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Safety Recommendation Details

Safety Recommendation A-92-120
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 OPERATORS, BY AIRWORTHINESS DIRECTIVE, TO INCORPORATE DESIGN CHANGES FOR THE B-737 MAIN RUDDER POWER CONTROL UNIT SERVO VALVE WHEN THESE CHANGES ARE MAKE AVAILABLE BY BOEING. THESE CHANGES SHOULD PRECLUDE THE POSSIBILITY OF RUDDER REVERSALS ATTRIBUTED TO THE OVERTRAVEL OF THE SECONDARY SLIDE.
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: 11/10/1992
Date Closed: 8/11/1994
Addressee(s) and Addressee Status: FAA (Closed - Acceptable Action)
Keyword(s):

Safety Recommendation History
From: NTSB
To: FAA
Date: 8/11/1994
Response: THE BOARD NOTES THAT ON 1/3/94, THE FAA ISSUED AD 94-01-07, WHICH REQUIRES REPETITIVE INSPECTIONS OF THE BOEING 737 MAIN RUDDER PCU AT 750 HOUR INTERVALS. THIS REPETITIVE INSPECTION WILL CONTINUE UNTIL THE PCUS ARE REPLACED. THIS ACTION BY THE FAA SATISFIES THE INTENT OF A-92-118, & -120. THE BOARD BELIEVES THAT IS THE INTEREST OF SAFETY, ALL BOEING 737 MAIN RUDDER PCUS SHOULD BE MODIFIED AT THE EARLIEST POSSIBLE DATE, & SINCE THE COMPLIANCE PERIOD APPEARS TO BE FOUNDED ON REASONABLE ESTIMATES OF EQUIPMENT AVAILABILITY, THE BOARD CLASSIFIES THESE RECOMMENDATIONS "CLOSED--ACCEPTABLE ACTION."

From: FAA
To: NTSB
Date: 7/14/1994
Response: THE FAA ISSUED AIRWORTHINESS DIRECTIVE 94-01-07 ON 1/3/93, APPLICABLE TO CERTAIN BOEING MODEL 737 SERIES AIRPLANES. THIS AD REQUIRES REPETITIVE TESTS OF THE MAIN RUDDER POWER CONTROL UNIT TO DETECT EXCESSIVE INTERNAL LEAKAGE OF HYDRAULIC FLUID, STALLING, OR REVERSAL, & THE EVENTUAL REPLACEMENT OF THE MAIN RUDDER POWER CONTROL UNIT WITH AN IMPROVED MODEL.

From: NTSB
To: FAA
Date: 6/10/1993
Response: THE BOARD NOTES THAT THE FAA WILL CONSIDER ISSUING AN NPRM TO ADDRESS THIS RECOMMENDATION AS SOON AS BOEING ISSUES THE SERVICE INFO MENTIONED IN RESPONSE TO A-92-118. PENDING FURTHER INFO, THE BOARD CLASSIFIES A-92-120 "OPEN--ACCEPTABLE RESPONSE."

From: FAA
To: NTSB
Date: 1/19/1993
Response: THE FAA AGREES WITH THE INTENT OF THIS RECOMMENDATION AND WILL CONSIDER THE ISSUANCE OF A NOTICE OF PROPOSED RULEMAKING TO ADDRESS THIS RECOMMENDATION AS SOON AS BOEING ISSUES THE SERVICE INFORMATION MENTIONED IN RESPONSE TO RECOMMENDATION A-92-118.