When Dalai Lama was to fly from Lhasa to India

The first Chinese plane landed in Damshung, north of Lhasa in 1955

An episode, not very not well-known, was the possibility to 'evacuate' the Dalai Lama from Lhasa soon after the Chinese troops crossed the Upper Yangtze river and entered Chamdo in 1950.
S.N. Haksar, Joint Secretary in the Ministry of External Affairs and Commonwealth Relations wrote a Top Secret Note on November 30, 1950.
The Note was addressed for Cabinet's Foreign Affairs Committee.
Apart from the Prime Minister, the members of the Cabinet's Committee for Foreign Affairs were Sardar Vallabhbhai Patel, Deputy Prime Minister; Maulana Abul kalam Azad, Minister of Education; C. Rajagopalachari, Cabinet Minister; N. Gopalaswami Ayyangar, Minister of Transport & Railways.

S.N. Haksar' note reads:
Subject: Flight of an aircraft to Lhasa to evacuate the Dalai Lama.

1. Although the Dalai Lama has no intention at present to leave Lhasa, an enquiry has been made by the Tibetan Government whether a plane could be sent by us to Lhasa to evacuate Dalai Lama should this become necessary at a later date, and alternatively whether we would allow the use of Indian air-fields (on the assumption that the Tibetan Government make their own arrangements for a flight).
2. The question has to be considered from the technical and the political aspects. Air Vice Marshal Mukherji is of the opinion that the flight does not present any serious difficulties. There is a good airport at Darbhanga and two air strips nearer Lhasa. Lhasa is well within range of a Dakota, and it can easily fly both ways without refueling. It is true that some of the highest peaks are in this area but it is possible to avoid them by flying over the valleys and the rivers which do not attain a height greater than 15 or 16 thousand feet. As for the landing at Lhasa, it has been reported by our Mission at Lhasa that there is a big plain which can be easily converted into a landing strip. This is probably the same plain as is mentioned as a possible airfield site in the Report of Tolstoy and Dolan, two American Army Officers who travelled from India to China via Tibet in 1942-43 on a mission to survey the routes and possible airfields. According to their report the plain is of hard-packed clay and allows an approach clean of mountains at both ends. All that seems to be needed is to clear it of stones. The Tibetan Government employ an Austrian as an engineer and this man should be quite competent to supervise the removal of the stones.
3. After consulting the maps and the report of the American Mission, Air Vice Marshal Mukherji is of the opinion that the flight to Lhasa does not entail any special hazard and is no more risky than, for instance, a flight to Leh.
4. Turning now to the political aspect, it may be mentioned that Robert Trumbull of the New York Times has taken a keen interest in this matter. Evidently, he wants to get a first class newspaper story out of it, and is prepared to pay the cost of the flight. He has been in communication with the Tibetan authorities at Lhasa and with Shakabpa. He has suggested Lessitor, an American pilot, to undertake this flight. Lessitor claims to have considerable experience of flying over the 'hump' and is at present the chief pilot of the Darbhanga Airways, whose plane he intends to use for the purpose. This is a purely private arrangement. The Tibetan Government charter a plane from an Air Company and the Government of India have no concern with it beyond granting permission to an air company to undertake a charter flight which is ordinarily granted as a matter of course. We have offered sanctuary to the Dalai Lama in India and it would be difficult to stand in the way of his evacuation by withholding the formal sanction to make the charter flight.
5. However, it should be possible to refuse permission to Trumbull’s plane if we can make alternative arrangements for the flight. Trumbull’s efforts should be checked because if he splashes to the world the news that he was the rescuer it would give the Chinese the much needed evidence in support of their allegation of Anglo-American influences being at work in Tibet. It would be preferable that we, and not Trumbull, arrange the plane. It should not be difficult to get an Indian pilot to undertake the flight but even if one is not available and Lessitor has to be employed, he should be engaged by us rather than by Robert Trumbull. In view of the importance of keeping the proposed flight of Dalai Lama a close secret, a search for an Indian pilot has not been made wet but it would begin if the principle of the flight is accepted. In that case a reply should go to Tibet that it would be possible to send a plane when necessary provided the weather permits and there is no apprehension of any resistance at the Lhasa air-strip then.

