RICHMOND, VA, USA
N221US
Boeing B-737-201
The Boeing 737-200 experienced a yaw/roll upset while operating at an airspeed of about 250 knots and an altitude of about 4,000 feet msl in visual flight rules (VFR) conditions. The pilots were able to regain control of the airplane and land at the destination airport without further incident. As a result of previous Boeing 737 rudder anomalies, including the fatal accident involving USAir flight 427, a Boeing 737-300, which entered an uncontrolled descent and impacted terrain near Aliquippa, Pennsylvania, on September 8, 1994, Safety Board investigators initiated an investigation into this incident. Investigators conducted detailed examinations of pertinent rudder control components, reviewed flight test data, performed a computer simulation, and completed an analysis of human performance data. As a result, the Safety Board concluded that the rudder reversed, moving to its right blowdown limit when the captain commanded left rudder, consistent with 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 complete report of this incident investigation, including all analyses, is contained in the final report of the USAir flight 427 investigation which can be found on the NTSB web site at the following link: http://www.ntsb.gov/publictn/1999/AAR9901.pdf
HISTORY OF FLIGHT On June 9, 1996, Eastwind Airlines flight 517, a 737-200, N221US, experienced a yaw/roll upset about 2200 Eastern Daylight Time near Richmond, Virginia. The airplane was operating at an airspeed of about 250 knots and an altitude of about 4,000 feet msl in visual flight rules (VFR) conditions when the yaw/roll event occurred. The pilots were able to regain control of the airplane and land at the destination airport without further incident. None of the 53 airplane occupants were injured, and no damage to the airplane resulted from the incident. As a result of previous Boeing 737 rudder anomalies, including the fatal accident involving USAir flight 427, a Boeing 737-300, which entered an uncontrolled descent and impacted terrain near Aliquippa, Pennsylvania, on September 8, 1994, Safety Board investigators initiated an investigation into this incident. The complete report of this incident investigation is contained in the final report of the USAir flight 427 investigation which can be found on the NTSB web site at the following link: http://www.ntsb.gov/publictn/1999/AAR9901.pdf . Additionally, all supporting factual information regarding this incident can be found in the public docket of the USAir flight 427 investigation. Excerpts of the factual narrative regarding the Eastwind incident from that report are presented here as follows: During postincident interviews with the Eastwind crew, the captain reported that he was flying the airplane with the autopilot disengaged [1] and his feet resting lightly on the rudder pedals during the descent to land at Richmond. Both the captain and first officer reported that they had not encountered any turbulence or unusual weather during the flight, which originated from Trenton, New Jersey, or the approach to land. However, the captain said that, as the airplane descended through about 5,000 feet msl, he felt a brief rudder "kick" or "bump" on the right rudder pedal but that the pedal did not move. The captain stated that he glanced at the first officer's feet to see if he had contacted the rudder pedals but that the first officer had his feet flat on the floor. Flight data recorder (FDR) information[2] and flight crew and flight attendant interviews indicated that, as the airplane descended through about 4,000 feet msl, the airplane yawed abruptly to the right and then rolled to the right. The captain stated that he immediately applied "opposite rudder and stood pretty hard on the pedal." The captain stated that, almost simultaneously with these rudder inputs, he applied left aileron.[3] Further, the captain consistently reported that the rudder pedal control felt stiffer than normal and did not seem to respond normally throughout the upset event. The first officer stated that he saw the captain "fighting, trying to regain control" and "standing on the left rudder." According to the captain, these flight control inputs slowed the yaw/roll event; however, the airplane "was still trying to roll," so he advanced the right throttle to compensate for the rolling tendency with differential power.[4] The captain stated that, after he made these inputs, the airplane appeared to move back toward neutral "for one or two seconds" and "might have momentarily banked left because of all the correction present" before returning abruptly to a right bank. The flight crew performed the emergency checklist, which included disengaging the yaw damper. Subsequently, the upset event stopped, and the airplane flew normally for the remainder of the flight. The pilots reported a delay of several seconds between the disengagement of the yaw damper and the end of the upset event. During postincident interviews, the lead flight attendant of Eastwind flight 517 stated that she was standing in the aisle near the rear of the airplane cabin before the upset began. At that time, she heard a distinct thump from below but not directly underneath her feet. (The rear flight attendant also reported hearing a thump sound while the airplane yawed to the right) She reported that, immediately after the thump occurred, the airplane began "rocking with a violent back and forth motion…. The motions…lasted no more than fifteen seconds, were violent from start to finish, and appeared to come in cycles." The FDR data revealed that the airplane rolled rapidly to the right about 10 degrees with a simultaneous heading change to the right of about 5 degrees per second. The FDR data also revealed that the airplane rolled back to the left, to a maximum left bank angle of approximately 15 degrees, while the right