It takes only a minute upon landing at the airport here to realize you have arrived at a military hub for the central government's intertwined wars against South Africa and Jonas Savimbi's guerrillas.

The airport is packed with Cuban officers and soldiers saying goodbye to colleagues -- Cuban and Angolan -- boarding the returning flight of the Angolan national airlines, TAAG, to Luanda, and welcoming others who have just arrived from the capital.

The engine roar of Soviet-built MiG21 and MiG23 jet fighters, piloted by Angolans newly returned from Moscow, fills the air. The planes race down the twin tarmac airstrips, shoot into the sky and circle for landings, only to take off immediately on another practice flight.

All morning, again in the early evening and on into the night, the sound of those MiGs reverberates over the town. It serves as a constant reminder that this is a major airbase as well as the headquarters of the Army's Fifth Military Region, covering Angola's entire southern border region.

Lubango lies on a line, running from Namibe on the coast through Matala and Menongue to Cuito Cuanavale in the east, that marks the southernmost deployment of Cuban troops. The line is 150 to 200 miles away from the border with Namibia and roughly along the 15th parallel.

The Cubans are not going farther south, according to Army chief of staff Col. Antonio dos Santos Ndalu, "to avoid problems of charges of foreign involvement." He was apparently referring to possible South African allegations.

"The Cubans are there to stop a big invasion from South Africa," he said.

There was no sign of any Cuban troops or even advisers south of Lubango's outskirts during a 250-mile trip by Land Rover down to Ondjiva, just 25 miles from the Namibian border.

The Angolans seem to be facing the South Africans and UNITA guerrillas alone down there. It is a cat-and-mouse game of war whose rules apparently include an Angolan avoidance of major clashes with the more powerful and mobile South African units roaming the savannah inside southern Angola.

In Lubango, however, the Cubans are very visible, together with an occasional Soviet adviser. They can be seen careering around town in Soviet-made jeeps and trucks in a town whose traffic today is mostly military.

The Cubans and Soviets are almost totally absent, however, from the Grand Hotel, Lubango's premier social center, living quarters and restaurant for Angolan Army officers and out-of-town visitors. The Cubans have separate quarters, one behind the hotel, and if the officers of the two allied armies mix socially, it is not obvious where they do it.

Col. Ndalu said the Angolan offer to send home most of the 26,000 to 27,000 Cuban troops attached to the Angolan Army in exchange for South Africa's withdrawal from Namibia was still valid. "But the Cubans in Cabinda Angola's oil center are not part of the calendar," he said. "We think if South Africa withdraws from Namibia, they will just put all UNITA people in Zaire and attack Cabinda."

The United States, for five years, sought to negotiate the withdrawal of the Cubans together with the independence of Namibia. It tried to negotiate a compromise between the South African demand that all Cuban troops be out in 12 weeks and the Angolan offer to withdraw most of them, but not those stationed in Luanda and Cabinda, over a two-year period.

Angola rejected the Reagan administration as mediator early this year after Washington began aiding UNITA. Angolan officials say the United States has lost all credibility as an impartial arbiter of the conflict and has asked the United Nations to take over the negotiations.

The talks are deadlocked.