Security workers who screen passengers at National Airport failed to find 27 of 134 imitation bombs and weapons planted in carry-on luggage by Federal Aviation Administration undercover investigators during two months last year, the agency said yesterday.

That 20 percent failure rate of security checkers at National was the same as the average posted by all workers tested during a four-month period at 28 major domestic airports, according to a General Accounting Office study.

However, the effectiveness of those checkers varied greatly from airport to airport. At one facility, all but 1 percent of the imitation weapons were detected, while at another only 34 percent were detected and the rest were carried through the security system.

National's failure rate "is not acceptable," said Ron M. Linton, a member of the board of the Metropolitan Washington Airports Authority, which this month assumed operational control of federally owned National and Dulles International airports. "I want to know why it's not a 100 percent success rate," said Linton, chairman of the board's operations committee, which plans to study security issues at both airports.

The FAA since 1973 has required commercial airlines to screen virtually every passenger and piece of carry-on baggage for weapons or explosives. The airlines hire private firms to conduct the screening, which is subject to periodic FAA testing.

The FAA tests airport security by hiding objects in carry-on luggage that passengers surrender for X-ray checks and on individuals who walk through metal detectors or are searched personally. All methods are subject to mechanical or human error, security specialists stress.

A rash of deadly terrorist incidents aimed at civilian airliners worldwide in 1985 brought renewed emphasis to airport screening and a strong commitment from Transportation Secretary Elizabeth Hanford Dole to beef up security and personnel checks at airports.

"These test results demonstrate the need for the FAA to set minimum screening requirements and to penalize airlines for loose security standards," said Rep. Cardiss Collins (D-Ill.), chairwoman of the House Government Operations subcommittee on government activities and transportation, which will hold a hearing on the GAO report today.

The GAO did not provide airport-by-airport results, but the FAA confirmed a WJLA (Channel 7) report that security workers at National did not detect 27 of 134 imitation weapons and bombs used in FAA tests in November and December. The FAA declined to release the figures for September and October, the other two months of the four-month national study.

The GAO study omitted Dulles, the fastest growing airport in the country, because "there wasn't sufficient data for us do any kind of analysis," said Tom Barchi, director of the division that compiled the report.

The FAA conducted 10 tests of the security workers at Dulles, well below the 25-test minimum required by the study, according to the House subcommittee staff. The FAA conducted fewer tests at Dulles because it has "very adequate, very efficient" security workers and "we are putting our resources where the problem is," said agency spokesman Fred Farrar.

Dulles security workers "passed every one" of seven FAA tests in November and December, Farrar said. However, the subcommittee staff disclosed that the Dulles workers failed at least one of the three tests conducted in the preceding two months.

Congressional sources and airline officials reported a year ago that Dulles security guards failed to recognize imitation weapons in 25 percent of the spot checks conducted by airlines.

In October, the subcommittee found the FAA "seriously remiss" in allowing Dulles security to reach an "unsatisfactory" level in 1986.

Airports General Manager James A. Wilding and authority board Chairman A. Linwood Holton Jr. had not seen the GAO report and declined comment.

The FAA would not release the results of security tests at Baltimore-Washington International Airport, but a congressional source said the airport ranked "above average" and was among the top 10 airports with the highest security ratings.

The airlines "are aware of the need to continue to improve" airport security, but would not want the GAO report "to indict a successful system that has met the actual threat" of hijacking, said Thomas M. Tripp, a spokesman for the Air Transport Association, an industry group.