|Date||3 March 1991|
|Summary||Loss of control due to rudder hardover|
|Site||Widefield Park, El Paso County|
near Colorado Springs Municipal Airport, Colorado Springs, Colorado,
|Aircraft type||Boeing 737-291|
|Flight origin||Greater Peoria Regional Airport|
|Stopover||Quad City International Airport|
|Last stopover||Stapleton International Airport|
|Destination||Colorado Springs Municipal Airport|
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 onboard were killed.
The 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
Flight 585 was operated by a Boeing 737-291, registered N999UA.: 7 The 737 was originally manufactured for the "old" Frontier Airlines in 1982 and was acquired by United Airlines in 1986 when the former went out of business (a new airline company with the same name formed eight years later).: 7 On the date of the accident, the aircraft had accumulated approximately 26,000 flight hours.: 7
The flight crew consisted of Captain Harold Green (52), First Officer Patricia Eidson (42), and 3 flight attendants. The captain, who had over 10,000 hours as a United Airlines pilot (including 1,732 hours on the Boeing 737), was regarded by colleagues as a conservative pilot who always followed standard operating procedures.: 5 The first officer had accumulated over 4,000 flight hours (including 1,077 hours on the Boeing 737), and she was considered by Captain Green to be a very competent pilot.: 6
On February 25, 1991, the aircraft was flying at 10,000 feet when the rudder abruptly deflected 10 degrees to the right. The crew onboard reduced power and the aircraft returned to normal flight. A similar event occurred two days later. Four days later, the aircraft crashed.
Flight 585 was a regularly scheduled United Airlines Flight from 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 and the now-decommissioned Stapleton International Airport in Denver, Colorado. On March 3, 1991, the flight operated from Peoria to Denver without incident.: 1
At 09:23 AM Mountain Standard Time, Flight 585 departed Denver with 20 passengers and 5 crew members on board and was scheduled to arrive in Colorado Springs at 09:46 AM.: 2 At 09:37 AM, the aircraft was cleared for a visual approach to runway 35.: 2 The aircraft then suddenly rolled to the right and pitched nose down. The crew tried to initiate a go-around by selecting 15-degree flaps and an increase in thrust. The altitude decreased rapidly and acceleration increased to over 4G until the aircraft crashed into Widefield Park, less than four miles (6 km) from the runway threshold, at a speed of 245 miles per hour (395 km/h). The aircraft exploded on impact and an enormous fireball erupted as a result. According to the accident report, the resulting crash and explosion carved a crater 39 by 24 feet (12 m × 7.3 m) and 15 ft (5 m) deep. Everyone on board was killed instantly, and an eight-year-old girl who lived nearby the tumbling jet was knocked to the ground by the force of the impact, suffering minor injuries.
The National Transportation Safety Board (NTSB) commenced an investigation, which lasted for 21 months.
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, but the data tape inside was also intact. However, the data tape had creases in it, resulting in the playback quality being poor.: 40 The CVR determined that the pilots made a verbal (and possible physical) response to the loss of control.: 102
The following is an excerpt of the last two minutes Flight 585 CVR, starting two minutes before impact (the full CVR recording started before Flight 585 took off from Stapleton):
|Transcript of the last two minutes of United Airlines Flight 585's Cockpit Voice Recorder (Times are expressed in MST)|
|# = Expletive deleted; * = Unintelligible word; () = Questionable text;  = Commentary; - = Break in continuity; Shading = Radio communication|
|09:41:20||Captain||Twenty five flaps.|
|United five eighty-five, after landing hold short of runway three zero for departing traffic on runway… three zero.|
|09:41:25||[Sound similar to that of an engine power increase]|
|09:41:30||Captain||Starting on down.|
|09:41:31||First officer (to Colorado Springs tower)||We'll hold short of three zero, United five eighty five.|
|09:41:33||First officer||That's all the way to the end of our runway not * doesn't mean a thing.|
|09:42:05||[Sound of "CO" ident on radio channel two]|
|09:42:08||First officer||The marker's identified now it's really weak.|
|09:42:29||First officer||(We had a) ten-knot change here.|
|09:42:31||Captain||Yeah, I know… awful lot of power to hold that… airspeed.|
|09:42:38||First officer||Runway is ah eleven thousand feet long|
|09:43:01||First officer||Another ten knot gain.|
|09:43:03||[Sound similar to that of flap lever actuation]|
|09:43:09||[Sound similar to that of an engine power reduction]|
|09:43:28||First officer||We're at a thousand feet.|
|09:43:32||First officer||Oh god (flip)-|
|09:43:34.7||Captain||Oh! [Exclaimed loudly]|
|09:43:35.5||[Click sound similar to that of a flap lever actuation]|
|09:43:36.1||[Click sound similar to that of a flap lever actuation]|
|09:43:36.5||Captain||No! [Very loud]|
|09:43:37.4||[Click sound similar to that of a flap lever actuation]|
|09:43:37.5||First officer||Oh #.|
|09:43:38.4||First officer||Oh my god... [unidentifiable click sound] oh my god! [A scream]|
|09:43:40.5||Captain||Oh no (#). [Exclaimed loudly]|
|09:43:41.5||[Sound of impact - end of tape]|
Thus, the first NTSB report (issued on December 8, 1992) did not conclude with the usual "probable cause". Instead, it stated:: 102
The National Transportation Safety Board, after an exhaustive investigation effort, could not identify conclusive evidence to explain the loss of United Airlines flight 585.
Main article: Boeing 737 rudder issues
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.
The NTSB reopened its investigation into Flight 585 in parallel with the 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
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.
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.