THE CAUSE OF the 8 September, 1994, crash of a USAir Boeing 737-300 at Pittsburgh, Pennsylvania, is unlikely ever to be known for certain, according to US National Transportation Safety Board (NTSB) investigators. The aircraft inverted and dived to earth from 6,000ft (1,800m), killing all 132 people on board.

Thomas Haueter, the NTSB investigator-in-charge, told a Pittsburgh board of inquiry that enough data may be gathered to yield a "probable cause", but even that is not expected to be delivered until later this year. The inquiry opened on 23 January.

The NTSB could not deliver even a probable cause for the March 1991 United Airlines 737-200 which also inverted and dived, but from a lower height.

Jim Hall, the NTSB chairman who chaired the proceedings, said: "The fact is that too large a segment of the airline fleet is flying around with inadequate flight-data recorders [FDR]. In this age of high technology, the American public are puzzled when we can lose a major aircraft like a 737 in Colorado Springs and not be able to determine what happened to it."

He was referring to the Pittsburgh aircraft's 11-channel FDR, which does not record the positions of the control surfaces.

During the weeklong proceedings several witnesses, including those from the Federal Aviation Administration, USAir and Boeing were called. Although some additional light was shed on the accident, the inquiry failed to yield a definite cause for the crash.

Boeing insists that a rudder malfunction alone could not have caused the Colorado Springs and Pittsburgh crashes. It was mentioned as a possible cause of the Colorado Springs accident.

A multiple-cause verdict is, therefore, expected. The USAir aircraft was known to have flown throughout the wake vortex of a Boeing 727, which might have precipitated a chain of events leading to the dive.

When the final report is issued, it is expected to contain recommendations for better FDRs fleet-wide, and also for the formulation of hydraulic fluid-purity standards.

The USAir 737's, hydraulic rudder power-control unit, was found by the investigators and it tested satisfactorily, however. The level of fluid contamination was no worse than in other units sampled, the investigators say.

Source: Flight International