India believes that China now has five airfields in Tibet (Gongar, Pangta, Linchi, Hoping and Gar Gunsa) that are bases for military aircraft. India is also seeing more Chinese warplanes being based full time in Tibet. This was somewhat unexpected, and it's all about thin air.
It was less than a year ago that, for the first time, Chinese J-11 jet fighters were seen training over Tibet. J-11s are the most modern Chinese made fighters. More than 200 have been built since they were introduced in the late 1990s, they are appearing in more unexpected places (like the Chinese naval air force). For a long time, the Chinese Air Force had no combat aircraft stationed in Tibet, except for some older (MiG-21 clones) J-7s that were flown in regularly, for temporary duty at major commercial airports. Some of these J-7s now appear to be there permanently.
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. In all of Tibet, there are only 58,000 kilometers of 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, especially in an emergency or combat.
Researchers recently discovered that most Tibetans evolved in the last 3-6,000 years to deal with this problem. It appears that most of the people moving to, and staying in, highland Tibet, were 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. 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.