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Safety Recommendation Details

Safety Recommendation A-96-110
Details
Synopsis: On March 3, 1991, at 0944 mountain standard time, United Airlines flight 585, a Boeing 737-291 airplane, crashed during an approach to the Colorado Springs, Colorado, airport. The crew of 5 and the 20 passengers were killed. The airplane was destroyed by the impact and a postcrash fire. The weather was clear with unlimited visibility. There were windshear reports during the day. At the time of the accident the surface winds were reported to be out of the northwest at 20 knots gusting to 28. The safety board has not determined the cause(s) of the accident and an investigation of airframe, operational and weather factors is continuing.
Recommendation: TO THE FEDERAL AVIATION ADMINISTRATION: Conduct a detailed engineering review of the B-737 yaw damper system, and require the Boeing Commercial Airplane Group to redesign the yaw damper system, as necessary, to eliminate the potential for sustained uncommanded yaw damper control events. After the B-737 yaw damper system is redesigned, issue an airworthiness directive to require the installation of the improved yaw damper system on all B-737 series aircraft.
Original recommendation transmittal letter: PDF
Overall Status: Closed - Acceptable Action
Mode: Aviation
Location: COLORADO SPGS, CO, United States
Is Reiterated: No
Is Hazmat: No
Is NPRM: No
Accident #: DCA91MA023
Accident Reports: Uncontrolled Descent and Collision with Terrain, United Airlines Flight 585, Boeing 737-200, N999UA, 4 Miles South of Colorado Springs Municipal Airport
Report #: AAR-92-06
Accident Date: 3/3/1991
Issue Date: 10/18/1996
Date Closed: 3/17/2000
Addressee(s) and Addressee Status: FAA (Closed - Acceptable Action)
Keyword(s):

Safety Recommendation History
From: NTSB
To: FAA
Date: 3/17/2000
Response: THE SAFETY BOARD HAS REVIEWED THESE DOCUMENTS AND FINDS THAT THEIR ISSUANCE MEETS THE INTENT OF A-96-110. ACCORDINGLY, IT HAS BEEN CLASSIFIED "CLOSED--ACCEPTABLE ACTION."

From: FAA
To: NTSB
Date: 11/17/1999
Response: Letter Mail Controlled 11/19/1999 12:53:14 PM MC# 991313 ON 6/23/97, THE FAA ISSUED AIRWORTHINESS DIRECTIVE (AD) 97-14-03 TO REQUIRE THE INSTALLATION OF A REDESIGNED YAW DAMPER SYSTEM AND A RUDDER LIMITER. SUBSEQUENTLY, BOEING PUBLISHED SERVICE BULLETIN 737-27A1206 TO PROVIDE INSTRUCTIONS FOR THE INSTALLATION OF THE NEW PRESSURE REDUCER AND YAW DAMPER COUPLER ON BOEING 737 AIRPLANES. THE SERVICE BULLETIN PROVIDES AN ACCEPTABLE METHOD OF COMPLIANCE WITH AD 97-14-03. I HAVE ENCLOSED A COPY OF THE SERVICE BULLETIN FOR THE BOARD'S INFORMATION. I CONSIDER THE FAA'S ACTION TO BE COMPLETED ON THIS SAFETY RECOMMENDATION, AND I PLAN NO FURTHER ACTION.

From: NTSB
To: FAA
Date: 2/2/1999
Response: THE FAA ISSUED AD 97-14-03 ON 6/23/97, TO REQUIRE THE INSTALLATION OF A REDESIGNED YAW DAMPER SYSTEM AND A RUDDER LIMITER ON B-737'S. PENDING THE FAA'S CERTIFICATION OF THE PROPOSED NEW YAW DAMPER SYSTEM, A-96-110 IS CLASSIFIED "OPEN--ACCEPTABLE RESPONSE."

From: FAA
To: NTSB
Date: 5/13/1998
Response: Letter Mail Controlled 5/18/98 3:54:43 PM MC# 980640 THE FAA ISSUED AD 987-14-03 TO REQUIRE THE INSTALLATION OF A NEW YAW DAMPER SYSTEM.

