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Safety Recommendation Details

Safety Recommendation A-96-114
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: Evaluate the adequacy of the chip shearing capacity for all sliding spool control valves used in transport-category aircraft flight control system and take appropriate action to correct any problems identified to preclude potential for actuator jamming, binding, or failure.
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: 8/16/2001
Addressee(s) and Addressee Status: FAA (Closed - Acceptable Action)
Keyword(s):

Safety Recommendation History
From: NTSB
To: FAA
Date: 8/16/2001
Response: The FAA enclosed with its March 20 response a report of the evaluations, as the Safety Board requested in its August 16, 2000, letter. The Board has reviewed the report of the relevant information extracted from several manufacturers' detailed evaluations, and notes that their respective regulatory authorities (for non-U.S. manufacturers) or the cognizant FAA aircraft certification office had concurred with the findings. Because the report represents an industry-wide consensus, the Board believes that the criteria recommended by the SAE A-6 Committee should be the basis for certification decisions by the FAA. Because the FAA has completed the actions requested by the Safety Board, Safety Recommendation A-96-114 is classified "Closed--Acceptable Action."

From: FAA
To: NTSB
Date: 3/20/2001
Response: Letter Mail Controlled 03/26/2001 8:50:41 PM MC# 2010264: On January 14, 2000, the Federal Aviation Administration (FAA) informed the Board that its aircraft certification offices had evaluated the adequacy of the chip-shearing capacity of sliding spool control valves for certain airplanes. A copy of the criteria that was used in the evaluation was provided to the Board. The FAA considered its action to be completed in response to this safety recommendation. On August 16, 2000, the Board expressed concern that although the FAA had completed the evaluation of chip-shearing of sliding spool control valves and concluded that all evaluated aircraft comply with the criteria, the Board remains concerned because individual manufacturers' chip-shearing specifications differ greatly. The Board asked that the FAA provide a report of the evaluations, so that the Board could verify how all transport-category aircraft evaluated were found to have adequate chip-shearing capacity given the wide variation in chip-shearing requirements. The FAA agrees with the Board that individual manufacturer's specifications differ greatly regarding chip-shearing requirements, but these differences are due to design flexibility allowed by the regulations to show compliance without the regulatory authority dictating design. The following are the criteria used to determine the chip-shearing requirements: SAE A-6 Committee's Recommended Criteria: For flight-critical design applications, the recommended minimum force is the greater of (1) 80 pounds or 50 pounds if valve jam detection and correlation is inherent, or (2) a force based on 75 percent fill of the maximum metering slot with 130 ksi stress material. (The SAE emphasized that the chip-shear capacity determination is both empirical and scientific in nature.) Design Philosophies: It should be noted that the chip-shear capability is typically a component-level requirement that ensures robust valve performance in the presence of contamination and not usually employed for continued safe flight and landing. Continued safe flight and landing due to numerous types of failures and jams, including valve jams from chips, are typically ensured through higher system-level requirements. The FAA has determined that the higher system-level requirement leads to architectures that ensure continued safe flight and landing despite the differing chip-shear requirements from manufacturer to manufacturer. In response to the Board's request for a report of the evaluations, I am enclosing the relevant information extracted from several detailed evaluations from applicants and concurred in by either their respective regulatory authorities (non-U.S. manufacturers) or the cognizant FM aircraft certification office. I believe that the FAA has addressed the full intent of this safety recommendation, and I consider the FAA's action to be completed.

From: NTSB
To: FAA
Date: 8/16/2000
Response: ALTHOUGH THE FAA HAS COMPLETED THE EVALUATION OF CHIP-SHEARING SLIDING SPOOL CONTROL VALVES AND HAS CONCLUDED THAT ALL EVALUATED AIRCRAFT COMPLY WITH THE CRITERIA, THE SAFETY BOARD REMAINS CONCERNED BECAUSE INDIVIDUAL MANUFACTURERS' CHIP-SHEARING SPECIFICATIONS DIFFER GREATLY. FOR INSTANCE, THE CHIP-SHEARING REQUIREMENTS FOR A DC-9 SERVO VALVE ARE MUCH GREATER THAN THOSE FOR A BOEING 737. THEREFORE, THE SAFETY BOARD WOULD APPRECIATE RECEIVING A REPORT OF THE EVALUATION SO THAT WE MAY VERIFY HOW ALL TRANSPORT-CATEGORY AIRCRAFT THAT WERE EVALUATED WERE FOUND TO HAVE ADEQUATE CHIP-SHEARING CAPACITY GIVEN THE WIDE VARIATION IN CHIP-SHEARING REQUIREMENTS. PENDING THE BOARD'S RECEIPT OF SUCH A REPORT, A-96-114 IS CLASSIFIED "OPEN--ACCEPTABLE RESPONSE."

