In 1980, for the first time in aviation history, foreign visitors flying to the United States may outnumber Americans traveling to foreign destinations by air.

Americans have always comprised a majority of those flying between the United States and foreign countries. But for the first time last year, the U.S. citizen share of airline passenger traffic between the United States and other countries began to fall below 50 percent on a monthly basis.

In both August and September, the last months for which figures are available, Americans comprised just 48 percent of the total number of air passengers in the U.S. world market.

As recently as February 1978 the U.S. citizen share was 61 percent, but the percentage has steadily declined ever since, according to the Department of Transportation.

While U.S. citizens still accounted for 52 percent of the U.S.-international air travel market for the first nine months of 1979, airline officials and analysts believe that foreign visitors to the United States may tip the balance this year.

A major factor in the change has been the weakness of the dollar relative to other world currencies. "It has become very expensive for Americans to go abroad but has become relatively more inexpensive for people from abroad to come here," says Edward Robinson of DOT's Office of Air Transportation.

Another factor cited by U.S. aviation officials may stem from the Carter administration's policy of promoting international aviation competition. Despite skyrocketing fuel and other increasing costs, general pressure and the signing of liberal bilateral air agreements have increased significantly the availability of lowfare air travel to and from the United States.

Total passenger traffic between the United States and other countries continued to grow last year -- 14.5 percent in the first nine months -- but the increase in the number of foreign travelers far outstripped the rise in American passengers. In the January-September period last year, travel by American citizens was up 4 percent in the U.S.-world market while travel by foreigners was up 28 percent.

DOT's monthly air traffic reports are based on statistics collected by the Immigration and Naturalization Service at each U.S. airport that offers international flights. The reports exclude U.S.-Canada air travel data.

Other highlights of the latest DOT report show that:

U.S.-Britain was still the leading air market in the first nine months of 1979, but overall traffic increased a weak 4 percent over the comparable 1978 period. U.S. citizen travel to Britain was down 8 percent while foreign traffic from Britain rose 21 percent.

U.S.-Mexico was the second leading market during the period with a 25 percent increase in traffic.

U.S.-Cuba air trffic, all on charters, had the largest increase -- well over 1,000 percent -- but on a very small 1978 base.