From: NTSB
To: FAA
Date: 8/7/1997
Response: Notation 6882: The National Transportation Safety Board has reviewed your Notice of Proposed Rulemaking (NPRM), "Airworthiness Directives; Boeing Model 737-100, -200, -300, -400, and -500 Series Airplanes," which was published in 62 FR 34185 on June 25, 1997. The notice invites comments on the adoption of an airworthiness directive (AD) that would require removal of the yaw damper coupler after a specified number of service hours, replacing the internal rate gyroscopes with a new overhauled unit, and performing a test to verify the integrity of the yaw damper coupler, and repair, if necessary. The specified actions are intended to prevent sudden uncommanded yawing of the airplane from potential failures within the yaw damper coupler system. The Safety Board notes that the NPRM is responsive, in part, to the Safety Board's Safety Recommendation A-96-110, which was issued to the FAA on October 18, 1996. The recommendation, which resulted from the investigation of a Boeing 737 (B-737) accident at Aliquippa, Pennsylvania, on September 8, 1994, and other roll upset incidents asked the FAA to conduct a detailed engineering review of the B-737 yaw damper system, and require the Boeing Commercial Airplane Group to redesign the yaw damper system, as necessary, to eliminate the potential for sustained uncommanded yaw damper control events. After the B-737 yaw damper system is redesigned, issue an AD to require the installation of the improved yaw damper system on all B-737 series aircraft. The Safety Board agrees with the FAA that replacement of potentially faulty yaw damper coupler rate gyroscopes after a specified number of service hours will help to reduce the frequency and consequences of B-737 yaw upsets and fully supports the NPRM. The Safety Board appreciates the opportunity to comment on this NPRM.

From: NTSB
To: FAA
Date: 7/15/1997
Response: A-96-110 ASKED THE FAA TO CONDUCT A DETAILED ENGINEERING REVIEW OF THE B-737 YAW DAMPER SYSTEM, & REQUIRE THE BOEING COMMERCIAL AIRPLANE GROUP TO REDESIGN THE YAW DAMPER SYSTEM, AS NECESSARY , TO ELIMINATE THE POTENTIAL FOR SUSTAINED UNCOMMANDED YAW DAMPER CONTROL EVENTS. AFTER THE B-737 YAW DAMPER SYSTEM IS REDESIGNED, ISSUE AN AD TO REQUIRE THE INSTALLATION OF THE IMPROVED YAW DAMPER SYSTEM ON ALL B-737 SERIES AIRPLANES. A-96-110 IS CLASSIFIED "OPEN--ACCEPTABLE RESPONSE."

From: NTSB
To: FAA
Date: 4/23/1997
Response: Notation Number 6744C: Attention: Rules Docket 97-NM-28-AD The National Transportation Safety Board has reviewed your notice of proposed rulemaking, "Airworthiness Directives; Boeing Model 737-100, -200, -300, -400, and -500 Series Airplanes," which was published in 62 FR 12121 on March 14, 1997. The notice proposes to require that a newly designed rudder-limiting device and yaw damper system be installed on all Boeing 737-100 through -500 series airplanes. On October 18, 1996, the Safety Board issued the following recommendations to the Federal Aviation Administration Require the Boeing Commercial Airplane Group, working with interested parties, to develop immediate operational measures and long-term design changes for the B-737 series airplane to preclude the potential for loss of control from an inadvertent rudder hardover. Once the operational measures and design changes have been developed, issue respective airworthiness directives to implement these actions. (A-96-107) Conduct a detailed engineering review of the B-737 yaw damper system, and require the Boeing Commercial Airplane Group to redesign the yaw damper system, as necessary, to eliminate the potential for sustained uncommanded yaw damper control events. After the B-737 yaw damper system is redesigned, issue an airworthiness directive to require the installation of the improved yaw damper system on all B-737 series aircraft. (A-96-11 0) The Safety Board believes that the proposed changes are responsive to those parts of the recommendations that had not yet been acted upon. The Board is encouraged by the FAA's response and urges the FAA to expedite installation of the new yaw damper system and rudder limiting device. The Safety Board appreciates the opportunity to comment on this proposed rule.

