Two 737s Crashed in the Exact Same Way With No Explanation. It Took a Decade To Find Out Why.
Here's what you'll learn when you read this story:
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In 1991, a Boeing 737, rolled over in the air and nosedived into an explosive crash. Three and a half years later, it happened again.
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An investigation was launched by the National Transportation Safety Board (NTSB), but tests kept coming up inconclusive until one investigator took a novel approach.
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That final test proved that the dual servo valve inside the power control unit (PCU) was jamming from exposure to extreme cold followed by extreme heat.
On the morning of March 3, 1991, United Airlines Flight 585 had just come in from Denver and was preparing to land in Colorado Springs when it ran into turbulence, began to spin uncontrollably, and then nosedived straight into the ground. The Boeing 737's fuel exploded instantly on impact, transforming the plane into a mangled pile of scrap metal. All 25 people on board the flight were killed.
Initially, Flight 585 was thought to have suffered from an explosion in midair. But that theory soon dissipated as the National Transportation Safety Board (NTSB) continued their investigation and found that the plane had been intact the entire time it was flying.
Every piece of the smoking rubble was examined. The black box from the cockpit was taken in to see if any evidence of what caused the crash had been recorded, but nothing suggested there had been any human error. Investigators could only confirm what witnesses had seen with their own eyes: the plane had unexpectedly rolled and flown inverted before losing all control and spinning into oblivion. When the engine turbines were examined, earth caked on the blades gave away that they had been fully operational, still whirring and sucking in air when the plane crashed. The position of the needles on the plane's hydraulic dials provided further evidence that the engine had not experienced a breakdown. Ultimately, with their findings still inconclusive after ruling out malfunctions in every other part of the aircraft (and analysis of weather data revealing only moderately turbulent winds), the initial investigation concluded without finding a definitive cause of the crash.
Then, three years later, a second plane fell from the sky.
On September 8, 1994, USAir Flight 427 , also a Boeing 737, was performing a landing maneuver to touch down at Pittsburgh International Airport when it crashed into a ravine, having rolled in nearly the exact same manner as United Flight 585. Emergency crews rushed to the scene, but they soon realized that no survivors remained amid the thick smoke. The crash killed 132 people.
Over time, more odd similarities between the two crashes began to emerge. Both planes were on the verge of landing when they lost control, their engines were still functioning when they slammed into the ground, and the hydraulic devices inside their tails had come out of both accidents with hardly a dent. Just as oddly, investigators discovered that the hydraulic fluid from Flight 427 contained unexpected fragments of metal, which caused the NTSB to grow suspicious of a manufacturing error.
Pressured to find an answer quickly, investigators sent the parts in question to their manufacturer, Parker Hannifin , for testing. Boeing and Parker Hannifin both claimed that filters would have stopped any stray debris from causing the slides to jam, but investigators insisted they be put through a battery of tests.
With test results coming up empty over and over, the NTSB's investigation was brought to a standstill—that is, until a pilot who survived the same phenomenon was interviewed by the NTSB , and explained that trying to manipulate the rudder with the pedal during landing felt pointless, as if the rudder were moving the opposite way. At this point, investigators understandably began to wonder if the plane's rudder and the power control unit, or PCU (which is similar to power steering in an automobile) could have been involved in the crash.
The PCU is essential throughout each flight and is utilized most intensively during landings—using pressurized hydraulic fluid, it converts pressure from the pilot pushing the rudder pedal into pressure that moves the rudder on the tail. Inside the PCU is a dual servo valve, a cylindrical contraption with two components that control hydraulic fluid flow as they slide past each other.
Armed with a new direction, investigator Greg Phillips decided to perform one last trial. Based on an idea from a hydraulics expert, Phillips pushed the PCU recovered from Flight 427 to its limit with a thermal shock test. The unit was immersed in dry ice before being hit with a blast of nitrogen gas, which was meant to expose it to the same freezing temperatures it would encounter at 30,000 feet. It was then injected with incredibly hot hydraulic fluid. When given the command to start, it jammed and ceased to function. Phillips then looked for scratches and found none, which he expected after finding none on either aircraft in the initial crash investigations.
The investigation revealed that the servo valve was not only prone to jamming, but capable of functioning in reverse, which explains why a pilot trying to correct for the problem would be powerless to stop the rudder turning in the opposite direction, causing the plane to roll over and eventually crash. And after reopening the investigation into Flight 585, investigators eventually concluded that it had fallen victim to the same culprit. After 10 years, the tragedies finally had an explanation.
Training protocols and manufacturing standards were revised in the wake of the twin Boeing 737 tragedies. The dual servo valve was redesigned to resist reversal, and Boeing spent hundreds of millions of dollars to replace the valves on every 737 that was in service. Pilots were also trained on how to react if the rudder began to reverse itself.
Nowadays, if a plane rolls mid-air, the pilot is likely doing it on purpose.
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