Boeing 737 rudder issues

Last updated

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. [1] 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.

Contents

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. [2] :13 [3] :ix

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. [4]

Rudder function

Three main flight control surface deflections - aileron, elevator, rudder Aeroplane Coordinates and Control.svg
Three main flight control surface deflections - aileron, elevator, rudder

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. [2] :14 The single rudder panel is controlled by a single hydraulic Power Control Unit (PCU). [2] :13 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. [2] :19 The PCU for affected Boeing 737 aircraft was designed by Boeing and manufactured by Parker Hannifin. [2] :20

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. [2] :ix The resulting crash destroyed the aircraft and killed all 25 people on board. [2] :ix

A United Airlines Boeing 737-200, similar to the one involved in the crash Boeing 737-291-Adv, United Airlines AN0474496.jpg
A United Airlines Boeing 737-200, similar to the one involved in the crash

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. [3] :47 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. [3] :47 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. [3] :48 The first officer was considered to be "very competent" by the captain on previous trips they had flown together. [3] :48 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. [3] :48–49

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. [2] :ix [3] :49 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. [5]

Second accident

N513AU, a USAir Boeing 737-3B7 that was involved in an accident in 1994 N513AU-LAX (edit).jpg
N513AU, a USAir Boeing 737-3B7 that was involved in an accident in 1994

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. [3] :4 The resulting crash killed all 132 people on board. [3] :9 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. [3] :44 As a result, the NTSB conducted additional testing on United Flight 585's Parker-Hannifin PCU servo during its Flight 427 investigation. [2] :73

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. [6]

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. [7] 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. [6]

Eastwind incident

N221US, the aircraft involved in the incident, ten months before the incident Eastwind Airlines Boeing 737-200; N221US, August 1995.jpg
N221US, the aircraft involved in the incident, ten months before the 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, to Richmond, Virginia. [2] :ix The aircraft experienced two episodes of rudder reversal while on approach to land in Richmond. Unlike the two prior incidents, the rudder issues on Flight 517 spontaneously resolved and the pilots were able to safely land the aircraft. One of the 53 people on board was injured. [3] :51

The NTSB investigated the Eastwind incident, and incorporated information from both United Flight 585 and Eastwind Flight 517 into its ongoing investigation of the Flight 427 crash. [3] :44

Renewed investigation and conclusions

Because Eastwind Flight 517 had landed safely and the pilots of Flight 517 had survived, the NTSB was also able to perform tests on a plane that had experienced problems similar to the accident aircraft, was able to interview them, and gain additional information on their experience. The captain told the NTSB in a post-accident interview that they had not encountered any turbulence during the flight, and that, during their landing descent, he felt the rudder "kick" or "bump" even though neither pilot had moved the rudder pedals. [3] :51 When the plane abruptly rolled to the right, the captain applied left aileron and attempted to move the rudder, but the rudder pedal controls felt stiffer than normal and did not seem to respond to his input; when his flight control inputs did not immediately resolve the roll upset, he also advanced the throttle of the right engine in an effort to compensate. [3] :51–52

The NTSB and Boeing engineers conducted a series of tests on the PCUs from Flight 517 and Flight 427, as well as PCUs used in other aircraft and a brand-new PCU that had not yet been used in flight (the PCU from Flight 585, although it had been recovered, was too badly damaged to test in this manner). [3] :71,81–85 Testing revealed that under certain circumstances, the PCU's dual servo valve could jam and deflect the rudder in the opposite direction of the pilots' input. [3] :81–85 Thermal shock testing revealed that the uncommanded rudder movement could be replicated by injecting a cold PCU with hot hydraulic fluid. Thermal shock resulted in the servo's secondary slide becoming jammed against the servo housing, and when the secondary slide was jammed the primary slide could move to a position that resulted in rudder movement opposite of the pilot's commands. [2] :79 [3] :294 The NTSB concluded that all three rudder incidents (United Flight 585, USAir Flight 427, and Eastwind Flight 517) were most likely due to the PCU secondary slide jamming and excessive travel of the primary slide, resulting in the rudder reversal. [3] :294

On March 24, 1999, after a four-year investigation, the NTSB issued its probable cause finding for Flight 427. The NTSB concluded that the probable cause of the Flight 427 crash was rudder reversal due to the PCU servo malfunction. [3] :295 Two years later, the NTSB published an amended accident report for Flight 585 that found the same probable cause for that accident as well.

As a result of the NTSB's findings, the Federal Aviation Administration ordered that the servo valves be replaced on all 737s by November 12, 2002. [8] The FAA also ordered new training protocols for pilots to handle in an unexpected movement of flight controls.

