Federal Express Corp. said its daily air express volumes for February rose 15.7 percent over February 1991 levels, with gains reported in its four main product lines.

The figures mark the fourth consecutive month that the Memphis-based carrier has reported double-digit traffic growth. The company handled a daily

average of 1.52 million packages and documents across its global system last month, compared with 1.31 million moved each night in February 1991.Federal Express reported strong gains in its next-afternoon and second-day businesses, and its International Priority product, a time-definite, door-to- door delivery service.

Perhaps more significantly, the company booked 5 percent more shipments last month on its higher-priced "Priority Overnight" service than it did in February 1991. Its January "Priority Overnight" volumes were p 6 percent over the same period last year.

Industry analysts view the movement in the company's overnight volumes as critical in determining if an economic recovery is in place. Generally, shippers will switch to the more time-sensitive form of shipping if they feel comfortable with the economic outlook.

For many months, the company experienced a substantial erosion in Priority Overnight volumes as cost-conscious shippers shifted to less-expensive products. Despite the gains in the last two months, daily "Priority Overnight" volumes are still off by 16,000 shipments for the first three quarters of its fiscal year.

Dan Copp, a Federal Express spokesman, said the February gains were skewed

because the first quarter of last year was unusually weak. For example, Federal Express moved only 718,862 Priority overnight shipments in January 1991, its lowest monthly level of the year.

Other observers agreed. Robert G. Brazier, Airborne Express president, said he "would look for a longer trend" before he could say the domestic air express industry had emerged from the recessionary doldrums.

"If we have numbers like this through March, then I think we're moving in the right direction," he said.

Unlike Federal Express, Airborne doesn't publish monthly traffic data. But Mr. Brazier said that Airborne's February volumes rose 20 percent to 30 percent over a year earlier and that the company appears headed for similar

gains this month.

Jim Parker, an analyst for Robinson-Humphrey Inc. in Atlanta, said the rise in Federal Express volumes "shows some pickup in business." But he attributed the increased activity in part to a more aggressive pricing strategy aimed at wooing large-volume shippers away from competitors like Airborne and United Parcel Service.

Mr. Brazier said he's spotted a similar trend. Federal Express "recently has been more inclined to match or undercut existing prices or be more aggressive on new bids," he said.