Have you ever looked out the window while flying, only to discover a small hole in the middle pane? While not found in all airplane windows, many do in fact have holes. Upon seeing this, some passengers assume that it’s a manufacturing defect, but there’s actually a sensible reason for them.
Window Bleed Holes Explained
The holes you see in airplane windows are called “bleed holes.” Normally, airplane windows are comprised of three individual layers or panes of lightweight, flexible material, such as Lexan polycarbonate or plastic acrylic. In the middle pane, there’s usually a single bleed hole at the bottom center. So, what’s the purpose of these bleed holes?
Bleed holes are designed primarily to balance air pressure as the airplane’s altitude increases. A typical commercial jet has a cruising altitude of about 28,000 to 35,000 feet — more than six and a half miles. At altitudes such as this, the air is thin with minimal pressure. However, the interior cabin of airplanes is pressurized to create a comfortable, safe environment for passengers and crew members. As a result, the high air pressure inside the cabin pushes against the windows in an attempt to balance itself with the low air pressure outside the airplane.
Airplane windows are designed to withstand significant stress, including the air pressure created at high altitudes. But they are often designed with a bleed hole to help alleviate some of this stress. Because it’s located in the middle pane, the bleed hole allows pressurized to reach the outer pane. Bleed holes are essentially work as a bleed valve by allowing pressurized air from inside the cabin to reach the outer pane of the window. Without a bleed hole, a window’s inner pane would be exposed to pressurized air. With a bleed hole, however, pressure is transferred to the outer pane, thereby alleviating the inner and middle panes of pressure. Furthermore, bleed holes act as a failsafe in the event that the outer pane ruptures or otherwise breaks.
The bottom line is that you shouldn’t be alarmed if you discover a hole in an airplane window while flying. Known as “bleed holes,” they are a feature of airplane windows and not a defect or sign of damage. Their purpose is to balance the air pressure so that the outer pane of a window is exposed to pressurized air rather than the inner pane. It moves pressurized air from inside the cabin to the outer pane, allowing the air to equalize.