Federal officials admitted yesterday that they wrongly assured area officials in 1981 that proposed noise standards for National Airport would shut down commercial jet traffic there shortly after 10 p.m.

Shortly after the assurances were given, the Federal Aviation Administration received test results indicating that a new DC-9 was quiet enough to fly into National despite the new noise standard, FAA officials said yesterday. "We were as surprised as anyone to find out that it the jet's noise was as low as it was," said Ed Sellman, manager of the FAA's noise technology branch.

His statements and those of other FAA officials conceding the agency inadvertently misled area officials came a day after American Airlines disclosed it would effectively skirt the airport's 10 p.m. curfew by using the quiet DC-9 Super 80 to make two daily late-night flights into National.

Trans World Airways said yesterday it may follow American by moving a scheduled 9:59 p.m. arrival of a St. Louis flight to past 10 p.m. Other airlines are expected to follow, especially because the airlines can add the late-night flights without using one of the valuable landing slots that National's daytime operations require.

One government official privately has estimated that as many as 24 flights may be booked into National between 10 p.m. and midnight, a popular arrival time at many major East Coast airports. If that prediction is correct, passenger traffic at National would be increased from the current level of about 13.5 million a year toward Transportation Department limit of 16 million.

Rep. Frank Wolf, one of the officials who said he had been led to believe in 1981 that National would have no late-night commercial flights, yesterday angrily called on the Transportation Department to stop the FAA, which runs National, from allowing the commercial planes to schedule the DC-9 landings after 10 p.m.

"It clearly violates the spirit and the intent of the policy," Wolf said. ". . . There would be a curfew for nighttime commercial aircraft -- that was the whole basis of the policy."

The FAA defended the flights yesterday, saying in a statement that the landings will be consistent with the airport's noise policies. "The aircraft will be no louder than propeller airplanes now landing at the airport after the noise curfew, and it will sound no louder to people within their homes," the FAA said.

"Noise levels at Washington National Airport reached their peak in 1981 and have been decreasing since--and are expected to continue to decrease--as manufacturers continue to spend significant amounts of money to reduce the noise generated by new aircraft," it said.

The FAA's Sellman said the curfew--actually a limit on the noise levels permitted during nighttime operations at National--were set on the basis of how badly sleep would be disrupted in homes along the airport's flight paths.

Tests indicate that under the rules adopted in 1981, noise inside homes closest to the paths would reach a maximum of 55 decibels, a level that awakens only about one in 10 sleepers. That was considered to be an acceptable level of noise, Sellman said.

Opponents of the new flights argued yesterday that top federal officials, including then transportation secretary Drew Lewis, gave repeated assurances during debate over National's future that the standards were so tight that no existing passenger jet would be able to operate, and said the fact that the DC9-80 later proved to be exempt was never made clear.

Linda Gosden, a spokeswoman for Lewis, now chairman of a major cable television company, acknowledged that Lewis had given these assurances and said he acted on information then available to him. When the updated information was received, she said, the FAA made it public. Sellman said the FAA noted the DC-9's noise limits in an advisory circular published Nov. 20, 1981.

Trans World spokesman David Venz said his carrier's move is not related to American's plans but is intended to put the flight closer to the actual time of arrival.

Currently, National's noise standards effectively prohibit commercial jet aircraft from operating there between the hours of 10 p.m. and 7 a.m., although there is a 30-minute grace period at night for jets that may be delayed en route. After that, arriving planes are diverted to Dulles or Baltimore-Washington International airports, FAA officials said.

American has scheduled a flight from Chicago to arrive at 10:55 p.m. daily and another from Dallas-Fort Worth at 12:19 a.m., after calling at BWI. The DC-9s are not quiet enough to meet the nighttime take-off standards and will have to remain at National overnight.

There are now 555 slots for air carriers between 7 a.m. and 10 p.m. Demand for them far exceeds supply, and currently the airline committee that is charged with distributing them by common consent is deadlocked.

The FAA has said that the noise policy was not set up to bar all airliners, only ones that exceeded certain noise levels -- 85 decibels on landing and 72 for takeoff. Any plane that meets these standards should be allowed to operate, it said.