Related Accidents / Incidents

United Airlines Flight 585

On March 3, 1991, while on approach to Colorado Springs, Colorado, United Airlines Flight 585, a Boeing Model 737-291 airplane, N999UA, rolled to the right and dove into the ground, killing all 25 people on board. The flight from Denver, Colorado had been uneventful, but conditions in the Colorado Springs area were very turbulent with high winds and reported low-level wind shear. The flightcrew was concerned about the low-level wind shears, querying the control tower about recent reports. While on final approach, in the landing configuration (flaps at 40 degrees and landing gear down), just after passing through 1,000 feet above the ground, the airplane suddenly rolled to the right. The captain called for 15 degrees of flaps, intending to initiate a go-around. However, control appears to have been lost, and the airplane impacted the ground approximately three miles from the end of the runway.

Initially, the NTSB could not conclusively determine a probable cause of the accident, but determined that the most likely causes were either a weather phenomenon known as a wind rotor, which is common in the area and well known to local sailplane pilots, or a malfunction of the rudder/yaw damper system based on damage to certain parts of the yaw damper and main rudder PCU. The airplane had a recent history of yaw damper anomalies, and maintenance crews had recently replaced the yaw damper coupler. At the time of the accident, the NTSB was not sufficiently familiar with either the rudder system of the Model 737 or with wind rotor phenomena, and was not able to conclude that either event was the cause of the accident. Until USAir Flight 427 and the investigation surrounding that crash, Flight 585 was considered unsolved. Following the investigation into USAir 427, the NTSB, based on information learned from Flight 427, eventually determined that the cause of Flight 585 was also a rudder reversal and subsequent loss of control.

A description of the events of Flight 585 is contained in the NTSB report concerning Flight 427, beginning on page 258. An animation of the flight path of Flight 585 is available at the following link: (Flight 585).

Eastwind Airlines Flight 517

While Eastwind Airlines Flight 517 did not crash, it was an important factor in the final outcome of the Flight 427 investigation.

On June 9, 1996, Eastwinds Flight 517, a Boeing Model 737-2H5 airplane, N221US, experienced a roll/yaw upset while in approach to land at Richmond Airport, Richmond, Virginia. The airplane was not damaged, and no one was injured. However, from flightcrew interviews and flight data recorder information, the NTSB determined that the airplane had experienced an "uncommanded rudder displacement that exceeded the normal operating limits of the yaw damper system."

At the time of the incident, the airplane was in initial approach at 4,000 feet and approximately 250 knots. The pilot reported afterwards that he had felt a rudder "bump" to the right followed by a roll to the right. He stated that he applied left rudder but the rudder felt stiff. He stated that he used left wheel and asymmetric thrust to control the airplane. Upon accomplishment of the emergency checklist, when the yaw damper was turned off, the airplane became controllable, but the flightcrew wasn't sure if the problem corrected itself coincident with yaw damper deactivation. The airplane had a previous history of rudder "bumps" and difficulty in trimming. The main rudder PCU had been replaced on May 14, 1996, approximately three weeks before the incident. NTSB examination of rudder components determined that the yaw damper linear variable displacement transducer (LVDT) was misadjusted such that rather than allowing a three-degree displacement in both directions, would cause a displacement of only 1.5 degrees to the left and 4.5 degrees to the right with no aerodynamic loads on the rudder. Simply stated, the yaw damper would deflect the rudder asymmetrically. The NTSB also discovered chafed wiring between the yaw damper coupler and the main rudder PCU, which they determined could have resulted in a short circuit and a yaw damper hardover (which in this case would result in a rudder deflection to the right, beyond the normal limits of the yaw damper's capability). A kinematic analysis of the flight data recorder information determined that the rudder required to create the resultant flight path was at least six degrees, which was beyond the capability of the misadjusted yaw damper LVDT.

Based on the results of flightcrew interviews, kinematic analyses, and computer simulation data, the NTSB determined that the event experienced by Eastwind Flight 517 was a rudder reversal, initiated when the pilot pushed on the left rudder pedal, resulting in a roll to the right. The event had been controllable because of the airplane's relatively high speed (well above the crossover speed).

A more complete description of the Eastwind incident is contained in the NTSB report concerning Flight 427, beginning on page 263. An animation of the incident flight path is available at the following link: (Flight 517).

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