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. There were no survivors.
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 (PCU).
On March 3, 1991, Flight 585 was operated using a Boeing 737-291, registered N999UA. The 737 was originally manufactured for Frontier Airlines in 1982, and purchased by United Airlines in 1986. On the date of the accident, the aircraft had accumulated 26,050 flight hours.
The flight crew consisted of Captain Harold Green, 52, First Officer Patricia Eidson, 42, and Flight Attendants Lisa Church, Anita Lucero, and Monica Smiley. The captain, who had over 9,900 hours as a United Airlines pilot, was regarded by colleagues as a conservative pilot who always followed standard operating procedures. The first officer had accumulated over 3,900 flight hours, and Captain Green considered her to be a very competent pilot.
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.
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. At 09:37 AM, the aircraft was cleared for a visual approach to runway 35. 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 from the runway threshold, at a speed of 245 mph.
The National Transportation Safety Board commenced an investigation, which lasted one year and nine months.
Although the flight data recorder (FDR) outer protective case was damaged, the data tape inside was intact and all 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 of powerful rotor winds from the nearby Rocky Mountains might have had, but there was not enough evidence to prove either hypothesis.
Thus, the first NTSB report (issued on December 8, 1992) did not conclude with the usual "probable cause". Instead, it found:
The National Transportation Safety Board, after an exhaustive investigation effort, could not identify conclusive evidence to explain the loss of United Airlines flight 585.
" This was only the fourth time in the NTSB's history that it published a final aircraft accident report with an undetermined probable cause.
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) were the result of a sudden malfunction of the aircraft's rudder power control unit. On March 27, 2001, the NTSB issued a revised final report, which found that Flight 585's pilots lost control of the airplane because of a mechanical malfunction:
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 (also called Air Crash Investigation or Air Disasters) dramatized the crash of Flight 585 and the subsequent 737 rudder investigation in a 2007 episode titled Hidden Danger (broadcast in some countries as Mystery Crashes).