For the first time, Chinese J-11 jet fighters have been training over Tibet. J-11s are the most modern Chinese made fighters. While fewer than 150 have been built since they were introduced in the late 1990s, they are appearing in more unexpected places (like the Chinese naval air force). The Chinese Air Force has no combat aircraft stationed in Tibet, although older (MiG-21 clones) J-7s appear to be flown in regularly, for temporary duty at major commercial airports.
J-11s are illegal Chinese copies of the Russian Su-27. This plagiarism has been a source of friction between Russia and China for over five years. It all began, legally, in 1995, when China paid $2.5 billion for the right to build 200 Su-27s. Russia would supply engines and electronics, with China building the other components according to Russian plans and specifications. But after 95 of the Chinese built aircraft were built, Russia cancelled the agreement. They claimed that China was using the knowledge acquired with this Su-27 program, to build their own copy of the Su-27, the J-11. Russia kept the piracy issue quiet for a while, and warned the Chinese that simply copying Russian technology would produce an inferior aircraft. Apparently the Chinese did not agree, and are continuing their work on the J-11, using only, what they claim is, Chinese technology.
The J-11 is believed to now include better electronics and some other Chinese design modifications. China can manufacture most of the components of the J-11, the one major element it must import are the engines. China believes it will be free from dependence on Russia for military jet engines within the next 5-10 years. Currently, China imports two Russian engines, the $3.5 million AL-31 (for the Su-27/30, J-11, J-10) and the $2.5 million RD-93 (a version of the MiG-29s RD-33) for the JF-17 (a F-16 type aircraft developed in cooperation with Pakistan.)
The main reason for not stationing fighter squadrons in Tibet probably has to do with the high altitude of the area, and the expense of moving the large quantities of fuel and other supplies needed to maintain air units. There is only one rail line into Tibet (recently built) and few heavy duty truck roads.
China also has a serious problem in Tibet with altitude sickness among its troops. This illness occurs when people who grew up near sea level (most of the world's population) move to altitudes greater than 2,100 meters (7,000 feet). Below that, the air contains 21 percent oxygen. Above that, the weaker air pressure lowers the amount of oxygen the body can absorb. That produces "altitude sickness", manifested by shortness of breath, disorientation, nosebleeds, nausea, dehydration, difficulty sleeping and eating, headaches and, if you stay up there long enough, chronic disability.
The average altitude of Tibet is 4,100 meters (14,000 feet). Most people can adapt, sort of, to the altitude sickness. Some can't. But the Tibetans have evolved to deal with it. The majority of Chinese soldiers coming to the Tibetan highlands (which is most of Tibet) require a few days, or weeks, to acclimate. But they are still susceptible to altitude sickness if they exert themselves, especially for extended periods. This makes Chinese military personnel in Tibet much less effective.
Researchers recently discovered that most Tibetans evolved in the last 3-6,000 years to deal with this problem. It appears that the most of the people moving to, and staying in, highland Tibet, where those with the rare genes that made them resistant to altitude sickness. These people became the dominant population in Tibet, mainly because they were healthier at high altitudes. Nearly all Tibetans have this gene (which controls how their red blood cells operate, to maintain sufficient oxygen levels). Very few lowland Chinese have these genes.
The Chinese military is spending a lot of time, effort and money trying to solve this problem. Currently, most of the troops in the Chinese Chengdu Military Region are in the eastern, lowland half. In the western portion (Tibet), they station the 52nd and 53d Mountain Brigades, and struggle to keep these 5,000 troops fit for duty. If there's an emergency, as there was two years ago, the nearby 13th and 14th Group Armies can send troops from their lowland bases. Over 20 percent of these troops will be hampered by altitude sickness once they reach the highlands, and commanders are trained to deal with that.
Chinese troops operating at the highest altitudes (4,500 meters, on the Indian border) now have access to exercise rooms (one of 1,000 square meters and another of 3,000 square meters) that are supplied with an oxygen enriched atmosphere. Troops exercising in these rooms increase the oxygen in the blood, and are much less likely to get hit with a case of altitude sickness. Thus the troops can stay in shape without getting sick. For border patrols at high altitudes, troops usually carry oxygen bottles and breathing masks.
So far, the Chinese have only been able to limit the attrition from altitude sickness, not eliminate it. Given the alertness required of aircraft maintenance personnel, and pilots preparing for flights, plus the logistical problems, the air force has declared Tibet fit to visit, but not to base aircraft units in. Still, the Chinese air force may one day have to fight in the air space over Tibet, so some training up there is in order.