Trial Graphics: USAir 427

TMBA created a series of 3D animations illustrating the final incident data for USAir flight 427. The animations were created for the TV series "Extreme Evidence". These 3D animations are based on the FAA and NTSB final accident data.

These graphics conceptually illustrate the failure of the servo valve. This valve regulates the flow of hydraulic fluid that travels from the tail of the aircraft to the cockpit rudder controls.

Why USAir Flight 427 suddenly flipped upside down and screamed to earth remains one of aviation's most disturbing mysteries. Authorities think the Boeing 737-300 jetliner's rudder swung to the extreme left and froze, tossing the plane into a tumbling, high-speed dive. The accident unfolded in clear, calm skies as Flight 427 descended routinely into Pittsburgh.

Despite 40,000 investigative man-hours and millions of dollars spent on lab and flight tests, investigators still can't explain why the rudder moved.

The National Transportation Safety Board determined that the probable cause of the USAir flight 427 accident was a loss of control of the airplane resulting from the movement of the rudder surface to its blowdown limit. The rudder surface most likely deflected in a direction opposite to that commanded by the pilots as a result of a jam of the main rudder power control unit servo valve secondary slide to the servo valve housing offset from its neutral position and overtravel of the primary slide.



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