Constant inspection and overhaul by commercial jet operators keep planes flying well past 20 years in some cases without compromising safety, experts said.

After declaring a short circuit likely ignited a fuel tank blast four years ago that killed 230 people on TWA Flight 800 --a 25-year-old Boeing 747 jumbo jet-- National Transportation Safety Board Chairman Jim Hall questioned the safety of older jets.

Hall Wednesday ordered a study of the last 20 years of commercial aviation accidents to check for links to aircraft age, particularly catastrophic accidents and in-flight fires.

"Most people understand that as you get older, things degrade," he said.

"I don't want to leave the impression that we've resolved that in this issue, age is not a factor, because I personally believe it might be."

But aviation experts said evidence does not support that conclusion.

Accidents do tend to increase on many jet models as they age, but that reflects resale of older jets to airlines in poorer nations that may not maintain them as carefully as carriers in the United States and other industrial powers.

"If you maintain that airplane it will be safe. If you don't maintain that airplane it will be less safe," said Liz Verdier, a spokeswoman for Seattle-based Boeing, the world's biggest plane maker.

And much like car owners, aircraft operators tend to scrap older jets with damage that might be repaired on a new plane, increasing the industry's "hull-loss rate" as planes age.

In fact, historically new jet models have suffered some of their highest accident rates in their first few years on the market as manufacturers iron out design bugs and operators become familiar with new controls and systems.

"If you factor all that stuff out it looks like the accident rate for these airplanes actually becomes pretty flat," said Kevin Darcy, a Seattle-based safety expert at Safety Services International.

Boeing has conceded that airplane wiring had been inadequately inspected and maintained in the past and has issued dozens of service bulletins recommending new procedures since the crash of Flight 800.

"Have we missed some stuff? Yes. We discovered a lot of things as a result of this (accident) that will only improve safety. We have implemented or are in the process of implementing them," Verdier said.

But industry experts stress that the NTSB was unable to conclude definitively that wiring caused the fuel tank explosion, calling it a best guess, not a certainty.

"You really get down to what was the condition of the wiring of that particular airplane before the crash and the fact is nobody knows. Nobody will ever be able to say conclusively what happened," said Jim McKenna, executive director at the Aviation Safety Alliance.

Several experts agreed that Hall's apparent determination to link age with airplane accidents was misguided at best.

"I don't think the chairman (Hall) has any evidence besides his personal feelings that the age of an airplane is directly related to problems that you might experience on that airplane," McKenna said.

The National Transportation Safety Board said older aircraft could not automatically be considered unsafe, but that more needed to be done to remove even the remote risk of a fuel-tank explosion such as the one aboard the TWA Boeing 747 that killed 230 people on July 17, 1996, off the coast of Long Island.

The board said it could not determine for certain what sparked the explosion, but agreed unanimously that "of the sources evaluated by the investigation, the most likely 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."

In other words, worn wiring likely caused a short circuit that allowed high-voltage electric current to come in contact with low-voltage wires running into the fuel tank.

Contributing to the crash was Boeing's design that places hot air-conditioning packs directly under the center fuel tank, and the design philosophy that calls for eliminating all potential sources of ignition from fuel tanks rather than assuring that vapors in the tank would never be flammable.

Members of the five-member board sometimes grew emotional as the two-day hearing neared an end. Member John Goglia, a former aircraft mechanic and union official, almost broke down as he told victims' family members how upset he was by his first plane crash as a mechanic.

Both he and board chairman Jim Hall urged that the aircraft hulk, painstakingly reassembled inside a hangar on Long Island, be preserved and moved to a proposed safety board training academy in Virginia.

"We must not let this airplane go," said Goglia, who has taken hundreds of airline mechanics and others to see the ghostly hulk as an object lesson on the importance of their jobs. "We must use it as a teaching tool for the aviation community."

Dozens of family members applauded Goglia and Hall, but they saved their loudest applause for Peter Goelz, the board's former managing director, and Betty Scott, the government affairs deputy director who effectively started the board's family affairs program after the crash. Congress has since given the board prime responsibility for seeing to the needs of families after a crash.

The investigation has led to improvements in aviation safety, including a new Federal Aviation Administration program to deal with aging aircraft systems such as wiring. Board members, who routinely are critical of the FAA, expressed pleasure with the FAA's reactions to board recommendations on fuel tanks and wiring.

