Happy De-Ice Season! (It’s winter somewhere)
When flying during the cold weather months, passengers often see aircraft being de-iced. Ever wonder why the airline chose to delay your flight to wash their airplane with slimy green goop?
Aircraft de-icing is really important. Here’s how and why airlines do it…
Aviation rules require that an aircraft’s wings and tail be free of snow, ice and frost before takeoff. I’ll explain with a quick beginner aerodynamics lesson.
Really easy aerodynamics… I promise!
The flow of air above and below an aircraft wing produces lift (high school students blame it on Bernoulli and Newton). Anything on the wing’s surface that changes the shape or texture of the wing will disrupt the airflow and the wing won’t provide the amount of lift that the aircraft designers promised.
Now, back to the snow, ice, and frost. Frozen precipitation is dangerous because it changes the shape and texture of the wing. The top of the wing needs to be smooth for optimum performance. Frost changes the wing texture to something like sandpaper. Snow is even worse, it changes the aerodynamic shape of the wing.
On a “clean” wing, the air flows smoothly. On a wing “contaminated” with frost or snow, the air has trouble sticking to the wing surface. The air separates and becomes turbulent. When this happens, we lose lift; and that could be the start of a very bad day. If you see airliners being de-iced on a clear, chilly morning, it’s probably because they have frost on the wings.
Aircraft de-icing procedures vary from airport to airport. Smaller airports will de-ice the aircraft at the gate (like the picture at the top) or just after the aircraft pushes back onto the taxiway. Big airports often have a “de-icing pad” where all aircraft are de-iced. De-ice pads are nice because the airport authority can collect the used fluid for proper disposal or recycling. The fluids used are typically complex concoctions based on propylene or ethylene glycol (similar to automotive antifreeze).
Aircraft De-icing is a one or two-step process depending on the weather.
- Step 1: “De-ice” removes ice, frost, and snow.
- Step 2: “Anti-ice” protects the wings from new ice, frost, and snow.
Step #1: De-Icing – Remove The Bad Stuff!
Spraying the aircraft with very hot, high pressure fluid removes snow, ice, or frost adhering to the wings. Fluids are tinted to make them easier for pilots and ground crews to identify.
The stuff typically used to remove existing snow is called “Type-1” and is tinted orange. Type-1 fluid is easy to spot because it’s hot and produces a lot of steam when sprayed.
If the snow has stopped falling, the process is complete after de-icing. The flight crew can taxi to the runway and safely takeoff. If it’s still snowing, there is more to do…
Step #2: Anti-Icing – Protection From NEW Bad Stuff!
The weather is crummy and the snow fall continues. While Type-1 fluid does a great job of removing snow and ice, it doesn’t protect the wings from new snow. Even during a short taxi, dangerous accumulations of snow can build up on the aircraft.
To protect the wings from further contamination, the de-ice crews will apply a coat of “Type-4” fluid (may vary internationally). Type-4 is super thick, super slimy, and green (think lime jello). Instead of being sprayed at high pressure, it dribbles out of the hose: glug, glug, glug.
Type-4 is interesting stuff. It grabs any new snow and ice and holds it until the takeoff roll. It’s designed to maintain its jello-like consistency until the aircraft reaches about 110 mph. At that point the fluid (and accumulated snow) begins to shear off the wing. By the time the aircraft nose lifts, all the bad stuff is gone.
If you get the “green stuff” on a flight, watch the wing during the takeoff roll. You’ll see it do its thing!
What happens to the excess fluid?
Aircraft de-icing crews do a great job of spraying the airplanes, but inevitably, some of the spray misses the aircraft and ends up on the ground. Airports collect this yucky runoff and either recycle, or dispose of it in an environmentally safe manner.
Now you’re totally AeroSavvy about aircraft de-icing! If you need a little visual stimulation to bring it all together, check out the two short videos below.
Here’s a video of a WestJet aircraft having frost removed in Winnipeg. It surprises many passengers that aircraft are de-iced for such small amounts of frost. The reason is that we really don’t know how much frost it takes to negatively impact performance (and we’d rather not find out!).
My sincerest apologies for the cheesy flight attendant announcements.
This video shows both steps of the de-ice/anti-ice process. During the first half of the video you can see the high pressure, hot & steamy Type-1 being applied. Then they switch to the low pressure, green Type-4.