Building on information gathered since the in-flight explosion of TWA Flight 800 three years ago, the Federal Aviation Administration yesterday proposed a mandatory design review of fuel tanks on nearly all of the nation's commercial air fleet--both jets and propeller-driven aircraft.

The proposal covers all commercial aircraft that carry 30 or more passengers, a total of about 6,000 aircraft. An FAA spokeswomen said the proposal covers more than 90 percent of U.S. commercial aircraft and is one of the largest such orders ever contemplated.

The impact of the FAA proposal, if adopted, is expected to be felt worldwide as foreign governments adopt similar rules. FAA officials said a number of foreign governments have already begun working with them on the possible fuel-tank problems.

FAA Administrator Jane Garvey called the proposal a "fundamental change in how fuel tanks are designed, maintained and operated."

For the first time, the federal government hopes to deal with all three sides of what is called the "fire triangle"--ignition, fuel and oxygen--rather than focusing all its efforts on preventing an ignition that would touch off an explosion.

To accomplish this, the FAA is seeking new fuel-system design standards for aircraft manufacturers that will minimize potential sources of ignition and also either minimize the potentially explosive vapors or protect the aircraft from any fire or explosion that such vapors might cause.

The government yesterday acknowledged that so far it simply wasn't feasible to try to eliminate all fuel vapors.

Also, manufacturers would have one year to come up with new maintenance programs for existing planes' fuel tanks and surrounding electrical wiring, with specific guidelines for aircraft operators to identify potential ignition sources before they develop into safety hazards. Airlines then would have six months to implement the new inspection and maintenance procedures.

The FAA estimated the cost of the proposal at $170 million over 10 years, but agency officials said they were certain the cost figures would be disputed by the airline industry as too low.

The agency said the new certification standards and mandatory review were prompted by the TWA 800 tragedy of July 1996, in which 230 people were killed when a fuel tank on a Boeing 747 jet exploded shortly after takeoff from New York City on a flight to Paris. The National Transportation Safety Board is not expected to issue its final report on the TWA crash until next spring.

"Significant lessons were learned from the TWA investigation," said Elizabeth Erickson, the FAA's director of aircraft certification.

She said investigators know the fuel tank exploded on TWA 800 but don't know the source of ignition.

Erickson said that as part of the investigation of the TWA crash, the FAA has issued directives to manufacturers and airlines dealing with fuel-tank problems as they were discovered. As a result, she said, the public should not be afraid to fly during the government rule-making process.

The public has 90 days to comment on the proposal. The FAA then has what amounts to an unlimited time to sift through the comments before issuing a final proposal.

An FAA spokeswoman estimated the entire process would take at least a year.

A spokeswoman for the Air Transport Association, the trade group that represents the airline industry, said airlines would have to study the FAA proposal. She said the individual airlines have already been working with both the manufacturers and the FAA on potential tank problems.

David Venz, vice president at Airbus Industrie, said the FAA proposal didn't break any new ground and was much in line with the work being done by the FAA's Aviation Rulemaking Advisory Committee, which includes various industry representatives. "They've been looking at the whole fuel-tank issue," he said.

Boeing spokesman John Dern said the company welcomed the proposal in principle and would be filing comments with the FAA. "In some cases, these are systems that have been in service for 30 or 40 years, so we're confident they're safe," Dern said. "But we've also learned some things in the last few years, so we're trying to figure out how to integrate what we've learned with the proven design concepts."

Yesterday's proposal also seeks to require that center fuel tanks not be heated and that they cool at a rate equivalent to that of fuel tanks in an aircraft's wing. As a result of its investigation of TWA 800, the NTSB heard scientific testimony that there was a huge difference between the amount of electrical energy required to set off an explosion inside a cool fuel tank and what was necessary in a tank that was only slightly warmer.

The potential combustibility of jet fuel is also under study.

Although jet fuel today is considered less explosive than older fuels, Erickson said the government thinks researchers are on the verge of developing a system of far less explosive aircraft fuels.