Why are aeroplane windows so tiny?

By Simon Jones

Why can’t a plane be more like a bus?

Here’s a little public transport riddle for you. Why is it that buses have really big windows, but aeroplanes have tiny windows? That’s what I’m going to explore in today’s post, including a nice little demonstration of why we want to keep them closed!

Buses have big windows so that all the passengers can easily see and laugh at pedestrians getting splashed by puddles, having their umbrellas turned inside out, or dropping their phones on the street.

Routemaster bus, an excellent way to view pedestrians (Photo: R.W. Calamar, CC-BY)

Routemaster bus, an excellent way to view pedestrians (Photo: R.W. Calamar, CC-BY)

But why can’t we have these lovely big Birmingham Double Glazing windows on aeroplanes? Well, the reason is one of stability. When an aeroplane is 30,000 feet or more in the sky, it is surrounded by an atmosphere of far lower pressure than it was on the ground, which means the air pushes with less force against the things it touches. However, inside our aeroplane we need a pressure similar to what we experience on earth in order to allow us to breath.

If you have an area of higher pressure inside the aeroplane, and an area of much lower pressure outside the aeroplane, that creates a big pressure difference. Air with a higher pressure will try to move to an area of lower pressure. That means the air on the outside is not pushing much and the air inside is pushing a lot. The plane wants to expand outwards.

To make sure the fuselage (the bit you sit in) can withstand that pressure difference and force, it needs to be made out of the strongest materials available. Glass is just not strong enough to be used for large pieces.

In the future, Airbus are aiming for with their concept plane, which gives an idea of what aeroplanes might be like in 40 or 50 years time.  Materials Engineers are working on new materials which offer strength along with other characteristics, like transparency!

Ok, but what about the cockpit windows? They’re nice and big and square!

The view from the cockpit (Photo: Andrew McMillan, PD)

The view from the cockpit (Photo: Andrew McMillan, PD)

This is true. However, from an engineering design point of view, this is a re-arrangement of priorities. The pilots have far more need to be able to see where they’re going than the humble passenger, and for this reason aeronautical engineers spend hundreds of thousands of dollars on ultra-strong windows; stronger even than the protective glass you see in banks.

These windows not only withstand the incredible pressure difference and the occasional unlucky bird, they are also held in very versatile frames designed to absorb and endure some of the strains of flying.

Is it true, that if a window on a plane breaks, a person can be sucked out of the tiny hole that’s left behind?

Boeing 777- Look at those tiny windows! (Photo: N509FZ, CC-BY-SA)

Boeing 777- Look at those tiny windows! (Photo: N509FZ, CC-BY-SA)

Sure, it’s true… if you live in a Hollywood movie. As far as I know, this has never actually happened. Here’s the thing: even if a passenger window were to pop out in a freak accident, the difference in air pressure would be reconciled very quickly, in a matter of seconds. So while there would be a strong suction, it wouldn’t last long enough to actually get someone all the way through a window. Despite the stories, humans don’t often get crushed and squash neatly through tiny holes under pressure; we are far more likely to block and seal the hole!

 

Of course, if an aeroplane window were to fall out, the high pressure air would quickly leave the cabin and a rapid and dramatic reduction in air pressure wouldn’t be good news. Let’s do an experiment to find out exactly what would happen.

Try it yourself

Not by flying in a depressurised plane, obviously!  If you have access to a Vacu Vin container with vacuum pump you can try this neat little demo.

Mr Marshmallow ready for depressurising

Mr Marshmallow ready for depressurising. Image: science made simple CC-BY-NC

You will need

1. Vacu Vin container and pump (or other method for creating a vacuum in a container)

2. Marshmallows

3. Cocktail Sticks

What to do

1. Make yourself a marshmallow man

2. Pop him in the container and pump out as much of the air as you can

3. Have a look at him, has he changed?

4. Release the valve to let the air back in an observe any changes in Mr Marshmallow as you do

Oh no!

Poor Mr Marshmallow puffed up when the air pressure decreased and shrank back down again when the air was let back in.  That’s not the sort of thing we want to happen to our bodies especially our lungs, but so long as you don’t hold your breath at the same time you can avoid it. Holding your breathe means the air is trapped so when it expands you would be in trouble. If you do carry on breathing the expansion is not enough to harm us, unless you were flying really high.

Trying to breath in low pressure air isn’t much fun though, it’s harder to get the oxygen you need to your brain and might make you faint. However, oxygen masks would automatically descend for us all to put on (after we’ve panicked for a little bit, of course), allowing us to breath at pressure for a minute or two, until the pilot flew the aeroplane down low enough to reach an area of higher pressure allowing us to breathe once more.

If you don’t have your own vaccuum sealed container and pump, you and your pupils can see it for yourself in our Tomorrow’s Engineers show. We discuss how engineers at Airbus work to ensure people don’t get sucked out of the window, and how these relates to swiss cheese! If you’re intrigued, click here to find out more…!

Curriculum Links

England

Design and Technology-Evaluate

  • Analyse the work of past and present professionals and others to develop and broaden their understanding
  • Investigate new and emerging technologies

Design and Technology- Technical Knowledge

  • Understand and use the properties of materials and the performance of structural
    elements to achieve functioning solutions

Biology- Gas Exchange Systems

  • The mechanism of breathing to move air in and out of the lungs, using a pressure model to explain the movement of gases, including simple measurements of lung volume

Physics- Pressure in Fluids

  • Atmospheric pressure, decreases with increase of height as weight of air above decreases with height

Wales

Chemical and Material Behaviour

  • The properties of a material determine its uses.

Scotland

Properties and uses of Substances

  • I have carried out research into novel materials and can begin to explain the scientific basis of their properties and discuss the possible impacts they may have on society.

SCN 4-16a

Topical Science

  • I have researched new developments in science and can explain how their current or future applications might impact on modern life.

SCN 4-20a

Northern Ireland

Chemical and Material Behaviour

  • Structures, properties, uses of materials

 

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We also work in partnership with Tomorrow’s Engineers to inspire the next generation of engineers!

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Posted in Activity, Engineering, Physics