Faulty Valve Behind Twin Boeing Crashes Uncovered After Decade-Long Probe

In the early 1990s, two Boeing 737s crashed in remarkably similar circumstances. The first incident occurred on March 3, 1991, when United Airlines Flight 585 plummeted into a ravine near Colorado Springs, killing all 25 people on board. Initially, investigators thought an explosion might have caused the crash, but they soon ruled out that possibility.

Three years later, USAir Flight 427 suffered a similar fate, crashing into a ravine at Pittsburgh International Airport and claiming 132 lives. Investigators were baffled by the similarities between the two crashes, which included intact planes with functioning engines and hydraulic systems.

A decade-long investigation eventually revealed that both crashes were caused by a faulty dual servo valve in the power control unit (PCU). The valve, similar to power steering in an automobile, is essential during landings. It was prone to jamming and could even function in reverse, making it impossible for pilots to correct the issue.

Investigator Greg Phillips performed a thermal shock test on a recovered PCU from Flight 427, which jammed when exposed to extreme cold followed by hot hydraulic fluid. The finding explained why pilots trying to manipulate the rudder during landing felt like they were fighting against an unresponsive system.