“Oxygen masks will drop in case of emergency…” is something we hear each time before take-off in an airplane. However, not many people ponder on how we can fly comfortably above 30,000ft in normal circumstances without any need for oxygen. Whether we fly in a small regional jet or a large airplane like the Airbus A380, most commercial airplanes are pressurized. For many of us, airplane pressurization is a complicated subject, but it is an important one, nonetheless.
So why are airplanes pressurized?
Airplanes are pressurized to ensure that sufficient air pressure is available in an airplane cabin at higher altitudes for the passengers to breathe easily, and to ensure passenger safety and comfort.
At altitudes above 30,000ft, the ambient pressure is less than 5PSI which is insufficient for us, human beings. In addition to difficulty in breathing, lack of oxygen can cause hypoxia and can even be fatal.
When it comes to the economic benefits of pressurizing aircraft, they shouldn’t be neglected either. At high altitudes, the air is thinner, and airplanes can cruise more easily with lesser fuel consumption.
Also, at higher altitudes, the weather does not affect the flight as much which saves passengers from turbulence and discomfort.
Additionally, during ascent and descent, the ambient pressure outside changes rapidly and can cause serious discomfort for passengers. Pressurization system of an airplane keeps the air pressure inside the airplane cabin constant even when ambient pressure is changing to ensure passenger safety and comfort.
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Why is pressurization needed in an airplane?
If you have visited mountainous areas, which are often more than 7000ft above sea level, you must have noticed that one gets tired very quickly. The heavy breathing after a little walk or hike in high altitude areas is an indication that our body is not getting sufficient oxygen.
Imagine what would happen to someone if you were to breathe in the open atmosphere above 30,000ft. The thin air and lack of oxygen at these altitudes would cause hypoxia resulting in fatigue or loss of consciousness.
Pressurization is needed to maintain comfortable air pressure and sufficient oxygen concentration in an airplane cabin, so that passengers and crew can embark on long air journeys with ease.
Modern airliners keep the airplane cabin at 11PSI which corresponds to 8000ft altitude during the cruise.
Pilots commonly refer to air pressure inside the cabin in terms of cabin altitude. In an airplane, pilots are provided information about the actual altitude of the airplane are well as the cabin altitude.
So, we have seen how pressurization is essential for commercial airplanes but is the entire airplane pressurized?
What sections of an airplane are pressurized?
Pressurization is required primarily in the cabin for people and crew and in cargo holds for animals and therefore the entire airplane is not pressurized.
The wings, landing gear compartments, horizontal stabilizer, and rudder of most airplanes are not pressurized.
To visualize the pressurized areas of an airplane, we can consider the cabin as a cylindrical capsule that is sealed and can, therefore, hold air pressure inside while the rest of the airplane is unpressurized.
How is an airplane pressurized?
Much like filling air pressure inside any object, an airplane is pressurized when air is compressed and forced inside the fuselage by pressurization system, commonly termed as ‘packs’.
In modern airplanes, these systems are complex, redundant mechanisms that ensure robust pressurization regulation during all phases of flight.
When an airplane is flying, the air inlet scoops on the fuselage skin allow the air to reach the packs at high velocity. This intake air passes through one or multiple heat exchangers (which control the temperature) and is conditioned to the temperate selected by the pilots.
This pressurized and air-conditioned air is then pushed into an airplane cabin through inlet valves.
However, the cabin pressure or cabin altitude is regulated by the outflow valve that determines the amount of air flowing out of a pressurized airplane.
To sum it up, we can simply say that pressurized air enters an airplane through inlet valves and the amount of pressure inside is controlled by the discharge of pressurized air using outflow valves.
As we have read above, the pressurization of airplanes is to cater to passenger comfort and safety; what about pressurization in cargo airplanes?
Are cargo airplanes pressurized as well?
