IN the annals of the National Transportation Safety Board, four airline accidents have never been solved. The investigative body is laboring hard to make sure that last September's crash of USAir Flight 427 doesn't become its fifth unsolved mystery. But as hearings here in Pittsburgh demonstrated this past week, the cause of the crash is still unknown.
''I don't have any leading suspect,'' says Thomas Haueter, deputy chief of the board's major-investigation division.
''We're frustrated. Super-frustrated,'' adds Jack Wurzel, flight-safety coordinator for the International Association of Machinists, a union of airline mechanics that is participating in the investigation. ''We just don't know.''
The lack of information in the USAir accident, which claimed the lives of all 132 people aboard, is pushing the safety board to recommend a major upgrade of commercial airliners' flight-data recorders.
Investigators have so far pieced together this much of the USAir flight's final moments: The plane, a Boeing 737, was making its final approach to Pittsburgh International Airport and coming out of a left-hand turn when its rudder moved suddenly and unexpectedly to the left. This caused the plane to yaw or skid forward with its nose pointing slightly leftward. This movement caused the plane to begin to roll to the left.
Under normal circumstances, these forces would not have been enough to cause the crash, investigators believe. But something else -- still unknown -- happened. Did the rudder stay stuck to the left? Was it mechanical failure or pilot error or both? And what caused the unexpected rudder movement in the first place?
''We know the control surface that caused it but we don't know how it caused it,'' says Don McClure, air-safety coordinator for the Air Line Pilots Association, a national union based in Washington.
Unexpected rudder movements have occurred before in Boeing planes. The safety board has documented at least 14 such incidents on 737s alone. But according to records of the Federal Aviation Administration, mechanics who have inspected the planes after such reports have been unable in almost all cases to determine the cause of the malfunction.
Although they remain optimistic they will solve the mystery of Flight 427, investigators say they would be much further along if they had more information from the flight. That is why the Safety Board has signaled that it will almost certainly recommend a major upgrade of the flight-data recorders found in commercial airliners.
The recorders log a plane's performance, such as altitude and airspeed, which can give investigators important clues as to the final moments before a crash. As airliners have become more sophisticated, so have their flight recorders. Virtually all commercial airliners have recorders that track at least 11 parameters. The Federal Aviation Administration (FAA) requires that the newest planes record at least 31 parameters and recommends more than a dozen more.
The Safety Board says it's considering recommending that the FAA require 80 or even 90 parameters to be logged. ''I don't know of anybody who's going to object to the addition of more parameters on the flight-data recorders,'' says Jack Gamble, a spokesman for Boeing Commercial Airplane Group.
The problem is money. While it won't be too difficult to upgrade the latest planes, which already monitor scores of parameters digitally, older planes present a challenge.
''It's an additional burden,'' says USAir spokesman Paul Turk. Some 235 planes, more than half USAir's fleet, are 737s. Retrofitting them to record addition parameters would cost an estimated $66,000 in labor alone and two weeks of downtime to rewire each airliner with the necessary electronics. A flight-data acquisition unit would cost another $40,000 or so, says one expert.