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 the Boeing 737 were killed on impact.
The National Transportation Safety Board, (NTSB), was initially unable to resolve the cause of the crash, but after similar accidents and incidents involving Boeing 737 aircraft, the crash was determined to be caused by a defect in the design of the 737's rudder power control unit.ix
Background.
Aircraft.
Flight 585 was operated by a Boeing 737-291, registered as N999UA with serial number 22742. It was manufactured in May 1982 for the original incarnation of Frontier Airlines, and was acquired by United Airlines on June 6, 1986, when the former went out of business (a new airline company with the same name formed eight years later). On the date of the accident, the aircraft had logged 26,050 flight hours in 19,734 takeoff and landing cycles.25
Crew.
In command was Captain Harold Green, aged 52.108 Green was hired by United Airlines on May 15, 1969, and had logged 9,902 hours as a United Airlines pilot (including 1,732 hours on the Boeing 737), and was regarded by colleagues as a conservative pilot who always followed standard operating procedures.5 The first officer was Patricia Eidson, aged 42.108 Eidson was hired by UAL on November 21, 1988 and had logged 3,903 flight hours (including 1,077 hours on the Boeing 737). She was considered by the captain to be a very competent pilot.
On February 25, 1991, the aircraft was flying at when the rudder abruptly deflected 10 degrees to the right. The crew on board reduced power and the aircraft returned to normal flight. A similar event occurred two days later. Four days later, the aircraft crashed.
Accident.
Flight 585 was a regularly scheduled flight, originating at General Wayne A. Downing Peoria International Airport in Peoria, Illinois to Colorado Springs, Colorado, making intermediate stops at Quad City International Airport in Moline, Illinois, with an intended final stop at Stapleton International Airport in Denver, Colorado, at 09:46 Mountain Standard Time (16:46 UTC). On March 3, 1991, the flight operated from Peoria to Denver without incident.1
At 09:23 (16:23 UTC), Flight 585 departed Denver with 20 passengers and 5 crew members on board.2 At 09:30:37 (16:30:37 UTC), the aircraft received Automatic terminal information service "Lima", that was about 40 minutes old, stating "Wind three one zero at one three gust three five; low level wind shear advisories are in effect, local aviation wind warning in effect calling for winds out of the northwest gust to and above." The flight crew added to their target landing airspeed based on this information.
At 09:32:35, First Officer Eidson reported to Colorado Springs Approach Control that their altitude was . Colorado Springs Approach then cleared the flight for a visual approach to runway 35 and instructed the flight to contact Colorado Springs Tower. 
At 09:37:59 (16:37:59 UTC) Colorado Springs Tower cleared Flight 585 to land on runway 35,2 notifying the flight that wind was 320 degrees at with gusts to . At this moment, the aircraft was at . First Officer Eidson inquired about reports from other aircraft about airspeed changes, and at 09:38:29 (16:38:29 UTC) the tower replied that another 737 had reported a loss at , a gain at , and a gain at , at approximately 09:20 (16:20 UTC), 17 minutes prior. Eidson replied, "Sounds adventurous... United five eighty-five, thank you."
At 09:40:07 (16:40:07 UTC), Flight 585 was informed of traffic, a Cessna at their eleven o'clock, northwest bound, landing at runway 30. The crew was unable to locate the traffic, but 37 seconds after it was reported, the tower informed the flight that the traffic was now behind them. This Cessna was located about northeast of the accident when it occurred, and he had also reported slight, occasional, moderate chop at . The Cessna pilot had also noted indicated airspeed fluctuations between and with vertical speed indications of approximately per minute.
At 09:41:23 (16:41:23 UTC), air traffic control directed Flight 585 to hold short of runway 30 after landing to allow for departing traffic. Eidson replied "We'll hold short of three-zero, United five eighty five." This was the last transmission received from the flight.
In the final minute of the flight, normal acceleration varied between 0.6–1.3g, with an airspeed of with 2 to 10 knot excursions.
At 09:42 (16:42 UTC), about 20 seconds prior to the crash, the aircraft entered into a controlled 20-degree bank and turn for alignment with the runway. Four seconds later, First Officer Eidson informed Captain Green that they were at .
