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Safety Recommendation Details
SINCE MARCH 1998, THE NATIONAL TRANSPORTATION SAFETY BOARD HAS LEARNED OF SIX AIRPLANES ON WHICH FLIGHT CONTROL CABLE TERMINALS HAVE FRACTURED. ADDITIONAL TERMINALS FROM SOME OF THESE SIX AIRPLANES AND TERMINALS FROM FOUR OTHER AIRPLANES WERE ALSO EXAMINED AND FOUND TO BE CRACKED. NINE OF THESE 10 AIRPLANES WERE PIPER AIRCRAFT THAT HAD AT LEAST ONE FRACTURED OR CRACKED MS21260-S4 TERMINAL, ILLUSTRATED IN FIGURE 1 (ATTACHED). THE 10TH AIRPLANE WAS A CESSNA AIRCRAFT ON WHICH AN AN669 TERMINAL, THE PRECURSOR TO THE MS21260 TERMINAL, WAS FRACTURED. METALLURGICAL EXAMINATIONS CONDUCTED BY THE SAFETY BOARD DETERMINED THAT MOST OF THE TERMINALS FRACTURED OR CRACKED THROUGH THE SHAFT ON THE THREADED END OF THE TERMINAL, CLOSE TO THE WRENCH FLATS. A FEW OF THE TERMINALS CONTAINED CRACKING IN THE SWAGED END. THE EXAMINATIONS IDENTIFIED CHLORIDE STRESS-CORROSION CRACKING, WHICH HAD ORIGINATED AT GENERAL CORROSION PITS ON THE SURFACE OF THE PART, TO BE THE FRACTURE MODE OVER MOST OF THE FRACTURE SURFACE OF THE TERMINALS.
THE NTSB RECOMMENDS THAT THE FAA: ISSUE APPROPRIATE AIRWORTHINESS DIRECTIVES, APPLICABLE TO PIPER PA-28, PIPER PA-44, AND CESSNA 172 SERIES AIRPLANES OLDER THAN 15 YEARS, TO REQUIRE RECURRENT VISUAL INSPECTIONS, ON AN APPROPRIATE, CALENDAR-BASED INTERVAL, FOR EVIDENCE OF CORROSION PITS OR CRACKING ON CONTROL CABLE TERMINALS THAT WERE OR MAY HAVE BEEN CONSTRUCTED FROM SAE-AISI 303 SE STAINLESS STEEL. THE INSPECTIONS SHOULD BE PERFORMED AFTER REMOVAL OF ANY SAFETY WIRE OR SAFETY CLIPS ON THE TERMINALS. REQUIRE THAT ANY TERMINALS WITH SIGNS OR CORROSION OR CRACKING BE REMOVED FROM SERVICE IMMEDIATELY.
Original recommendation transmittal letter:
Closed - Acceptable Alternate Action
Addressee(s) and Addressee Status:
FAA (Closed - Acceptable Alternate Action)
General Aviation, Maintenance
Safety Recommendation History
Notation 8437: The National Transportation Safety Board (NTSB) has reviewed the Federal Aviation Administration’s (FAA) notice of proposed rulemaking (NPRM), titled “Airworthiness Directive; Piper Aircraft, Inc. Airplanes,” which was published at 76 Federal Register 45979 on August 2, 2012. The NPRM proposes to adopt a new airworthiness directive (AD) applicable to Piper models PA-28, PA-32, PA-34, and PA-44 series airplanes that would require inspecting the horizontal stabilator control system for damage to control cables and turnbuckles and replacing parts as necessary to prevent failure of the stabilator control system. This proposed AD was prompted by reports of control cable assembly failures that may lead to failure of the horizontal stabilator control system and could result in loss of pitch control. The NTSB has previously recommended the proposed action. Between March 1998 and April 2001, the NTSB learned of six airplanes on which flight control cable terminals had fractured. Terminals from four other airplanes were also examined and found to be cracked. Nine of these 10 airplanes were Piper aircraft that had at least one fractured or cracked terminal in the stabilator control system, and three of the fractures occurred in flight. As a result, on April 16, 2001, the NTSB issued Safety Recommendations A-01-6 and -7: Issue appropriate airworthiness directives, applicable to Piper PA-28, Piper PA 44, and Cessna 172 series airplanes older than 15 years, to require recurrent visual inspections, on an appropriate, calendar-based interval, for evidence of corrosion pits or cracking on control cable terminals that were or may have been constructed from SAE-AISI 303 Se stainless steel. The inspections should be performed after removal of any safety wire or safety clips on the terminals. Require that any terminals with signs of corrosion or cracking be removed from service immediately. (A-01-6) Determine which currently certificated aircraft models are authorized to have control cable terminals made from SAE-AISI 303 Se stainless steel, evaluate the need to require recurrent visual inspection, on an appropriate, calendar-based interval, of the terminals on these aircraft, and require such inspections if needed. (A-01-7) In a January 30, 2002, response to these recommendations, the FAA indicated that it found 18 reported cases of corrosion in the terminals of Piper PA-12, -14, -18, -28 and -28R airplanes in a fleet of 14,564. For the Cessna 172, the FAA found 11 reported cases of broken/frayed cables in a fleet of 24,925 aircraft. All aircraft certification offices also confirmed with their respective aircraft manufacturers that most aircraft use SAE-AISI 303 Se stainless steel terminals and they are an industry standard design. Based on the results of its evaluation, the FAA did not believe that action by way of an AD was warranted at that time. On November 1, 2001, the FAA issued Special Airworthiness Information Bulletin (SAIB) CE-02-05, which recommended that owners or operators of rotary and fixed-wing aircraft inspect the flight control cables at 100-flight-hour intervals or at each annual inspection and, if corrosion or pitting is found, that the control cable attach fittings be replaced, even if the manufacturer’s maintenance manual does not recommend