Other suspected 737 rudder PCU malfunctions

The following Boeing 737 aircraft incidents were also suspected of being caused by a rudder PCU malfunction:

SilkAir crash

SilkAir Boeing 737-300, similar to the one involved in the crash 9V-TRD B737-3M8 Silk Air MAN MAY92 (6181415754).jpg
SilkAir Boeing 737-300, similar to the one involved in the crash

On December 19, 1997, SilkAir Flight 185 crashed in Indonesia, killing 104 people. Because the crash involved a Boeing 737-300 rolling and diving toward the ground at a steep angle, investigators considered the possibility of rudder hardover due to PCU servo malfunction. [14] The Indonesian National Transportation Safety Committee, the lead investigating agency, concluded in its December 14, 2000 final report that it had found "no evidence to explain the cause of the accident." [15] :24 However, on the same day the U.S. NTSB, which also participated in the investigation, issued its own final report that contradicted the Indonesian NTSC findings. [15] :24–25 The NTSB's report found that there was sufficient evidence to rule out mechanical failure (based on NTSB/Parker-Hannifin's own examinations of the suspected PCU/dual-servo unit recovered from the SilkAir crash), and that the probable cause of the accident was "intentional pilot action," most likely the captain, purposely crashing the aircraft by applying sustained nose-down control pressure. [15] :24–25 [16]

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, the PCU/dual-servo component manufacturer, to pay US$44 million to the plaintiff families. [17] Parker Hannifin subsequently appealed the verdict, which resulted in an out-of-court settlement for an undisclosed amount.

The Discovery Channel Canada TV Series Mayday (called Air Crash Investigation or Air Disasters in some markets) dramatized the NTSB's 737 rudder investigation in a 2007 episode titled "Hidden Danger" (broadcast in some countries as "Mystery Crashes"). [5]

Mayday separately dramatized the SilkAir crash investigation and lawsuit, including its connection to the 737 rudder controversy, in a 2012 episode titled "Pushed to the Limit" (broadcast in some countries as "Pilot Under Pressure"). [14]

See also

Related Research Articles

<span class="mw-page-title-main">SilkAir</span> Defunct Singaporean regional airline (1989–2021)

SilkAir Singapore Private Limited, operating as SilkAir, was a Singaporean airline with its head office in Changi, Singapore. It was a wholly owned subsidiary of Singapore Airlines and in 2017, operated scheduled passenger services from Singapore to 54 cities in Southeast Asia, the Indian subcontinent, East Asia, and Northern Australia. As the regional wing of Singapore Airlines, it served the short to medium-haul destinations in the Singapore Airlines Group network.

<span class="mw-page-title-main">Parker Hannifin</span> American technology firm

Parker Hannifin Corporation, originally Parker Appliance Company, usually referred to as just Parker, is an American corporation specializing in motion and control technologies. Its corporate headquarters are in Mayfield Heights, Ohio, in Greater Cleveland.

<span class="mw-page-title-main">SilkAir Flight 185</span> 1997 aviation incident

SilkAir Flight 185 was a scheduled international passenger flight operated by a Boeing 737-300 from Soekarno–Hatta International Airport in Jakarta, Indonesia to Changi Airport in Singapore that crashed into the Musi River near Palembang, Sumatra, on 19 December 1997, killing all 97 passengers and seven crew on board.

<span class="mw-page-title-main">United Airlines Flight 585</span> 1991 aviation accident in Colorado

United Airlines Flight 585 was a scheduled passenger flight on March 3, 1991, from Denver to Colorado Springs, Colorado, carrying 20 passengers and 5 crew members on board. The plane experienced a rudder hardover while on final approach to runway 35 at Colorado Springs Municipal Airport, causing the plane to roll over and enter an uncontrolled dive. All 25 people on board were killed.

<span class="mw-page-title-main">Copa Airlines Flight 201</span> 1992 aviation accident in Panama

Copa Airlines Flight 201 was a regularly scheduled passenger flight from Tocumen International Airport in Panama City, Panama, to Alfonso Bonilla Aragón International Airport in Cali, Colombia. On 6 June 1992, the Boeing 737-204 Advanced operating the route rolled, entered a steep dive, disintegrated in mid-air, and crashed into the jungle of the Darién Gap 29 minutes after takeoff, killing all 47 people on board. The in-flight break-up was caused by faulty instrument readings and several other contributing factors, including incomplete training.

Aircraft upset is an unacceptable condition, in aircraft operations, in which the aircraft flight attitude or airspeed is outside the normally intended limits. This may result in the loss of control (LOC) of the aircraft, and sometimes the total loss of the aircraft itself. Loss of control may be due to excessive altitude for the airplane's weight, turbulent weather, pilot disorientation, or a system failure.

