PITTSBURGH -- The USAir jetliner that crashed on Thursday, killing all 132 aboard, had been repeatedly inspected in recent months to make sure that it did not have a rudder problem that has caused safety concerns about the Boeing 737, a federal aviation official said yesterday.
Nothing was found to be out of order in the inspections that were performed by USAir after being ordered last March for all similar planes until the manufacturers change the design of a rudder mechanism and replace the part, said Donald Riggin, the manager of the FAA office in Seattle that oversees safety rules for Boeing aircraft.
As investigators seek to determine why the USAir jet suddenly rolled to the left and nose dived straight down from a mile high, one subject of the preliminary investigation has been the possibility of a rudder problem.
On planes like the 737, a malfunctioning rudder can orient the wings in a way that causes one to lift and the other to sink.
Ordinarily a pilot could compensate, but in some cases, a pilot could bank sharply to one side and lose control of the plane, government and industry researchers have found.
Malfunctioning rudders have never been blamed for a crash of a 737.
But the theoretical possibility has been enough to warrant continued attention.
Ever since a fatal, nose-diving United Airlines crash in Colorado Springs in 1991, also involving a Boeing 737, the National Transportation Safety Board, the Federal Aviation Administration, and Boeing have been investigating incidents in which rudder mechanisms have malfunctioned on the jets.
The crash in Colorado Springs is one of only a few that the safety board has investigated but remains unable to explain.
The plane in that accident had experienced rudder problems in the days just before the crash.
The safety board and the aviation agency have, therefore, continued to look closely at rudder problems in the Boeing 737.
Rudder power control valve mechanisms are manufactured for Boeing 737s by Parker Hannifin, a manufacturer of vehicle, aviation, and marine components based in Cleveland.
They will be replaced throughout the fleet of thousands of planes over the coming five years when a new design becomes !B available, Mr. Riggin said.
In the meantime, planes must be inspected at least every 750 hours.
The rudder mechanism on the USAir jet had been inspected three times since March, most recently about 100 flights before the crash, Mr. Riggin said in a telephone interview.
No problems with its rudder mechanism were ever detected.
The problem with the rudder valve mechanism involves two cylinders, one fitting inside the other, in a hydraulic fitting that helps push the rudder, a piece of the tail fin that helps steer a plane.
If the two cylinders do not fit together precisely, hydraulic fluid can leak within the mechanism.
In extreme cases, a pilot might then command the rudder to steer to the right and it could instead steer to the left, according to government documents.
Safety board officials are just beginning to piece together the fragments of wreckage scattered in a heavily wooded area near Pittsburgh International Airport, and have not yet examined rudder parts, although they said they intended to do so.