On a day in early November, security officials from the sprowling Boeing aircraft company in Seattle took a disturbing piece of information to the fraud division of the King County, Wash., police. In the course of investigating "problems with employees," Boeing security found that a number of tools and blueprints for airplane parts had vanished.

In the following days people were interviewed, documents obtained.By early December, the Seattle office of the Federal Aviation Administration was notified of the dark possibility that airplane parts - fraudulently labeled Boeing and supposedly certified as safe - had apparently been made elsewhere and sold.

Airlines were notified. Soon two suspected parts were returned to Boeing, and, with FAA officials watching, Boeing engineers opened a landing gear accessory module. Immediately they knew it had not been built by Boeing. An absolutely unthinkable thing had indeed happened: airplane replacement parts that were never certified as safe and had been installed on U.S. passenger jetliners.

Worse, the FAA's announcement of the scheme marked the second disclosure in days of a bogus airplane parts operation. A California company was earlier accused of selling helicopter parts that supposedly were made by Bell Helicopter Co. and the Sikorsky Helicopter Division of United Aircraft Corp.

Despite an elaborate system designed to ensure the safety of U.S airplanes, the FAA now concedes that that system is vulnerable to bogus parts schemes. Indeed, an FAA official cannot even say whether the agency has nipped the flowering business of counterfeit parts or whether there is a more extensive - and undiscovered - trafficking in uncertified parts.

"That's what's got me worried," said James O. Robinson, acting chief of the engineering and manufacturing division of FAA. "With all these cunning minds, the unscrupulous person seeking to make a dollar can get around" the FAA safety certification process, he said.

All those cunning minds. For sure, the bogus Boeing business was the third known incident in which the West Coast aerospace belt had harvested counterfeit parts. Besides those for helicopters, the National Aeronautics and Space Administration's space shuttle was fitted with counterfeit - but high quality - transistors, NASA said.

(FAA regulations do not apply to space projects.)

FAA, essentially, sets technical standards for airplane parts, authorizes production by a manufacturer who proves he can meet those standards, and then licenses an inspector, usually an employee of the same company, to assure the quality.

These inspectors, or the firm itself, is monitored by FAA with varying scrutiny. A big company like Boeing, which builds planes with tens of thousands of parts in each one, will have at least one full-time monitor; a small company gets infrequent review.

In the past, this system has been criticized for the obvious - a company employee does the inspecting for FAA.

Underlying this intricate structure of widespread self-policing is the shared interest - even if only financial - that air travel should be safe travel. "The industry tends to squeeze out those who don't conform," Robinson observed.

And generally it has worked well. Mechanical problems today seldom account for aviation accidents. Most often the error is human. "It's a good system," says Robinson, "if it's followed."

But now replacement parts, in two known cases, have been built outside that system by unauthorized people. Stamped with false identification and serial numbers, the Boeing parts were then marketed through two Seattle area firms, ADS Supply Co. and Air Repair Inc., engaged in otherwise proper aviation business, the FAA says. ADS, for example, buys used parts, has them reconditioned, and sells them, according to a co-owner.

Besides these two, sources say that perhaps three other companies may be involved, possibly including former or current Boeing employees. FAA feels it has located all of the Boeing 727 and 737 bogus parts, and the agency emphasizes that it believes that none ever caused or could cause an accident.

A spokesman for Frontier Airlines said it had jets flying for three years with the bogus parts and never had trouble with any of them. How had Frontier come to do business with ADS?

"It's a competitive market," said a Frontier spokesman. "Parts are available from a number of suppliers. (ADS) had a good price and a good delivery schedule."

"It frequently costs more, but we always buy from the manufacturer when we can," said a spokesman for Delta Airlines, which had none of the counterfeit parts, "It pays to buy straight from the manufacturer."

An FAA spokesman said the agency will review its investigations of the helicopter and Boeing parts schemes with an eye to revising its inspection system. But a longtime expert on aviation safety commented, "I don't know how they could stop it, unless you say 'All parts must be bought from the manufacturer,' and then you'd find yourself in court for creating a monopoly.

"And FAA would need 500,000 inspectors to check everything."