The Federal Aviation administration issued New York Air its operating certificate yesterday, clearing the way for the airline to begin low-fare service between Washington and New York today.

The airline, an affiliate of Texas International Airlines, had hoped to begin service last Sunday but was forced to postpone the start-up because of the work -- largely paperwork -- still remaining to satisfy federal safety standards.

In another development, the FAA announced that it is seeking to close what it is seeking to close what it considers a loophole in its rules in order to limit New York Air to nine roundtrip flights a day. The airline, using the "loophole," currently plans to operate ten flights in each direction a day weekdays.

The FAA said it will publish a proposal in Monday's Federal Register that would prohibit scheduled airlines operating out of four airports subject to "high-density rules" -- including Washington's National Airport -- from operating any flights for which it had not received "slots" -- permitted takeoffs and landings.

New York Air officials decided to take advantage of the loophole in the high-density rules when they were able to get only 18 slots for their operations at National. They felt 20 was necessary to operate a daily schedule that could compete reasonably with the hourly Eastern Airlines Air-Shuttle.

Because slots are essentially airline reservations for landings and takeoffs in bad weather conditions -- conditions that don't often prevail at National -- New York Air scheduled the two flights for which they didn't have slots as "subject to Air Traffic Control conditions." The caveat means that the flights would operate during good weather when the so-called visual flight rules prevail because the high-density rule allows extra flights to land in those conditions. However, the flights would be canceled during bad weather when instrument flight rules (IFR) prevail -- when the airport bars airlines and private pilots from landing and taking off unless they have the slot reservations.

In its proposed rule, however, the FAA said the provision New York Air is using was never meant to be used in such a way. "The purpose of this section was not to expand the hourly IFR reservations available to air carriers and scheduled air taxis, but was simply a means to provide some flexibility for those 'other' operators who could not obtain or had no need for daily slots," the FAA said.

To allow scheduled operators to utilize this section on a regular basis as a means of supplementing its authorized slots, as proposed by New York Air, would amount to an amendment of the hourly slot limitations imposed on the airport, the FAA said.

The proposal would make it clear that scheduled operations are ineligible for additional slot reservations; the proposal would not affect private pilots which routinely exceed the slot limitations granted that flying sector. Comments on the FAA proposal are due Jan. 2.

Right now, at least, New York Air plans to operate 10 roundtrip flights a day during the week, eight on the weekend. It will charge $49 each way on eight of its weekday flights, and $29 for a lunchtime and night flight. All seats on the weekend flight will cost $29.

In contrast with the Eastern Airlines Air-Shuttle, which guarantees a seat to any passenger at the gate in advance of the scheduled flight time, passengers must book reservations on NYA. The regular fare on the shuttle is $59 although it is charging $29 betwen noon Saturday and noon Sunday.