Starting in 1991, a number of accidents and incidents involving the Boeing 737 were the result of the airplanes' unexpected movement of their rudders. The rudder is controlled by the Power Control Unit (PCU). Inside the PCU is a dual servo valve which helps direct hydraulic fluid in order to move the rudder. The PCU is manufactured by Parker Hannifin.
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On March 3, 1991, United Airlines Flight 585, a 737-200, crashed in Colorado Springs, CO, killing 25 people.
On September 8, 1994, USAir Flight 427, a 737-300, crashed near Pittsburgh, PA, killing 132 people.
During the course of the investigation of Flight 427, the National Transportation Safety Board (NTSB) discovered that the PCU's dual servo valve could jam as well as deflect the rudder in the opposite direction of the pilots' input, due to thermal shock, caused when cold PCUs are injected with hot hydraulic fluid. As a result of this find, the Federal Aviation Administration (FAA) ordered that the servo valves be replaced and that new training protocol for pilots to handle unexpected movement of flight controls be developed. [1]
On April 11, 1994, Continental Airlines pilot Ray Miller reported his aircraft rolled violently to the right; it landed safely.[2]
On June 9, 1996, Eastwind Airlines Flight 517, a 737-200, experienced loss of rudder control while on approach to Richmond, VA.
On February 23, 1999, MetroJet Flight 2710, a 737-200, experienced a slow deflection of the rudder to its blowdown limit while flying at 33,000 feet above Salisbury, Maryland.
On December 19, 1997, SilkAir Flight 185 crashed in Indonesia, killing 104 people. While the Indonesian National Transportation Safety Committee, the lead investigating agency, could not determine the cause, the U.S. NTSB, which also participated in the investigation, concluded in a report issued in 2000 that there was no mechanical failure, and that accident was caused by a pilot, most likely the captain, intentionally crashing the aircraft by applying sustained nose-down control pressure.[3][4]
In 2004, a Los Angeles jury, which was not allowed to hear or consider the NTSB's conclusions about the accident, ruled that the 737's rudder was the cause of the crash, and ordered Parker Hannifin, a rudder component manufacturer, to pay US$43 million to the plaintiff families. Parker Hannifin subsequently appealed the verdict, which resulted in an out-of-court settlement for an undisclosed amount.[5][6][7]
The FAA ordered an upgrade of all Boeing 737 rudder control systems by November 12, 2002.[8]
The Boeing 737 rudder control issue, and the two documented crashes associated with it, were profiled in The History Channel show Engineering Disasters 19 and on the National Geographic Channel's Air Emergency episode, "Deadly Components", first aired on May 12, 2008.
The TV series Mayday also profiled these crashes in the episode "Hidden Danger". The Mayday episode included details on Eastwind Airlines Flight 517, which led to NTSB investigators issuing a finding in the earlier events.
