Investigation Summary
The investigation into Flight 585 is one of the longest in NTSB history, spanning a decade. Initially, investigators were split between a 'rotor' (a powerful mountain-wave wind gust) and a mechanical failure. Because the flight data recorder (FDR) on the older 737-200 only recorded five parameters, there was insufficient data to prove a rudder malfunction. It was not until the 1994 crash of USAir Flight 427 and a 1996 non-fatal incident on Eastwind Airlines Flight 517 that investigators gained the evidence needed to test the PCU under extreme conditions. They discovered that a thermal shock could cause the dual-servo valve to jam, and if the pilot applied significant pressure to the pedals during such a jam, the rudder could move in the opposite direction of the command.
Final Conclusions
The NTSB determined the probable cause was a loss of control resulting from the movement of the rudder surface to its blowdown limit. The rudder most likely deflected in a direction opposite to that commanded by the pilots due to a jam of the main rudder power control unit (PCU) servo valve secondary slide and an overtravel of the primary slide.
Video Analysis
Investigation of United Airlines Flight 585 Rudder Hardover — This video examines the 1991 crash of a Boeing 737 in Colorado Springs caused by a mechanical failure in the rudder power control unit. It covers the initial unsolved inquiry and the eventual discovery of the PCU design flaw.
Watch on YouTube ↗If playback is blocked, the owner has disabled embedding — use the link above.
Systems & Failure Modes
Rudder Power Control Unit (PCU)
A hydraulic actuator that moves the rudder; the dual-servo valve inside was found to be susceptible to jamming and reversal.
Flight Data Recorder (FDR)
The older model installed recorded only heading, altitude, airspeed, vertical acceleration, and time, missing critical control surface positions.
Interesting Facts
- 01The aircraft had experienced two prior unexplained rudder deflections in the week leading up to the crash.
- 02The NTSB's initial 1992 report was 'inconclusive,' a rare occurrence for the agency.
- 03The crash site was extremely localized, indicating the plane was intact upon impact.
- 04The investigation was officially reopened and concluded only after the USAir 427 and Eastwind 517 incidents provided new evidence.
- 05Flight load acceleration increased to over 4G during the final dive prior to impact.
Safety Actions & Advisories
Rudder PCU Redesign
Following the investigation, the FAA mandated a complete redesign of the Boeing 737 rudder Power Control Unit. The new design replaced the dual-servo valve with a system that eliminated the possibility of a single-point failure causing a rudder reversal or jam.
FDR Upgrade Mandate
The NTSB recommended and the FAA later mandated that older flight data recorders be upgraded to record additional parameters, including control column, pedal, and control surface positions, to ensure investigators could accurately diagnose mechanical failures.