replacement of corroded fittings. The FAA also determined that no additional inspections were needed, as recommended in Safety Recommendation A-01-7. On May 17, 2002, the NTSB acknowledged that, although the FAA did not issue an AD as recommended in Safety Recommendation A-01-6, the FAA’s issuance of the SAIB was a positive action that satisfied the recommendation’s intent in an alternative manner; consequently, Safety Recommendation A-01-6 was classified “Closed—Acceptable Alternate Action,” and Safety Recommendation A-01-7 was classified “Closed—Acceptable Action.” Within the past 2 years, the NTSB has investigated two accidents and one incident involving Piper aircraft where control cable assembly failures due to stress corrosion cracking led to failures of the horizontal stabilator control system. That these events continue to occur more than 10 years after the SAIB was issued shows that the SAIB was not effective. The NTSB considers the potential loss of a primary flight control a significant safety hazard that warrants mandatory action as proposed in the NPRM and fully supports the proposed AD. The NTSB appreciates the opportunity to comment on this NPRM.
The Safety Board has reviewed the FAA's actions regarding this safety recommendation, and although the FAA has not issued an appropriate AD as recommended, the Board does appreciate the FAA's reasoning process in determining its final action. The FAA notes that aircraft manufacturers, type flying clubs, and the Aircraft Owners and Pilots Association (AOPA) reported that the number of failures that occurred is insufficient to validate the need for an AD. The AOPA suggested that the FAA issue an Alert Service Bulletin or a Special Airworthiness Information Bulletin (SAIB) as an alternate course of action. The FAA concurred and issued an SAIB on November 1, 2001. The Board acknowledges the positive action of issuing an SAIB concerning these instances. Accordingly, Safety Recommendation A-01-6 is classified "Closed--Acceptable Alternate Action."
Letter Mail Controlled 02/07/2002 9:50:55 AM MC# 2020126 The Federal Aviation Administration (FAA) has received the information from aircraft manufacturers and type clubs on their review of this safety recommendation. All aircraft manufacturers have confirmed that most aircraft use SAE AISI 303 Se stainless steel material for flight control systems. The review revealed factors like water, exhaust gases, dissimilar metals, and battery off-gasses may contribute to corrosion problems. The review found no evidence of corrosion or pitting in the aircraft fleet, but found corrosion in isolated specific airframe applications in some Piper models where the battery is located very close to the area in which control cables enter the bottom of the fuselage. The Piper PA-12, -14, -18, -28 and -28R have batteries installed in the tail cone and have reported 18 cases of corrosion in a fleet of 14,564 aircraft. There were no other reports of cable problems on any other Piper models. Cessna 172 reported 11 cases of broken/frayed cables in a fleet of 24,925 aircraft. All aircraft manufacturers mandate an inspection of control cable attach fittings at annual or l00-hour inspection intervals. None of the occurrences resulted in any serious accidents, and most of the occurrences were discovered during maintenance activity. Based on the results of the evaluation, the FAA does not believe airworthiness directive action is warranted. The Aircraft Owners and Pilots Association (AOPA), based on input from its aircraft owners/operators, suggested that the FAA issue an Alert Service Bulletin or a Special Airworthiness Information Bulletin (SAIB) as an alternate solution. The FAA concurs with AOPA and on November 1, 2001, issued SAIB CE-02-05 recommending that owners or operators of rotary and fixed-wing aircraft inspect the flight control cables at 100 flight hour intervals or at each annual inspection. If corrosion is found, it is recommended that the control cable attach fittings be replaced even if the manufacturer's maintenance manual does not recommend replacement of corroded fittings. I have enclosed a copy of the SAIB for the Board's information. I consider the FAA's action to be completed on this safety recommendation.
The Safety Board is pleased with the FAA's rapid response to this safety recommendation and, pending the FAA's recommended actions following analysis of responses from manufacturers and flying clubs, Safety Recommendation A-01-006 is classified "Open--Acceptable Response."
Letter Mail Controlled 06/08/2001 11:54:36 AM MC# 2010473 The Federal Aviation Administration (FAA) has requested The New Piper Aircraft, Inc., and the Cessna Aircraft Company to investigate the areas outlined in this safety recommendation and to inform the FAA of their proposed actions. The FAA is also seeking input from the affected type clubs. The FAA's Small Airplane Directorate will develop an appropriate course of action to address this safety recommendation after it has evaluated the information provided by the aircraft manufacturers and the type clubs. I will keep the Board informed of the FAA's progress on this safety recommendation.
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