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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To make sure he hadn’t made a mistake, Kikta showed the results to the other engineers in the room. They agreed with his interpretation. It appeared that the valve had reversed. Kikta looked up and saw that his boss, Jim Draxler, was putting his coat on, getting ready to leave. Kikta stopped him.

“I think I’ve found something in the data,” Kikta said. “We might have a problem here.” Draxler took his coat off, set down his briefcase, and listened to what Kikta had to say. The consequences of his discovery were enormous. If he was right, the PCU was not performing the way Boeing had promised. The valve-within-a-valve was supposed to provide redundancy if one slide jammed. But this result meant a single jam could cripple a plane.

The next morning Draxler convened a group that he called his “grizzled veterans,” engineers who had lots of experience with flight controls. Kikta explained his findings and showed them the charts. Draxler went around the room, asking each engineer about the significance of Kikta’s discovery. They were unanimous: It was a serious problem that needed to be fixed quickly.

Boeing sprang into action. The company ordered Parker Hannifin to run its own tests to check Kikta’s conclusions. Parker engineers confirmed the results and discovered that when they jammed the outer valve, the levers in the PCU appeared to flex slightly, which allowed the inner valve to line up with the wrong holes.

Boeing was notorious for being the slow-moving “Lazy B,” but not this time. Fear was a powerful motivator. Engineers usually needed weeks to get an airplane for a test, but now they got one off the assembly line in just 24 hours. The plane landed at Boeing Field and was pulled into a company hangar. As a cold rain fell outside on the night of October 29, 1996, the 737 was rigged with the special device that Parker had built to simulate the jam. Michael Hewett, a Boeing test pilot, climbed into the cockpit while Kikta stood on a platform on the tail of the plane, watching the rudder and the PCU. Hewett pushed on the pedals, moving the rudder from side to side. The first two tests went smoothly, and the rudder operated as intended.

Then came a more rigorous test. Hewett slowly stepped on the left pedal and the rudder moved properly. He then jammed his foot on the right pedal as hard as he could. It kicked back with tremendous force.

The rudder swung in the wrong direction.

Further tests showed that the likelihood of the rudder reversing depended on where the outer slide jammed. If it jammed closer to its neutral position, the rudder was less likely to reverse. But if it jammed when it was farther from neutral, a reversal was more certain.

It was about midnight now and everyone was exhausted. They all drove home worrying about what they should do to fix the plane.

The next day, Boeing notified the FAA that the company had found a problem with the rudder PCU but wanted 24 hours to figure out how to deal with it. The FAA agreed.

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