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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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The Parker technicians took the valve apart, measuring and documenting each piece. They put them under a microscope, examining each surface for scratches or scrapes. They found none and no evidence of a jam. Phillips breathed a sigh of relief.

They had proved that the valve could jam—and leave no evidence behind.

Six weeks later, Phillips’ group reconvened in a Boeing laboratory in Seattle. This time, instead of testing the PCU in the Coleman cooler, they used a specially designed foam box with a window on top. The box’s cooling system was more powerful and precise, with temperatures closely monitored by a computer.

They ran through the same tests they had done in Valencia, starting with the factory PCU at room temperature and then trying a variety of thermal shocks. Once again, the factory PCU passed every test.

The technicians removed it and replaced it with the PCU from Flight 427. It passed the first tests with no trouble. Then came a repeat of the most extreme test in the Coleman cooler. They removed hydraulic pressure from the PCU and let it soak in the cold air until it reached –40 degrees. The hydraulic fluid was heated to 170 and shot directly into the PCU. The technician moving the valve back and forth felt it slow down. He didn’t notice it bind, but a computer showed it had jammed momentarily. He repeated the action, and felt the lever kick back when he tried to move it to the right. When he tried again, he felt it stick to the left and then jam.

Once again they had shown that the 427 valve was unique. It jammed when the factory unit did not.

Yet Boeing was right. The extreme temperature range necessary for a thermal shock just wasn’t present in real life. And there was no proof that it had happened on the USAir plane. Despite their skepticism, Boeing engineers said they would examine the charts from the tests for anything unusual.

A few days later, in a building overlooking Paine Field in Everett, a young Boeing engineer named Ed Kikta sat at his desk, reviewing the charts. He could see the test data on his computer screen, but he liked to print the results so he could study them more closely. The charts showed the flow of hydraulic fluid during each test: higher when it was pushing the rudder and down to zero when it was not. Kikta expected that when the outer valve jammed during the thermal shock, the inner valve would compensate and send an equal amount of fluid in the opposite direction, which would keep the rudder at neutral. That was the great safety feature of the 737 valve. It could compensate for a jam.

But as Kikta studied the squiggly lines for the return flow, he saw dips that were not supposed to be there. When he matched them to another graph showing the force on the levers inside the PCU, he made an alarming discovery. When the outer valve had jammed, the inner valve had moved too far to compensate. That meant the rudder would not have returned to neutral, the way it was supposed to.

The rudder would have reversed.

That could be catastrophic. A pilot would push on the left pedal, expecting the rudder to go left, but it would go right.

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