• Pressurisation in aeroplanes

    Indispensable in commercial flights

Pressurisation in aeroplanes is an essential mechanism to guarantee the viability of commercial flights. As we ascend, pressure decreases and we need a system to maintain the required pressure within the aircraft.

But how does it work? Why is pressurisation necessary in aviation? What happens if the cabin is depressurised?

In this post, we take on various issues you may have wondered about (or not) about the pressurisation of aeroplanes; be warned , though, it’s going to be very, very interesting. Let ‘s get started!

Origins:

Why was pressurisation started in the cabins of aeroplanes?

Throughout the first years of aviation, aeroplanes weren’t able to reach high altitudes and flight duration was very limited.

As time went on and systems became more sophisticated, the height at which planes fly was the natural evolution the sector was after. Why did this happen? It’s quite simple:

  1. The higher the altitude, the less fuel needed and speed increases considerably.
  2. Bad weather happens in the atmosphere’s lower layers.
  3. Geographical accidents impose the need to fly at a certain height in order to avoid them.

And what drawbacks came up when flight altitude was increased? As we ascend in the atmosphere, partial pressure of the different gasses decreases, generating two problems.

Firstly, piston engines needed great amounts of oxygen to combust so from a certain altitude there was a risk of engines stopping.

Secondly, the human factor. As the partial pressure of oxygen was reduced, passengers were exposed to hypoxia, a condition in which reaction and reasoning are undermined.

In the early 1940s, therefore, aircraft manufacturers started to create the first aeroplanes with pressurised cabins.

The first pressurised flight in history

The Boeing 307 Stratoliner belonging to TWA, Trans World Airlines, carried out the first flight in history with a pressurized cabin on 8th July 1940.

The flight departed from Burbank in California, bound for La Guardia in New York. It took 12 hours and 22 minutes and carried 33 passengers.

You can see a Boeing 307 like the one which performed the `feat´ in the picture.

How is a plane pressurised?

An aeroplane’s fuselage is basically a cylinder that moves through air carrying passengers from one place to another, so pressurising it is not too complicated.

On the one hand, we have air inlets to the pressurisation system. Planes’ engines are great fans that can move up to 1900 litres of air per second; a small proportion of this is used up to supply fresh air to the inside of the plane.

On the other one, valves allow air to come in and out, regulating the amount of air let out of the aircraft. When pressure needs to be increased, valves close up and the other way round too, when it has to be reduced, they’re opened.

During cruising, in case you were wondering, although air comes in through the engines, we needn’t worry about its quality as it never comes into contact with fuel and is therefore, totally clean and odourless.

Ways of pressurising aeroplanes

Current airplanes have modern systems that allow pressurization of cabins based on two factors:

Vertical speed

Vertical speed is used in ascent and descent. To give an example, an aeroplane can be ascending at 3500 feet per minute but thanks to the pressurisation system, vertical speed inside the cabin will be 1000 feet per minute. As you can imagine, this makes the flight much more comfortable for passengers.

Pressure altitude

The other factor is pressure altitude. The difference between exterior and interior pressure is limited because of aeroplane’s structural features . An aeroplane flying at 40,000 feet has a pressurised cabin at 8000 feet in order not to go over this value, which is usually 9 PSI.

  • Image: aeroprints.com

What happens if a plane’s cabin is depressurised?

Although it’s really difficult for this to happen, planes are prepared for the possibility of a depressurisation in the cabin. It can be one of the following three types:

  • Explosive depressurisation: is when depressurisation occurs in under 0.1 seconds. Non secured objects will shoot off pushed by the difference in pressure and a mist would be formed inside the aeroplane due to the cooling air and condensing of humidity.
  • Fast depressurisation: Happening in over 0.5 seconds.
  • Gradual depressurisation: As its name suggests, it is produced in a very progressive manner so the only way to detect it is through instruments.

Oxygen systems come into play

In the improbable case of a depressurisation occurs in the cabin, oxygen masks would automatically pop out and pilots would initiate an emergency descent to a height under 10000 feet or 3000 metres.

In aeroplanes we find two independent oxygen systems:

  • Pressurised oxygen: It is oxygen which has been introduced inside pressurised bottles. This system is used by pilots and provides more autonomy.
  • Chemical oxygen: Solid oxygen is used by passengers. It’s an easy system to keep and is longlife.

The evolution of cabin pressurisation

In most aeroplanes, cabins are pressurised at about 8000 feet presently, but according to studies published by The New England Journal of Medicine, if changed to 6500 feet, human bodies would absorb an 8% extra oxygen.

This means we wouldn’t feel as tired in long haul flights as hydration would also be reduced and it would, undoubtedly, be a great advance in terms of flight comfort.

Some aeroplanes are already programmed for the change, such as the Boeing 787 Dreamliner, which has now incorporated this technology. What are your thoughts about that?