Have you ever sat there at 30,000 feet and wondered, “how is this possible?” Have you ever wondered why airplanes are able to do what they can? Well, I am here to help. In a series that I will call Aircraft 101, I will try to explain how physics gives – what appears to be – the magic of flight. These posts will be targeted at the common traveler, and I will try to avoid too many complicated details. That being said, forgive me if I get carried away.

In this post we will look at: Aircraft Windows.

Once you have boarded your flight, the windows are most likely the first the to gain your attention. Whether it’s to have a look outside at the scenery, watch the airport workers toss around your precious luggage, or just to simply shut the window shade. Once on board, the windows of an aircraft provide a link to the outside world. They may seem simple, but they have more of a background than you may realize.

Connecting Us To The Outside World

Studies have shown that most airline passengers prefer the window seat. Even those who don’t will probably admit to stealing a glance out of the window during take-off and landing. People enjoy being able to see what’s around them. This is true in all vehicles, from cars to spacecraft.

When building a vehicle, windows can pose a challenge for engineers. Typically, the desire is to build a vehicle that is extremely tough and really reliable, with limited failure points. In this sense, windows are useless and only weaken the structure you are trying to create. This is especially true when it comes to aviation. Discussions have persisted for years about using virtual windows on future aircraft designs, but so far this has not become reality (although Emirates has unveiled virtual windows in the 777 first class suites).

Windows on Apollo 11 spacecraft. Large opening is where the hatch would have been.

I remember being told once that there was a push to remove the windows in the new Orion spacecraft and replace them with virtual windows. The astronauts did not like this and demanded there be windows. Screens, unlike windows, don’t provide the same connection to the outside world.

Shape Is Important

When staring out the windows on your flight, have you ever wondered why they are shaped that way? Well, the answer to that question is actually really important. The shape of the window is designed to keep you safe, but at the beginning of air travel this wasn’t necessarily the case.

Pressurized Cabins

The first pressurized airliner to enter commercial service was the Boeing 307 Stratoliner. The use of a pressurized cabin allowed the aircraft to fly around 20,000 ft. This allowed the aircraft to fly over many weather disturbances. This was a revolutionary and led to air travel as we know it today.

During flight, the pressure on the interior and exterior of the aircraft will change. At sea level both the exterior and the interior pressure are equal to the standard atmospheric pressure of 14.7 psi.

Once in flight, the aircraft will climb to cruising altitude. At this altitude the atmospheric pressure drops significantly. In this example we are using a cursing altitude of 35,000 feet, which would mean the exterior pressure of the aircraft drops to 3.5 psi (atmospheric pressure associated with this altitude). In order for passengers survive in these conditions, the aircraft cabin is pressurized. The aircraft is actually pressurized to about 11 psi, which is less than standard atmospheric pressure (this is equivalent to the pressure at about 8000 feet).

Pressuring up the cabin to only 11 psi provides a reasonable comfort level to the passenger while at the same time reducing the stress on the aircraft. As the aircraft descends for landing, the pressure inside the cabin is returned back to the atmospheric pressure until the pressures on the exterior and interior of the aircraft are once again equal. This is what is known as the “pressurization cycle” of the air-frame, and plays a significant role in the windows shape.

The Early Years

In total, ten Stratoliners were built. Pan Am would operate three of these aircraft, while TWA would operate five. The aircraft entered service in 1940 with Pan Am, but both Pan Am and TWA would lend the aircraft to the US government for use in WWII. The aircraft would be retired from service in the US by 1951, but served as a good stepping stone for future aircraft.

Pan Am Stratoliner Windows, photo by Mike Peel, licensed under CC BY-SA 4.0

Next we have the De Havllind Comet. The Comet was the first jet airliner to enter commercial service. It was sleek-looking and intriguing to the entire airline industry, offering a relatively quiet flight and great passenger experience. One of the items the Comet offered was relatively large windows. The Comet entered service in 1952, but was immediately plagued with issues. After multiple fatal crashes, the aircraft was grounded in 1954.

Early version of the de Havilland Comet (Comet 1) showing the square window design

It was determined that metal fatigue was the cause of the crash. One of the causes of this fatigue was high stress concentrations at the corners of the large windows. As part of the response and re-certification of the aircraft, the square windows were replaced with round ones, which resulted in much lower stress in the air-frame. The redesigned aircraft returned to service and flew with several airlines for many years before being retired.

Redesigned windows of later Comet aircraft, photo by Ian Dunster, licensed under CC BY-SA 4.0

Stress Fatigue

The issue with square windows lies in the pressurization cycle. As the aircraft undergoes a pressurization cycle, the air-frame essentially grows and shrinks. Looking at the graphics below, you can see how the differing pressures on the interior and exterior of the aircraft cause the airframe to be stressed.

Equal pressures place the air-frame in a “relaxed” state

Differing pressures place the airframe in a “stressed” state

In an exaggerated example, think of the aircraft as a balloon. You take this balloon and you blow it up. You then let all the air out, and repeat these steps over and over. As you blow up the balloon, the plastic material of the balloon stretches because the pressure is increasing. As the plastic expands, it is stressed. This is called hoop stress. The aluminum airframe of an aircraft is not as flexible as the plastic of a balloon, so this hoop stress has to be managed.

As this “stretching” occurs, any relatively sharp corners (such as the corners of square windows) in the airframe concentrate this stress in small areas. After several pressure cycles, stress fractures will begin to form in these areas. As the aircraft continues to undergo pressure cycles, these fractures, or cracks, in the air-frame will then began to propagate throughout the airframe. This will continue until the air-frame fails, most likely resulting in a complete loss of the aircraft.

Window on a modern Boeing 787 aircraft

Replacing the sharp corners of the square windows that were used on early aircraft with a more blended, or rounded, corners significantly reduces the air-frame stress in those locations. This is the reason for the switch from square to round windows in the Comet. This is also the reason that the window you look out of in modern aircraft is shaped the way it is.

Modern Windows

Learning from the past developments of their predecessors, modern aircraft makers have continued to use the modified window design. Using the larger square window layout, but implementing rounded corners, has allowed for larger windows while still keeping stress concentration low enough for safe air-frame pressure cycling.

So, as I said, there is a little bit more history in that window than you may realize. Who would have known that so much relies on those rounded corners of your window. Well, now you do!

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