Yesterday, the board added several recommendations, including a review of the design specifications for aircraft wiring, new electrical bonding practices inside fuel tanks, and development of ways to eliminate the ignition risk of silver-sulfide deposits that grow inside fuel tanks and can conduct current.

The board also recommended better training of maintenance personnel, improved reporting of unsafe wiring found by mechanics, and the use of new technology such as automated wire test equipment and new "arc-fault" circuit breakers to halt electrical current immediately if a short circuit is detected.

Board staff members said examinations of other aircraft wiring found many of the same problems as were on TWA 800, including cracked and worn wiring, a build-up of flammable lint around electrical connections, the presence of foreign objects such as metal shavings, and the routing of low-voltage and high-voltage wires in the same bundles.

Part of second day of hearings was spent explaining what hundreds of witnesses saw, and why those who thought they saw streaks rising to the airplane actually saw burning fuel from the airplane itself.

800 and recommended changes in aircraft wiring and better fuel systems.

The board formally concluded on Wednesday that the Boeing 747 was destroyed by an explosion in its center fuel tank, probably triggered by a short circuit. It asked the Federal Aviation Administration to examine aircraft wiring practices, review wiring design specifications and require improvements.

The FAA has 90 days to respond to the recommendations. Board Chairman Jim Hall said that during the TWA investigation his agency has sent 11 other recommendations to the FAA, and it has acted or is taking action on most of them.

``An additional level of safety can be added by looking for ways to reduce the flammability of fuel tanks themselves,'' said Boeing spokesman Russ Young. Protecting the fuel tanks from heat is among options that Boeing, other manufacturers, the FAA and airlines are looking at, he said.

``We're going to continue trying to eliminate any potential ignition sources within those fuel tanks, which is a long-standing design philosophy,'' Young said Thursday. ``Those two things together we think will take what is already a very low risk and take it even lower.'' Speculation on the cause of the crash off the coast of New York's Long Island, which killed all 230 aboard, has ranged from maintenance problems to a bomb, a missile, and even a meteorite. In a two-day hearing concluding its investigation the board struggled to disprove those theories.

Joseph Lychner of Houston, who lost his wife, Pam, a former TWA flight attendant, and his two young daughters, Shannon and Katie, in the disaster, found little comfort at the hearing.

``The presentation by the NTSB has been excellent but there have been no surprises. The center fuel tank blew up and killed my family, and the loved ones of the other family members. That is not supposed to happen, is it?'' Lychner said.

Michel Breistroff from Paris, whose son Michel, 24, had been about to join the French Olympic hockey team, is not convinced the plane was brought down by a mechanical problem. He said he had lingering doubts about a missile or some other non-mechanical means.

``They have been explaining to me but inside my head I don't fully believe what they are saying. I wish I could be convinced so I could be free,'' Breistroff said.

The board spent much of Wednesday afternoon focusing on the missile theory, stressing that no radar returns show the presence of a missile.

Eyewitness reports generally support the evidence of an internal explosion, though a few mention a streak of light before the plane went down.

Investigator David L. Mayer said that the first explosion in the moving airplane would have looked like a small, moving light followed, a few seconds later, by a fireball as the plane broke up and fuel from other tanks went into the blaze. The fireball then would break up as the pieces fell into the ocean.

``There is a remarkable consistency of the accounts and most seem to be describing the break-up of the accident airplane,'' Mayer said.

Some witnesses who reported seeing a streak from the ground didn't include that in their first description, only adding it later after the idea had been raised in the news, he added.

``Witness statements only help solve the puzzle, physical evidence is almost always the key,'' said board member John Goglia.

Earlier Wednesday the board raised the question of aging aircraft. The TWA Flight 800 plane was 25 years old.

``The longer an airplane is around, the more changes and modifications it needs,'' said Hall, adding that it is not clear the industry is paying enough attention to these problems.

The Federal Aviation Administration and the aviation industry have launched aging aircraft programs to study the needs of older planes and how to deal with them.

Goglia, a former aviation mechanic, stressed that the mere age of an aircraft does not mean it's not safe. ``A properly maintained airplane can last forever,'' he said.

Robert Swaim of the board's aviation engineering division said, however, that inspections of a number of commercial aircraft showed increasing problems as the planes got older.

``We looked at other planes from other carriers and other countries,'' Swaim said. Inspectors found worn insulation, improper wiring repairs, open splices that should have been sealed and lint on circuit breakers.

Bernard Loeb, the board's aviation safety director, cautioned that investigators were not saying the problem had reached the level of planes being unsafe.