Nigeria's newly elected president, Shehu Shagari, the moderate leader of a powerful African nation, has called for a non-negotiable deadline for independence in Namibia by 1981.

The 50-nation Organization of African Unity has also focused its attention on Namibia (South West Africa), a protectorte entrusted by the League of Nations to South Africa following World War I. The United Nations in 1966 revoked that mandate, and a 1971 decision by the International Court of Justice called upon South Africa to withdraw immediately.

President Shagari has now proposed a break-off of airline travel to and from South Africa should the South African government refuse to comply with U.N. resolutions calling for Namibian independence. Considering the strong-arm tactics by Nigeria to force the British government to refuse recognition of the Muzorewa "internal settlement" in Zimbabwe last year, few will doubt that Shagari's call for airline sanctions can be backed up with action.

The case of Zimbabwe also suggests international pressures might be effectively applied to the question of Namibia.

The ability of the Carter administration to maintain economic sanctions against Rhodesia through some tough battles with racist and Cold War hardliners in Congress enabled Britain's Lord Carrington to negotiate free and fair elections that have virtually checked Soviet influence in a new Zimbabwe. The new government there has not only sought ties to the West, but so far has refused to accept a Soviet ambassador in Salibury. Prime Minister Robert Mugabe has made himself available to a variety of private sector and governmental delegations from the West.

South Africa and the Soviet Union were somewhat restrained in their involvement in Zimbabwe, but both were sure that the West would settle for something less than genuine majority rule. The Russians figured that such a cop-out would strengthen their military role in the area. The South Africans were willing to risk expanded Soviet influence in the belief that they could then challenge the Cubans who were close to their own borders and that such an escalation would arouse support from the West in hidden, if not more overt, forms.

Now the scene turns to Namibia. The United States must be determined that South Africa face up to the harsh realities of the world. If there are no free and fair elections in Namibia, South Africa must be forced to face sanctions.

Negotiations to bring Namibia to independence began in April 1977, with the then-five Western members of the U.N. Security Council (Britain, Canada, France, West Germany and the United States) launching a diplomatic initiative involving South Africa, the liberation movement SWAPO (South West African People's Organization), the neighboring front-lie states (Angola, Tanzania, Mozambique, Botswana and Zambia) and the U.N. secretary general.

Three years of quiet but continuous talks have produced a plan for withdrawal of South African troops, their replacement by a U.N. peace-keeping force, a demilitarized zone along the borders and a six-month election period culminating in an independent Namibia.

But after receiving every possible concession from both SWAPO and neighboring Angola, the South African government continues to stall, hoping for a shift in Western resolve. They were at first convinced that they could count on greater understanding from a Thatcher government in the United Kingdom, but Carrington's skillful diplomacy on Zimbabwe brought them little comfort.

I would assume that South Africa currently awaits elections in West Germany and, more important, in the United States, with the prospect of a Reagan victory seen as a return to the days when Africa was viewed as little more than a Cold War theater. Foreign policy advisers close to Reagan have already discussed renewing strategic military links with South Africa, and John Connally, a likely Reagan secretary of state, is a long-time favorite of racist South Africa.

All international airlines fly under charters granted by the International Civil Aeronautics Organization. Landing rights and flight plans are subject to its monitoring, so an international air travel sanction against South Africa -- unlike an oil embargo or economic sanctions -- would be enforceable.

Nearly 2,000 businessmen and tourists depart from South Africa daily for cities in Europe and North America. If airline sanctions were invoked, the South African government and its citizens would have to decide whether they want to be cut off from the rest of the world.

The purpose of such airline sanctions would not be to cripple South Africa, but to provide an outside moral authority enabling more enlightened leadership to prevail. Such an authority might be necessary to allow South African Prime Minister Pieter Botha to pass the blame for compliance to the West, much as some southern politicians in the United States reluctantly capitulated to Supreme Court decisions as a means to facilitate peaceful desegregation.

U.S. Africa policy is one of Jimmy Carter's success stories. Relations with African nations have never been better. Africa supplies 40 percent of U.S. oil imports and has remained a stable and developing supplier. Increased trade and investment have great potential in this virgin market of 400 million people now in the throes of industrialization.

And despite widespread political turmoil among African nations, there is no dissent on the continent regarding Southern African liberation. A front-page story in the Nigerian Daily Times, known to voice official government policy, was headlined "Shuu Wants Namibia Free Next Year."

This is an issue that clearly separates Carter from Reagan and can shore up lagging black and liberal support for the president.

Clearly some tough decisions must be made, but if the West does not choose non-violent pressure, continued violence could spread through southern Africa and a great opportunity for peace, trade and development will have been lost.