Here's What That Tiny Hole In Airplane Windows Is For

The stellar view from an aircraft window makes window seats the most coveted spots in the airplane cabin, although there are some seats you'd definitely want to avoid. While some mockingly refer to airplanes as "flying tin cans," these engineering marvels incorporate multiple layers of redundancy even in the smallest of the components, making them our safest mode of transportation.

Airplane windows might seem like a simple glass barrier (it is actually stretched acrylic) between the cabin and the outside air, but they hide a treasure trove of carefully planned and well-executed engineering. The tiny hole you may have noticed in airplane windows is not a design or manufacturing flaw, but a feature that helps keep the integrity of the cabin structure, among other things.

You may recall from high school science that atmospheric pressure decreases with increasing altitude. Planes typically cruise at an altitude of 30,000 feet, where the pressure drops to just over a quarter of that at sea level. While breathing at such low pressures, most people would quickly feel dizzy, lightheaded, and lose consciousness due to hypoxia. To prevent this, airplane cabins are pressurized to maintain comfortable levels. Since the cabin has much more pressure than the outside environment while cruising, the entire aircraft structure must handle an enormous amount of outward pushing force. The tiny hole in the windows is commonly known as the "bleed hole" or "breathing hole" and makes sure the difference in pressure is mostly handled by the strongest parts of the window.

To understand why there's a hole in airplane windows, let's first understand the basic science behind them. The aircraft window is a three-pane structure and each layer is made to serve a different purpose. The outer and middle layers are both made of stretched acrylic, with the outer layer being thicker to withstand most of the pressure differential. The middle layer acts as a fail-safe in case the outer layer is compromised, while also preventing condensation (traditional home and office window panes use a similar approach to avoid fogging). The middle layer is generally thinner than the outer layer. The innermost layer isn't meant for handling the pressure at all; it acts as a barrier for protecting the middle and outer layers while providing a surface for passengers to rest their heads.

As the middle layer is a fail-safe, engineers designed the outer layer to face the pressure differential. However, this wouldn't have been possible if the middle and outer layers were completely sealed together. That's where the bleed hole comes in, allowing for pressurized cabin air to pass between the two layers, gradually exerting most of the pressure on the outer layer. This gradual increase in the pressure differential helps maintain the structural integrity of the outer window pane.

As mentioned earlier, the middle layer of the window would come to the rescue even if the outer layer was compromised. However, if both these layers fail, the airplane would run into a serious emergency known as depressurization. That's when the airplane cabin loses its ability to maintain comfortable pressure for the people on board. The inside pressure equalizes to what it is outside, and the people on board would have to breathe the oxygen-scarce air at such low pressures. If other systems on the aircraft are working fine, you would see the oxygen masks deploy for every passenger — a scenario depicted in various movies like "Snakes on a Plane" and "Final Destination."

Most planes have 12 to 15 minutes of oxygen per passenger, allowing pilots to descend the aircraft to 10,000 feet or below where the air is oxygen-rich and comfortable for most people. That said, cabin depressurization is a fairly uncommon phenomenon considering the number of planes flying every day. Moreover, pilots are heavily trained to handle such emergencies efficiently. We have a wishlist of high-tech features we'd like to see on commercial planes, but this relatively low-tech system makes flying safer and more enjoyable.