Cabin Depressurization

Reading Time: 10 min.

Most people think that during a decompression event the inside of a plane’s cabin looks like this:

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Fortunately, this is not the case. Actually, we experience decompression in every single flight and we barely notice it. This happens because the pressure drops at a rate which is almost unnoticeable from the human body and it’s always regulated by special instruments in order to be within certain limits.

We can classify decompression according to its duration, in three different types:

  1. Explosive Decompression (<0.5 sec)
  2. Rapid Decompression (>0.5 sec)
  3. Slow or Gradual Decompression

To better understand the difference between rapid and explosive decompression you have to know that, during the Explosive Decompression, the cabin pressure changes so fast that even our lungs can’t follow. However, the most dangerous scenario is when the pressure loss stays unnoticeable. I’m talking about the case of the Slow Decompression.

Why does an airliner have to be pressurized?

The main reason is low oxygen concentration. As the plane climbs to higher altitudes the density of air decreases. The Earth’s atmosphere, which contains about 21% oxygen, is concentrated really close to our planet’s surface. To be more precise, 3/4 of its mass lies within 11 km of the sea level due to gravitational forces.

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Air pressure and density decrease with altitude. Different shades of blue roughly denote the layers of the atmosphere.

If we weren’t able to take the necessary oxygen to function normally, then our bodies would suffer from hypoxia. Our brain, which requires almost 20% of the oxygen we inhale, would start to become dumber! However, this wouldn’t be a painful condition. On the contrary, we would experience euphoria, which would make the whole situation even more dangerous since we wouldn’t be in the position to realize the imminent threat.

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As we reach higher altitudes we have less time to react in a case of cabin decompression. (Source: SmarterEveryDay 157)

So, during a decompression event, every second counts! The higher you are the less time of useful consciousness you have. This is the reason why during the pre-flight safety briefing they insist to “secure your own oxygen mask before assisting others“! After few seconds without oxygen, you might be so dumb that you will no longer be in a position to do anything either for yourself or anyone else around you.

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“Secure your own oxygen mask before assisting others”

According to the  Federal Aviation Administration (FAA), pilots begin to need oxygen when they fly above 3800 meters (12500 feet) for more than 30 minutes and passengers, have to use oxygen masks above 4600 meters (15000 feet). By pressurizing the air inside the cabin of an aircraft, we simply increase the density of air and therefore the oxygen concentration.

Federal Aviation Regulations, Part 91, §91.211

“No person may operate a civil aircraft of U.S. registry at cabin pressure altitudes above 15000 feet (Mean Sea Level – MSL) unless each occupant of the aircraft is provided with supplemental oxygen.”

There are many interesting videos on YouTube with people who voluntarily experienced hypoxia in controlled environments. These people were fully monitored by specially trained doctors in order to understand how low oxygen affects the human body.

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The brain of this man can no longer perform even the simplest functions due to lack of oxygen. Check out the smile on his face! (Source: SmarterEveryDay 157).

Then, why do our ears pop during ascending?

The answer is quite simple. Airliners never increase the cabin pressure up to 101,325 KPa which is the air pressure at MSL. In reality, the equivalent altitude of the air pressure inside a cabin, while an aircraft is cruising, is close to 1800 – 2400 meters (6000 – 8000 feet) . This is the reason why your ears pop and why an empty plastic bottle, which was closed during the flight, is crashed after the descent.

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An empty water bottle is crushed during descending due to the increase in air pressure.

“Then why don’t they increase the pressure?”, you may rightly ask. Well, according to aeronautical companies… it isn’t needed. A healthy human can easily handle the pressure at an altitude of 2400 meters without acclimatization. Acclimatization is a known term to most mountain climbers. It’s the process in which the human body adjusts to the environmental changes and it’s mandatory for everyone who wants to ascend above 2400 meters. So if you are healthy you will probably not have a problem. On the other hand, if you are not, maybe the situation will be a little bit different:

World Health Organization (WHO), International Travel and Health

“Individuals with ear, nose and sinus infections should avoid flying because pain and injury may result from the inability to equalize pressure differences. If travel cannot be avoided, the use of decongestant nasal drops shortly before the flight and again before descent may be helpful.”

As you already know, high-cost is always a better motivator for not doing stuff! So the truth is that it would cost a lot of money to build and fly an aircraft having the same safety but with higher pressure inside the cabin. Don’t forget that the pressure differential at this altitude is already really high. It is like flying inside an inflated balloon. Do you know what happens if you inflate the balloon too much? … Explosive Decompression!

cross-section-of-737
Air pressure inside a plane increases dramatically with altitude. This stress weakens the plane’s skin over time.

The aeronautical companies design their airplanes taking into account that the fuselage must take all this additional stress. If we wanted to pressurize the air inside the cabin even more, then the fuselage would require more reinforcements, which are equal to more weight, which is equal to more fuel, which is equal to more money!

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A320 Pressurization Diagram. Notice that the pressurization cycle starts before take off and ends after landing in order to avoid pressure surge during rotation.

An airplane, during its lifecycle, will be pressurized and depressurized thousands of times. In fact, the forces due to these compressions and decompressions are so extreme that the lifespan of an airliner is measured in pressurization cycles! Not in flight hours, not in years, but in how many times it was pressurized and depressurized!

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The air inside the cabin is pressurized by the engines. Each turbine compresses and diverts a percentage of the intake air into the cabin (bleed air).

What happens during explosive decompression?

If we ever found ourselves in the extreme scenario of an explosive decompression, we would first hear a loud bang. Then, air would be sucked out of the airplane violently. Small and light objects would fly towards the breach. Due to low temperatures, the water vapor in the cabin would condense into liquid forming a thick fog. When the cabin altitude reaches 14000 feet (4200 meters) the oxygen masks would be automatically released.

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Chemical oxygen generator. The retaining pin is removed by just pulling the mask.

However, this whole situation would last only for a few seconds, until the pressure outside and inside the cabin would be equalized. During this period it would be critical to ensure our personal safety and only then offer our assistance to others. The oxygen is supposed to last for about 15 minutes. During this time a burning smell may be noticed but probably the reason would be the chemical reaction that occurs for the generation of oxygen.

If any decompression event arises, while you are in flight,  you have to stay calm and:

  • Immediately wear the nearest oxygen mask
  • Sit down and fasten your seat belt or grasp a fixed object
  • Hold on

For more information, you can read the briefing notes about “Cabin Decompression Awareness“, by Airbus, and the “Aircraft Depressurization” bulletin, by the Australian Transport Safety Bureau. I would also recommend you to check out the Aviation Herald which has news about the occured incidents and accidents in commercial flights.

If you want to have access to more interesting stuff about Aviation, then you should follow this blog and find me in the social media at the end of this page. If you liked this post don’t forget to leave your comment below as well as any questions you want me to answer in my upcoming posts.

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