On any commercial airline, despite there being emergency doors with latches, every single passenger window is sealed and often doubly protected with a layer of plexiglass. Even in the event of fires, where clearing smoke out of the cabin could be essential, the windows will not open.
While it is somewhat obvious that the wind and noise from having a window open and cruising at 600 miles-per-hour are uncomfortable and undesirable, the question remains why will they not open at all.
Why don’t airplane windows open? Because in the event of a window opening while at altitude, the resulting loss of cabin pressure would cause hypoxia (oxygen deprivation) amongst the passengers, a rapid change in temperature, and a potentially dangerous ‘suction’ point as the pressure rapidly equalizes.
As altitude increases, pressure decreases, and the amount of air we take into our lungs with each breath is a fraction of the air molecules we breathe on the ground.
This means that supplemental oxygen is recommended above 10,000 feet. The cruising altitude of a 747 Airbus is around 35,000 feet above sea level.
In this article, we’re going to discuss what would happen if an airplane window were able to be opened during flight. To do this, we must understand how Boyle’s law works, how cabin pressure is maintained, what happens to our bodies from breathing less oxygen, and the effects of a sudden pressure differential. We’re also going to discuss what would happen if someone were to force open an emergency door in flight as well as attempts from passengers to disrupt cabin pressure in the past.
Table of Contents
How does Boyle’s Law affect an airplane?
To understand why windows aren’t an option on planes, we must first understand Boyle’s Law.
Boyle’s Law essentially states that all things being equal, the less pressure a gas is under, the more volume a gas will take up. (It actually states “The absolute pressure exerted by a given mass of an ideal gas is inversely proportional to the volume it occupies if the temperature and amount of gas remain unchanged within a closed system.”, but we’ll keep it simple)
Think of it like this. If you had an empty balloon and somehow ‘caught’ only 20 molecules of air in it, then brought the balloon to a much lower atmospheric pressure, those same 20 molecules would be less condensed, and the balloon would get bigger as the molecules moved away from one another.
The image below shows the relationship between pressure and volume.
The gravity of the earth causes pressure to be higher at sea level, so as altitude increases, pressure decreases. This means when an airplane is flying at 35,000 feet, it is flying through much ‘thinner’ air than it is at, say, 8000 feet. This thinner air means less drag and makes it more efficient to travel long distances quickly.
However, the tradeoff is that thinner air doesn’t provide enough oxygen per breath for a human. At sea level, atmospheric pressure is around 14.7 psi. At the cruising altitude of an airliner, it is 3.46. This means that for every breath you take at altitude, you’re only taking in about a fourth of the oxygen you need to function and survive. This is only remedied by cabin pressurization, the mechanical act of forcing air pressure into the cabin so it is the equivalent pressure of being around 8000 feet above sea level.
This means that if a window were suddenly (or even slowly) opened mid-flight, the forced pressure inside the cabin would equalize to the outside pressure, causing hypoxia (oxygen deprivation) amongst the passengers and crew. This has actually happened before and is a commonly practiced drill amongst aircrew, though it isn’t caused by an open window. So how exactly is cabin pressurization achieved and how can it be lost?
How is cabin pressurization achieved and how can it be lost?
How is that we can sit 35,000 feet above the ground comfortably? And if cabin pressure isn’t lost through a window being opened, how does it happen?
First, cabin pressure is achieved by utilizing what every jet engine airliner already has on it; the jet engines.
A jet engine is already working as a giant compressor. The blades inside suck air in and compress it. When fuel is injected into compressed air, the resulting expansion (or explosion) turns another set of blades and flows out of the exhaust, creating thrust.
Cabins are pressurized by air that is redirected from between the compression and combustion stages. However, this air is very hot after being suddenly compressed, so it must be cooled before it is forced into the cabin. This is accomplished through the ventilation system which is allowing air from outside (the temperature of outside air at 35,000 feet is approximately -65F/-54C) to flow in with the compressed air. This brings the cabin pressure to approximately equal to what ambient pressure is at 8000 feet above sea level.
The pressure is kept there because if it were higher, say, equal to that of sea level, a greater amount of pressure would be applied to the skin of the aircraft from the inside, and risk damage on the fuselage. Bringing the pressure to the equivalent of what it is at 8000 feet allows minimal hypoxia among passengers (maybe some mild sleepiness) and minimal pressure on the aircraft.
So, what causes depressurization in flights when it does happen? Historically, a variety of situations have caused it. Examples include a slow leak through a cracked window, sudden structural damage to the fuselage, or a mechanical error in the compression system. Whatever causes it, the results can be devastating if not dealt with in a correct manner.
How long can passengers survive in a depressurized cabin?
This question has many parts, so we’ll break it down to what happens when a cabin does suddenly depressurize.
Deployment of oxygen masks
First, automatic sensors will sense the loss of cabin pressure and immediately deploy oxygen masks. Hypoxia takes place very quickly and accelerates with altitude, so in a worst-case scenario, the time before a loss of consciousness could be seconds.
Simultaneously, the sudden loss of pressure can cause pain and damage to the eardrums and mucous membranes. This is because small pockets of air in the human body (such as in the middle ear and sinuses) will suddenly expand, often causing pain and minor bleeding.
Air Rush and temperature drop
The temperature is also going to drop dramatically, as the temperature equalizes with the low temperatures outside.
Air will rush from inside the cabin to wherever the breach is, potentially taking with it large objects (even passengers) if the hole, and corresponding air rush, is large enough. This will stop as the pressure equalizes.
Descent
The pilots will immediately begin a controlled descent to below 10,000 feet. This will be bumpier, as the air is now a lot thicker than normal cruising altitude, but the air is there is dense enough for passengers to breathe. This must be done rapidly because the oxygen masks will only last each passenger anywhere from 10-30 minutes. Once at a safe altitude, the pilot will redirect to the nearest airport and make an emergency landing.
So how long can passengers survive in a depressurized cabin?
To answer the original question, how long can passengers survive in a depressurized cabin without oxygen masks, the answer ranges from just a couple of minutes (in high altitudes) to indefinitely (generally any altitude below 12,000 feet).
Historically, during a pressure loss, the greatest danger is the pilots not getting supplemental oxygen quickly enough, causing them to lose consciousness.
Can airplane emergency doors be opened mid-flight?
The short answer is no, they cannot. The emergency exit doors are mechanically locked and controlled from the cockpit. When the aircraft lands, the pilot switches this to manual control in case the aircrew needs to open them for an emergency on the ground. There have been multiple reported instances of individuals having panic attacks and unsuccessfully attempting to open the emergency exits mid-flight.
So, why not have the same system for airplane windows? The extra cost and weight have no added benefits. Applying an electronic locking mechanism to each window would throw the gross weight of the aircraft off, require extra maintenance, and the only time it would be practical to use would be on the ground.
Summary
Windows don’t open on airplanes because keeping the aircraft sealed is paramount to passenger safety and comfort during the flight. Airplanes use their own jet engines to compress air and keep the cabin pressurized to safe pressure, approximately equal to the pressure at 8000 feet about sea level. During a loss of cabin pressure, oxygen is immediately supplied to everyone on the airplane, and the pilots will rapidly descend to safer altitudes, both for higher oxygen levels and higher temperatures.
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