From: NTSB
To: FAA
Date: 3/3/1997
Response: Notation 6744B: Attention: Rules Docket No. 96-NM-266-AD The National Transportation Safety Board is pleased to submit comments to the Federal Aviation Administration (FAA) docket 96-NM-266-AD, Airworthiness Directive (AD) 96-26-07, published in Volume 62 Federal Register 15 on January 2, 1997. This AD, applicable to all models of the Boeing 737 airplane (B-737), was issued by the FAA as a final rule, and the operational procedures specified in the AD became effective on February 17, 1997. The FAA invited comments on this rule and has indicated its willingness to consider amending the AD, based on the comments received. The Safety Board has conducted numerous examinations and tests during its investigations of the B-737 accidents that occurred near Colorado Springs, Colorado, on March 3, 1991, and near Aliquippa, Pennsylvania, on September 8, 1994, and the incident that occurred near Richmond, Virginia, on June 9, 1996. Although the Board's investigations are not complete, the test results indicate that under certain rudder system failure conditions, a rudder input from the flight crew (and possibly from the yaw damper system) can result in movement of the rudder opposite to that commanded. While there is no documented in-service experience of an uncommanded or reversed rudder movement resulting from these failure conditions, the existence of this failure mode means that the B-737 is exposed to a potential loss of control stemming from a single failure that can no longer be considered an extremely improbable or remote event. As the Safety Board expressed in safety recommendation letters to the FAA Administrator of October 18, 1996, and February 20, 1997 (enclosed), the Board believes that design modifications and fleet retrofits of the B-737 rudder power control unit (PCU) and associated systems are necessary to ensure an adequate level of safety in B-737 operations. Further, the Safety Board has recommended, in these letters, that operational procedures and supportive flight crew training measures be developed to assist B-737 flight crews in maintaining aircraft control during a rudder system malfunction. These actions are needed as interim measures to provide an adequate level of safety, pending retrofit of the redesigned PCV and associated systems to the existing B-737 fleet. The subject AD specifies two sets of operational procedures relevant to an uncommanded rudder movement that have been incorporated into the FAA-approved Airplane Flight Manual for the B-737. The first details immediate action (recall) procedures to be executed by flight crews in the event of an uncommanded yaw or roll event. The second is a set of followup procedures to be executed by flight crews after reestablishing control of the aircraft. The Safety Board concurs with the immediate action procedures, which stress the need for flight crews to regain lateral and directional control of the airplane by using all available control inputs, increasing airspeed, and reducing the angle of attack. The Safety Board agrees that these actions are likely to enable pilots to recover control of the aircraft under most conditions. The Safety Board also supports the AD's specification to move the System B flight control switch to the "Standby Rudder" position to aid in recovery. However, the AD specifies that this action take place as a followup procedure, not an immediate action item. If rudder system malfunctions were to occur at a relatively low altitude and at an airspeed near or below the "crossover speed" (at which the roll rate commanded by the rudder exceeds that available from lateral controls), or if there were to be a delay of only a few seconds in the flight crew's recovery attempt, resulting in an extreme aircraft attitude or high roll rate, the flight crew may not be able to regain control of the airplane without immediately moving the System B flight control switch to the "Standby Rudder" position. Turning on the standby rudder PCU turns off the System B hydraulic power to the PCU, thereby reducing the hydraulic power to the rudder by half The standby rudder PCU has slightly more authority than a main PCU operating only on System A hydraulic power, thus allowing the pilots to neutralize the rudder position. It is the Safety Board's understanding that immediately initiating this action can significantly enhance the effectiveness of the pro-recovery rudder flight control inputs in regaining control in the event of a reversing rudder malfunction. The System B flight control switch in the B-737 is located in the overhead panel, above and behind the captain's head, next to three switches (System A flight controls, System A spoilers, and System B spoilers) that are identical in shape. The similarity among these switches makes it difficult to readily identify and move the correct switch during a dynamic yaw/roll situation that may have devolved to an unusual pitch and roll attitude. Further, the flight control switches are guarded, and the System B flight control switch has a three-position ("On," "Off," and "Standby Rudder") design. To select the "Standby Rudder" position, the pilot must lift the guard and move the switch through the "Off' position. The potential for selecting the incorrect switch can be reduced by retrofitting a redesigned System B flight control switch distinguishable in both size and color from nearby controls, allowing the switch to freely move between "On" and "Standby Rudder," and guarding against inadvertent movement to the "Off' position. In its letter to the FAA of October 18, 1996, the Safety Board cited the history of yaw damper malfunctions in the B-737 and requested, in Safety Recommendation A-96-l10, that the FAA conduct a detailed engineering review of the B-737 yaw damper system, require the Boeing Commercial Airplane Group to redesign the yaw damper system in accordance with this review, and