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.

Air & Space Magazine

After USAir Flight 427, a Boeing 737-300, crashed outside Pittsburgh on September 8, 1994, St. Petersburg Times reporter Bill Adair was granted unprecedented access to the National Transportation Safety Board’s investigation. Boeing claimed the crash—which killed all 132 people on board—was caused by pilot error; the pilots’ union claimed the Boeing 737 was defective. A series of clues unearthed through meticulous detective work pointed to a problem in Flight 427’s power control unit, a hydraulic device that controls the movement of the rudder.

When the investigation concluded in March 1999, Adair published a vivid four-part narrative in the St. Petersburg Times. An expanded account of the investigation, including the players and politics involved, was published in April 2002 by Smithsonian Institution Press in the book The Mystery of Flight 427. In this excerpt, investigators zero in on a suspicious servo valve in the power control unit.

A HYDRAULIC VALVE HAD TO pass a battery of tests to get accepted by Boeing. One test shook it violently, like a can of house paint in a mixer. Another test moved the valve back and forth five million times. The most brutal test froze the valve to –40 degrees Fahrenheit and injected it with hot hydraulic fluid. That represented the worst imaginable condition—a hydraulic pump overheating when the plane was in frigid air at 35,000 feet. Hot fluid would shoot into the frozen valve, causing it suddenly to expand. The test was called thermal shock.

Boeing did not manufacture its own valves, just as it didn’t build most of the parts for its planes. Instead, it relied on hundreds of suppliers such as Bendix Electrodynamics. The company was bidding to make a similar one for Boeing’s giant new plane, the 747. Bendix engineers built a prototype of the valve to undergo the standard battery of tests—the paint shaker, the marathon, and thermal shock.

The tests for the 747 valve were conducted in a gray stucco building in an industrial section of North Hollywood, not far from the Burbank airport. The lab, which took up most of the first floor, was filled with a thick, oily smell from all the hydraulic fluid. The room was a veritable torture chamber for a hydraulic valve. The lab even had special steel containers called crash boxes that were used the first time a valve was pressurized, in case it exploded.

Upstairs was a man named Ralph Vick, an engineer who worked on some of the company’s most important projects. Vick was not directly involved in the bid for the 747 valve, but he kept close tabs on the tests because he—like everyone else in the company—desperately wanted to win the big Boeing contract.

The torture tests on the 747 valve were no different from hundreds of others performed in the Bendix lab that year. The technicians placed the valve in a tiny freezer and hooked up the hydraulic lines. Once the valve had cooled to sub-zero, they flipped a switch and heard the steady whine of the hydraulic pumps. They moved the valve back and forth, as if a pilot were stepping on the pedals. Then someone flipped another switch, and piping hot fluid shot inside. Usually the valve kept moving. But this one strained and then stuck for a few seconds.

It had failed the test.

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.

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