Recent Failures

The European Aviation Safety Agency (EASA) has issued a number of Emergency Airworthiness Directives (EADs) along with a Safety Information Bulletin regrading the use of MS21042 & NAS1291 nuts. The conclusion of many of these EADs is that MS21042 nuts failed due to hydrogen embrittlement that was introduced during heat treat or plating.

However, the reported failure rate of MS21042 nuts is minor as compared to quantity of nuts manufactured and used on a yearly basis. Hydrogen Embrittlement (HE) typically manifests within 24 hours of installation. Parts that are affected by HE have a high failure in excess of 30% which is not reflected in current findings. Furthermore, many of these nuts were used in critical applications which is in violation of MS33588.

EASA Reports

EASA_EAD 9/20/2013

EASA stated that a technical investigation reported that cracks in a MS21042L4 nut connecting a Thomas coupling with the splined adapter on AW100SP helicopters were believed to have been caused by hydrogen embrittlement.

EASA_EAD 10/30/2013

EASA stated that two MS21042-4 nuts connected to the swashplate support to the upper case of the main transmission of a A190S helicopter were found cracked. They stated the nuts in use were deficient with evidence of hydrogen embrittlement.

EASA_EAD 5/02/2014

Observed a cracked MS21042L4 which connected a flexible coupling with the coupling adapter on a AB 206 helicopter. EASA stated the deficiency was due to hydrogen embrittlement.

EASA_EAD 5/07/2014

EASA expanded the EAD above to include AB 212, AB 412, and AB 412EP helicopters.

EASA_EAD 5/13/2014

EASA expanded the EAD to include MS21042-5 nuts found on the flexible coupling. Pictures are included showing the location of the cracked nuts.

FAA bulletin & AD

FAA Airworthiness Bulletin

The Federal Aviation Administration (FAA), released a special airworthiness information bulletin on April 28, 2014.

Airworthiness Directive

In response to EASA's EADs starting in October of 2013, the FAA has issued an Airworthiness Directive stating that all MS21042-4 nuts, when connected to the swashplate support to the upper case of the main transmission should be replaced within 25 hours of time in service. The airworthiness directive prohibits the installation of MS21042-4 nuts in the upper case of the main transmission on any helicopter.