Yes, most of the time, cargo airplanes are pressurized as well. The main reason for this stems from the initial design of an airplane. Most passenger and cargo airplanes have a similar basic structural design. In typical airplane design, the cockpit and airplane cabin is part of the pressurized capsule. Even if passengers are not traveling on a cargo airplane, it still needs to be pressurized for the Pilots and any onboard crew.
Cargo planes are also utilized for live cargo such as transport of animals, birds, and even fish. Animals and birds also require sufficient air pressure and oxygen to be able to breathe and therefore, the pressurized and temperature-controlled environment is provided to them in cargo airplanes.
Can an airplane fly without pressurization?
In the good old days, passengers were flown in unpressurized, turboprop aircraft that flew at low altitudes.
However, as we learned more about flying, we understood that flying at higher altitudes was more comfortable and we could avoid turbulence and bad weather, in addition to getting better fuel consumption from jet engines.
One crucial element that can be extracted from the above-noted fact is that airplanes can fly without pressurization. Pressurization is only required to compensate for the low air pressure at altitudes above 10,000ft.
During the flight, when pilots observe a loss of pressurization, their checklists focus on descending safely below 10,000ft as quickly as possible. Below 10,000ft, sufficient air pressure is available for us to breathe.
Now, as pressurization systems provide adequate oxygen for us during flight, do we really need oxygen systems in airplanes as well?
Why do airplanes need oxygen masks if the airplane is already pressurized?
Modern airplanes have separate oxygen generation or supply systems as well, but these are only for emergency conditions when a rapid loss of pressurization occurs.
In case of emergencies, in the air or on-ground, oxygen masks are deployed for all passengers. The pilots have separate oxygen masks called quick donning masks available with them in the cockpit.
Risks involved in aircraft pressurization
Like any other system, the pressurization system can fail as well. But modern commercial airplanes have multiple pressurization systems i.e: multiple ‘packs’ for redundancy.
Over pressurization of an airplane
We have read above that pressurization is regulated through outflow valves which discharge air into the atmosphere and therefore keep a balanced air pressure inside the cabin. But just like a balloon, an airplane structure can disintegrate if it is filled with pressure beyond its limits.
To make sure this does not happen, safety valves that are built into an airplane fuselage operate when the air pressure inside an airplane cabin goes beyond specified limits.
In addition to ensuring the structural integrity of an airplane, these safety valves are crucial in saving the lives of passengers and crew on-board. Over pressurization can be fatal for human beings but that is another discussion for another day. Let us discuss what happens when pressurization is lost.
Pressurization loss
In case of any leaks in the pressurized cabin or malfunction of the pressurization system, pressurization is lost. As we have discussed above, as a Pilot notices rise in cabin altitude, his checklists direct him to bring the airplane below 10,000ft and he can continue flying.
If pressurization loss goes unnoticed by the pilots, it can lead to hypoxic conditions and loss of consciousness for all people on-board.
Explosive depressurization
Explosive depressurization is perhaps, the scariest condition associated with pressurization systems. This condition can occur when the air pressure inside an airplane cabin is discharged at an extraordinary rate, through for example a broken window or a hole in the fuselage.
https://www.youtube.com/watch?v=4yG2h1aDB6k
The risks of pressurization we have noted above are quite chilling, but they are mitigated though intelligent airplane design and effective training of pilots.
Conclusion and Summary
- Modern commercial air travel has become a lot safer, cheaper, and more comfortable due to the pressurization systems.
- The pressurization systems of modern airliners ensure that all passengers and crew onboard are provided with sufficient oxygen and a comfortable environment i.e. air temperature and pressure onboard through all phases of flight.
- Pressurization of airplane cabin in commercial passenger and cargo planes is achieved by the intake of compressed, conditioned air and its controlled discharge in the atmosphere.
- Nowadays, passengers can travel around the globe in long-haul flights that have a flight time up-to 19hours. These long flights that operate above 35,000ft, are possible due to pressurization systems, both in terms of passenger comfort and fuel efficiency.
The next time, you are comfortably cruising above the clouds, you will remember that you have pressurization systems to thank for convenience. Safe travels and happy flying!
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