On any commercial flight, windows are always sealed and often equipped with an additional layer of plexiglass for extra protection. In the case of a fire, where clearing smoke out of the cabin seems essential, the windows remain closed. Keeping the windows closed on aircraft protects passengers and aircrew from the wind and noise produced from cruising at 600 miles-per-hour.
As altitude increases, pressure decreases, and the quantity of air we take into our lungs is reduced to about a fraction of the air molecules we breathe on the ground. In the event that a window opens while at high altitudes, the loss in cabin pressure from such an action can cause hypoxia, which is characterized by oxygen deprivation. Furthermore, opening aircraft windows results in a rapid change in temperatures, causing a potentially dangerous suction point as pressure rapidly equalizes.
To better understand the importance of keeping windows sealed during all stages of flight, this blog is going to discuss a few critical aspects. First, we must outline how Boyle’s Law works, which is essentially an experimental gas law. Then, we will detail how cabin pressure is maintained, the consequences of breathing less oxygen, and the effects of a sudden pressure differential.
Boyle’s Law states that all things being equal, the less pressure a gas is subjected to, the more volume a gas will take up. In more technical terms, the absolute pressure applied by a given mass of an ideal gas is inversely proportional to the volume it takes up if the temperature and amount of gas remain the same within a closed system. For example, if an empty balloon only has 20 molecules of air in it and is brought to a much lower atmospheric pressure, then those same 20 molecules are less compact and the balloon would get bigger as the molecules move away from one another.
Another important consideration is the fact that the gravity of the earth causes pressure to be higher at sea level, so with increases in altitude, pressure decreases. For instance, when an aircraft is flying at 35,000 feet, it is technically flying through “thinner” air than it would at 8,000 feet. Thinner air is ideal for flights, as it produces less drag and makes it easier to travel long distances. However, the major trade off is that thinner air makes it harder for humans to get enough oxygen. At cruising altitude, the atmospheric pressure is around 3.46 PSI, which is only about a ¼ of the oxygen necessary to function and survive. This is remedied with cabin pressurization, the mechanical act of forcing air into the cabin so that it has the same pressure as it would at around 8,000 feet above sea level.
In jet engine airliners, cabin pressure is achieved by using the jet engines. As a jet engine is essentially a giant compressor, the blades suck air in and compress it. When fuel is injected into compressed air, the resulting explosion turns another set of blades and exits out of the exhaust, generating thrust. Put more simply, cabins are pressurized by air that is redirected from the compression to combustion stages. As the air is very hot, it is directed into a ventilation system to cool it down prior to being forced into the cabin.
Consequences of Depressurization
When a cabin suddenly depressurizes, the automatic doors sense the loss in cabin pressure and immediately deploy oxygen masks. As hypoxia occurs very quickly and accelerates with altitude, loss of consciousness can happen in seconds. The loss of pressure can also cause pain and damage to the eardrums and mucous membranes.
Depending on where the breach is located and if the hole is large enough, the rush of air into the cabin can potentially take large objects with it. Controlled descent is harder to achieve as the air becomes thicker, so pilots must land promptly because the oxygen masks will provide breathable air for a maximum of 30 minutes.
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