Thoughts Aloft |||

It’s been a little while since I’ve done an aviation related post and thought that aircraft de-icing would be an approriate topic given the time of year.

We have all seen pictures of airplanes being de-iced during the winter months and unless you only fly in warm climates, there’s a good chance you have been on a plane while it was being de-iced. Having to de-ice can be just as frustrating for the pilots as it is for the passengers in the back, especially if we’re delayed, but it’s something that has to be done to ensure the safety of the aircraft and everybody on board. The majority of the time one of the pilots will make an announcement saying the plane has to be de-iced and approximately how long the process will take but may not include any information as to why. While it’s not uncommon to hear complaints about the delay the deicing process can cause, passengers understand that it is for safety but not necessarily why it has to be done. I hope this article will help you understand the why.

Before getting into the details, I want to give you a brief description of lift and how ice can affect the amount of lift a wing produces. Lift is developed by the faster airflow over the top of the wing creating a lower pressure than the slower moving beneath the wing. Pressure always travels from high to low, so the higher pressure air beneath the wing pushes up on the wing trying to get to the lower pressure air on top creating lift. The main threat with ice, frost, of snow is the reduction in lift that they can cause. They reduce the amount of a lift a wing produces in two ways, they create a rough surface on the wing slowing down the air moving over the top of the wing  and by also changing the shape of the wing in a way that could disrupt the airflow over the top of the wing, thereby reducing lift. Quick sidebar: yes, the slats and flaps (the portions of a wing that lower during takeoff and landing) do change the shape of wing, but they are designed to do that and change the shape of the wing in a uniform way. Airlines use a clean aircraft concept which means the aircraft has to be free of all ice, snow, and frost before taking off and is where the de-icing and anti-icing  process comes in. What do we mean by de-ice and anti-ice? It’s simple, de-ice is removing ice, snow, or frost, and anti-ice is preventing it from forming.

The first, and sometimes only, step in the process is applying de-ice fluid to the aircraft to remove any ice, snow, or frost that is present on the aircraft. We use a heated propylene-glycol mixture, known as Type I fluid which is typically orange in color, to do this. It is applied heated to help melt the ice, snow, or frost off the aircraft. If frost, ice, or snow already on the aircraft is the only concern or if there is no precipitation falling and sticking to the aircraft, Type I fluid is all that is necessary. This step creates a clean surface on the aircraft but also preps the aircraft for the next step, if necessary.

The next step is preventing any more snow sticking or ice forming on the airplane. We use a different propylene-glycol mixture, known as Type III or Type IV, to do this. The only difference between Type III (typically light yellow in color) and Type I V fluids (typically green in color) is the holdover time which will be discussed shortly. (There is a Type II fluid but it’s not used for reasons I don’t know.) The Type III or Type IV fluid is applied cold and is designed to adhere to the surface of the aircraft to form a barrier that prevents snow from sticking and ice from forming. As the aircraft accelerates down the runway and rotates, the fluid slides off the aircraft.

Where the frustration comes in for us pilots, beyond the obvious delay, is making sure we don’t exceed our holdover time. This holdover time is the length of time that the Type I, Type III, and Type IV fluid is effective for. The length of the holdover time depends on the outside air temperature, type and intensity of precipitation that is falling, the type of fluid used, and even the brand of fluid (some brands of Type IV have longer holdover times than others) and we have charts that determine the holdover time.  Type I has the shortest holdover time, around 15 minutes, but typically isn’t an issue.  This is because we can exceed Type I holdover times if there is no active precipitation or if there is no chance of ice forming or snow sticking to the aircraft. Examples of this would be removing  frost on a clear morning or removing ice or snow already present on the aircraft on a clear day.  Type IV has the longest holdover time and is the primary type of anti-ice fluid used. Unlike with the Type I fluid, we cannot exceed the holdover time for Type IV fluid, even if the precipitation has stopped. This means that if we get sprayed with Type IV fluid and do not takeoff within our holdover time, we have to get the aircraft re-sprayed with Type I and possibly Type IV depending on the conditions.  If at any point after getting sprayed we notice snow sticking to the aircraft, ice starting to form, or any other indication that the fluid is no longer doing its job or is losing its effectiveness, even if we are still inside our holdover time, we have to get the aircraft resprayed and the process starts over agian. Occasionally the outside conditions are so bad that fluid starts losing its effectiveness even before the entire aircraft has been sprayed. When this happens our options are pretty much limited to waiting for the conditions to improve or delaying and possibly canceling the flight.

Once we takeoff, we use the aircraft’s de-icing and anti-icing systems to keep the aircraft clean. These systems typically use either electrical power or hot engine bleed air (excess air) to do their job. The pitot tubes (the probes that measure airspeed) are electrically heated to provide anti-ice protection and the windshield is typically electrically heated as well, again providing anti-ice protection, propeller driven aircraft use electrical power to protect the propellers. To protect the wings, vertical stabilizer (the vertical tail surface), and horizontal stabilizer (the horizontal tail surface), and engine inlets, hot engine bleed air is typically used to either heat these areas (anti-ice) to prevent ice from forming or to inflate de-ice boots which inflate rapidly to pop off any ice that has formed on these surfaces. Some smaller jets and the Boeing 787 use electrical power to protect these surfaces. The systems are more complicated than this but this is the basic concept of how aircraft are protected from ice in flight and I wanted to briefly explain it.

So next time you see some orange or green slime on an aircraft during the winter time, you hopefully know what it is. I hope this post was easy to understand and gives you a good idea as to what is actually happening during the de-ice process on the ground and a basic idea of how aircraft are protected from ice in flight.

I debated whether to bring this up as I don’t want to add unnecessary fear for something that is completely safe but with recent events I thought would try to calm these fears.  With the recent crash of AirAsia 8501, the media is trying to draw similarities to the 2009 crash of Air France 447 where icing was a CONTRIBUTING factor to the crash, not the SOLE cause of the crash as much media portrays it. It is too early in the investigation and there is simply not enough information yet to determine the cause of AirAsia 8501 and I’m not going to speculate on the cause. Flying in icing conditions is safe, airplanes do it all the time.The only reason I even brought it up was because of the media attention on the role that icing MAY have played in the crash and to hopefully alleviate and calm any fears created by this.

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