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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.

“I think we should look at this,” Vick said. “It may be something.”

The NTSB had not done a thermal shock test on Flight 427’s valve because there had been no comments on the cockpit tape about a hydraulic problem. If one of the pumps had broken, it would have triggered a warning light in the cockpit and the pilots would likely have mentioned it. But Phillips agreed to try the test. He was open to any suggestion.

The power control unit from the USAir crash, manufactured by Parker Hannifin, would be frozen to –40 degrees, similar to the outside temperatures at 30,000 feet, and then would be pumped with hot hydraulic fluid.

No one expected a breakthrough. The 737 valve had passed its own thermal shock test when it was certified in the 1960s. Besides, the temperature range was far more extreme than anything the PCU encountered in real life. Boeing officials viewed the test as a waste of time. Boeing’s Jean McGrew, chief engineer for the 737, said the airplane would encounter thermal shock conditions only if it flew to the moon.

On August 26, the Greatest Minds in Hydraulics and Phillips’ systems group gathered at Canyon Engineering, a tiny hydraulics company in an industrial park in Valencia, California. They had chosen Canyon because the chairman of the hydraulics panel worked there, but the company did not have the sophisticated test equipment that Boeing and Parker Hannifin, the unit’s manufacturer, did. Phillips brought the PCU in a sturdy navy blue chest, like a violinist carrying his prized Stradivarius. He took the 60-pound case to his hotel room each night to make sure that no one could tamper with the device.

At Canyon, the PCU was placed in a big white Coleman cooler, the same kind you would take on a picnic. Holes were cut in the cooler for pipes and tubes and then sealed with gray duct tape. John Cox, the pilots’ union representative in the investigation, and several others in the room said they were concerned that the temperatures were not controlled closely enough to produce legitimate results. But they forged ahead with the tests to see what would happen.

The group tested two PCUs—a new one straight from the factory and the one from the crash. To make sure that the hydraulic fluid was similar to Flight 427’s, they used fluid drained from other 737s. They used a pneumatic cylinder to act like the pilot’s feet, pushing the valve back and forth. The room filled with a steady rhythm of clicks and hisses as the cylinder moved the valve left and right.

Click, hiss, click, hiss. They put the factory PCU through its calisthenics at room temperature, testing it 50 times. It responded normally. They let gaseous nitrogen into the cooler and watched the temperature gauges plummet to –30 or –40 degrees to simulate the air at 30,000 feet. Click, hiss, click, hiss. Finally, they tried two tests to simulate an overheated hydraulic pump, heating the fluid to 170 degrees. Click, hiss, click, hiss. The hot fluid hit the cold valve, but there were no problems. The factory PCU worked great.

They removed it from the Coleman cooler and installed the PCU from Flight 427. Click, hiss, click, hiss. No problems at room temperature. Click, hiss, click, hiss. The frigid unit was blasted with hot fluid, but it still worked fine.

It was the investigators’ last day in Valencia, and the tests were going so smoothly that several people started to pack up and say goodbye.

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