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Safety Recommendation Details

Safety Recommendation A-92-119
Details
Synopsis: THE DUAL SERVO VALVES REMOVED FROM THE B-737S THAT CRASHED IN COLORADO SPRINGS, COLORADO, ON MARCH 3, 1991, AND IN THE DARIEN PROVINCE OF PANAMA ON JUNE 6, 1992, WERE ALSO TESTED. THE RESULTS SHOW THAT A 50 PERCENT PRESSURE DROP COULD HAVE DEVELOPED ON THE COLORADO SPRINGS UNIT IF A FAILURE MECHANISM PRODUCED AN OVERTRAVEL OF THE SECONDARY VALVE SLIDE. AS UNDERSTOOD THUS FAR, IF SUCH A PRESSURE DROP OCCURRED, THE MAIN RUDDER PCU COULD ONLY DEVELOP 50 PERCENT OF THE RUDDER HINGE MOMENT CAPABILITY, WORKING IN THE PROPER DIRECTION. THE PRESSURE DROP WOULD BE SIMILAR TO LOSING EITHER A OR B REDUNDANT HYDRAULIC SYSTEMS. MOREOVER, THE RESULTS SHOW THAT A COMPLETE PRESSURE DROP, WITHOUT REVERSAL, COULD HAVE DEVELOPED ON THE PANAMA UNIT ONLY IF A FAILURE MECHANISM PRODUCED AN OVERTRAVEL OF THE SECONDARY SLIDE VALVE. THE UNIT WOULD LOSE HINGE MOMENT CAPABILITY, BUT MOVEMENT OF THE RUDDER IN THE OPPOSITE DIRECTION BEYOND NEUTRAL WOULD NOT OCCUR.
Recommendation: THE NTSB RECOMMENDS THAT THE FEDERAL AVIATION ADMINISTRATION: REQUIRE THAT BOEING DEVELOP AN APPROVED PREFLIGHT CHECK OF THE RUDDER SYSTEM TO BE USED BY OPERATORS TO VERIFY, TO THE EXTENT POSSIBLE, THE PROPER OPERATION OF THE MAIN RUDDER POWER CONTROL UNIT SERVO VALVE UNTIL A DESIGN CHANGE IS IMPLEMENTED THAT WOULD PRECLUDE THE POSSIBILITY OF RUDDER REVERSALS ATTRIBUTED TO THE OVERTRAVEL OF THE SECONDARY SLIDE.
Original recommendation transmittal letter: PDF
Overall Status: Closed - Acceptable Alternate 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: 11/10/1992
Date Closed: 8/11/1994
Addressee(s) and Addressee Status: FAA (Closed - Acceptable Alternate Action)
Keyword(s):

Safety Recommendation History
From: NTSB
To: FAA
Date: 8/11/1994
Response: THE BOARD NOTES THAT THE FAA MAINTAINS THAT A QUANTITATIVE PREFLIGHT TEST CANNOT BE DEVELOPED. HOWEVER, THE BOARD CONSIDERS THE FAA'S REQUIREMENT FOR REPETITIVE INSPECTIONS OF THE PCU AT 750 HOUR INTERVALS UNTIL TERMINATING ACTION SUFFICIENT TO ADDRESS THE INTENT OF RECOMMENDATION A-92-119. THEREFORE, BOARD CLASSIFIES THE RECOMMENDATION "CLOSED--ACCEPTABLE ALTERNATE ACTION."

From: FAA
To: NTSB
Date: 7/14/1994
Response: THE FAA HAS RECONSIDERED ITS PREVIOUS POSITION ON THIS RECOMMENDATION & REMAINS CONVINCED THAT CURRENT PREFLIGHT CHECK PRECEDURES ADEQUATELY ENSURE PROPER RUDDER OPERATION. THE FAA AGREES THAT RAPID RUDDER INPUTS ARE A FACTOR IN UNCOVERING RUDDER CONTROL ANOMALIES. HOWEVER, AS NOTED BY THE BOARD, A RAPID RUDDER INPUT DURING EVERY PREFLIGHT CHECK INCREASES THE POSSIBILITY OF STRUCTURAL RUDDER DAMAGE. ADDITIONALLY, IT WOULD BE IMPOSSIBLE TO CONDUCT THIS CHECK WITH ANY DEGREE OF CONSISTENCY BECAUSE OF VARIANCES AMONG PILOTS. FINALLY, THE FAA DOES NOT AGREE THAT ALL RUDDER CONTROL ANOMALIES DUE TO SECONDARY SLIDE OVERTRAVEL CAN BE DETECTED DURING PREFLIGHT CHECKS.

