Aircraft De-Icing

 

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.

 

FAA MAGIC!

     With the Christmas and Holiday season upon us, many of us will be flying to visit family and friends. If you are going to be flying, there is a good chance at least one of your flights will be an “Express” or “Connection” flight on a regional aircraft flown by a regional airline partner of your major airline of choice and you may be told to “gate-check” your larger carry on bags because they will not fit in the overhead bins. There are times where these flights are weight restricted to ensure that any aircraft limitations are not exceeded or these flights encounter an issue of being overweight.  Have you ever been on one of these flights and seen the crew members bringing your gate-check bags into the cabin and told it was to help the “weight and balance” of the aircraft and thought “What’s the point of bringing the bags into the cabin” or “How does this help with weight? A bag is a bag.” It’s because of what I jokingly refer to as FAA magic.  Allow me to explain. 

     When we say “weight and balance” we are actually referring to two separate but related things, the weight of the aircraft and the balance of the aircraft or location of the center of gravity (CG) of the airplane. If either one or both of these is outside the certified limits or performance limits of the aircraft, we can’t fly. If the CG is out of limits, it’s an easy fix of moving weight around or adding ballast to get the CG within limits. This is why you may see the crew members asking some passengers to move to a different part of the aircraft. Weight issues are typically caused by the aircraft exceeding one or more of a few different weight limitations, the specifics of which are beyond the purpose of this post because overweight is overweight.

     At this point you may be wondering how we know how much each passenger or bag weighs and the truth is, which may be surprising to some is, we don’t. We use an average passenger weight and an average bag weight in our computations. The passenger weights we use account for the passenger, their carry-on bag, and their personal item. For my airline those weights are 190 pounds per passenger during May through October and 195 pounds per passenger November through April (the extra five pounds accounts for the heavier clothing and jackets work during the colder months) and 30 pounds per bag and 60 pounds per bag for larger or heavy bags. 

     When we do get into an overweight situation there are a few ways to remove the extra weight. Removing passengers and/or bags is an obvious, though not a customer friendly, solution especially if we have to remove passengers. One thing that can help with the extra weight is if we have any kids (13 or younger in their own seat) onboard because we can remove 100 pounds off the total weight per kid. You may also hear the crew or gate agents refer to a half-weight and they are simply referring to kids on board. Another option we have, and the reason for this post, is bringing gate-check bag into the cabin to eliminate weight and is where the FAA Magic comes into play. Those average passengers weights mentioned earlier are based on the presumption that your carry-on bag and personal item are either in the overhead bin above you and/or underneath the seat in front of you. When those bags are gate-checked and put into a baggage compartment, those bags are no longer near where you are seated, they add another 30 pounds per bag that we have to account for in our weight and balance calculations. In essence these bags are being counted twice, once in the passenger weight and once in the baggage compartment. By bringing them into the cabin, the weight of that bag poofs and disappears (that’s the FAA Magic) and removes 30 pounds per bag from our total weight. The way I explain it to my passengers is that it’s FAA Magic, that if the bag goes in the baggage compartment it weighs 30 pounds but if it’s in the cabin it weighs nothing. Explaining it this way not only gives the passenger an understanding of why we’re doing it but also typically gives them a little laugh or at least a smile. 

     With the busiest travel season of the year, at least in the US, just starting I thought this was an appropriate time for this post. All aircraft can run into weight and balance issues regardless of size, though they’re most common on smaller aircraft. Unfortunately there are times when, even when bringing bags into the cabin and accounting for kids, that we are still overweight and have to remove passengers. If this happens on a flight that you’re on, be assured that the crew has done everything they can to accommodate as many passengers as possible, and that your patience and understanding in the matter TRULY is appreciated. Hopefully this post will help with that understanding part.  

Safe flying!!!

Flying with Fountain Pens

     We’ve all heard the horror stories of exploding or leaking fountain pens in flight due to pressure changes from higher pressures on the ground, and therefore in the pen itself, to the cruising altitude where cabin pressure is lower. Knowing that pressure travels from high to low, it’s easy to see why this can and does happen. Well I flew recently (not uncommon for me being a pilot but that’s besides the point) with a fountain pen and used it the entire flight, gate to gate, with no issues whatsoever. 

     Now for a (hopefully) simple, short, easy-to-understand explanation of why I think I had no issues using my pen the whole flight. The way a pressurization system on a plane works is (I’m going to really simplify it) is that bleed air (excess air) from the engines is pumped into the cabin to pressurize it, very similarly to blowing up a balloon. There is an outflow valve that regulates the pressure of the cabin by letting some of that air escape at a certain rate, similarly to letting some air out a ballon. This outflow valve is important because it allows some air to escape to prevent over pressurizing the cabin of the airplane. You may be thinking, “Why is this important?” or “Why should I care?” It’s important because it’s about regulating pressure, which is why I think I had no issues on my flight. 

     By using the pen the whole time, the pressure inside the pen never really had a chance to build up because by writing with it, you’re giving that pressure a chance to escape, keeping the pressure differential between the two (plane and pen) relative to what it was on the ground.  It’s the same reason why if you open a bottle of water on the ground before taking off and don’t open it again until reaching cruising altitude the bottle has expanded some (higher pressure inside the bottle from being on the ground trapped inside trying to escape to the lower pressure in the cabin at cruising altitude). However, if you open that bottle at regular intervals during the climb, you are allowing the pressure inside to escape, therefore keeping the pressure differential the same relative to what it was on the ground, and at cruising altitude the bottle is the same size and shape it was on the ground. Same is true for the descent, where if the bottle is left untouched after being opened at cruise, the bottle will be crushed in on the ground (higher pressure from being on the ground trying to get to the lower pressure inside the bottle from it being at cruise altitude). If opened at regular intervals during the descent, the bottle will have the same shape on the ground as it did at cruise, again because opening the bottle allows the pressure differential between it and the plane to remain the same.  Okay, maybe it wasn’t that short but hopefully it’s simple and easy enough to understand. 

     Short story, by constantly allowing the pressure inside the pen to stay relative to the cabin pressure, the pen should perform with no issues (in theory) as mine did on that flight.