Negative pressure in the airplane

Under pressure

Tim Takeoff
22.01.2019
3 pictures
5 minutes

Moving around the world in a large flying tin can is a major challenge for man and material as, amongst other things, humans need the air they breathe to survive. This is provided through a pressurised cabin in an aircraft, but how does it actually work?

Humans need to breathe approximately eight to nine litres of air on average every minute – on the ground. But the air pressure changes once we ascend to higher altitudes. In principle, we can compare air to a large tank full of water: the deeper you dive into the water, the higher the pressure. So if you are at the bottom of the tank, or on the ground, the pressure is at its highest and just below the surface of the water, or at the edge of our atmosphere, the pressure is at its lowest.

Partial pressure

So when you climb up into the air, the pressure drops, but our lungs need a certain amount of oxygen partial pressure so that they can transfer the oxygen into our blood. At a mere 1,500 metres, just a fraction of the altitude of a commercial aircraft, people only have access to around three-quarters of the normal amount of oxygen in the air. A healthy person can climb to an altitude of approximately 2,500 metres without having to worry about major problems but above this level they experience fatigue and headaches. The symptoms increase the higher they climb and deteriorate up to the point at which they become unconscious.

Time of useful consciousness

A Cola can

How do you now create the conditions in a large space like an aircraft where so many passengers can breathe comfortably without any professional oxygen equipment? All for a very long period of time?

So as to get a better idea, let’s imagine that the fuselage is a type of drink can as this is very similar in its basic design. The round shape and body structure generally allow it to be put under pressure. The external pressure is therefore different to the cabin pressure and the maximum difference at cruising speed is approx. 8.9 PSI. At this point there is approximately six tons of pressure per square metre on the aircraft cabin. But how is this achieved?

The “packs”

Fresh air is of course continuously fed into the aircraft cabin at the outset. This is done through the so-called packs (air conditioning system) that draw air from the engines. The aircraft cabin cannot be made fully airtight but this is not necessary. The fuselage in the rear section of the aircraft has so-called “outflow valves” so as to build up controlled pressure at this point. These are special large valves that are opened and closed automatically in modern aircraft. When the aircraft climbs, the air pressure decreases in the cabin in a controlled manner, in other words creating negative pressure, up to a maximum cabin altitude of approximately 8,000 feet (2,438 metres) at cruising altitude. The outflow valve then automatically maintains a constant pressure.

Doors open at negative pressure?

The outflow valve closes further again during the descent and the pressure in the cabin increases while the flying altitude decreases. The air pressure inside the cabin must be modulated to that of the ambient air before opening the doors on the ground. The outflow valves are fully opened on the ground – which is also clear to see. The doors cannot be opened at cruising speed, even if anyone wanted to do so, due to the negative pressure.

Structural limits

The doors, windows and pressure bulkhead that is installed at the rear of the cabin are subjected to considerable levels of stress. If one of these components or the outflow valve should fail, the aircraft has some emergency valves so as not to exceed the structural limit of the aircraft cabin. The oxygen masks are of course released from their compartments in the event of an uncontrollable loss of pressure and the crew initiates an immediate descent to an altitude at which you can breathe normally without the masks. In the event of an explosive decompression, the crew has about 15 minutes to reach this altitude because this is how long the flow of oxygen lasts in the system for the masks.

Dry air

Another problem with air at high altitude is its dryness. Since the air at high altitude is very cold, it is barely able to retain water vapour and the humidity diminishes further through the warming of the air within the air conditioning system. Optional humidification systems resolve this problem but they use energy and damage the aircraft structure in the long term. Since most jets still consist of a metal structure, the fuselage must continuously be checked for corrosion and is preserved with a very resistant paint. Humid air is therefore always a problem.

“Pop” and Valsalva

Our ears also “work” inside a pressurised cabin. A person’s eardrum acts as a natural barrier between the pressure in the middle ear and the ambient environment. Some people can experience discomfort especially during an aircraft’s descent when the pressure is increasing. The so-called Valsalva manoeuvre is generally recommended in this case. You need to close your mouth and pinch your nose while exhaling air forcefully. If you are lucky, your ears will pop and the pressure you feel in your ears will reduce significantly.

A sweet or chewing gum is also helpful as a swallowing action fully opens the Eustachian tubes (connecting the ear and the nasopharynx).

The future of cabin air

The Boeing 787 is a modern example of what the future might look like. This aircraft no longer draws the cabin air off the engines, but in an electrical compressor. New materials, such as carbon and glass fibre, help to significantly increase the humidity in the air. What’s more, the pressure in modern aircraft can be slightly higher, not only in the Boeing 787 but also in the Airbus A380. Their fuselage structure allows a maximum cabin altitude of just 5,000 or 6,000 feet as opposed to 8,000 feet.

And tomato juice?

Wikimedia Commons – NatiSythen 

by Tim Takeoff

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