All airplanes have a maximum altitude. The highest certified altitude of an airliner was Concorde's 60,000 feet. The record altitude for a jet plane is 123,520 feet, set by Alexandr Fedotov in 1997 in a military Soviet MiG-25M.
Today some of the corporate jets can fly at 51,000 feet, but most commercial airliners are limited to 45,000 feet or less. If a plane were to climb beyond its ceiling limit then the engines would eventually lose the ability to produce enough thrust to continue the climb.
The optimum altitude is also linked to the weight of a commercial jet and heavier planes fly lower, and lighter higher. Light aircraft do not have pressurized cabins and must remain below 10,000 feet. Any higher and the pilot is required to wear an oxygen mask to remain conscious.
One of the main reasons behind aircraft altitude is that, as the air gets thinner with altitude, planes can fly more easily and therefore faster and burn less fuel, which saves money. Normally commercial flights fly between 35,000 and 42,000 feet.
At these higher altitudes aircraft avoid much of the bad weather. The troposphere, the layer closest to the ground, is where most of the world's weather occurs. Usually measured up to 36,000 feet, this is where clouds are most likely, as well as heavy rains and high winds. Aircraft prefer life in the stratosphere, which means less turbulence.
In case of an emergency, say engine failure, aircraft at 35,000 feet, allows for much more time to deal with the situation. It also means that if all engines fail, the glide distance is much further.There are minimum altitude restrictions, but they apply much more to light aircraft than airliners. It is illegal to fly below 1,000 feet over a built-up area or 500 feet over any person, vehicle or structure. There are also vertical distance restrictions.
The elite group of 35-50 of the world's U-2 pilots routinely fly at 70,000 feet are subject to altitude-induced decompression sickness which is known by its common name, the bends. This is the same condition suffered by divers who rise too quickly from the high pressure at depth to the lower pressure near the surface.
U-2 pilots are especially at risk, not just because of their extreme altitude but also because their cockpits are only partially pressurized. The pressure in a U-2 cockpit is equivalent to the atmosphere at 29,000 feet, as high as Mt. Everest so U-2 pilots breathe pure oxygen for an hour before their flight and wear a pressurized suit.
Pre-breathing oxygen helps purge nitrogen from their bodies. If any nitrogen remains and the pilot climbs to altitude, just like when you open a can of soda, the thin atmosphere will cause the nitrogen dissolved in the body’s blood and tissue to boil which can cause extreme pain, bruising, brain damage and without treatment death.
In 2006, one U-2 pilot several hours into a combat mission started experiencing symptoms of knee pain and over the next five hours, an intense headache, nausea, and extreme fatigue. At one point he also hallucinated that his plane was in a steep bank and be began to feel disoriented and almost like he was intoxication, at one point even falling asleep.
It progressed to the point that he had essentially forgotten how to fly the plane. After vomiting into his pressure-sealed helmet he had to take it off he could see, but that exposed him to he much lower pressure which only made matters worse. He tried setting the autopilot for home, but he had forgotten how to use the device. It took three hours of careful instruction from his squadron commander to talk him back home because all he could remember was how to perform only the most basic tasks.
Landing was critical because he had developed several blind spots in his vision and could not even see the landing gear handle, but had to fumble for it in the area where he knew it was. Warnings were going off, but he couldn't remember what they meant. The pilot had no recollection of the last 45 minutes of his flight and during landing he was flying towards structures on the ground. It wasn't until a couple of jet fighters were launched that he was able to be guided towards the runway.
When he finally managed to land after blacking out at least one, his limp body had to be dragged out out the aircraft. But, he wasn't out of the woods. The nitrogen gas had caused havoc with his body. Normally the bends is treated early on when the symptoms begin. He was placed in a pressurized room where a gradual shift from low to high pressure gives nitrogen gas time to dissolve back into the blood. In this pilot's case, he was in and out of the room for several days as he was in a haze and could not remember how to simple, routine tasks. He fully recovered, but there are on his skin that was caused by bubbling nitrogen gas.
However, in many cases symptoms don't show up until hours, or even days, later and there can be long-lasting or permanent brain damage. Some pilots have reported the sensation of free-falling, being completely awake, but unable to move. Additionally some have experienced problems with short-term memory and reasoning.
U-2 pilots fly for longer stretches and so seem especially susceptible because their flights can last up to 12 hours. There is also another reason...a study found that even mild exercise, including motions used in flying, can increase the risk of getting the bends. Another factor is stress. In the old days U-2 pilots essentially did nothing more than fly over a target and take pictures often times leaving the plane on autopilot, they could even snooze. Today they must quickly react to threats from missiles etc, act as relay for communications and serve as an eye in the sky for troops on the ground.
The problems for U-2 pilots saw a significant increase during the wars in Iraq and Afghanistan because of the long missions and the stress. The military has taken some act, but in typical fashion, has not moved very fast to solve the problem.
The Northrop Grumman Global Hawk, an unmanned, remotely piloted drone, can fly almost as high and do almost everything a U-2 can do, but the Pentagon cut back on the program because U-2s are cheaper and, as all military personnel know, people are expendable.