require the installation of the improved yaw damper system on all B-737s. Further, in Safety Recommendation A-96-118, the Safety Board requested that the FAA require Boeing, working with other interested parties, to develop procedures that require B-737 flight crews to disengage the yaw damper in the event of an uncommanded yaw upset as a memorized or learned action. In response to Safety Recommendation A-96-110, the FAA and Boeing have announced plans to redesign the yaw damper system and retrofit the improved designs in the existing B-737 fleet. Pending this retrofit, however, the Safety Board is concerned that pilots experiencing yaw damper malfunctions in the B-737 may misinterpret them as a more serious uncommanded yaw/roll associated with a reversing rudder. While the planned yaw damper system improvements and fleet retrofits are being implemented, the procedures recommended by the Safety Board in Safety Recommendation A-96-118 need to be coordinated with those of the current AD by including the selection of the yaw damper switch to the "Off' position as an immediate action response to uncommanded yaw/roll in the B-737. This item could be specified in the AD following the disengagement of the autopilot and auto throttles. In the AD, the first thing pilots are instructed to do if they experience an uncommanded roll or yaw is to "maintain control of the aircraft with all available flight controls." If the rudder is jammed, the AD specifies as a followup procedure that B-737 flight crews should ''use maximum force (combined effort by both pilots) to overpower the rudder system." The Safety Board is concerned that these procedures may cause pilots experiencing an uncommanded yaw/roll in the B-737 to become overly fixated on attempting to break the rudder free with maximum force rudder pedal inputs, which would be counterproductive in correcting the reversing rudder malfunction. A more effective way of correcting the malfunction would be to continue with the next steps of the immediate action procedure if maximum force pedal inputs did not immediately work. The Board is also concerned that a pilot handling the controls of a B-737 during the initial stages of an uncommanded yaw/roll may experience difficulty communicating the proper control response to obtain the assistance of the non-flying pilot. For example, a simple request of "Get on the rudder with me" during an uncommanded or reversing rudder situation may result in the non-flying pilot applying opposite input to that of the flying pilot, if the non-flying pilot perceives the rudder deflection (albeit one not desired by the flying pilot) and attempts to add more. This possibility of the two pilots inadvertently struggling against each other with simultaneous, opposite flight control inputs suggests that a review of the procedures currently specified in the AD may be warranted. The Safety Board notes that the preamble to AD-96-07 refers to the test results that indicated the potential for a reversed rudder. However, the regulatory text of the AD, which has been placed in B-737 pilots' Airplane Flight Manuals, does not address the reversing rudder situation. In Safety Recommendation A-97-l7 issued to the FAA on February 20, 1997, the Safety Board addressed shortcomings in the AD related to this issue. Also, the Safety Board staffs informal conversations with U.S. air carrier training officials have indicated that there is an inadequate level of understanding about the rudder reversal situation among personnel who are currently implementing the relevant procedures in the AD. This includes, but is not limited to, misunderstandings about the direction of rudder pedal movement with respect to rudder deflection in a reversing rudder malfunction. Some pilots believe that in this malfunction, the pedal moves in one direction while the rudder moves in the opposite direction. In fact, rudder pedal movement follows the direction of rudder travel even though the pilots oppose that movement. The Safety Board is concerned that implementation of the procedures specified in the subject AD may be ineffective without accompanying enhancements in B-737 pilot training. In Safety Recommendation A-97-18, issued to the FAA on February 20, 1997, the Safety Board expressed the need for B-737 pilots to receive specific initial and periodic training in the recognition of and recovery from unusual attitudes and upsets caused by reverse rudder response. This training could be enhanced by emphasizing recognition of the reverse rudder problem (including the differential recognition of a yaw damper malfunction) and practice in responding to the more serious uncommanded yaw/roll involving full rudder travel under a variety of flight conditions. Finally, the Safety Board is aware that some operators of B-737s have suggested additional procedures for reacting to the reversing rudder situation: (I) adding 10 knots to the minimum airspeed for operating in partial-flap airplane configurations, which provides a greater airspeed margin above the crossover speed applicable to each airplane configuration; and (2) using differential thrust to aid in recovering from an uncommanded yaw/roll event. It is the Board's understanding that the research necessary to completely identify the crossover speeds applicable to all flap settings and to the different models of the B-737 series has not yet been completed. Similarly, a thorough evaluation of the human performance considerations relevant to the use of differential thrust during an unusual attitude recovery has not been undertaken. If airspeed adjustment and differential thrust procedures were shown to be beneficial and safe in such evaluations, their addition to the AD could enhance the ability of a flight crew to regain control of the airplane in the event of an uncommanded yaw/roll or rudder reversal malfunction. Thank you for the opportunity to comment on this final rule.

