YOU are cruising at 35,000 feet, peering out at the clouds, when you notice something that looks like it definitely should not be there: a small hole in your airplane window.
It seems like the kind of thing that would send a nervous flyer into a full spiral, but it has been there on every flight you have ever taken.
Does that tiny hole actually serve an important function?
Verdict:
TRUE
The small hole, known as a bleed hole or breather hole, plays a critical safety role on every commercial aircraft, allowing air to move between the window's inner air gap and the cabin interior to prevent dangerous pressure imbalances.
When an aircraft reaches cruising altitude, there is a significant difference in pressure between the thin air outside and the pressurised cabin within, which is kept at conditions mimicking the atmosphere at a much lower altitude.
An airplane window is not a single pane of glass but a multi-layered structure made from polymethyl-methacrylate acrylic, a tough transparent plastic.
The outermost layer is structural and sealed against the external environment.
The second layer, separated from the outer one by an air gap, acts as a fail-safe in the event the outer layer is damaged or compromised.
The third and innermost layer exists to protect the other two from dirt, scratches and accidental damage by passengers.
The breather hole sits in the inner layer, regulating airflow through that gap and ensuring that pressure does not build up unevenly across the window structure.
This design was not always the case.
In the early days of aviation, jets flew at lower altitudes and windows were square-edged, which posed no particular structural problem at the pressures involved.
But window design changed in the 1940s, when higher-flying planes created the need for pressurised cabins, beginning with the Boeing 307 Stratoliner.
In the 1950s, United Kingdom's de Havilland Comet experienced a series of catastrophic accidents, and the planes’ failures were eventually traced back to square window corners that caused metal fatigue, according to a report by Reader's Digest.
Engineers then determined that rounded windows with breather holes were necessary to reduce pressure issues and structural stress caused by repeated pressurisation and depressurisation cycles.
This tiny tweak was practised for other airplanes, and this made flying a lot safer.
Interestingly, there is an aesthetic reason for the breather hole as well - it releases moisture from the gap between the inner and outer panes, allowing the window to remain mostly fog-free.
So, the next time you are in an airplane and view the clouds, you now know why you are able to see the wonderful view, as well as fly safe to your vacation destination.
Source:
1. https://time.com/3903109/
2. https://www.rd.com/article/
3. https://www.afar.com/magazine/
