The Reason Airplane Windows Have Tiny Holes in Them

During a long flight, have you ever idled away the hours by taking a closer look at the aircraft cabin? If so, you might have noticed some unique and unusual details. For instance, unlike windows found elsewhere, the windows on an aircraft are oval instead of square. Find out why that is and the significance of that tiny hole you see at the base of each window.

Aircraft Windows Haven’t Always Been Round

Introduced during the early days of commercial aviation, the De Havilland Comet was a pioneering British plane that was the first to be fitted with jet engines. In contrast, rival planes at the time, such as the Lockheed Constellation, were powered by propellers. The Comet, launched in 1952, flew at higher altitudes and faster than its predecessors. When it first went into service, the Comet had square windows. Though no one knew it at first, this would prove to be a devastating flaw.

At first, there were no issues with the Comet, but as the planes racked up more mileage, fatigue fractures started to appear. Several Comets crashed, including one that plunged into the Mediterranean Sea on January 10, 1954, shortly after takeoff from Rome. When experts inspected the wreckage, they noticed that an 8-foot-long fracture had occurred in the corner of one of the windows that was positioned where the radio aerials were located, impacting another window frame and contributing to the breakup of the fuselage after takeoff.

Improving Airline Safety

Throughout the history of commercial aviation, investigators have used the knowledge gained from the findings in the aftermath of plane crashes to inform changes that should be made to improve safety. Though metal fatigue was officially the cause of those early De Havilland Comet crashes, the square design of the windows, coupled with the pressurized cabin, was considered a contributing factor. 

As a direct consequence of those tragic early crashes, Comets were grounded indefinitely, until they could be retrofitted with oval windows to resolve the problem of stress concentrations at the corners. Not only is a rounded shape stronger, but this new design was also better able to withstand the differences in pressure found on the inside and outside of the aircraft. (Aircraft cabins are pressurized to ensure the concentration of oxygen is high enough for us to breathe while traveling at high altitude.)

What Are the Tiny Holes for in Aircraft Windows?

The rounded shape of the windows you see on modern airplanes isn’t the only important aspect of their design. The internal layers you see on the window beside you are made of acrylic rather than glass. Look closely, and you’ll see a tiny hole in the nearest of the three layers. It’s known as a bleed hole or a breather hole. Though tiny, it plays an important part in keeping us safe while we’re in the air. 

Because air pressure varies with altitude, plane windows need this tiny hole to allow sufficient air to pass through in order to equalize the pressure. The stress remains greatest on the outside, which prioritizes passenger comfort and safety. If there was no hole, the outer pane could theoretically crack or even pop out completely. The hole’s secondary function is to prevent frost or condensation from building up, so it helps to ensure you always have an unobstructed view of what’s outside the window.

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