What’s the Reason Behind the Circular Shape of Airplane Windows?

In most buildings, square or rectangular windows are the norm, though rounded windows — known as oculi in architecture — add a unique and captivating touch to designs.

Consider airplanes and commercial jets. Their windows are always circular. Imagine boarding a plane and fastening your seatbelt beside a square window — it would feel oddly out of place.

In the early days of commercial aviation, square windows were standard in airplane design. For centuries, they had served homes and buildings well, so why alter a proven design?

Tragically, it took several accidents for engineers to reconsider airplane window designs. During the 1950s, as commercial planes grew larger, some began disintegrating mid-flight. Notably, two de Havilland Comets met this fate in 1953 and 1954. Investigations revealed the culprit: the square windows.

Engineers discovered that the sharp corners of square windows acted as weak points, leading to "metal fatigue failure." These edges were prone to stress and further compromised by high-altitude air pressure.

By the 1950s, aircraft such as the de Havilland Comets were flying higher and faster than ever before. Over time, repeated pressurization cycles caused the square windows to fracture under the strain.

In contrast, rounded windows evenly distribute pressure since they lack corners where stress can accumulate, minimizing the risk of cracks or fractures. Their circular design also enhances strength and prevents deformation, enabling them to endure the constant pressure changes between the aircraft's interior and exterior.

You may also observe multiple acrylic layers (not glass) separating you from the outside environment. These layers provide extra defense against weather conditions such as rain, wind, and fog. And that small hole at the base? Known as "bleed holes," they play a crucial role in maintaining stable cabin pressure by allowing air to flow between the window layers.

So, the next time you’re on a plane, take a moment to appreciate those rounded windows, whether you’re marveling at the view or avoiding it to spare yourself the reminder of your altitude.