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Starting in 1991, a number of accidents and incidents involving the Boeing 737 were the result of the airplanes' unexpected movement of its rudder. 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.


On March 3, 1991 United Airlines Flight 585marker, 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 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 FAA ordered that the servo valves be replaced and that new training protocol for pilots to handle unexpected movement of flight controls be developed.

Other Suspected 737 Rudder PCU malfunction incidents

On April 11, 1994 a Continental Airlines pilot reported his plane rolled violently to the right, landed safely.

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, Marylandmarker.

SilkAir controversy

On December 19, 1997, SilkAir Flight 185marker crashed in Indonesiamarker, killing 104 people. While the Indonesian NTSC, 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.

In 2004, a Los Angeles jury, which was not allowed to hear or consider the U.S. 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$43M to the plaintiff families. Parker Hannifin subsequently appealed the verdict, which resulted in an out-of-court settlement for an undisclosed amount.

Boeing 737 rudder upgrade directive

The FAA has ordered an upgrade of all Boeing 737 rudder control systems by November 12, 2002.

Cultural references

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.

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