Aircraft. In airliners, cabin altitude during flight is kept above sea level in order to reduce stress on the pressurized part of the fuselage; this stress is proportional to the difference in pressure inside and outside the cabin.
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The Cabin Altitude of a pressurised aircraft is normally maintained at and altitude of 8,000 ft or less as a compromise between the physiological needs of the crew and passengers and the structural limitations of the aircraft. At 8,000 ft the use of supplemental oxygen is not required.
For operations conducted under Parts 121 and 135, the flight crew must use oxygen when cabin altitudes are above 10,000 up to 12,000 feet after 30 minutes and at all times when above 12,000 feet. The general aviation pilot flying an unpressurized airplane will not normally operate above 25,000 feet.
Cruising at several thousand feet is perfectly safe. But idling on an airport runway might not be. Flying is now safer than ever. In 2013 only 265 people died in airplane accidents—out of 31 million commercial flights worldwide.
The general rule is that planes should have cabin pressurization when they go above 10,000 to 14,000 feet. Can a pilot depressurize a cabin? Most aircraft cabins are pressurized to an altitude of 8,000 feet, called cabin altitude.
Is it at all even possible for it to just drop? According to my company's training materials, an FAA study in the 1960s of depressurization events in business, airline, and military jet transport aircraft determined that the odds of experiencing cabin depressurization were one in 54300 flight hours.
If airplanes didn't pressurize their cabins, it could lead to insufficient oxygen as well as related medical problems like hypoxia. Airplanes need pressurized cabins because it ensures passengers, as well as crew members, receive an adequate amount of oxygen in the air they breathe.
On the ground, the airplane is unpressurized and the outflow valve is wide open. During preflight, the pilot sets the cruise altitude on a cabin pressure controller. As soon as the weight is off the main wheels at takeoff, the outflow valve begins to close and the cabin starts to pressurize.
At the normal stratospheric cruising altitudes of 30,000–38,000 ft, the outside pressure is 0.3–0.2 atm, respectively, while the cabin pressure is maintained at a level equal to that found at altitudes between about 5500 ft and 8000 ft, or between about 0.8 and 0.7 atm.
Whether flying at night or during the day, pilots need to see some kind of horizon. They use this to determine the airplane's attitude. At night pilots will turn their gaze from outside to inside and use the artificial horizon. The artificial horizon is normally a simply globe split into two hemispheres.
The reason planes cruise at high altitudes is that they burn less fuel and can fly faster, as the air is less dense. At 30,000 feet and higher, it is also possible for aircraft to avoid weather systems, making it more comfortable onboard.
Almost all large aircraft impacting the sea surface in an emergency or uncontrolled will break up immediately and catastrophically. One notable exception was US1549, an A320, which was landed on water without breaking up. It was described as still virtually intact though partially submerged and slowly sinking.
The biggest reason for flying at higher altitudes lies in fuel efficiency. The thin air creates less drag on the aircraft, which means the plane can use less fuel in order to maintain speed. Less wind resistance, more power, less effort, so to speak.
As long as the plane has communication to ATC or other planes, the pilot would report the problem and his/her next cause of action. This would include the intention to divert to the closest airport or to do an emergency water landing if there is no other option.