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.
- By Bill Adair
- Air & Space magazine, July 2002
(Page 2 of 11)
When Vick heard about the results, he knew it was a setback but not a catastrophe. The valve was an amazingly tight device, with only a few millionths of an inch between each slide, so a very tiny design error could cause a jam. The Bendix engineers went back to their drawing boards and redesigned the tolerances. The new valve passed without problems.
Thirty years later, Vick unpacked his suitcase in his hotel room and sat down at the desk with a legal pad. He had come to Washington for the first meeting of the “Greatest Minds in Hydraulics” to review the work of the National Transportation Safety Board on the Flight 427 case. The safety board had hit so many dead ends in the case that the panel had been assembled to look for new tests that the investigators should try. At 67, Vick was a quiet, serious man, a good choice for the group because he had designed dozens of valves and had been awarded 25 patents. He was quite familiar with the unique valve-within-a-valve used for the 737 rudder.
Sitting in his hotel room, he recalled the Bendix test 30 years earlier, when hot fluid hit cold metal and the prototype valve stuck for a few seconds. That jam turned out to be no big deal—a redesign took care of the problem. But he wondered if the rudder valve on the USAir plane had stuck the same way. He sketched a brief outline of the test on a piece of paper and gave it to NTSB investigator Greg Phillips the next day.
“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.