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Australians taking to the skies for the first time since the Covid pandemic ended are noticing the tiny ‘hole’ on their plane windows for the first time – and have questioned what it does.
Known as the ‘bleed hole’, the small opening in the window of commercial airliners is actually an important safety feature, according to aviation experts.
The hole helps to regulate how much pressure from the cabin is applied to the window panes and it ensures that if the pane breaks, the outermost pane breaks first.
So it is best not to touch the vent hole or cover it in any way so that it can do its job effectively while in the air.
Known as the ‘bleed hole’, the small opening in the window of commercial airliners is actually an important safety feature, according to aviation experts
This information appeared to ‘blow the minds’ of keen travelers online, with a post about the holes going viral.
While hundreds of others revealed they had ‘never noticed’ the holes.
“I’ve never noticed it like that, but I’ll look when we fly up,” said one woman.
While others wondered what would happen if everyone ‘blocked the gaps’ without realizing they are in the center pane of a three-pane system.
The question was posed by Robbie Gonzalez of iO9 to Chief Technology Officer Marlowe Moncur of GKN Aerospace.
A patent filed by Daimlerchrysler Aerospace Airbus in 1997 explained that this ‘air channel’ helps maintain ‘external atmospheric pressure inside’ the panes.
On an airplane, the air is pressurized by engines that compress it as it moves through a series of fans.
In order to maintain the cabin pressure, even at high altitudes, this incoming air is kept inside the cabin by means of a so-called outflow valve.
In order to maintain the cabin pressure, even at high altitudes, this incoming air is kept inside the cabin by means of a so-called outlet valve
It works in the same way as a tire is inflated – high pressure air is ‘pumped’ into the cabin and this air comes from the compression stage of the engines.
Sensors measure how much pressure is in the cabin, and this valve releases the air at a rate that maintains that pressure.
For example, when the aircraft is stationary, this valve is open. It only starts to close when the plane takes off.
Air at sea level is said to be about 14.7 pounds per square inch (PSI).
In comparison, a typical airplane cruises at between 9,150 meters and 12,200 meters, and at this altitude the pressure is approximately 4.3 PSI.
Due to the lack of oxygen at high altitudes, the aircraft must be pressurized in such a way as to make it comfortable and safe for the passengers.
Philip Spiers, head of the Advanced Structural Testing Center at the University of Sheffield Advanced Manufacturing Research Center (AMRC) with Boeing told MailOnline that at high altitudes ‘there are not enough oxygen molecules to sustain life.
‘Low pressure lowers the boiling points inside the body and on the edge of the room this can cause blood and tears to boil.’
The planes don’t go that high, but their flying height is usually higher than Mount Everest, and they take air to the cabin from the engines.
These engines spend their time compressing air up front to generate thrust, but Mr Spiers continued that they also vent some of that air in the process, dehumidify it and pump it into the cabin to provide the thrust.
“Airplanes have a higher pressure inside than outside,” Mr Spiers said.
‘It’s like a bottle of Coca Cola – shaking a bottle makes it stiff and hard, but when you undo it, it becomes discreet again. This stretches the skin around the plane.’
Typically the air inside the cabin is kept at around 11PSI, they experienced pressure levels of around 2,130 metres. And this change in pressure is what causes a person’s ears to pop.
To maintain this pressure, the structure of the aircraft and its windows must be able to handle the difference between the cabin pressure and the outside of the aircraft.
As a result, windows on commercial aircraft typically have three panes—outer, middle, and inner—made of acrylic and glass.
Spiers continued that the plastic pane on the inside of the plane is to prevent passengers from accessing the glass panes in the middle and on the outside.
There is a middle pane with a hole in it, then an air gap, followed by an outer pane of glass.
This hole helps maintain the pressure difference and directs it towards the outer pane instead of the inner pane.
This hole helps maintain the pressure differential and directs it towards the outer pane instead of the inner pane (stock image)
‘If the window was sealed [and didn’t have a hole in it]all the pressure in the cabin would act on the inner glass pane,’ continued Mr Spiers.
‘You want [this pressure] to act on the outside pane because if there is a problem with the outside it would be possible to see it during inspection.
‘If this pressure blows the pane out, the inner pane is still strong enough to withstand the pressure. You don’t want to see the inner pane fail first as the inspectors wouldn’t see it.’
‘Plus, this gives enough time for the aircraft to descend to a lower altitude to clear the problem.’
Michal Weiszer, researcher at the School of Engineering at the University of Lincoln added:
‘During the flight, the cabin is under pressure, and therefore it is necessary to equalize the pressure between the inner pane and the window itself, so that the outer window holds the load of the pressure difference.
‘Furthermore, the hole prevents moisture from building up between the panes.’
And Dr. Raf Theunissen, associate professor of aerodynamics at the Department of Aeronautical Engineering at the University of Bristol, said: ‘You can understand why the airlines put in these extra windows just by looking at the number of scratches on them.
“We don’t want to have scratches on the window itself because that reduces the strength.”
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