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Safety Recommendation Details

Safety Recommendation A-92-121
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: CONDUCT A DESIGN REVIEW OF SERVO VALVES MANUFACTURED BY PARKER HANNIFIN HAVING A DESIGN SIMILAR TO THE B-737 RUDDER POWER CONTROL UNIT SERVO VALVE THAT CONTROL ESSENTIAL FLIGHT CONTROL HYDRAULIC POWER CONTROL UNITS ON TRANSPORT-CATEGORY AIRPLANES CERTIFIED BY THE FEDERAL AVIATION ADMINISTRATION TO DETERMINE THAT THE DESIGN IS NOT SUSCEPTIBLE TO INDUCING FLIGHT CONTROL MALFUNCTIONS OR REVERSALS DUE TO OVERTRAVEL OF THE SERVO SLIDES.
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: 6/10/1993
Addressee(s) and Addressee Status: FAA (Closed - Acceptable Action)
Keyword(s):

Safety Recommendation History
From: NTSB
To: FAA
Date: 6/10/1993
Response: THE BOARD NOTES THAT THE FAA HAS COMPLETED A DESIGN REVIEW OF THE SERVO VALVES MANUFACTURED BY PARKER HANNIFIN ON ALL TRANSPORT-CATEGORY AIRPLANES. A PROBLEM WAS FOUND TO EXIST ONLY IN THE MAIN RUDDER POWER CONTROL UNIT ON THE BOEING 737 MODEL AIRPLANES. BASED ON THIS INFO, THE BOARD CLASSIFIES A-92-121 "CLOSED--ACCEPTABLE ACTION."

From: FAA
To: NTSB
Date: 1/19/1993
Response: THE FAA AGREES WITH THIS RECOMMENDATION, AND A DESIGN REVIEW OF THE SERVO VALVES MANUFACTURED BY PARKER HANNIFIN ON ALL TRANSPORT CATEGORY AIRPLANES WAS COMPLETED. THE PROBLEM WAS FOUND TO EXIST IN THE MAIN RUDDER POWER CONTROL UNIT ONLY ON THE BOEING 737 MODEL AIRPLANES.