Flying Safe

I love flying, looking down on the fluffy clouds and wondering about the lives of the people below. But have you ever wondered about the tiny hole at the bottom of the window? It turns out that this “breather hole” is vital…

An aeroplane window is made up of three panes, originally named the outer, middle and inner. The one nearest the passenger, the inner, isn’t structural. Instead it’s job is to be a scratch pane, for all those tired heads leaning against it. It is the the outer and middle panes which are there to keep us safe. Both are mounted into a rubber seal, with the outer pane a bit thicker than the middle one.

Pressure is usually measured in pounds per square inch, or psi. On the Earth’s surface we exist in 14.7 psi, just right for breathing oxygen. At a cruising altitude of 40,000 feet the air is only 3.3 psi, which would lead to both the pilot and passengers passing out. Therefore the inside of the plane is kept pressurised at roughly 11.1 psi. So during a flight effectively the cabin only climbs to an equivalent of 7000 feet. This pressurisation is vital for allowing us all to breathe on board. (In case you were wondering, the reason it is not 14.7 on the inside is that the pressure difference would require a heavier plane – not so handy for take-off!).

The outer window pane keeps this difference in pressure between the sky and the cabin, earning it the name of “primary structural window”. The middle pane is a back-up, in the unlikely event that the outer window fractures. Keeping this emergency middle window pane safe is the reason why it contains a hole. The tiny hole allows air from the cabin to bleed through into the cavity between the middle and outer panes, making the outer pane carry all of the pressurisation. The middle pane is under no stress, so in the event of the outer one cracking it is ready to take over and hold the pressure in the cabin. Such is the importance of the middle pane, that should a single one be cracked the plane will be grounded!

Apparently the hole is also meant to help alleviate fogging, although I’m not sure that is always the case. I’ve had plenty of windows mist over on a flight. The panes themselves are made from acrylic or plexiglass materials; whilst lightweight the material can be prone to “crazing” where hairline cracks appear. However, any scratches or misting doesn’t bother me – the only thing I’ll be checking from now on is that the middle pane comes with a hole!


If you want to read more, has some great diagrams and covers plane cabin pressurisation.