United Airlines, the world's largest air carrier, said yesterday that it will add four to five inches of legroom to the first six to 11 rows of economy class as a perk for its frequent fliers.

This feature, dubbed Economy Plus, will provide seats with 35 to 36 inches of legroom compared with the standard 31 inches. Economy Plus "will boost our customer satisfaction and clearly differentiate United from our competitors," said Rono J. Dutta, president of United Airlines Inc.

Advanced assignment of Economy Plus seating will be available on a first-come, first-served basis to United's Mileage Plus members who have "premier" status or above. To receive a premier status, the passenger must have flown 25,000 paid miles on United or one of its alliance airlines, or 30 or more segments in one calendar year.

United, which offers 2,370 flights each day, said customers paying full fare in economy class also will be able to receive pre-assigned seats in Economy Plus. Star Alliance Gold and Silver travelers also will qualify for Economy Plus.

The number of seats in Economy Plus will vary from 36 to 89, depending on the type of aircraft. Economy Plus seating should be available in 450 planes by April, United said.