Federal officials are investigating an incident in which a private plane landing at National Airport last week passed illegally close to an Air Virginia commuter plane preparing to take off, according to Harry Hubbard, the chief of National's tower.

Hubbard said a controller allowed a twin-engine Aerostar to land at about 10 p.m. on Sept. 12, even though the Air Virginia aircraft, a twin-prop Hawker Siddeley, was in position on the same runway waiting for permission to take off.

The Aerostar went over the other plane to land on the runway pavement in front of it, violating rules of air traffic that call for clear runways when planes land or take off.

Hubbard estimated the Aerostar was "several hundred feet" above the other plane, though the precise distance was not known. Neither crew considered the incident serious enough to report it to the Federal Aviation Administration, he said.

Hubbard blamed the incident on "human error" by a controller, who has been put on noncontrol duties pending completion of the investigation.

The incident came two weeks after another "operational error" at National. In that one, an American Airlines Boeing 727 and a New York Air DC-9 passed within one or two miles of each other on Aug. 27 while preparing to land.