The National Transportation Safety Board is taking twice as long to investigate plane crashes as it did five years ago, primarily because of the efforts it is still devoting to the daunting mysteries of the crashes of TWA Flight 800 in 1996 and a USAir flight near Pittsburgh in 1994.

Safety board officials and other experts say the delays, which are also the result of more complicated airplanes and intense legal pressures, raise questions about whether a flaw in equipment, training or procedures could cause more than one plane crash before being diagnosed and corrected."When you can't get out a probable cause finding and promptly issue your recommendations, that can really threaten to set back the fundamental soundness of the system," said Kenneth P. Quinn, a former general counsel of the Federal Aviation Administration who is in private law practice. Quinn praised the safety board as "lean and mean" but he added that air safety "relies on getting to the bottom of accidents just as fast as we can and as best we can."

Officials at the safety board concede that getting to the bottom of accidents is taking far longer. Despite relatively few accidents requiring board investigations this year and last, the span of the board's inquiries - from the time of a crash to the agency's final report - has doubled. What took six or nine months in the early to mid-1990s now takes twice that, the board acknowledges.

And many local, state and federal agencies, like the FAA, cannot issue findings - or even publicly discuss an accident - until the NTSB acts. Charged by Congress to investigate all fatal air crashes, the board is limited to issuing recommendations. But those rulings command widespread attention, and as the waiting worsens, some other agencies have become increasingly impatient and frustrated.

Some unanswered patterns worry aviation experts, who say that an unsolved crash can come back to haunt investigators. One example they give is the crash of a Comair turboprop on its way to Detroit in January 1997. The plane, an Embraer 120, began an ordinary left turn in frigid conditions and then fell out of the sky, killing all 29 people on board.

Whatever brought down the Detroit plane, for instance, might be related to an incident near Sacramento, Calif., in March, when another Embraer plunged more than a mile before the crew regained control. No one was hurt, but it raised a nightmare possibility for NTSB professionals: failing to learn the lessons from a crash before a similar one occurs.

The board has made some preliminary recommendations in the Detroit crash but is still working on the case 15 months later.

Safety board officials acknowledge that the investigative delays are frustrating, to agencies, airlines and private companies but contend that they have their hands full.

"Have we had more work than we can do?" said James Hall, the NTSB chairman. "The answer is yes." He compared his agency, with its staff of 400, with a fire department that can fight only a certain number of blazes at a time.

A large share of the board's investigative force has been consumed with the crash of TWA 800 and that of USAir near Pittsburgh, Flight 427, because both involved popular plane models whose possible mechanical failures raised broad questions about airline safety, Hall and others said. The crashes also involved great human tragedy that brought official and personal demands for answers.

Hall and others said that these complicated crash investigations were the tip of the iceberg. Airplanes with increasingly sophisticated technology require far more expansive investigations that take longer and longer, he said.