The four-year investigation of the crash of Trans World Airlines Flight 800, one of the most exhaustive in aviation history, came to an end yesterday with a warning that other older aircraft have wiring in as poor condition as the doomed aircraft's.

The National Transportation Safety Board said older aircraft cannot automatically be considered unsafe, but that more needs 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 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 a design philosophy that calls for eliminating all potential sources of ignition from fuel tanks rather than ensuring that vapors in the tank would never become 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 family members of the crash victims how upset he was by his first plane crash as a young mechanic.

Both he and NTSB Chairman Jim Hall urged that the aircraft hulk, painstakingly reassembled inside a hangar on Long Island, be preserved and moved to a proposed new 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 former NTSB managing director Peter Goelz and current deputy director for government affairs Betty Scott, who effectively started the board's family affairs program in the aftermath of the crash. Congress has since given the safety board prime responsibility for seeing to the needs of families after a crash.

The investigation already 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 responses to past board recommendations on fuel tanks and wiring.

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

Board staff members said careful examinations of other aircraft wiring found many of the same problems that were found on TWA Flight 800, including cracked and worn wiring, a buildup 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 the afternoon of the second day of hearings was spent carefully explaining what hundreds of eyewitnesses saw, and why those who thought they saw streaks rising to the plane from the surface actually saw streaks of burning fuel from the plane itself.