From: NTSB
To: FAA
Date: 1/12/1994
Response: THE BOARD NOTES THAT THE FAA HAS ISSUED A NOTICE OF PROPOSED RULEMAKING (DOCKET NO. 93-NM-79-AD) PROPOSING REQUIRED PERIODIC INSPECTIONS OF THE RUDDER SYSTEM IN ACCORDANCE WITH BOEING SERVICE LETTER 737-SL-27-82-B, DATED 7/13/93, UNTIL THE MAIN RUDDER SERVO VALVE IS REWORKED IN ACCORDANCE WITH BOEING SERVICE BULLETIN 737-27-1185, DATED 4/15/93. THE FAA'S PROPOSED RULE MEETS THE INTENT OF A-92-119, WHICH IS NOW CLASSIFIED "OPEN--ACCEPTABLE RESPONSE," PENDING IMPLEMENTATION OF THE FINAL RULE.

From: FAA
To: NTSB
Date: 12/2/1993
Response: THE FAA ISSUED A NOTICE OF PROPOSED RULEMAKING DOCKET NO.93-NP-79-AD) PROPOSING TO REQUIRE SPECIALLY TRAINED OPERATORS TO PERFORM A PERIODIC INSPECTION (INTERVALS MUST NOT EXEED 750 FLIGHT HOURS) OF THE RUDDER SYSTEM IN ACCORDANCE WITH BOEING SERICE LETTER 737-SL-27-82-B, DATED 7/13/93, UNTIL THE MAIN RUDDER SERVO VALUE IS REWORKED IN ACCORDANCE WITH BOEING SERVICE BULLETIN 737-27-1185, DATED 4/15/93. THE RUDDER PEDALS WILL BE CYCLED AT THE MAXIMUM RATE DURING THE INSPECTION, & SPECIAL INSTRUMENTATION & ADDITIONAL OBSERVERS WILL BE AVAILABLE TO DETECT PROPERLY ANY ANOMALY. MANDATORY MODIFICATION OF THE SERVO VALUE WOULD BE REQUIRED WITHIN 5 YEARS AFTER THE EFFECTIVE DATE OF THE FINAL RULE.

From: NTSB
To: FAA
Date: 6/10/1993
Response: THE BOARD RECOGNIZES THAT RAPID MOVEMENT OF THE RUDDER PEDALS ON THE GROUND COULD RESULT IN DAMAGE TO THE AIRPLANE. THE BOARD ISSUED A-92-119 TO REQUIRE BOEING TO DEVELOP A SAFE & EFFECTIVE PROCEDURE TO UNCOVER THE TYPE OF PROBLEM PRESENT ON THE UNITED AIRLINES BOEING 737-300. BASED ON THIS INFO, THE BOARD REQUESTS THAT THE FAA RECONSIDER ITS POSITION CONCERNING A-92-119 & CLASSIFIES IT "OPEN--UNACCEPTABLE RESPONSE."

From: FAA
To: NTSB
Date: 1/19/1993
Response: THE FAA DOES NOT AGREE WITH THIS RECOMMENDATION. THE UNITED AIRLINES BOEING 737-300 MAIN RUDDER POWER CONTROL UNIT (S/N 2228A), WHICH STOPPED MOVING AT APPROXIMATELY 25 PERCENT LEFT PEDAL TRAVEL, WAS DISCOVERED DURING A ROUTINE PREFLIGHT CHECK. THE FAA BELIEVES THAT THE CURRENT PERFLIGHT CHECK PROCEDURES ADEQUATELY ENSURE PROPER RUDDER OPERATION.