Boeing 737 rudder issues
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Starting in 1991, a number of accidents and incidents involving the Boeing 737 were the result of uncommanded 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.
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.
On June 9, 1996, Eastwind Airlines Flight 517 experienced two episodes of rudder reversal (which spontaneously resolved) while on approach to land. This incident occurred during the course of the investigation of Flight 427, and, as Flight 517 had landed safely, the National Transportation Safety Board (NTSB), now being able to perform tests on a plane that had experienced similar problems to the accident aircraft, but had landed safely, discovered that the PCU's dual servo valve could jam and 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 finding, the Federal Aviation Administration (FAA) ordered that the servo valves be replaced and new training protocol for pilots to handle unexpected movement of flight controls. 
Other suspected 737 rudder PCU malfunctions
On June 6, 1992, Copa Airlines Flight 201, a 737-200 Advanced, flipped and crashed into the Darién Gap jungle, killing 47 people. Investigators initially believed that the airplane experienced a rudder malfunction, but after an exhaustive and extensive inquiry, they concluded that the crash was caused by faulty instrument readings.
On March 8, 1994, a Sahara Airlines that had 3 trainee and one supervising pilots on board crashed after performing a "Touch-and-go landing" at New Delhi Airport, and slammed into a Russian jet. The four pilots were killed, as were five ground workers. Although the repairs done to the PCU were not with authorized parts, the incident is still thought to be, in part due to the plane's rudder reversing both right and left.
On June 9, 1996, Eastwind Airlines Flight 517, a 737-200, experienced two episodes loss of rudder control (uncommanded right bank due to rudder) while on approach to Richmond, VA. The plane landed safely. This incident was a turning point in the investigation as the NTSB had access to a 737 that had experienced rudder problems but had survived.
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 (based on Parker-Hannifin's own examination of the suspected PCU/dual-servo unit recovered from the crash), and that accident was a murder-suicide by a pilot, most likely the captain, intentionally crashing the aircraft by applying sustained nose-down control pressure.
In 2004, following an independent investigation of the recovered PCU/dual-servo unit, 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.
Boeing 737 rudder upgrade directive
The FAA ordered an upgrade of all Boeing 737 rudder control systems by November 12, 2002.
- NTSB summary
- The Seattle Times: 737 - The Crash In Colorado Springs, October 28, 1996
- The Seattle Times: Safety at issue: the 737 "Nightmare over Honduras", October 28, 1996
- SilkAir 185 - Flight Safety Australia Feb 2008
- ASN Accident description of SilkAir Flight 185
- Jury blames rudder, not pilot
- Jones Day Parker Hannifin settlement
- B-737 rudder design defect upgrade, , November 12, 2002.