Boeing 737 rudder issues

During the 1990s, a series of rudder issues on Boeing 737 aircraft resulted in multiple incidents. In two separate accidents, pilots lost control of their Boeing 737 aircraft due to a sudden and unexpected movement of the rudder, and the resulting crashes killed everyone aboard. A total of 157 people aboard the two aircraft were killed. Similar rudder issues led to a temporary loss of control on at least one other Boeing 737 flight before the problem was ultimately identified. The National Transportation Safety Board ultimately determined that the accidents and incidents were the result of a design flaw which could result in an uncommanded movement of the aircraft's rudder. The issues were resolved after the NTSB identified the cause of the rudder malfunction and the Federal Aviation Administration ordered repairs for all Boeing 737 aircraft in service.

Unlike other twin-engine large transport aircraft in service at the time, the Boeing 737 was designed with a single rudder panel and single rudder actuator. The single rudder panel is controlled by a single hydraulic Power Control Unit (PCU). Inside the PCU is a dual servo valve which, based on input from the pilot's rudder pedals or the aircraft's yaw damper system, directs the flow of hydraulic fluid in order to move the rudder. The PCU for affected Boeing 737 aircraft was designed by Boeing and manufactured by Parker Hannifin.

On March 3, 1991, United Airlines Flight 585, a Boeing 737-200, crashed while attempting to land in Colorado Springs, Colorado. During the airplane's landing approach, the plane rolled to the right and pitched nose down into a vertical dive. The resulting crash destroyed the aircraft and killed all 25 people on board.

Although the NTSB investigated the accident, it was unable to conclusively identify the cause of the crash. The rudder PCU from Flight 585 was severely damaged, which prevented operational testing of the PCU. A review of the flight crew's history determined that Flight 585's captain strictly adhered to operating procedures and had a conservative approach to flying. A first officer who had previously flown with Flight 427's captain reported that the captain had indicated to him while landing in turbulent weather that the captain had no problem with declaring a go-around if the landing appeared unsafe. The first officer was considered to be "very competent" by the captain on previous trips they had flown together. The weather data available to the NTSB indicated that Flight 585 might have encountered a horizontal axis wind vortex that could have caused the aircraft to roll over, but this could not be shown conclusively to have happened or to have caused the rollover.

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