Eastwind Airlines was a start-up airline formed in mid-1995 and headquartered in Trenton, New Jersey, United States, and later in Greensboro, North Carolina. Jim McNally, a former Price Waterhouse analyst who had headed that firm's recovery teams when several other airlines sought management and investment help, founded the airline. McNally's paper airline found a benefactor in UM Holdings, a Haddonfield, New Jersey-based investment company, which provided investment capital.

<span class="mw-page-title-main">Garuda Indonesia Flight 200</span> 2007 passenger plane crash in Yogyakarta, Indonesia

Garuda Indonesia Flight 200(GA200/GIA 200) was a scheduled domestic passenger flight of a Boeing 737-400 operated by Garuda Indonesia between Jakarta and Yogyakarta, Indonesia. The aircraft overran the runway, crashed into a rice field and burst into flames while landing at Adisucipto International Airport on 7 March 2007. Twenty passengers and one flight attendant were killed. Both pilots survived, and were fired shortly after the accident occurred. It was the fifth hull-loss of a Boeing 737 in Indonesia within less than six months and was the most recent accident with fatalities involving the airline.

<span class="mw-page-title-main">TWA Flight 841 (1979)</span> 1979 aviation incident

TWA Flight 841 was a scheduled passenger flight from John F. Kennedy International Airport in New York City, en route to Minneapolis-Saint Paul International Airport in Minneapolis, Minnesota. On April 4, 1979, at or around 9:48 p.m. Eastern Standard Time, while flying over Saginaw, Michigan, the Boeing 727-31 airliner began a sharp, uncommanded roll to the right, and subsequently went into a spiral dive. The pilots were able to regain control of the aircraft and made a successful emergency landing at Detroit Metropolitan Airport.

<span class="mw-page-title-main">Eastwind Airlines Flight 517</span> 1996 aviation incident

On June 9, 1996, while operating a passenger flight from Trenton, New Jersey to Richmond, Virginia, the crew of Eastwind Airlines Flight 517 temporarily lost control of their Boeing 737-200 because of a rudder malfunction. The crew were able to regain control and land the aircraft successfully. One flight attendant was injured.

A quick access recorder (QAR) is an airborne flight recorder designed to provide quick and easy access to raw flight data, through means such as USB or cellular network connections and/or the use of standard flash memory cards. QARs are typically used by airlines to improve flight safety and operational efficiency, usually in the scope of their flight operational quality assurance plans. Like the aircraft's flight data recorder (FDR), a QAR receives its inputs from the Flight Data Acquisition Unit (FDAU), recording over 2000 flight parameters. The QAR is also able to sample data at much higher rates than the FDR and, in some cases, for longer periods of time. Unlike the FDR, the QAR usually is not required by a national Civil Aviation Authority on commercial flights and is not designed to survive an accident. Despite this, some QARs have survived accidents and provided valuable information beyond what was recorded by the FDR.

<span class="mw-page-title-main">Continental Airlines Flight 1404</span> 2008 aviation accident

Continental Airlines Flight 1404 was a Continental Airlines domestic flight from Denver International Airport in Denver, Colorado, to George Bush Intercontinental Airport in Houston, Texas. On the evening of December 20, 2008, the flight crashed while taking off from Denver, resulting in two critical injuries, 36 noncritical injuries, and a hull loss of the Boeing 737-524 aircraft.

<span class="mw-page-title-main">USAir Flight 427</span> Aviation accident in 1994

USAir Flight 427 was a scheduled flight from Chicago's O'Hare International Airport to Palm Beach International Airport, Florida, with a stopover at Pittsburgh International Airport. On Thursday, September 8, 1994, the Boeing 737 flying this route crashed in Hopewell Township, Pennsylvania while approaching Runway 28R at Pittsburgh, which was USAir's largest hub at the time.

<span class="mw-page-title-main">American Airlines Flight 331</span> 2009 aviation accident

On 22 December 2009, an American Airlines Boeing 737-800, operating American Airlines Flight 331 and carrying 148 passengers and six crew, overran runway 12 on landing at Kingston in poor weather. The plane continued on the ground outside the airport perimeter and broke apart on the beach, causing injuries.

In aeronautics, loss of control (LOC) is the unintended departure of an aircraft from controlled flight and is a significant factor in several aviation accidents worldwide. In 2015 it was the leading cause of general aviation accidents. Loss of control may be the result of mechanical failure, external disturbances, aircraft upset conditions, or inappropriate crew actions or responses.

<span class="mw-page-title-main">Northwest Airlines Flight 85</span> 2002 aviation incident involving a Boeing 747-400 near Anchorage, Alaska, USA

Northwest Airlines Flight 85 was a scheduled international passenger flight from Detroit Metropolitan Wayne County Airport in the United States to Narita International Airport in Japan. On October 9, 2002, while over the Bering Sea, the Boeing 747-400 experienced a lower rudder hardover event, which occurs when an aircraft's rudder deflects to its travel limit without crew input. The 747's hardover gave full left lower rudder, requiring the pilots to use full right upper rudder and right aileron to maintain attitude and course.

