all - it was designed for smaller commuter aircraft but it turns out that Type IV is equally effective, so type III became irrelevant.
So how do we know what fluids to apply in what situations?
In Canada, Transport Canada publishes an updated manual called the HOT guidelines (not nearly as fun as it sounds), which lists the length of time you can expect your de-icing or anti-icing fluid to provide protection.
Anyway, if the airplane is just covered in ice but the skies are clear then we generally would only use Type I, but if it was snowing or freezingly raining then we'd add a coat of Type IV for added protection.
For example, if it was -5c with freezing fog at the airport we wanted to depart from, Type I fluid might give us up to 13 minutes of protection against refreezing, but if we added a coat of Type IV, it would provide us up to 80 minutes of protection against refreezing.
At large airports during bad-weather days there can be long lines of aircraft waiting to depart, so the longer you have protection, the greater your chances of being cleared for takeoff while your de-ice and anti-ice fluids are still working. If the lineup is too long and/or the precipitation is too bad (freezing rain, etc) then it's entirely possible the aircraft will have to pull out of the departure lineup and go back to get de-iced again.
And now, here's some of this stuff being put to use in real life:
We only took Type I de-ice fluid because it wasn't actually snowing in Omaha, so we didn't need to worry about any additional ice while we were taxiing for departure. In the video it looks snowy, but that's just snow blowing around the ramp.
In the 5 minutes it took to de-ice us this morning, they used 42 gallons (158L) of heated Type I Oh, did I mention it's $16.95/gallon ($4.49/L)? That adds up when they fire the stuff out of a cannon, in this case to the tune of $712. That's about $20 for every second they sprayed us.
I know that sounds outrageous, but it was actually a lot cheaper to deice in Omaha than in our home base of Toronto Pearson, where the airport association has a monopoly on de-icing facilities and they jack the price outrageously because they can. Last time we got de-iced in Toronto, the bill was $2,500.00. Still, it's better than trying to fly an iced-up plane and littering the countryside with aircraft parts, so that's what we'll continue to do.