If you’ve flown an aircraft in high summer temperatures, you know how hot the interior of your cabin can become. As such, you may be tempted to take every measure to keep the inside of the cabin as cool as possible. Despite this, using materials not designed for use on aircraft will likely only cause a bigger problem in the future. This blog will offer some advice in dealing with the heat and its effect on aircraft windows.
First, let’s look at what aircraft windows are made from. Most of the aircraft windows designed for pressurized aircraft are built from a material called stretched acrylic. This is made through a manufacturing process wherein acrylic material is heated to temperatures over 230 degrees, at which point it begins to melt and becomes easily pliable. It is pulled in all directions until it is stretched to approximately three times its original size. After this, the acrylic is cooled, machined, and polished to the appropriate optical standards. This process significantly increases its strength and rigidity, making it uniquely suited to aircraft windows. Aircraft windows are never made from glass, simply because glass does not have the structural integrity to withstand the extreme pressures it would face at the high altitudes many aircraft reach. However, stretched acrylic does come with certain cautions to be aware of. Namely, when it becomes too hot, it will try to shrink back to its original shape.
Aircraft cabin windows consist of three panes of stretched acrylic: an outer pane flush with the outside of the fuselage, an inner pane with a small hole in it, and a thin, non-structural plastic pane called the scratch pane. The thickness of the inner and outer panes vary from aircraft to aircraft. Inner panes are generally around 0.2” thick and are only present as a backup if the outer pane fails. The outer panes are much thicker, about twice the thickness of the inner pane. These panes must be thick, as they carry the pressure load for the life of the window. The increased thickness allows for engagement with the airframe structure, while maintaining the required strength.
As your aircraft gains altitude, the pressure acting on the outside of the plane decreases. Because aircraft cabins are pressurized to around 6,000 feet, there is more pressure inside the plane than outside. This pressure is acting on both the fuselage and cabin windows. The small hole within the inner pane allows some of the cabin air to escape into the pocket between the inner and outer panes, where it regulates itself. This forces the outer pane to take the entire load of pressure, though it is a very slow process. The hole in the inner pane is extremely small, so as the plane ascends, the pressure slowly equalizes.
While they may not seem that significant, aircraft windows are just as important as many other parts in terms of their effect on the structural integrity of the aircraft as a whole. By having a series of several interlocking windows, the chances of breaking are greatly reduced. Furthermore, aircraft windows help create a more comfortable environment for passengers within the cabin. Proper construction and care will ensure your windows remain in tip-top shape throughout the aircraft’s service life. To get the very best from your aircraft windows and your aircraft as a whole, there are a couple things you never want to do to your windows.
There are two big mistakes you can easily avoid when caring for your aircraft windows. The first mistake is using automotive window tint. This may seem like a clever idea, but automotive tint was not designed for use on aircraft. The problem with these products is that they can ultimately cause your window to absorb excessive heat. As stated earlier, overheating the stretched acrylic can cause it to shrink back to its original size. If the window is clamped in, this shrinking could cause it to fall out of the aircraft. Other windows are fastened in, so when the material begins to shrink, all it can do is tear. This can result in a hole in the cockpit window big enough to stick your head through. A second problem with automotive window tint is that it is designed to be applied on top of glass, and the adhesive used on the tint could have a negative effect on the acrylic, leading to premature failure.
The second mistake is using cling-on sunscreens. Many pilots use these to block the sun as they can be moved as needed and generally work very well. However, they are designed only for temporary use. It is strongly recommended that they be removed when the aircraft is parked to prevent overheating the acrylic.
New aircraft, such as the Boeing 787 Dreamliner, are taking exciting strides in cabin window technology. Though they are still made from the traditional stretched acrylic, Dreamliners do not have physical window shades for passengers to use. Instead, the windows utilize electrochromic technology which uses electricity to change the color and amount of light that passes through the window. This is done through the use of a gel that darkens or lightens depending on the change. Applying a small electrical voltage across the gel causes it to darken, while removing the voltage allows the gel to return to its natural transparent state. The voltage can be precisely controlled and adjusted in small increments to allow intermediate states of light transmittance to be selected.
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