engine thrust increased. [5] (The airplane was in a 15-degree left bank for approximately 3 seconds and remained in a left bank for an additional six seconds while the engine thrust increased; however, the FDR recorded little heading change.) While the right engine pressure ratio (EPR) increased, the airspeed increased from about 250 to about 254 knots. The airplane's heading changed to the left; hesitated at about 242 degrees; and began a series of heading oscillations of decreasing magnitude, including a left heading excursion of 4.1 degrees and a right heading excursion of 5.6 degrees (both in 1 second). During the heading oscillations, the airplane's roll attitude also oscillated between an approximate wings-level attitude and 10 degrees left wing down (LWD). The heading and roll oscillations decreased while the airplane maintained an approximate constant heading of about 240 degrees. AIRCRAFT INFORMATION Postincident examination of the airplane's maintenance records revealed three flight crew-reported rudder-related events during the month preceding the incident. The first event occurred on May 14, 1996, when the captain of the June 9 Eastwind incident flight experienced a series of uncommanded "taps" on the right rudder pedal just after takeoff, which he stated felt "like someone hitting their foot on the right rudder." The captain returned to the departure airport and landed without further incident. As a result of the uncommanded rudder movements reported to have occurred on May 14, the main rudder PCU was replaced that same day,[6] and the airplane was returned to service.[7] During a May 21 overnight inspection, rudder sweep and PCU leak examinations were conducted. ADDITIONAL INFORMATION Previous Event Experienced by the Captain. The captain reported that the rudder pedal bumps he experienced on May 14 felt identical to the rudder pedal bump he felt at the onset of the yaw/roll event on June 9. Additionally, the Eastwind flight 517 lead flight attendant was a cabin crewmember on the May 14 flight, during which the captain experienced the uncommanded rudder "taps." The flight attendant stated that she did not hear any sounds during the May 14 event and reported that the event was much less intense than the June 9 incident. She was in the front of the cabin during the May 14 event but was near the rear of the cabin during the June 9 incident. The other two uncommanded yaw/roll events were reported to have occurred on June 1 and June 8, 1996. [8] As a result of these reports, the yaw damper transfer valve and the yaw damper linear variable displacement transducer (LVDT) were removed and replaced on June 8. The incident pilots performed a postmaintenance test flight on the morning of June 9 and reported that the airplane performed normally, with no rudder system anomalies noted during the test flight. Because the airplane performed satisfactorily during the test flight, it was returned to service. When Safety Board investigators examined the rudder system and the main rudder power control unit (PCU) after the June 9 incident, they observed that the rudder's yaw damper system had been adjusted such that the rudder neutral (at rest) position was 1.5 degrees to the left when the yaw damper system was engaged and the rudder trim was set at zero. The active yaw damper could move the rudder 1.5 degrees farther to the left of this neutral position and 4.5 degrees to the right of this neutral position with no aerodynamic loads. Postincident PCU testing at Parker's facility indicated that the yaw damper LVDT neutral position was incorrectly set. (The normal limit of yaw damper authority on the rudder, if properly set, would have been 3 degrees to the left and 3 degrees to the right of the rudder's neutral position.) Additional examination and testing conducted by the Safety Board, Eastwind, and Boeing revealed that the wiring from the yaw damper coupler to the main rudder PCU was chafed and could have resulted in a short circuit, causing a full yaw damper command left or right. Additionally, examination of the yaw damper system revealed damage from infiltration of fluid that was consistent with, but not conclusive evidence of, an electrical fault. The main rudder PCU and yaw damper coupler were removed and replaced, new wiring was installed between the PCU and the yaw damper coupler, and the airplane was returned to service. To date, no further pilot complaints or maintenance write-ups regarding rudder "bumps" or other anomalous rudder motions have been reported on the incident airplane. Flight Tests. On June 22 through 24, 1996, the Safety Board conducted flight tests in the Eastwind flight 517 incident airplane, with Boeing, FAA, and Eastwind Airlines participation. The flight tests were to document the operation and limits of the airplane's yaw damper system, test and record the airplane's responses to various rudder inputs, and expose the captain of Eastwind flight 517 to various rudder inputs and document his reactions to and insights on the inputs. For the flight tests, the airplane's yaw damper system bias remained misadjusted so that it could command 1.5 degrees to the left and 4.5 degrees to the right of the rudder's trimmed position (as it was at the time of the incident). As with the wake vortex tests, additional test equipment and instrumentation were installed on the incident airplane to record and document the flights. [9] During the ground and flight tests,[10] the incident airplane was operated with a Boeing flight test pilot in the left seat and an FAA flight test pilot in the right seat; the captain of Eastwind flight 517 and additional Boeing and FAA personnel were seated in the cabin. The first flight test was conducted at altitudes between 8,000 and 13,000 feet msl, at an airspeed of 250 knots, and with the yaw damper engaged and the flaps and landing gear retracted. Attempts were