United Airlines Flight 585

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 737 were killed on impact.

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.

Aircraft and flight crew
Flight 585 was operated by a Boeing 737-291, registered. The aircraft was manufactured in 1982 for the original incarnation of Frontier Airlines, 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). On the date of the accident, the aircraft had accumulated approximately 26,000 flight hours.

The captain of the flight was Harold Green, age 52. Green had over 10,000 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. The first officer was Patricia Eidson, age 42. Eidson had accumulated over 4,000 flight hours (including 1,077 hours on the Boeing 737), and was considered by the captain to be a very competent pilot.

On February 25, 1991, the aircraft was flying at 10,000 ft 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 the now-decommissioned 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.

At 09:23 (16:23 UTC), Flight 585 departed Denver with 20 passengers and 5 crew members on board. 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 40 knots and above." The flight crew added 20 knots 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 11000 ft. 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, notifying the flight that wind was 320 degrees at 16 knots with gusts to 29 knots. At this moment, the aircraft was at 8000 ft. 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 15 knot loss at 500 ft, a 15 knot gain at 400 ft, and a 20 knot gain at 150 ft, 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, 5 mi 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 4 mi northeast of the accident when it occurred, and he had also reported slight, occasional, moderate chop at 7000 ft. The Cessna pilot had also noted indicated airspeed fluctuations between 65 knots and 105 knots with vertical speed indications of approximately 500 ft 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 155 knots 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 1000 ft.

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 4 mi from the runway threshold, at a speed of 245 mph. The aircraft was destroyed on impact and by the post-crash fire. According to the accident report, the crash carved a crater 39 x and 15 ft 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, and a little girl standing in the doorway of one of these apartments was knocked backwards by the force of the explosion, hitting her head, but she was released from a local hospital with no further issues after treatment.

Initial investigation
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. 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. The data available proved insufficient to establish why the plane suddenly went into the fatal dive. 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.

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. The CVR determined that the pilots made a verbal (and possible physical) response to the loss of control.

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):

Thus, the first NTSB report (issued on December 8, 1992) did not conclude with the usual "probable cause". Instead, it stated:

"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:

"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.