From: FAA
To: NTSB
Date: 1/16/1997
Response: IN OCTOBER 1994, THE FAA ORGANIZED A TEAM TO CONDUCT A CRITICAL DESIGN REVIEW OF THE FLIGHT CONTROL SYSTEMS INSTALLED ON BOEING 737 SERIES AIRPLANES IN AN EFFORT TO CONFIRM THE CONTINUED OPERATIONAL SAFETY OF THESE AIRPLANES. THE FORMATION OF THE CRITICAL DESIGN REVIEW TEAM WAS PROMPTED BY QUESTIONS THAT AROSE FOLLOWING THE ACCIDENT OF A BOEING 737-200 SERIES AIRPLANE NEAR COLORADO SPRINGS, COLORADO,& ONE INVOLVING A BOEING 737-300 SERIES AIRPLANE NEAR PITTSBURGH, PENNSYLVANIA. THE BOEING 737 YAW DAMPER SYSTEM WAS PART OF THE CRITICAL DESIGN REVIEW. THE CRITICAL DESIGN REVIEW CONCLUDED THEAT ADDITIONAL MAINTENANCE CHECKS, HARD-TIMING THE INTERNAL RATE GYROSCOPE OF THE YAW DAMPER COUPLER, & INSTALLING IMPROVED ENGAGE SOLENOID VALVES WOULD SIGNIFICANTLY INCREASE THE RELIABILITY OF THE YAW DAMPER SYSTEM. ON 8/21/96, THE ISSUED A NOTICE OF PROPOSED RULEMAKING (NPRM) PROPOSING TO ADOPT AN AD APPLICABLE TO ALL BOEING 737-100, -200, -300, -400, & -500 SERIES AIRPLANES. THE NPRM PROPOSES TO IMPLEMENT THE RECOMMENDATIONS OF THE CRITICAL DESIGN REVIEW. THE ACTIONS PROPOSED BY THE NPRM ARE INTENDED TO PREVENT SUDDEN UNCOMMANDED YAWING OF THE AIRPLANE DUE TO POTENTIAL FAILURES WITHIN THE YAW DAMPER SYSTEM BY REQUIRING INSPECTION & REPLACEMENT OF CERTAIN COMPONENTS, IF NEEDED.