Q. How does de-icer on planes work?

D.M.

Dedham

A. Tony McClary, manager of safety and training for Hudson General, a major de-icing contractor at Logan International Airport, has de-iced thousands of planes, and says with no small amount of pride that it's a complicated procedure that involves a lot more than squirting hot water on a plane so its surfaces will be smooth and perform the way they're supposed to.

In fact, he says, de-icing is only one step. Step two is anti-icing, to keep the frozen stuff from building back up as the plane sits in the snow/sleet/freezing rain, waiting to take off. Both steps use fluids that contain glycols, chemicals that have a lower freezing point than water.

De-icers use either ethylene glycol or propylene glycol in a 50-50 mix with water, heated to 180 degrees Fahrenheit and sprayed from a power hose onto the plane. The heat and powerful spray do some of the de-icing work. But the key are those glycols, which lower the freezing point of moisture on the plane's surface to as low as 60 degrees below zero.

After spraying the de-icing fluid on the plane, the ``holdover time'' during which the plane stays clean is only 6 to 15 minutes, depending on weather conditions. So sometimes they do a second step, called anti-icing, in which they spray the plane with a mix of glycol and non-Newtonian fluids. That's just a fancy name for fluids too thick to run by themselves, like ketchup or honey.

The thicker fluid stays on the plane longer and lets the glycols keep things from refreezing while the plane is waiting. Holdover time after a step two anti-ice treatment is as much as an hour and twenty minutes, depending on the specific fluid used and the weather conditions. Usually they only spray the thicker stuff on the critical control surfaces of the plane, the wings, and the tail.

Non-Newtonian fluids designed for anti-icing are not really like ketchup or honey. ``They have the unique ability to change viscosity when confronted with aerodynamic forces acting on the fluid,'' McClary says. That's a fancy way of saying that when the plane speeds up as it takes off, the force of the wind on the fluid makes it get thinner, making it more like water than honey, and it drips right off the plane. By the time the plane exceeds 156 miles per hour, the fluid is gone, the plane's surface is clean, and takeoff (at 180 miles per hour for your average jetliner) is safe.

Step One fluids are pinkish. The Step Two fluids are greenish. Both consume oxygen when they get into aquatic ecosystems. Both biodegrade quickly. In five days they're 70 percent gone. After 20 days they're 96 percent broken down.