<span class="mw-page-title-main">Aeroflot Flight 1802</span> 1976 aviation accident

Aeroflot Flight 1802 was a commercial flight from Vinnytsia to Moscow that crashed after the rudder deflected sharply and the propellers feathered on 15 May 1976. All 52 passengers and crew aboard the aircraft perished in the crash.

<span class="mw-page-title-main">Southwest Airlines Flight 345</span> 2013 aviation incident in New York City, US

Southwest Airlines Flight 345 was a scheduled flight from Nashville International Airport, Tennessee, to New York City's LaGuardia Airport. On July 22, 2013, the Boeing 737 operating the route suffered a front landing gear collapse while landing at LaGuardia Airport, injuring 9 people on board. The aircraft, which was worth an estimated $15.5 million at the time, was written off and scrapped as a result of the accident.

<span class="mw-page-title-main">Tatarstan Airlines Flight 363</span> November 2013 aircraft accident in Kazan, Russia

Tatarstan Airlines Flight 363 was a scheduled domestic passenger flight, operated by Tatarstan Airlines on behalf of Ak Bars Aero, from Moscow to Kazan, Russia. On 17 November 2013, at 19:24 local time (UTC+4), the Boeing 737-500 crashed during an aborted landing at Kazan International Airport, killing all 44 passengers and 6 crew members on board, making it 2013's worst plane crash.

References

  1. "Report says Boeing 737 rudder problems linger". TimesDaily . September 12, 1999. Retrieved July 10, 2016.
  2. 1 2 3 4 5 6 7 8 9 10 11 Uncontrolled Descent and Collision With Terrain, United Airlines Flight 585, Boeing 737-200, N999UA, 4 Miles South of Colorado Springs Municipal Airport, Colorado Springs, Colorado, March 3, 1991 (PDF). National Transportation Safety Board. March 27, 2001. NTSB/AAR-01-01. Retrieved January 17, 2016.
  3. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Aircraft Accident Report - Uncontrolled Descent and Collision With Terrain, USAir Flight 427, Boeing 737-300, N513AU, Near Aliquippa, Pennsylvania, September 8, 1994 (PDF). National Transportation Safety Board. March 24, 1999. NTSB/AAR-99-01. Retrieved July 10, 2016.
  4. "Fatal Flaws". Why Planes Crash. Season 2. 2016. MSNBC.
  5. 1 2 "Hidden Danger". Mayday . Season 4. 2007. Discovery Channel Canada / National Geographic Channel.
  6. 1 2 Equinox - "The Box", Channel 4, 2001.
  7. The quick access recorder had been pioneered by BA's predecessor airline, BEA, on its Hawker Siddeley Trident aircraft back in the 1960s. Subsequently, QARs were fitted to all BEA, and then BA, aircraft.
  8. "Boeing Model 737 Series Airplanes". www1.airweb.faa.gov. Archived from the original on October 7, 2018. Retrieved May 21, 2016.
  9. 1 2 "737: The crash in Colorado Springs". The Seattle Times . October 28, 1996. Archived from the original on August 8, 2019. Retrieved May 21, 2016.
  10. "ASN Aircraft accident Boeing 737-2R4C VT-SIA Delhi-Indira Gandhi International Airport (DEL)". Aviation Safety Network . Flight Safety Foundation . Retrieved May 21, 2016.
  11. "Accident to Sahara India Airlines B-737-200 Aircraft During the Training Flight on 8.3.1994 at IGI Airport, Delhi Report of the Court of Inquiry". Court of Inquiry, High Court of Delhi. March 22, 1995. Retrieved May 25, 2020.
  12. NTSB Office of Public Affairs (February 24, 1999). "Metrojet B-737 In-flight Event - February 23, 1999" (Press release). National Transportation Safety Board . Retrieved May 21, 2016.
  13. Phillips, Don (February 26, 1999). "Aging Data Recorder Impedes Jet Probe". The Washington Post . Retrieved May 21, 2016.
  14. 1 2 "Pushed to the Limit". Mayday . Season 12. 2012. Discovery Channel Canada / National Geographic Channel.
  15. 1 2 3 "Final Flight: SilkAir" (PDF). Civil Aviation Safety Authority. January–February 2008. Archived from the original (PDF) on June 3, 2016. Retrieved May 21, 2016.
  16. "ASN Aircraft accident Boeing 737-36N 9V-TRF Palembang". Aviation Safety Network . Flight Safety Foundation . Retrieved May 21, 2016.
  17. "$43.6 Million Awarded in Silk Air Crash Cases" (PDF). www.malaysianflightmh370lawyer.com. August 17, 2004. Retrieved May 21, 2016.