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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They had reached the most extreme condition. The PCU was depressurized, frozen with the nitrogen gas, and then injected with piping-hot fluid.

The hot fluid hit the cold valve. Click, hiss, click, hiss, click, hiss, click, hisssssssssssssssssss.

The hissing changed pitch. The valve had jammed.

“It didn’t come back,” said someone in the room.

“That’s interesting,” said someone else. “Reeeeaaalllll interesting.”

A second later, the arm went back to neutral and began cycling again. Click, hiss, click, hiss.

They stopped the test and talked about what had happened. Did they have a breakthrough? The test conditions were so poorly controlled that any result was questionable. A computer operator who had been collecting test data had mistakenly deleted everything, so the team had little evidence of what they had seen. Everyone agreed to try it again.

Click, hiss, click, hisssssssssss. Click, hisssssssssss. The valve was moving slower than it was supposed to. Click, hisssssssssssssssssssssssss. It stuck again.

The group agreed that the test should be done again in a more controlled setting. The Boeing team criticized the tests, saying they were too extreme and that the valve could have been damaged. So the next morning, Phillips woke up at 4 a.m. and drove to Parker Hannifin in order to perform a test to make sure the valve was okay.

The test was crucial. When the group had first examined the valve after the crash, they had not found any scratches inside it. If they found scratches now, it would prove that a jam had occurred, which would indicate there had not been a jam on Flight 427. Also, a scratch would mean that the valve had been altered since the crash, which would rule out any further tests. The whole theory about a valve malfunction would go down the drain.

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