Boeing 737 rudder issues

During the 1990s, a series of issues affecting the rudder of Boeing 737 passenger aircraft resulted in multiple incidents. In two separate accidents, pilots lost control of their aircraft due to a sudden and unexpected rudder movement, and the resulting crashes killed everyone on board, 157 people in total. Similar rudder issues led to a temporary loss of control on at least one other Boeing 737 flight before the cause of the problem was ultimately identified.

The National Transportation Safety Board determined that the incidents were the result of a design flaw that could result in an uncommanded movement of the aircraft's rudder.

The issues were resolved after the Federal Aviation Administration ordered modifications for all Boeing 737 aircraft in service. In 2016, former NTSB investigator John Cox stated that time has proven the NTSB correct in its findings because no additional rudder-reversal incidents have occurred since Boeing's redesign.

Rudder function

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 that, 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.

First accident and investigation

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 near-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 585'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.

On December 8, 1992, the NTSB published a report that identified what the NTSB believed at the time to be the two most likely causes of the accident. The first possibility was that the airplane's directional control system had malfunctioned and caused the rudder to move in a manner that caused the accident. The second possibility was a weather disturbance that caused a sudden rudder movement or loss of control. The Board determined that it lacked sufficient evidence to conclude either theory as the probable cause of the accident. This was only the fourth time in the NTSB's history that it had closed an investigation and published a final aircraft accident report where the probable cause was undetermined.

Second accident

On September 8, 1994, USAir Flight 427, a Boeing 737-300, crashed near Pittsburgh, Pennsylvania. While on approach to Pittsburgh International Airport, Flight 427 suddenly rolled to the left. Although the pilots were briefly able to roll right and level the plane, it rolled left a second time and the pilots were unable to recover. The resulting crash killed all 132 people on board. The NTSB realized early into its investigation that the crash of Flight 427 might have been caused by an unintended or uncommanded rudder movement, similar to the suspected (but not yet established) cause of the Flight 585 crash. As a result, the NTSB conducted additional testing on United Flight 585's Parker-Hannifin PCU servo during its Flight 427 investigation.

One of the problems facing investigators was the relative lack of precision in the data produced by the flight data recorder (FDR), which only recorded control inputs at periodic intervals with significant time gaps between samples, gaps during which no data was recorded no matter what the pilot did with the controls. This lack of precision led to it being possible for Boeing to interpret the data differently from the way the NTSB did, leading the manufacturer to suspect and insist that the pilot had responded incorrectly to a wake turbulence incident.''Equinox'' - "The Box", Channel 4, 2001.

At the request of the NTSB, data from the Penny & Giles quick access recorder (QAR) of a British Airways (BA) Boeing 747-400 was supplied to the NTSB by BA. The data was from a London-Bangkok flight in which the aircraft suffered an uncommanded elevator movement and momentary elevator reversal on take-off, the aircraft then landed safely. Operating alongside the FDR system, the QAR on BA's 747-400s, in conjunction with a Data Management Unit, received and recorded more aircraft parametric data, including control input values at a higher rate. This BA data led to renewed suspicion of the similar valve design used on the 737 rudder. As a result of this earlier BA incident, Boeing had, in fact, modified the design of the 747 elevator servo system, and the modified system had been retroactively fitted to all 747-400s in service.

Eastwind incident

On June 9, 1996, while the NTSB's investigation of Flight 427 was still ongoing, the pilots of Eastwind Airlines Flight 517 briefly lost control of their aircraft, a 737-200, while flying from Trenton, New Jersey