WICHITA, KAN., JUNE 6 -- Air Midwest grounded one-third of its fleet for maintenance and repairs requested by the Federal Aviation Administration but will have its planes back in the air by Monday, a spokesman said today.

The airline, which serves 60 airports in 15 states, grounded 18 of its 54 planes and canceled 20 percent of its flights Friday. By early this afternoon, all but nine planes were back in service, said Mike Scheidt, Air Midwest senior vice president and chief operating officer.

The planes were removed from service because the FAA told the airline it could no longer delay certain repairs. All the grounded planes were Fairchild Metro IIs, a 17-seat turboprop.

Although the FAA did not order Air Midwest to take the action, an agency spokesman said the groundings were related to a recent six-week inspection by the agency.

"We're not going to nitpick over the legitimacy of their allegations," Scheidt said. "We're just going to do whatever they want. That's what you need to run an airline." He described the areas in need of repair as "small cracks and loose rivets."

Jerald D. Mertens, manager of the FAA's flight standards office in Wichita, said the airline had been delaying maintenance and repairs unduly. The airline needed to fix small cracks on the planes' flaps, elevators and rudders -- repairs that are not uncommon, he said.