Within the next four seconds, at 09:43:33 (16:43:33 UTC), the aircraft suddenly rolled to the right, heading rate increasing to about 5-degrees per second as a result, nearly twice that of a standard rate turn, and pitched nose down. First Officer Eidson stated "Oh God, [flip]!", and in the same moment Captain Green called for 15-degrees of flaps while increasing thrust, in an attempt to initiate a go-around. The altitude decreased rapidly and acceleration increased to over 4G until, at 09:43:41 (16:43:41 UTC), the aircraft crashed at an 80-degree nose-down angle, yawed 4-degrees to the right, into Widefield Park, less than from the runway threshold, at a speed of . The aircraft was destroyed on impact and by the post-crash fire. According to the accident report, the crash carved a crater and deep. Segments of the 737 were buried deep within this crater, requiring excavation. Everyone on board was killed instantly. The aircraft narrowly missed a row of apartments; a girl standing in the doorway of one of the apartments was knocked backwards by the force of the explosion, hitting her head, but was released from a local hospital with no further issues after treatment.
Investigation.
Initial investigation.
The National Transportation Safety Board (NTSB) has released its concluding report into the crash that occurred on March 3, 1991, involving United Airlines Flight 585. Based on the findings of the research, it was determined that the crash was brought about by a mechanical breakdown in the rudder power control unit of the aircraft. 
The NTSB's initial report on the crash was released in December 1992, but it ruled the probable cause as undetermined. The agency reopened the investigation in September 1994 after another crash of USAir Flight 427 that was under similar conditions. The NTSB's investigation considered data from the crash of Flight 585, as well as other incidents, including a non-fatal crash in 1996 of Eastwind Airlines flight 517 The NTSB finalized it's report on United 585 Report #AAR-01-01 on March 03, 2001. 
Although the flight data recorder (FDR) outer protective case was damaged, the data tape inside was intact and all of the data were recoverable.38 Five parameters were recorded by the FDR: heading, altitude, airspeed, normal acceleration (G loads), and microphone keying. The FDR did not record rudder, aileron or spoiler deflection data, which could have aided the NTSB in reconstructing the plane's final moments.100 The data available proved insufficient to establish why the plane suddenly went into the fatal dive.102 The NTSB considered the possibilities of a malfunction of the rudder power control unit servo (which might have caused the rudder to reverse) and the effect that powerful rotor winds from the nearby Rocky Mountains may have had, but there was not enough evidence to prove either hypothesis.102
The cockpit voice recorder (CVR) was also damaged, though the data tape inside was intact. However, the data tape featured creases that resulted in poor playback quality.43 The CVR recorded the pilots making a verbal (and possible physical) response to the loss of control.102
The first NTSB report (issued on December 8, 1992) did not conclude with the usual "probable cause". Instead, it stated:102
 ;templatestyles src="Template:Blockquote/styles.css" / ;The National Transportation Safety Board, after an exhaustive investigation effort, could not identify conclusive evidence to explain the loss of United Airlines Flight 585.
Intervening events.
Following the failure to identify the cause of Flight 585's crash, another Boeing 737 crash occurred under very similar circumstances when USAir Flight 427 crashed while attempting to land in Pennsylvania in 1994.
Renewed investigation and probable cause.
The NTSB reopened its investigation into Flight 585 in parallel with the USAir Flight 427 investigation, due to the similar nature of the circumstances.
During the NTSB's renewed investigation, it was determined that the crash of Flight 585 (and the later Flight 427 crash) was the result of a sudden malfunction of the aircraft's rudder power control unit. Another incident (non-fatal) that contributed to the conclusion was that of Eastwind Airlines Flight 517, which had a similar problem upon approach to Richmond on June 9, 1996. On March 27, 2001, the NTSB issued a revised final report for Flight 585, which found that the pilots lost control of the airplane because of a mechanical malfunction. The renewed investigation concluded with a "probable cause" that stated:114
 ;templatestyles src="Template:Blockquote/styles.css" / ;The National Transportation Safety Board determines that the probable cause of the United Airlines flight 585 accident was a loss of control of the airplane resulting from the movement of the rudder surface to its blowdown limit. The rudder surface most likely deflected in a direction opposite to that commanded by the pilots as a result of a jam of the main rudder power control unit servo valve secondary slide to the servo valve housing offset from its neutral position and overtravel of the primary slide.
Memorial.
A memorial garden honoring the victims is located at Widefield Park. The garden consists of a gazebo and 25 trees planted in honor of the victims.
In popular culture.
The Discovery Channel Canada / National Geographic TV series "Mayday" dramatized the crash of Flight 585 and the subsequent 737 rudder investigation in a 2007 episode titled "Hidden Danger".
The crash is dramatized in the episode "Fatal Flaws" of "Why Planes Crash".
References.
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 This article incorporates public domain material from 