American Airlines has asked the Federal Aviation Administration to grant it a six-month exemption from the FAA rule barring flights of more than 1,000 miles in and out of National Airport.

The airline wants to avoid the practice, devised to circumvent the rule, of having flights from National "hop" to Dulles International Airport before flying to Dallas/Fort Worth.

If approved by FAA Adminstrator Donald Engen, American's request could be a precedent-setting exception to the FAA's 1,000-mile "perimeter" rule that likely would prompt other airlines to ask for the same treatment, airline industry officials said.

An exemption for American would also further establish that airline's preeminence in flights between the Washington area and Dallas, one of the country's most lucrative airline routes.

Gene Overbeck, American's senior vice president for government affairs, said he expects critics of National Airport and anti-jet noise activists to oppose the move.

"Many of American's Dulles flights are now sold out well in advance of departure time," according to American's petition to the FAA, filed last week. Overbeck said that if direct flights are permitted between National and Dallas, American plans to add new roundtrip flights between Dulles airport and Dallas.

Currently, because of the 1981 1,000-mile "perimeter" rule, American Airlines has routed four roundtrip flights a day leaving National on an initial "hop" to Dulles before continuing to Dallas/Fort Worth. The practice is expensive for the airline, industry officials said.

The airline is proposing four roundtrips a day between Dulles and Dallas/Fort Worth and another four between National and Dallas/Fort Worth.

The FAA established the 1,000-mile rule in 1981, when it established a passenger "cap" at National of 16 million, in part to maintain the airport's short-haul character.

Some airlines wanting nonstop flights to distant cities have grumbled about the perimeter rule, but American's attempt to obtain an exception is apparently the first, industry officials said.

American officials have been especially upset by the rule because the Dallas/Fort Worth airport is the airline's "hub," the one where American schedules about one-half of its more than 1,000 daily flights, Overbeck said. American is the only major airline whose "hub" airport is beyond 1,000 miles from National.

With Braniff International cutting its service between Dulles and Dallas from four flights to one a day, American dominates the route as a near monopoly.