NTSB · Safety Recommendation
Safety Recommendation A-01-006
Recommendation
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.
Safety Issue and Context
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.
Response History
September 17, 2012 · NTSB Correspondence · FAA
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.