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.

At 09:40:07 (16:40:07 UTC), Flight 585 was informed of traffic, a Cessna at their eleven o'clock, 5 miles (8.0 km) northwest bound, landing at runway 30.

This Cessna was located about 4 miles (6.4 km) northeast of the accident when it occurred, and he had also reported slight, occasional, moderate chop at 7,000 feet (2,100 m).

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.

[5] 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.

[1]: 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.

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.

[11] 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:[1]: 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.A memorial garden honoring the victims is located at Widefield Park.

[6] 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.

This article incorporates public domain material from websites or documents of the National Transportation Safety Board.