Long-term air cargo growth will average 6.6 percent a year over the next 20 years, with the long-term downward spiral in cargo industry profits coming to a smooth landing, according to Boeing Co.'s annual world air cargo forecast.

World air cargo traffic increased over 12 percent last year, but there appears to be a slowdown in the Europe-to-Asia and North America-to-Latin America markets, the report said.The highest air freight market growth over the next 20 years will occur on Asian routes, said David F. Pierce, regional director, revenue analysis-cargo for Boeing's Commercial Airplane Group.

"Asia is rapidly becoming industrialized and by 2014 it will be producing 35 percent of the world's manufactured goods," he said. Volume between North America and Asia was up nearly 14 percent in 1994.

He said declining passenger yields have forced airlines to turn to cargo for additional revenue opportunities. Cargo and passenger yields have declined by about 3 percent a year since 1970, reflecting productivity gains, increased competition and technical improvements.

But cargo yield "reversed its general direction from the last two successive years of 7 percent declines with a marginal increase (in 1994) of one half of one percent," the forecast said.

Further increases are anticipated this year, the report continued, but long-term profit growth is expected to remain equal with inflation.

Mr. Pierce puts together the Boeing forecast each year from a host of sources, including government statistics, airline data, various consultants and Boeing's own trade database. It is one of the most eagerly anticipated and widely respected air cargo projections published.

"Based on what I've seen, it's the most comprehensive forecast as to what's going on," said Robert V. Dahl, project director for Air Cargo Management Group, a consulting firm in Seattle. Reports by other manufacturers ''tend not to go into the detail that Boeing does," he said.

"I feel we're the most conservative soothsayers in the industry," Mr. Pierce said in a recent interview. "The future looks very strong."

One of the key trends he said he's noticed is that the international air express market is beginning the follow the large growth pattern evident in the U.S. domestic market.

"If you look at international express today, it's 5 percent of the international air cargo market," Mr. Pierce said. But it grew 25 percent last year and "I'm forecasting the market will grow 18 percent a year for the next 20 years." That means that over that period that 5 percent share will grow to over 30 percent, which could cause big changes in the industry and its infrastructure, he said.

It's an important point that airport facility planners, for example, should review, he said.

In 1994, total air cargo capacity increased 2 percent while the market grew 12 percent, but over the long term, that disparity will even out, with cargo capacity forecast to grow 6.1 percent a year while traffic grows 6.6 percent a year. Freighter capacity is projected to average 6.7 percent a year, the Boeing report said.

The total jet freighter fleet declined slightly in 1994 to 1,003 aircraft, but over the next 20 years Boeing said the fleet will more than double, with nearly 600 large capacity freighters required. Large freighters - with capacity over 50 tons - will become the largest segment of the fleet, comprising 38 percent of the fleet by 2014, as opposed to 18 percent last year.