The Australian Transport Safety Bureau (ATSB) was requested by the Civil Aviation Safety Authority (CASA) to assist in the examination of a number of aircraft flight control cable terminal fittings, to identify if they had been affected by stress corrosion cracking (SCC). The terminals had been manufactured from SAE-AISI 303 Se stainless steel (part numbers AN669 and MS21260) and were installed on Piper, Cessna and Beech aircraft. SCC is an environmental failure mechanism resulting from a combination of a susceptible material, applied (or residual) stress and a corrosive environment.
What the ATSB found
While several of the 54 submitted cable terminals showed evidence of surface pitting corrosion, none revealed any evidence of SCC when examined visually and using non-destructive testing techniques.
During the course of the ATSB examinations, CASA received fractured cable terminals from a Piper PA32 and an amateur built aircraft; these fittings were also submitted for examination. The failure mechanism in both cases was confirmed as SCC that had initiated on the external terminal surfaces. The ATSB was also advised of the additional failure of a terminal that was investigated by the New Zealand Civil Aviation Authority. The failure mechanism was also confirmed as SCC, however the cracking had initiated from the internal surface of the swaged terminal sleeve where it was in contact with the wire cable.
What's been done as a result
A number of actions have been taken in Australia and internationally to address the issue of SCC in control cable terminals manufactured from SAE-AISI 303 Se. The latest CASA airworthiness bulletin (AWB 27-001 Revision 3) updated owners on the potential for SCC of flight control cable terminals and urged operators to consider replacing the cables before they reach 15 years in service. A recent Federal Aviation Administration Airworthiness Directive (AD-2013-02-13) required the inspection of the stabilator control system on certain Piper aircraft and replacement of parts as necessary. CASA has also initiated a project that is seeking to amend Civil Aviation Order 100.5, ‘General requirements in respect of maintenance of Australian Aircraft’, to mandate a recurring inspection of terminals manufactured from SAE-AISI 303 Se which have a total time in service of 15 years or greater.
The ATSB encourages owners, operators and maintainers of aircraft that may be fitted with cable terminals with part numbers AN669 and MS21260, to familiarise themselves with the issues surrounding terminal fitting corrosion and the associated risks to continued airworthiness. Personnel should familiarise themselves with CASA Airworthiness bulletin, AWB 27-001 and the US National Transportation Safety Board Safety Recommendations A-01-6 through -8 released on April 16, 2001. Both documents highlight the risk to continued safe operation associated with SCC of flight control cable terminals and provide a comprehensive background on the failure mechanism and experiences. The ATSB urges operators to consider replacement of the cables in line with the regulators guidelines, as experience has shown that inspection alone is not a complete defence against SCC failures.