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Probable Cause

It took 28 seconds for USAir Flight 427 to plummet from the sky. It took the National Transportation Safety Board five years to figure out why.

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Anomalous rudder motion: It was a Boeing euphemism for a catastrophic situation—a rudder jam and a reversal.

John Cox of the Air Line Pilots’ Association’s John Cox heard rumblings about the discovery on Halloween night but didn’t hear the news until the morning of November 1, when the alert was issued. He had spent an extra day in Pittsburgh and was summoned to the office of William Barr, USAir vice president of flight operations. A group of pilots and safety officials were meeting to discuss the service bulletin and how USAir should respond. The airline had the third-largest fleet of 737s in the world.

Barr asked Cox point-blank, “Is the airplane safe?”

“Yes,” Cox said. He was convinced that a jam was still highly unlikely and that, even if one occurred, pilots could recover. USAir had been the first airline to raise its minimum speed above 190 knots (220 mph), so that a rudder hardover could be countered by turning the wheel; therefore, USAir pilots had an extra cushion of safety. And the airline’s pilots were already doing a rigorous rudder check, so they were effectively conducting the test before every flight.

Just before Thanksgiving, Phillips went back to the Parker plant in Irvine to compare the valve from the USAir plane with the ones from an Eastwind Airlines 737 that had had a rudder anomaly and the factory PCUs. He wanted to find out if there was something that made the Flight 427 valve jam when the others would not.

Every rudder valve was slightly different. All valves had to meet certain Boeing and FAA standards, but none of the holes for hydraulic fluid on each one were exactly the same. The tests so far suggested that some valves could be more prone to reverse than others.

At Parker, the three valves were each disassembled and examined and then hooked into a test rig to see how far off neutral they had to be moved before the rudder would reverse. The factory valve performed the best. It would not reverse until the outer slide was 38 percent extended. But the USAir and Eastwind valves would reverse more easily, when the slides were extended 12 and 17 percent, respectively. Also, a measurement of the distance between the valve slides found that the USAir unit was considerably tighter than the other two.

That was the final piece of evidence that Haueter had waited for. At last he had proved that the USAir valve was unique. After three years and hundreds of tests, he now had a scenario for what had happened to Flight 427.

It went like this: It was a smooth flight from Chicago to Pittsburgh, so there was not much movement of the yaw damper, which automatically moves the rudder to compensate for the onset of yaw. That lack of movement might have allowed particles to build up in the hydraulic fluid. There could have been a modest thermal shock to the PCU because of overheated fluid from a hydraulic pump—not enough to set off a warning to the pilots but enough to make the cold valve suddenly expand.

The PCUs on other 737s might have tolerated that without trouble. But the valve on this particular USAir plane was especially tight. The thermal shock and the contaminants caused a jam. And the jam happened when one slide inside the valve was slightly off center and more likely to reverse. The pilot or the yaw damper was commanding the rudder to go right, but it went hard over to the left.

All of this occurred at the most vulnerable speed for a 737: 190 knots. The pilots compounded the problem when they pulled back on the control column, which made the plane lose speed and stall. The plane spiralled down and crashed into a hill.

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