The National Transportation Safety Board has adopted a revised final report on the 1991 crash of a Boeing 737 near Colorado Springs, Colorado that killed all 25 persons aboard. The Board said that the most likely cause of the accident was the movement of the rudder in the direction opposite that commanded by the flight crew. The decision tracks information learned from the investigation of two fatal 737 accidents - including this one - and a non-fatal incident.
On March 3, 1991, United Airlines flight 585, a Boeing 737-291 (N999UA), crashed on approach to Colorado Springs. The aircraft departed from controlled flight approximately 1,000 feet above the ground and struck an open field. After a 21-month investigation, the Board issued a report on the crash in December 1992. In that report, the NTSB said it "could not identify conclusive evidence to explain the loss of" the aircraft, but indicated that the two most likely explanations were a malfunction of the airplane's directional control system or an encounter with an unusually severe atmospheric disturbance.
On September 8, 1994, a USAir Boeing 737-300 operating as flight 427 crashed on approach to Pittsburgh, Pennsylvania in the town of Aliquippa, killing all 132 persons aboard. On June 9, 1996, an Eastwind Airlines Boeing 737-200 experienced control difficulties while on approach to Richmond, Virginia. The crew was able to maintain control and landed the aircraft safely. Based on all three occurrences and following extensive and unprecedented tests on the rudder power control unit of the Boeing 737 model airplane, on March 24, 1999 the Safety Board issued its final report on the crash of USAir flight 427, citing a malfunction in the plane's rudder system as the most likely cause of all three events.
The Board's revised report on the crash of United Airlines flight 585 cites the same probable cause as that of flight 427, that is:
".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."
In its revised report on flight 585, the Board noted that since the upset occurred less than 1,000 feet above the ground, the pilots had very little time to react to or recover from the event. The Board concluded that the flight crew of United 585 "could not be expected to have assessed the flight control problem and then devised and executed the appropriate recovery procedure for a rudder reversal under the circumstances of the flight." Although training and pilot techniques developed in recent years show that it is possible to counteract an uncommanded deflection of the rudder in most regions of the flight envelope, "such training was not yet developed and available to the flight crews of United flight 585 and USAir flight 427."
Since the crashes of United 585 and USAir 427, Boeing has redesigned and retrofitted new power control unit servo valves for all 737s operating in this country. In addition, Boeing announced a new redesign of the rudder system that it says will provide the reliable redundancy called for in NTSB recommendations contained in the USAir 427 report.
Adoption of the revised United 585 report removes one of four transport category aircraft accidents in the NTSB's 34-year history for which the Board could not determine a cause. The three remaining crashes for which the probable cause is officially listed as undetermined are the January 6, 1969 crash of a Convair 440 in Bradford, Pennsylvania that killed 11; the November 14, 1970 crash of a DC-9 in Huntington, West Virginia that killed 75, including the Marshall University football team; and the March 13, 1974 crash of a Convair 340/440 in Bishop, California, that killed 36, including a film crew.
The Board's revised report is available on the NTSB's web site at http://www.ntsb.gov <www.ntsb.gov>, under "Publications," "Aviation." Printed reports will be available in several weeks for purchase through the National Technical Information Service at (800) 533-NTIS.