Have you ever noticed that the windows in an airplane cabin are round, without sharp corners like the windows in buildings? Perhaps you enjoy sitting by the window seat on a plane to admire the scenic views along the way. But why are airplane windows round and not square?
According to aviation news website “Simple Flying,” all cabin windows on airplanes are now either circular or have rounded corners, rather than sharp angles. However, this was not always the case. The earliest jet airliners were equipped with square windows, but after several mid-air disintegration accidents in the 1950s, aircraft manufacturers changed to round windows.
The Federal Aviation Administration (FAA) pointed out that the square windows were a contributing factor in three mid-air disintegration accidents involving the de Havilland Comet 1 aircraft produced by the British aircraft manufacturer de Havilland in 1953 and 1954.
These accidents resulted in the sudden disintegration of the aircraft while in flight, leading to fatalities. Investigations revealed that the stress concentrations caused by the square windows far exceeded previous calculations. The FAA stated, “These stress concentrations led to material fatigue at the corners of the windows, ultimately resulting in rapid structural failure of the aircraft.”
As a result, aircraft manufacturers like de Havilland switched to using round windows, and the aviation industry as a whole recognized that airplanes should only use round or rounded rectangular windows.
The sharp angles of square windows create local stress, whereas round or rounded windows help distribute stress more evenly, aiding the aircraft in resisting cracking under cyclical loads.
Aircraft cabins are pressurized, causing the fuselage to bear outward expansion stress. Every opening on the fuselage, such as windows, can compromise the overall structure, leading to stress concentrations. Corners act as magnifiers, amplifying stress. Stress concentrations at the corners of square windows could be three times higher than those in curved windows, depending on various factors.
Cyclic loading, not flight hours, significantly influences aircraft aging. The stress from a 30-minute short-distance flight is comparable to that from a 12-hour long-haul flight. This affects the fuselage skin, fasteners, lap joints, and frames. Among these, window and door openings endure some of the most severe pressure differentials.
In 1988, an Aloha Airlines Boeing 737-200 aircraft experienced an upper fuselage rupture incident, an example of cyclical fatigue. The aircraft did not fully disintegrate, and the pilots successfully made an emergency landing, saving all onboard.
The National Transportation Safety Board (NTSB) attributed the incident to severe detachment and fatigue damage in the fuselage skin joints, caused by cyclic pressurization. Notably, the round windows used in the aircraft were not the structural failure points in this incident.
In addition to utilizing round windows, another solution to avoid fuselage disintegration is to eliminate windows entirely. Spike Aerospace, an American aerospace company, is developing a supersonic business jet without windows, replacing them with full-size high-definition digital screens. This aids the aircraft in withstanding the pressures of supersonic flight.
Traditionally, ship windows also feature round designs for similar structural reasons. However, the physical characteristics of ships at sea and aircraft in the sky are not identical. Modern cruise ships typically have large rectangular windows that are reinforced and feature large radius corners.
While ships do not face rapid pressurization cycles like aircraft, windows at different levels on the ship endure varying pressures due to continuous wave impacts at the lower hull and strong wind forces higher up.
Square windows commonly seen on cruise ships are often not part of the hull structure but rather a less critical upper deck component.
For space shuttles, portholes are typically round for structural integrity, though they may adopt other shapes like trapezoids to aid crew visibility during docking maneuvers. While the portholes in regular airplane cockpits may not always be round, they tend to have rounded edges.
It’s worth noting that Christine Cancar, a female pilot, has revealed that passengers seated by the window may face higher health risks during flights as airplane windows do not filter out harmful UV rays from sunlight.
Due to the increased risk of skin cancer in the elderly, it is advisable for them not to sit by the window when traveling by plane. Additionally, Linda Ferguson, a senior flight attendant, emphasizes the importance of not resting your head on the window or walls when napping for hygiene reasons.