In response to the identification of hydrogen-embrittled self-locking nuts, CASA issued airworthiness bulletin (AWB) 14-002 on 12 October 2011 alerting aircraft owners, operators and maintenance personnel to the possibility of in-situ failures of MS 21042 and NAS 1291-series self-locking nuts. The AWB provided background information on previous occurrences and the mechanism and hazards associated with hydrogen embrittlement, and recommended that:
(a) Pilots and maintenance personnel closely monitor the occurrence of hydrogen-induced delayed cracking in high-strength steel standard aircraft hardware, such as nuts via close inspection following installation and thereafter at Daily / Preflight and periodic inspections.
(b) Before simply replacing cracked/failed nuts with new items, consider contacting the manufacturer for advice regarding replacement of associated fasteners which may have suffered over-loading as a result of the failure of one or more nuts.
(c) Report all MS 21042 and NAS 1291 series nut failures to CASA via the SDR [Service Difficulty Reporting] system.
The ATSB is satisfied that the action taken by CASA adequately addresses the safety issue.