A few comments:
The Darbhanga Airport, mentioned by Air Vice Marshal Mukherji was the longest runway airport of Bihar after Independence. It is still today operated by the Indian Air Force. It was then built for the use of Maharajah of Darbhanga's aeroplanes.
Darbhanga Aviations was a private Indian airline started in 1950 by Maharaja Kameshwar Singh of Darbhanga. He had three aircraft; the airline's operations stopped in 1962.
Brooke Dolan and Ilia Tolstoy were US intelligence officers working for the Office of Strategic Services (OSS), the precursor of the CIA.
The duo traveled to Lhasa in the summer of 1942.
The National Archives blog explains:

The Allies were desperate to find a land route that would reconnect China and India. The task fell to two OSS men—Ilia Tolstoy, the grandson of Leo Tolstoy, and explorer Capt. Brooke Dolan. To complete the land route would require traversing Tibet, and to traverse the hidden country required the permission of a seven-year-old boy, the Dalai Lama.
When the two men arrived in Lhasa, the remote capital of Tibet, these spies were received as ambassadors. A military brass band played, and they were treated as guests of honor in a city that only a few decades earlier had forbidden Westerners to enter.
They came carrying a message from President Franklin Delano Roosevelt. On December 20, at 9:20 in the morning, they were granted an audience with His Holiness. As a further sign of his respect for these two emissaries, the men were allowed to ride horses up the Potala to the quarters of the Dalai Lama.

The Austrian engineer, mentioned by Haksar is Peter Aufschnaiter, the companion of Heinrich Harrer, who spent 8 years in Tibet.
Regarding the remark of Air Vice Marshal Mukherji that the flight to Lhasa does not entail any special hazard and is no more risky than a flight to Leh, it has to be noted that the first flight to Leh occurred on May 24, 1948 only, when Mehar Baba landed a Dakota at Leh, on an unprepared surface at 11,540 ft. Incidentally, his passenger was Maj Gen K.S. Thimayya, then GOC 19 Division and later Indian Army Chief.
The Dakota had no heating facilities, no pressurisation and did not carry any surveyed route map.
A flight to Lhasa would have been similarly hazardous.
It has to be noted that in 1949, the Tibetan National Assembly agreed to the idea of an air link between Lhasa and India. A British company, the General Electric Co. was given a contract to built a small hydropower station to supply electricity to Lhasa. The Company was the first to be permitted to land in Lhasa. The permission was given under the Regent’s seal.
It was an important strategic decision to open Tibet to air traffic. Due to the high altitude of Lhasa and the rarefied atmosphere, the flying and more importantly the landing of planes posed many problems.
The fact that the landing strip needed to be much longer in higher altitudes and the airplane engine more powerful, was an interesting challenge for the Indian Government. The permission given to General Electric also had important strategic consequences in the event of military intervention in the Himalayan region, though for India defending Tibet was more a question of political decision.
The first flight to Lhasa happened in 1955 at Damshung Airport, north of Lhasa near the Nam tso lake.
Damshung  was then the world's highest airport.

According to China Tibet Online:

There was even no telephone in the Damshung Airport. The civil aviation flight team of Beijing Administration used Ilyushin Il-18s. The plane flied from Beijing to Chengdu, and must arrive at Damshung at the forenoon of the next day and left as soon as possible, because the strong wind in Damshung will blow after the midday, making the airport filled with stones.
The earliest flights were not on a regular basis, and then it gradually became one flight every one month or half month. The ticket was quite hard to get.
The staff of the Damxung Airport at that time lived in the simple bungalow built of iron sheet, enduring hot summer and cold winter. There was no electricity and telephone, and they could only use candle for light in the evening. The unique machine connecting with the outside world was radio station.

The Dalai Lama's flight never materialized and a few weeks later, the Tibetan leader left the Tibetan capital by road (read horse-back and palanquin) for Yatung in Chumbi near the Indian border, where he took refuge for several months.