They had proved that the valve could jam—and leave no evidence behind.
Six weeks later, Phillips’s 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. There were no clicks and hisses this time because the pneumatic system had been replaced by a hydraulic one. In some tests, the technicians just pulled on a lever to move the valve. 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 Condition G, 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 minus 40 degrees. The system A 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. They repeated the action, and the technician felt the lever kick back when he tried to move it to the right. He tried again, 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 didn’t happen 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. They might learn how the valve and rudder reacted to a jam.
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.
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, it meant 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 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 one 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 twenty-four hours. The plane landed at Boeing Field and was pulled into the B-52 hangar where the fat guy tests had been held. As a cold downpour 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. Hewett, the 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 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. “Everyone was concerned,” Draxler recalled later. “We didn’t know what it meant, how it all fit together.”
The next day, Boeing notified the FAA that the company had found a problem with the rudder PCU but wanted twenty-four hours to figure out how to deal with the problem. The FAA agreed.
Intense meetings went on all day in Renton and Everett as the Boeing engineers discussed how to respond. They broke into two teams, one to come up with a plan for how pilots could 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 ten 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, which could mean that the plane no longer met certification standards.
About twenty-five Boeing and FAA officials gathered in a conference room. McGrew took a seat just beneath a smiling photo of FAA administrator David Hinson. In the corner were a TV and a VCR, with the clock display flashing “12:00” throughout the meeting. 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.
Did this match what had happened to the USAir plane? an FAA official asked.
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 thirty 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.