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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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Intense meetings went on all day in Renton and Everett, Washington, as the Boeing engineers discussed how to respond. They broke into two teams, one to come up with a plan to enable pilots to detect and respond to a jam, and another to look at long-term design changes to the PCU. They worked into the night. By 11 p.m., they got approval from senior management for a pilot test and some short- and long-term changes to the PCU.

The next day, Halloween, about 10 Boeing officials drove to the FAA office in Renton, a big mirrored cube of a building beside Interstate 405. They weren’t sure what the FAA would do. Would the agency want to ground the airplane? The PCU no longer protected against jams the way it was supposed to, so the plane might no longer meet certification standards.

About 25 Boeing and FAA officials gathered in a conference room. Draxler began by explaining what they had found in the tests, with Hewett frequently interrupting to give his perspective. It took a unique kind of windup to trigger the phenomenon, they said. You had to press on one pedal and then stomp hard on the other to make the primary slide line up with the wrong holes and cause the reversal.

An FAA official asked: Did this match what had happened to the USAir plane?

The Boeing engineers said all they knew from the test was that if you jam the outer slide, you could get a reversal. Jams were extremely unlikely because of the many filters in the hydraulic system, which removed particles before they caused problems. In 30 years and more than 50 million flights, there had been only seven confirmed jams. None had resulted in an accident or injury. And there was no evidence that one had occurred on the USAir plane.

Another FAA official pointed out that the new evidence seemed to counter Boeing’s claims that the pilots had caused the crash.

Jean McGrew spoke up. “We’ve received a lot of public criticism about hiding things and not wanting to spend a lot of money,” he said. “But I frankly don’t care [what it costs]. If there is something wrong with the airplane, I want to fix it.”

The meeting ended. Boeing said it would issue a bulletin to warn airlines about the condition. The bulletin would require mechanics to perform a test every 250 hours, stomping on the pedals to check for jams. The FAA planned to issue an emergency airworthiness directive that mandated the tests. Boeing also said it would develop a long-term plan to redesign the valve to prevent a reversal. That fix was likely to take several years.

These emergency directives were more symbolism than real action, designed to reassure the public that the FAA was taking action. The engineers knew the tests would not be very effective. They would catch a jam if it occurred at the precise moment of the test, but a jam could still occur at any time.

Despite Boeing’s discovery, FAA officials say they did not give serious thought to grounding the 737 fleet. The plane had a good safety record, they said, and a jam was still considered highly unlikely.

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