From: FAA
To: NTSB
Date: 1/14/2000
Response: Letter Mail Controlled 01/20/2000 1:02:33 PM MC# 2000093 THE FAA'S AIRCRAFT CERTIFICATION OFFICES HAVE EVALUATED THE ADEQUACY OF THE CHIP SHEARING CAPACITY OF SLIDING SPOOL CONTROL VALVES FOR CERTAIN AIRPLANES. I HAVE ENCLOSED A COPY OF THE CRITERIA THAT WAS USED IN THE EVALUATION FOR THE BOARD'S INFORMATION. AS STATED IN THE FAA'S LETTER DATED 6/29/98, THE FAA CONCLUDED THAT ALL SLIDING SPOOL CONTROL VALVES USED IN THE FOLLOWING TRANSPORT-CATEGORY AIRPLANES FLIGHT CONTROL SYSTEMS MEET THESE EVALUATION CRITERIA: BOEING 707 (EXCEPT THE RUDDER), BOEING 727, BOEING 737, BOEING 747, BOEING 757, BOEING 767, AND BOEING 777; MCDONNELL DOUGLAS DC-9, DC-10, MD-80, MD-90, AND MD-11; LOCKHEED L-1011 AND L-382; GULFSTREAM V; SAAB 340 AND 2000; DORNIER DO-328; FOKKER F-28 (ALL MODELS); EMBRAER EMB-120 AND EMB-145; AND APPLICABLE MODELS FOR CESSNA, LEARJET, RAYTHEON, AND SABRELINER. CONSEQUENTLY, THE FAA PLANS NO FURTHER ACTION WITH THESE AIRPLANES. THE FAA IS WAITING FOR DATA FROM AEROSPATIALE. HOWEVER, SINCE THE FAA'S EVALUATION HAS DETERMINED THAT THE MAJORITY OF TRANSPORT-CATEGORY AIRPLANES MEET THE EVALUATION CRITERIA, I ASK THAT THE BOARD CLASSIFY THIS RECOMMENDATION IN A "CLOSED" STATUS. (SPECIAL NOTE: SINCE RECEIPT OF THIS LETTER, AEROSPATIALE HAD RETURNED THEIR EVALUATION AND IT HAS CONCLUDED THAT THEIR SYSTEMS DO NOT USE THIS TYPE OF CHIP SHEARING SLIDING SPOOL CONTROL VALVE, PER NOTE FROM J. LEACH, FAA).

From: NTSB
To: FAA
Date: 2/2/1999
Response: THE BOARD WOULD LIKE TO RECEIVE A COPY OF THE FAA'S EVALUATION CRITERIA FOR DETERMINING THE ADEQUACY OF THE CHIP SHEARING CAPACITY OF THE SLIDING SPOOL CONTROL VALVES. PENDING OUR VIEW OF THE CRITERIA AND THE FAA'S COMPLETION OF THE EVALUATION PROJECT, A-96-114 IS CLASSIFIED "OPEN--ACCEPTABLE RESPONSE."

From: FAA
To: NTSB
Date: 6/29/1998
Response: Letter Mail Controlled 7/7/98 3:54:45 PM MC# 980844 THE FAA AIRCRAFT CERTIFICATION OFFICES HAVE EVALUATED THE ADEQUACY OF THE CHIP SHEARING CAPACITY OF SLIDING SPOOL CONTROL VALVES FOR CERTAIN AIRPLANES. THE FOLLOWING CRITERIA USED IN THE EVALUATION WERE RECOMMENDED BY THE SOCIETY OF AUTOMOTIVE ENGINEERS (SAE) A-6 COMMITTEE AND THE INFORMATION FROM THE 8/12/97, BOEING COMMERCIAL AIRPLANE GROUP LETTER: FOR FLIGHT CRITICAL DESIGN APPLICATIONS, THE RECOMMENDED MINIMUM FORCE IS THE GREATER OF (1) 80 LBS., OR 50 LBS. IF VALVE JAM DETECTION AND CORRECTION IS INHERENT, OR (2) A FORCE BASED ON 75 PERCENT FILL [WITH CONTAMINATION] OF THE MAXIMUM METERING SLOT WITH 130 KSI STRESS MATERIAL. THE CRITERIA ALLOW ACCEPTABILITY TO BE SHOWN BY CONTROL SURFACE REDUNDANCY OR SYSTEM DESIGN FEATURES THAT MITIGATE THE EFFECT OF VALVE JAMS TO A NON-CATASTROPHIC CONDITION, AS WELL AS PERMIT CONTINUED SAFE FLIGHT AND LANDING, EVEN IF A VALVE JAM OCCURS. TO DATE, THE FAA CONCLUDED THAT ALL SLIDING SPOOL CONTROL VALVES USED IN THE FOLLOWING TRANSPORT-CATEGORY AIRPLANES FLIGHT CONTROL SYSTEMS MEET THESE EVALUATION CRITERIA: BOEING 707 (EXCEPT THE RUDDER), BOEING 727, BOEING 737, BOEING 747, BOEING 767, AND BOEING 777; MCDONNELL DOUGLAS DC-9, DC-10, MD-80, MD-90, AND MD-11; LOCKHEED L-1011 AND L-382; GULFSTREAM V; SAAB 340 AND 2000; DORNIER DO-328; FOKKER F-28 (ALL MODELS); EMBRAER EMB-120 AND EM13-145; AND APPLICABLE MODELS FOR CESSNA, LEARJET, RAYTHEON, AND SABRELINER. CONSEQUENTLY, THE FAA PLANS NO FURTHER ACTION WITH THESE AIRPLANES. THE FAA IS WAITING FOR DATA FROM EUROPEAN MANUFACTURERS, CANADIAN MANUFACTURERS, ADDITIONAL DATA FOR THE BOEING 707 RUDDER SYSTEM, AND DATA FOR THE DOUGLAS DC-8 AIRPLANES.

