By Jim Wolf

 WASHINGTON (Reuters) - Design flaws in the Boeing 747, the first wide-bodied jumbo jet, contributed to the mid-air explosion that destroyed TWA Flight 800 four years ago, the National Transportation Safety Board ruled on Wednesday.

 Capping the costliest investigation of its kind, the five-member board said the accident's probable cause was an explosion in the plane's center wing fuel tank caused by ignition of flammable vapors.

 Although the NTSB could not pinpoint the cause of the blast with certainty, the most likely source was ``a short circuit outside of the center wing tank that allowed explosive voltage to enter it through electrical wiring associated with the fuel quantity indication system,'' the board ruled.

 It ruled out a bomb or missile strike as the source of ignition and termed ``very unlikely'' a long list of other possible causes, including meteorites, lightning, static electricity or a malfunctioning fuel pump.

 Boeing, the world's largest aircraft builder, had told the safety board it found nothing to support the idea that a ''specific electrical system or component of the 747-100 fuel quantity indicating system ignited a fuel/air explosion.''

 ``None of the recovered fuel system components inspected and analyzed showed any evidence of being the ignition source that initiated the accident,'' Boeing said in an April 28 submission after its own $32 million investigation.

 All 230 people aboard the New York-to-Paris flight were killed when it blew up off Long Island on July 17, 1996, 14 minutes after takeoff from Kennedy International Airport.

 The safety board faulted the ``design and certification concept that fuel tank explosions could be prevented solely by precluding all ignition sources'' -- a swipe at both Boeing and the Federal Aviation Administration, which regulates the aviation industry.

 Another ``contributing factor'' to the accident was situating heat-generating equipment beneath the Boeing 747's center wing tank ``with no means to reduce the heat transferred into the center wing tank or to render the fuel vapors in the tank nonflammable,'' the board said.

 Boeing safety director Ron Hinderberger praised the NTSB for helping identify potential safety improvements, but lamented the ``combative'' nature of its report.

 ``It is unfortunate that we would take the information that we've gained from the investigation and say decisions made 30 years ago, or that people have been making on a worldwide basis, were flawed,'' Hinderberger told Reuters.

 ``We look at each accident and look what we can do to enhance safety and prevent a recurrence. We could wind the clock back 10 years ago and find planes weren't as safe as planes today,'' he added.

 The company's shares closed up 1-1/4 at 50-9/16, a two-year high. Seattle-based Boeing has delivered more than 1,250 747s to airlines worldwide since the first went to the defunct Pan American airways in 1969.

 The safety board -- an independent body that investigates accidents but can only make recommendations -- often criticizes the FAA, a Transportation Department arm that mandates safety changes and certifies production processes.

 NTSB Chairman Jim Hall said the accident was caused by a chain of events ``set in motion years before by the manufacturer's and the FAA's design and certification policy.''

 ``Boeing's design practice of permitting parts less than 3 inches (7.6 cm) in any direction to be electrically unbonded may not provide adequate protection against potential ignition hazards as a result of static electricity generated by lightning and other high-energy discharges,'' the board ruled.

 The board issued four new safety recommendations in addition to those that had already led to some 40 FAA safety directives growing out of TWA 800.

 It urged the FAA to examine the bonding of components in fuel tanks to eliminate potential ignition sources; to review wiring systems of all U.S.-certified aircraft and require any changes needed to ensure ``adequate separation;'' to work to eliminate the ignition risk posed by silver-sulfide deposits inside fuel tanks; and to act on training issues related to the repair of potentially unsafe wiring.

 ``We're committed to completing our work so that none of the lessons learned will go unheeded,'' FAA spokeswoman Drucella Andersen said.

 Alarmed by the possible role that aged wiring played in the disaster, Hall ordered a study on Wednesday into possible links between older aircraft and accidents and whether passengers should be told the age of planes they board.

 He ordered a review, by Nov. 15, of the past 20 years of commercial aviation accidents to check for ``any correlation between the age of the aircraft and the accidents' cause, particularly as that pertains to catastrophic accidents and in-flight fires.''

 ``I want you to determine, based on this information, whether there is a safety benefit to providing information on the age of the aircraft in commercial service to the American people,'' he told staff after an investigation that cost Boeing and the NTSB a combined total of $67 million.

 But Liz Verdier, a Boeing spokeswoman, said proper maintenance could prolong a jet's life for decades. ``If you maintain that airplane it will be safe. If you don't maintain that airplane it will be less safe,'' she said.

 The crashed 747, manufactured in November 1971, had accumulated 93,303 flight hours with 16,869 takeoff and landing ''cycles.''