made to induce an in-flight yaw damper failure and subsequent hardover command through a series of rapid and abrupt rudder pedal and control wheel inputs; however, the flight test pilots were unsuccessful in inducing a yaw damper hardover. Before the second test flight, the incident yaw damper coupler was removed, and a different yaw damper coupler, a yaw damper fault insertion box, and associated wiring were installed to allow the flight test pilots to command a yaw damper hardover condition using an electrical signal. The second flight test was also conducted at altitudes between 8,000 and 13,000 feet msl; at an airspeed of 250 knots; and with the yaw damper engaged, autopilot disengaged, and flaps and landing gear retracted. Yaw damper hardovers to the left and right were electronically commanded by the flight crew via the cockpit switchbox, and the maximum rudder and control wheel positions needed to stabilize the airplane were noted. Additionally, rudder pedal release tests were conducted using the following procedures:-- While maintaining straight and level flight using control wheel and rudder pedal inputs, right rudder trim was added in 1 degree increments, from 0 to 6 degrees trailing edge right rudder position. -- Rudder pedal inputs were released. -- Rudder position and control wheel input needed to control bank angle were noted. During portions of the second flight test, the captain of Eastwind flight 517 occupied the right pilot seat previously occupied by the FAA flight test pilot [11] and controlled the airplane during a series of yaw damper hardover insertions and rudder pedal release conditions (including four yaw damper hardovers of 4.5 degrees right rudder, three rudder pedal releases from the 6 degrees right rudder trim position, and three rudder pedal releases from the 4 degrees right rudder trim position). Recorded FDR and PADDS data indicated that the captain responded to the first yaw damper hardover 0.6 seconds after its initiation by stepping on the left rudder pedal. The flight test FDR data indicated that the airplane's bank angle increased to a maximum of about 4.5 degrees right wing down (RWD) and that its heading changed about 2 degrees (both in 1 second) before the airplane responded to the Eastwind flight 517 captain's recovery efforts. During the three subsequent yaw damper hardovers, the Eastwind flight 517 captain, at the direction of the Boeing flight test pilot, allowed the airplane to respond to the hardover condition for a few seconds before the captain responded with rudder pedal input. When the Eastwind flight 517 captain was exposed to the 6 degrees right rudder pedal release test condition (during which FDR and PADDS equipment recorded a 4 degrees right heading excursion and a bank angle increase to 8 degrees RWD, both within 2 seconds), he stated "that was more like it." (The incident FDR data indicated a 4.1 degrees right heading change within 1 second and a maximum bank angle increase to 10 degrees RWD within 2 seconds.) The Eastwind flight 517 captain indicated that the motion of the airplane during the portion of the second test flight, for which he was seated in the right pilot seat in the cockpit, was similar to the airplane motion he recalled experiencing during the incident and that the yoke pressure felt the same. However, the captain indicated that the rudder response during the first and second tests seemed different from what he experienced during the incident. He stated that the rudder felt stiffer and less effective during the actual incident. Rudder Actuator Reversals During Servo Valve Secondary Slide Jams. After the Safety Board's October 1996 thermal PCU tests, Boeing engineers began an independent detailed examination of the test data. Their review of the data indicated that the PCU servo valve responded slowly and erratically to the input commands when the secondary slide was jammed to the housing by the thermal shock and an input was applied to the external input arm. Boeing subsequently conducted tests using a new-production PCU that had been modified to simulate a jam of the secondary slide to the servo valve housing at various positions and then to simulate the application of a full rudder input to the PCU. These tests revealed that, when the secondary slide was jammed to the servo valve housing at certain positions, the primary slide could travel beyond its intended stop position because of bending or twisting of the PCU's internal input linkages (compliance). This deflection allowed the primary slide to move to a position at which the PCU commanded the rudder in the direction opposite of the intended command (reversal). Specifically, the tests revealed that, when the secondary slide was jammed at positions greater than 50 percent off neutral toward the extend or retract position and a full-rate command was applied to the PCU, the rudder would move opposite to the commanded position. After studying the thermal test conditions in which the USAir flight 427 main rudder PCU jammed, the Safety Board attempted to determine the combined effects of PCU servo valve secondary slide jamming and input linkage deflections (compliance) to determine if the USAir flight 427 PCU was more susceptible to reversal than other servo valves. These tests were conducted in November 1996 on three PCUs: a new-production PCU, the USAir flight 427 PCU, and the Eastwind flight 517 PCU. For this series of tests, a tool was used to mechanically jam the secondary slides of all three PCUs to their respective servo valve housings. Manual inputs were then applied to the PCUs with the yaw damper energized and deenergized (no yaw damper command was applied in both cases). When inputs at a less-than-maxi
The yaw/roll upset 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.
Source: NTSB Aviation Accident Database
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