Delta Air Lines reports its passenger revenue miles, the rate of one passenger flown for one mile, jumped 18.9 percent in April over the same month a year ago.

Delta spokesmen have downplayed the role the strike at its major competitor Eastern has played in its increased business this spring, saying much of Delta's new business is in markets that Eastern had not served. But the company has posted consistent increases in passenger revenue miles since early March, when Eastern machinists walked off the job, followed by the airline's pilots.Delta flew a total of 4.98 billion revenue miles this April compared to 4.19 billion reported in April 1988.

For the first four months of the year, Delta's revenue passenger miles have increased 16.5 percent to 18.5 billion from 15.9 billion flown in the same period in 1988.

Delta flew 5.8 million passengers in April, 16.6 percent more than the 5 million it had flown last April. During the four month period, the number of passengers boarding Delta planes increased 10.9 percent, from 19 million to 21 million in 1989.

Whit Hawkins, Delta's senior vice president for marketing, said the airline's merger earlier this year with Western Air Lines also contributed to the dramatic increases.

"The Delta-Western merger has begun to pay handsome dividends, which is showing up as traffic increases in every segment of the Delta system," he said. "For example, traffic at Los Angeles increased 11.2 percent, Salt Lake City 16.2 percent and Dallas/Fort Worth 18.2 percent."