For foreign visitors to China, Duration to travel must be limited. So flying to Tibet is an ideal way. It will take less time and be more comfortable than other options. Belows are details about Tibet Flights.
Now, there are several big cities in China manage flights to Lhasa. Which you should keep in mind. Cities like Beijing; Shanghai; Chengdu; Shangri-La Etc., have direct flights to Lhasa. From Kathmandu also manage flight to Lhasa.
By far, there are 5 civilian airports in Tibet: Lhasa Airport, Chamdo Airport and Nyingchi Airport, Ngari Airport and Shigatse Airport.