Why Are Airplane Windows Oval?

Have you ever sat in a window seat on an airplane? What a better way to fly compared to aisle seats, watching the white clouds go by flying over natural landscapes!

Now, have you noticed the shape of airplane windows? You’d probably know even if you’ve not paid attention. As you know, they’re oval shaped and cornerless. Did you know that shape resulted from trial and error? Let’s dive into the special story of airplane windows with Incheon Airport!

Truth behind accidents with Comet, the first jet passenger

De Havilland DH-106 Comet with square windows (Source: Wikipedia)

The story about oval airplane windows starts with the de Havilland DH-106 Comet, the world’s first commercial jet airliner. Comet, which began operating in 1951, is a very small passenger airplane by today’s standards as it can accommodate 40 passengers. But it was a symbol of advanced technology at that time. This passenger airplane made up more than half of the passenger air travel market.

From 1953, however, there were three accidents where it crashed midair. From May 1953 to April 1954, the accidents claimed 99 lives. At first, they could not figure out what caused the accident. After two more similar accidents, it was becoming convincing that there was something wrong with DH-106 Comet.

In the end, the UK government launched an investigation committee to identify the cause and conducted a simulation test. In the test, DH-106 Comet was placed in a huge water tank to reenact how the airplane broke down by adding and removing water and applying pressure. In the simulation test, the airplanes broke down in a similar fashion, and they found what caused the accidents!

Remains of an airplane with square windows (Source: Wikiwand)

There were two causes for the accidents! It was because of rivet screws and square windows used in airplane assemblies.

The first cause was rivets, which are still used today. At that time, there was no drilling before screwing rivets, so they were very unstable. Small cracks appeared in loose rivets, and fatigue (stress) snowballed due to an air pressure difference between the inside and outside of the airplane.

Here square windows further exacerbate cracks and stress. Square windows apply pressure around their edges, causing stress to be concentrated. As stress was introduced to airplane windows, cracks appeared from their edges, which were the weakest points, and led to an explosion midair.

It was revealed that square windows are most vulnerable to airplane fatigue, and this airplane model was modified and has been operated until 1997.

De Havilland DH-106 Comet modified with oval windows after accidents (Source: Wikipedia)

How have airplanes changed after the Comet accidents?

After unfortunate accidents with Comet, how have airplanes changed?

Square windows most vulnerable to stress turned into oval ones. If window edges are not sharp, but oval, it can minimize impact on windows despite stress from an air pressure difference.

Since then, all airplane windows have been designed as oval shapes. All doors in airplanes are made with round, not sharp, edges. A similar example is a spaceship or submarine’s windows, which are also oval and undergo huge pressure.

There is another way to minimize stress due to air pressure other than oval windows. It is to use acrylic plates, which are light and flexible, instead of glass. Acrylic plates have the advantage that it can withstand a pressure difference.

Did you know that there is a small hole in windows? The so-called breather hole reduces the temperature difference between the inside and outside of the airplane and balances pressure. The breather hole can prevent fogging no matter how big a difference there is between inside and outside temperatures.

Isn’t it surprising that a seemingly unimportant window shape has a huge effect on safety? We will come back with another interesting story about airplanes! J

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