National Airport officials are investigating an incident last Friday in which a New York Air DC9 and an American Airlines 727 came within one to two miles of each other while preparing to land.

Tower chief Harry Hubbard said there is no question the two planes flew with "less than standard separation," the official term used by officials when aircraft fly closer together than they should under Federal Aviation Administration rules.

An FAA spokesman said that while there was no indication that either pilot needed to take evasive action to avoid a collision, the planes apparently did violate the FAA requirement that aircraft maintain a distance of three miles horizontally or 1,000 feet vertically.

Both planes were flying at an altitude of 3,000 feet and were about 10 miles northwest of the airport, where they were being directed to positions for a down river approach to National's runway 18.

The FAA spokesman said the New York Air plane, approaching from the east, was making a left turn to line up with a localizer, a navigational guidance device, and appeared to have overshot the line by about a mile. The American Airlines plane was making a right turn onto the same path.

While it has not been determined exactly how close to each other the two planes came, an FAA spokesman said it is the air controllers's responsibility to see that proper distance is maintained between aircraft.

A spokesman for American Airlines said he was aware of the investigation but that the incident itself was considered so minor that the pilot apparently did not file an incident report. A New York Air spokesman said the airline has no record of the incident and only last night was informed that an investigation is under way.