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Self-locking nut failure

Issue number: AO-2011-016-SI-01
Who it affects: Owners and operators of Robinson Helicopters
Issue owner: Robinson Helicopter Company
Operation affected: Aviation: General aviation
Background: Investigation Report AO-2011-016
Date: 30 April 2012

Safety issue description

A number of self-locking nuts from other aircraft, of the same specification as that used to secure safety-critical fasteners in VH-HFH, were identified to have cracked due to hydrogen embrittlement.

During the course of the investigation the ATSB was provided with three self-locking nuts from other aircraft that had cracked in service. Detailed examination of those nuts identified that they had failed due to hydrogen embrittlement. In response to that finding, the ATSB notified the helicopter manufacturer, the Civil Aviation Safety Authority (CASA) and the United States National Transportation Safety Board and Federal Aviation Administration.

Proactive Action

Action organisation: Robinson Helicopter Company
Action number: AO-2011-016-NSA-010
Date: 30 April 2012
Action status: Closed

In response to the identification of hydrogen-embrittled self-locking nuts during this investigation, the helicopter manufacturer issued service letters: SL-58, SL-38 and SL-01 that were applicable to the R22, R44 and R66 helicopter types respectively. These service letters detailed the hydrogen-embrittlement risk, including the expected failure characteristics. The service letters reminded pilots and maintenance personnel of the importance of serviceable hardware and advised that any cracked or corroded nuts should be replaced. The manufacturer also requested that they be advised of any identified instances of cracked locknuts.

ATSB response:

The ATSB is satisfied that the action taken by the helicopter manufacturer adequately addresses the safety issue.

Proactive Action

Action organisation: Civil Aviation Safety Authority
Action number: AO-2011-016-NSA-011
Date: 30 April 2012
Action status: Closed

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.

ATSB response:

The ATSB is satisfied that the action taken by CASA adequately addresses the safety issue.

   
Current issue status: Adequately addressed
Status justification:

Given the prolific number of these locknuts, and how widely dispersed they are throughout the aviation industry, the communication of the potential for embrittlement failure was considered the only feasible safety action that could be taken. Although considered to be ALARP, the risk of a failure of a critical fastener remains significant.

 
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Last update 17 July 2014