The Boeing Co. studied fuel tank problems in one of its jumbo jets in 1980--16 years before similar problems in a similar jet apparently contributed to the explosion of TWA Flight 800--but failed to give its report to safety investigators until this June.

Congressional investigators are probing why Boeing never told the National Transportation Safety Board that it had completed a report on center fuel tank overheating in its E-4B, the military version of the civilian 747 that went down over the Atlantic Ocean in July 1996.

Safety board officials believe the report could have helped focus the TWA 800 investigation on mechanical failure much earlier, and one senator argued that its conclusions could have helped prevent the tragedy if they had been released after a 1990 fuel tank explosion.

The board released a terse statement yesterday expressing "dismay" and "displeasure" about Boeing's failure to produce its study, and attorneys for families of the 230 people killed aboard TWA Flight 800 warned that the March 14, 1980, report is sure to resurface in their multimillion-dollar lawsuits against the manufacturer.

Boeing declined official comment. Sources said company officials have acknowledged that they should have produced the report earlier, but have contended that it would not have affected the TWA 800 investigation.

The 1980 Boeing report focused on one of the key issues that preoccupied TWA 800 investigators--the possibility that excess heat from the jet's air conditioning bay could create highly flammable fuel vapors in the central tank--and it even made one of the safety recommendations that the board issued in December 1997.

Sen. Charles E. Grassley (R-Iowa), chairman of a subcommittee that oversees airline disaster investigations, said Boeing was under a legal as well as moral obligation to produce the study after TWA 800 went down.

He said that if the company had volunteered its four-volume report after a center fuel tank explosion in a different kind of Boeing jet--a Philippines Airlines 737--killed eight people at the Manila airport in 1990, American safety officials might have recommended changes that could have prevented the TWA 800 tragedy.

"The government and the manufacturers need to have an ethic that overreporting is better than underreporting," Grassley said. "This was too important to fall through the cracks."

In recent months, the Office of Special Investigations arm of the General Accounting Office, which conducts probes for Congress, has interviewed more than a dozen Boeing employees from the civilian and military wings of the company, and more than a dozen federal employees at the Air Force and the NTSB. Sources say that the investigators have no evidence of any intentional coverup at Boeing or at the Air Force, which commissioned the original report, but that Boeing officials admitted to the investigators that they should have produced the report earlier.

Safety board officials say that Boeing was legally bound to surrender the document and let them decide whether it was relevant to their investigation. And in fact, they say, the document was relevant. "It would have been very helpful to us," one official said. "I think even Boeing would admit that they should have told us about what they had."

In an interview, a Boeing official who spoke on condition of anonymity noted that the E-4B, which is used for special aircraft such as Air Force One, is not the same as the 747; for one thing, it has four air conditioning units in its main bay, vs. the 747's three. And he argued that the Boeing report was not "directly" related to the TWA 800 investigation, because it focused on testing the temperatures of different fuels in different situations, instead of searching for the ignition source of an explosion.

"I wouldn't say it's irrelevant, but I don't think there's a direct correlation between the results of this study and the conclusions of the TWA investigators," the official said.

The E-4B report did foreshadow the 747 investigation. In 1980, the Air Force directed Boeing "to provide a detailed analysis of the [center fuel tank] heating problem" with the E-4B; the board has focused on the same problem with the 747.

In 1980, Boeing concluded that "the dominant source of heating into the [center fuel tank] is . . . the air conditioning equipment bay" of the E-4B; the safety board has also focused on that air conditioning bay, which is directly below the 747's central fuel tank.

In fact, after TWA 800 went down, Boeing officials bolstered investigators' suspicions of sabotage, arguing that there was no way the central tank could get hot enough to become susceptible to an explosion; the E-4B report could have cast doubt on that argument. And ultimately, the safety board recommended adding insulation between the air conditioning bay and the fuel tank in the 747; in 1980, Boeing had made the same recommendation for the E-4B.

Nevertheless, the safety board did not learn about the Boeing report until March, when it was listed on the agenda for a meeting of an Air Force task force studying the safety implications of the TWA 800 crash for the E-4B. And it did not get the report until June. Sources say Boeing officials told the investigators that they could not have turned it over to the board because they considered it an Air Force report, but that miscommunications between the company's civilian and military wings may have contributed to the problem.

"We of course were dismayed that we did not receive a copy of this report in the early weeks of the TWA Flight 800 investigation," the safety board said in its statement. "We have expressed our displeasure to Boeing about their failure to inform us about the study and the three-year time span it took for us to receive it. . . . Its contents will be considered in the formulation of our final accident report on the crash of Flight 800."

The board hopes to finish that report next spring; it has yet to determine the source of the spark that caused the blast. But it has located the explosion in the center fuel tank, which would not have been possible if overheating had not produced flammable vapors. Steven Pounian, an attorney on the plaintiff's committee representing the families of TWA 800 victims, said that if Boeing knew about the problem, it should have done something about it.

"They definitely should have taken some action," Pounian said. "This report shows they knew the air conditioning bays were causing too much heat. So they knew the problems that could occur."