From: NTSB
To: FAA
Date: 7/15/1997
Response: A-96-14 ASKED THE FAA TO EVALUATE THE ADEQUACY OF THE CHIP SHEARING CAPABILITY FOR ALL SLIDING SPOOL CONTROL VALVES USED IN TRANSPORT-CATEGORY AIRCRAFT FLIGHT CONTROL SYSTEMS & TAKE APPROPRIATE ACTION TO CORRECT ANY PROBLEMS IDENTIFIED TO PRECLUDE POTENTIAL FOR ACTUATOR JAMMING, BINDING, OR FAILURE. PENDING FURTHER DISCUSSION ON THESE ISSUES & REVIEW OF THE FAA'S FINAL ACTION, A-96-114 & -116 ARE CLASSIFIED "OPEN--ACCEPTABLE RESPONSE."

From: FAA
To: NTSB
Date: 1/16/1997
Response: IN AUGUST 1995, THE FAA REQUESTED THE SOCIETY OF AUTOMOTIVE ENGINEER (SAE) A-6 COMMITTEE--AEROSPACE FLUID POWER, ACTUATION, & CONTROL TECHNOLOGIES--TO DEVELOP AEROSPACE STANDARDS FOR HYDRAULIC SYSTEM CLEANLINESS LEVELS, COMPONENTS SENSITIVITY, & CHIP SHEAR CAPABILITY. SPECIFICALLY, THE FAA REQUESTED THE DEVELOPMENT OF THE FOLLOWING FOR TRANSPORT CATEGORY AIRPLANES: MINIMUM ACCEPTABLE HYDRAULIC FLIGHT CONTROL SYSTEM & COMPONENT SENSITIVITY (JAM POTENTIAL & ACTUATOR PERFORMANCE) TO PARTICULATE & CHEMICAL CONTAMINATION, AS WELL AS METHODS TO DETERMINE THE SENSITIVITY. MAXIMUM ALLOWABLE PARTICULATE & CHEMICAL CONTAMINATION LEVELS OF HYDRAULIC FLUID. MINIMUM ACCEPTABLE ACTUATOR CHIP SHEAR CAPABILITY. IN OCTOBER 1995, THE SAE ESTABLISHED A HYDRAULIC FLUID CONTAMINATION TASK FORCE CONSISTING OF ONE FAA REPRESENTATIVE & SEVERAL REPRESENTATIVES FROM THE AIRLINES, FLUID SUPPLIERS, AIRCRAFT MANUFACTURERS, ACTUATOR/SERVO VALVE MANUFACTURERS, PUMP MANUFACTURERS, & FILTER MANUFACTURERS. SEVEN SUBTASK GROUPS WERE IDENTIFIED TO WORK ON THE FOLLOWING ISSUES: (1) JAMS DUE TO PARTICLE CONTAMINATION; (2) JAMS DUE TO CHEMICAL CONTAMINATION; (3) PERFORMANCE DEGRADATION DUE TO PARTICLE CONTAMINATION; (4) PERFORMANCE DEGRADATION DUE TO CHEMICAL CONTAMINATION'; (5) SENSITIVITY TO PARTICLE CONTAMINATION; (6) SENSITIVITY TO CHEMICAL CONTAMINATION; & (7) CHIP SHEAR FORCE LEVELS & THEIR RELEVANCE TO SERVO VALVES/ACTUATORS. THE SAE TASK FORCE PLANS TO SUBMIT ITS RECOMMENDATIONS TO THE FAA IN THE JANUARY-FEBRUARY 1997 TIMEFRAME. THE FAA WILL REVIEW THE SAE RECOMMENDATIONS & TAKE APPROPRIATE ACTION. I WOULD ASK THAT THE BOARD PARTICIPATE IN THE VARIOUS SAE ACTIVITIES SO THAT THE BOARD'S POSITIONS ARE PROPERLY REFLECTED IN THE OVERALL EFFORT. IN ADDITION TO THIS ACTIVITY, THE FAA WILL CLOSELY REVIEW THE CHIP SHEAR CAPABILITY OF THE NEWLY DESIGNED RUDDER PCU SLIDE VALVES DURING THE TYPE CERTIFICATION WITH AN OBJECTIVE OF IMPROVING THE CHIP SHEAR CAPABILITY OF THE NEW DESIGN. I WILL KEEP THE BOARD APPRISED OF THE FAA'S PROGRESS ON THESE RECOMMENDATIONS.