This seemingly insignificant hole is one of the core components of the porthole safety system. Engineers usually call it a "breathing hole" or "exhaust hole."

       Liu Jianhui: Aircraft windows have three layers. The outermost layer is called the structural layer, which carries approximately 90% of the pressure at cruising altitude. The middle layer bears some of the pressure, primarily protecting the outer layer. The layer closest to the passengers is the decorative layer and anti-wear layer. When in the air, the outside air pressure is relatively low, approximately 0.2 Pa. The small hole is actually called a pressure relief vent. The pressure difference between the inside and outside is large, and the small hole balances the pressure between the inner and outer layers.

As the aircraft climbs and the outside air pressure drops dramatically, the high-pressure air in the cabin flows through this small hole and slowly flows into the gap between the inner and outer panes of glass. This ensures that the majority of the cabin pressure is borne by the stronger outer pane, thus protecting the inner and middle panes from deformation or cracking due to excessive pressure. This makes the outer pane the true "load-bearing" pane. If the outer pane is damaged, the middle pane can still temporarily maintain cabin pressure, buying the pilot valuable time to respond and allow the aircraft to descend to a safe altitude.

　　Jin He, an engineer at the Airbus Beijing Engineering Center, said: " The second key is balancing temperature and humidity. As the aircraft ascends, the air outside becomes thinner and the temperature drops. We know that in winter, when the outside temperature is very low and the inside is very hot, condensation forms on the windows. On an aircraft, we prevent this by using air flow to maintain a more even temperature and humidity, thus preventing the formation of condensation."

A large temperature difference between the inside and outside of the cabin can cause condensation, or even ice. Once fog or ice forms, not only can passengers lose sight of the scenery outside, but the crew also needs to observe the wings and engines through the windows. If the windows are obscured and unable to see, flight safety is affected.

Furthermore, this small hole can help engineers determine if an aircraft's window is leaking. The aircraft cabin is a pressurized environment. If a tiny crack or leak develops somewhere in the outer window, the cabin air will be ejected through the hole, creating a small stream of air. This airflow will scour the inner wall of the outer window, leaving a noticeable mark at that location over time. Maintenance personnel can simply detect these marks and determine if the window is leaking and roughly estimate the severity of the leak.

　　Will the "breathing holes" in aircraft window windows affect flight safety?

In addition, many passengers are very concerned about the small "breathing hole" on the porthole, which carries multiple functions. Will this small hole become a weak link in the porthole and affect flight safety?

　　Jin He: This small hole is intentionally designed. It not only does not affect the safety of the aircraft, but also makes it safer.

According to the engineer, although the "breathing hole" is only the size of a needle tip, its size and position have been carefully measured and cannot deviate at all.