Wilbur and Orville Wright flew an open wooden biplane, exposing pilots to outside air. Modern commercial airliners cruise around 30,000 feet where temperatures often drop to -40°F or lower, requiring insulated cabins. Cabin windows are typically three or four layers of plexiglass or stretched acrylic rather than glass. Each window contains a single breather or bleed hole located in the center of the inner pane near the bottom. The hole sits between inner and outer panes and does not open to the outside, so covering it does not cause airflow into the cabin. Altitude changes cause pressure differences that the hole helps address.
Their modest aircraft, a wooden biplane made of spruce and ash covered by cotton muslin, was open to the outside air. Fortunately, their history-making flight in 1903 lasted just 12 seconds and barely reached 10 feet off the ground. Today's commercial planes spend most of their time at 30,000 feet (or more) where the temperature hovers around -40 degrees Fahrenheit, and temps as low as -70 degrees aren't unheard of. For that reason, we need insulated aircraft to keep from freezing to death.
While we don't get the same killer views the Wright Brothers had in their open-air design, we can book a window seat. And if you've ever sat in one and noticed that the window has a tiny hole in the bottom of it, hopefully you didn't panic. It's not a manufacturing mistake. It's an intentional feature that serves two key purposes-and it even has a special name.
Airplane cabin windows aren't made of glass; they're typically made of three or four layers of plexiglass (which is a bit of a misnomer since it's actually a form of plastic) or stretched acrylic. There is one bleed hole per window, and it's usually found in the center at the bottom of the window. If you've ever placed your finger over the hole, you'll notice that no air comes in or out.
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