737S CALLED LESS SAFE THAN OTHER JETS.

Byline: Matthew L. Wald The New York Times

The Boeing 737, the most popular airliner in history, is less safe than other airliners because the failure of a single valve, the one that controls the rudder, can cause it to crash, the National Transportation Safety Board said Thursday.

But the board stopped short of grounding the planes or even declaring them unsafe. Investigators have noted that failure of the valve is rare, and recent changes in operating procedures by the airlines could compensate for the problem.

Boeing and the Federal Aviation Administration promptly disagreed that the plane was less airworthy than any others. A spokeswoman for the Boeing Co. called the 737 ``absolutely'' safe, and the FAA has called the 737's safety record ``very good'' compared to other airliners.

Only last month, in a decision announced by Vice President Al Gore, the aviation agency said it planned to propose replacing the valves. That proposal has not officially been made, however, because Boeing is still designing the replacement valves.

In the meantime the airlines, under orders from the aviation agency, have been making changes in their emergency manuals and instructing their pilots about how to react to the valve problem should it occur.

Although the safety board did not say so explicitly, its conclusions about the rudder valve, contained in a 10-page letter to the FAA, makes it clear that it believes it has solved the mystery of two 737 crashes: one in Pittsburgh in 1994 and another in Colorado Springs in 1991; a total of 155 people were killed in the two crashes. Safety investigators have long suspected that unusual rudder malfunctions were responsible for the crashes.

The board's letter also said that if Boeing were applying today for permission to sell the plane, the request would not be approved because the 737 is in violation of a fundamental design criteria: that no single failure be capable of causing a crash unless the failure is ``extremely improbable.''

The safety board said the valve could jam and then reverse, causing the rudder to move in the opposite direction commanded by the pilot. As a result, pilots react in a way that actually exacerbates the problem, rather than corrects it.

These failures ``can no longer be considered an extremely improbable or an extremely remote event, and thus raise serious questions about the validity of the certification of the existing B-737,'' the safety board said.

Asked whether 737s are safe and whether he would fly on one, Ted Lopatkiewicz, a spokesman for the board, said its statement would have to speak for itself. He also said all five members of the board had approved the report and decided not to issue further statements.

The safety board does not regulate, only recommend. But it is extremely influential.

The FAA, which is in charge of aviation safety but is also supposed to balance new rules against additional costs, said it believed the chance of a jam and reversal was, in fact, extremely remote, and that the steps it was taking would compensate for the mechanical problem presented by the rudder valves.

Thomas McSweeny, the FAA's director of aircraft certification, said this afternoon: ``We believe the rudder system is commensurate with that level of safety on every other airplane out there. We believe the actions we have taken at this point are sufficient to insure compliance of that airplane with the safety requirements, until final replacement of the power control unit can be made.''

He added, ``We're looking at an event that we believe is extremely improbable.''