1. How Airplane Windows Differ from Home Windows: No Corners
Airplane windows are made with curves that have no corners, whether viewed from above or the side. Some are close to a circle, while others resemble tall ovals. [1]
Many windows also have a very small hole near the bottom edge. This feature is just as connected to lessons from past accidents as the window shape itself. The role of this small hole will be explained in a later section. First, let us look at the jet airliner accidents from the 1950s that are said to have prompted this widespread change in design.
2. The Comet Jet Airliner Crashes of 1954
The Comet was introduced as the world's first passenger aircraft powered by jet engines. Its selling point was that it could fly fast at high altitudes. [1]
However, on January 10, 1954, BOAC Flight 781 crashed in mid-air. On April 8 of the same year, South African Airways Flight 201 also broke apart and fell from the sky. [1]
The accident involving Flight 783 on May 2, 1953, is also counted as part of this series of crashes. To organize these without mixing up years or flight numbers, there were three main incidents. High in the sky, the aircraft structure failed. Engineers had to find out why.
3. How Repeated Cabin Pressurization Causes Metal Fatigue
Air is thin at high altitudes, making it impossible to breathe without help. Passenger planes pump air into the cabin to maintain pressure similar to ground level. This process is called pressurization. The aircraft fuselage is pushed from the inside like an inflating balloon. [1]
The committee investigating Flight 781 concluded that the cabin failed around a window for an antenna on the roof, known as an ADF window. The cause is said to be metal fatigue resulting from repeated cycles of pressurizing and depressurizing. [2]
Metal fatigue occurs when metal develops cracks after receiving small forces repeatedly, even if a single force would not break it. It is similar to how a wire or paperclip eventually snaps if you bend it straight and back again many times.
4. The Farnborough Water Tank Test: Inflating an Aircraft Again and Again
The Royal Aircraft Establishment at Farnborough in Britain conducted tests by submerging an aircraft of the same type as the crashed plane, registration G-ALYU, in a large water tank. They pumped water inside to repeat the pressurization process. [4]
Using water is explained as making the test relatively safer because if the structure breaks, it does not expand rapidly like air would. The aircraft had already undergone 1,221 pressurization cycles during actual flights. Adding about 1,800 more cycles in the tank brought the total to approximately 3,000 cycles before the fuselage ruptured, according to records. [4]
Because different sources cite slightly different totals like 3,057 or 3,060, we use "about 3,000." This is said to correspond to about 9,000 hours of actual flight time. [6]
Manufacturer tests reportedly showed no damage up to 18,000 cycles. This means the real aircraft failed much sooner than expected.
5. Did Square Windows Really Cause the Comet Crashes?
You often hear the explanation that "the windows broke from their square corners." However, looking at the records reveals some discrepancies. [1]
The passenger windows on the Comet were not perfect squares; they were rectangles with rounded corners. Still, the curve of the corners was sharp enough that forces concentrated there more than designers had predicted. [3]
Furthermore, the investigation committee did not identify a passenger window as the starting point of the failure. The crack is understood to have grown from a rivet hole for a countersunk screw at the rear-right corner of an antenna window (rear ADF window) above the cockpit. [3]
The investigation also showed that the design company was working within the scientific limits of the time, and that rivet holes punched out could also be starting points for cracks. It seems we cannot say the shape of the window alone was the cause. [4]
6. Why Sharp Corners Concentrate Force
The small notches on snack bags are there to concentrate force and make the bag easier to tear open. When a plate has holes or cutouts, much greater force concentrates on parts of the edge than in the surrounding area. This is called stress concentration. [5]
Theoretically, if a round hole exists in an infinitely wide plate, the maximum force at the edge of the hole becomes three times the original force. For elliptical holes, this factor increases as the shape becomes more elongated and oriented poorly. Sharp internal corners also locally increase forces. [5]
Rounding a corner does not reduce force to zero. The key point is that even at three times, it is less than what occurs at a sharp angle. Generally, rounding corners helps suppress the concentration of force. Japanese Wikipedia explains that after the Comet accidents, adding corners to openings in aircraft became something to be avoided. [1] Please read this as separate from the direct cause of the accident or specific design changes.
In the 1950s, aircraft with "onigiri-shaped" windows (rounded squares) were attempted but did not become widespread. There are also examples of US Air Force transport aircraft painting the edges of round windows black to make them look square. [7]
7. The Bleed Hole: The Small Hole in an Airplane Window
Airplane windows are described as having a structure with three layers of acrylic panels stacked together, sharing the pressure difference between inside and outside the aircraft. [8]
The middle layer has a small hole called a bleed hole. This hole allows cabin pressure to act on the outer pane, meaning the outer pane usually takes the pressure load while the middle pane serves as a backup that receives less force. It is also said to help prevent fogging. [8]
The position and number of these holes vary depending on the source and aircraft model. Therefore, this is introduced here as a "common mechanism."
8. Try It Yourself: Experiments to Investigate Further
What you can do today: Compare snack bags with notches to paper without notches. Prepare two pieces of paper the same size, one with a small cut made by scissors at the edge and one without. Pull them left and right with both hands to compare how easily they tear. Use scissors together with an adult.
What you can do when out: If you have a chance to ride in an airplane, look for the rounded corners of the windows and the small hole near the bottom. Even just looking at the row of windows on the aircraft from an airport observation deck can lead to discoveries.
Official information: Airlines, aviation museums, and introductory books on aeronautical engineering available in Japanese discuss stress concentration and metal fatigue.