The cabins of airplanes are pressurized and the temperature is internally controlled. Why are all planes constructed this way and what would happen if these systems failed?

For passenger comfort and so you wo not die of the cold and lack of oxygen at high altitudes. The temperature is way below zero when you're up there, and there is not enough oxygen to live for more than a few minutes, depending on how high up you are. When the system fails, you have to use the oxygen masks that are in the plane. Otherwise you will fall asleep and die. It does not happen very often.

Look up Payne Stewart, & that will show u what happens when the environmental controls fail. It is pretty rare, & if one person had gone on Oxygen, it probably would have turned out differently.

Not sure why u ask why all planes r constructed this way. l suppose so people will survive the trip. How else could u do it? l guess we could all wear spacesuits that r pressurized & air conditioned. Or, u could all wear oxygen masks for the whole flight. It would be a lot less comfortable, though.

Well here r the basics. Let is start with the oxygen issue. As altitude increases, the amount of air, or density decreases, which means the higher u go, the less air there is & therefore the less oxygen. All planes r not pressurized, nor do they all have supplemental oxygen. It is been a while since I've read the FAR is But here is what l remember. A light plane without supplemental oxygen is allowed to fly at an altitude of 12,500 feet. It can exceed this altitude up to 14,000 feet for no more than thirty minutes. Within the thirty minute time frame the aircraft must be back down to 12,500 or below. If these limits r exceeded a condition called hypoxia will affect the crew & passengers. It is deadly. The symptoms of hypoxia r disorientation, lack of good judgment, & finally incapacitation.
OK for the temperature part of ur question. The standard adiabatic lapse rate is 2 degrees per 1000 feet of altitude. That means that on a 90 degree day, it is below freezing at 30,000 feet. The pressurization is necessary in high altitude flight, for the same reasons that scuba divers have to come up slowely to prevent a condition called the benz. The lack of atmospheric pressure causes the oxygen & the nitrogen in ur blood to expand. It also lowers the boiling point of all liquids.

for a number of reasons. The first & most important of all is so u can breathe, my friend! You need that air. Second, u would not want to be cruising along at a chilly -72 degrees celcius in the cabin, now would you? Another reason is because at that great altitude, ur ears would build with great pressure & cause TREMENDOUS discomfort on the cruise & the descent, possibly even ascent. Last one l could think of is so the plane wo not collapse in on itself. It is not built like superman, it is just pumped with lots of air to be inflated like a football, otherwise the pressure would cause the plane to collapse in on itself like a pop can being crushed in.

People turn into Popsicles pretty quick at -30 below 0 Centigrade. Hypoxia develops quickly too.

Just to add to what is already been said, all planes are not constructed that way. Most light single-engine airplanes are not pressurized and seldom go above 14,000 feet or so.

Most here have good info. However, an aircraft will not collapse in on itself if not pressurised as the pressure internally & externally would be the same - exactly as it is on the ground. Nor would u get the 'bends' if the aircraft is flying at normal altitudes (35-41000 feet). Bends occur when nitrogen is disolved into the bloodstream by the effects of greater positive pressure (dive in the sea, & every 32 feet u decend doubles the pressure on ur body) The pressure at 36000 is approx 1/4 that of sea level. That, & the time taken to get to altitude means no bends. There is the same percentage of oxygen in the air at high altitude as there is at low, it is just that the amount of air in a given volume (ie a lung full) is less, therefore the amount of oxygen in less. This causes the hypoxia. Pressurising the aircraft cabin (usually to a level equivilent to being at 8000 feet) force more air into that volume (lung full) & therefore gives the person enough oxygen to work with. Should the pressurisation fail, in a commercial plane, then the oxygen masks should drop out of the ceiling (normally when the pressure in the cabin makes it seem like u r higher than 14000 feet) & give pure oxygen (either from a chemical oxygen generator or a compressed oxygen cylinder) for long enough for the pilot to get the plane down to 10000 feet, usually a minimum of 12 minutes supply.

The aircraft is pressurized because at flight levels where airliners operate ( around 30,000 ft) the air is too thin (low atmospheric pressure) for humans to breathe.

The air conditioning is there not only for temperature comfort.
It is most important purpose is to bring the air inside the cabin, which is virtually air sealed. And believe it or not the air we breathe inside the cabin is from the engine compressors.
(Do not worry that is the stage before the fuel injection & ignition. -it is just compressed air.) And for all of the modern airliner not a single freon or any synthetic coolant is used. It just uses air to air heat exchange & a little water for cooling.

And if the pressurization & air conditioning system fails the pilots bring the aircraft down to a flight level where human can breathe comfortably. this descent will only take a few minutes even (as in 7mins. max comfortable angle) if u r flying at 32,00 feet. And at this stage the oxygen masks r dropped to help u breathe, as an additional safety precaution.

FYI: The air conditioning & pressurization belongs to the same aircraft system. if there is no air con system there will be no air inside the cabin to pressurize.

- A & P guy

The aircraft fuselage is pressurized to stop the onset of hypoxia (lack of oxygen to the brain). The cabin is temperature controlled for comfort as the air temperature at cruising altitude is around -54degrees Celsius. If the system failed at cruising altitude in around 90 seconds u would lose consciousness, of which after 60seconds ur decision making abilities r severely impaired.

The emergency oxygen systems r installed as redundancy in case of failure. The standard oxygen generator passenger service unit will provide 12 minutes of pure oxygen to allow the aircraft to descend to a safe altitude.