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ASN Wikibase Occurrence # 226868
Last updated: 1 February 2021
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Date:07-APR-2016
Time:
Type:Silhouette image of generic A139 model; specific model in this crash may look slightly different
AgustaWestland AW139
Owner/operator:Chevron USA Inc
Registration: N639NA
C/n / msn: 41326
Fatalities:Fatalities: 0 / Occupants: 9
Other fatalities:0
Aircraft damage: Substantial
Category:Accident
Location:Galliano, LA -   United States of America
Phase: En route
Nature:Offshore
Departure airport:Galliano, LA (GAO)
Destination airport:Galliano, LA (GAO)
Investigating agency: NTSB
Narrative:
During a business flight over the Gulf of Mexico, the crew felt a slight vibration of the tail rotor with no other anomalies noted and landed the helicopter uneventfully at their planned destination. A postflight inspection revealed that one of the tail rotor blade erosion shield extensions, also known as a lightning strip (LS), was missing and that only its side tabs remained attached to the blade. One of the main rotor blade tips sustained substantial damage when the LS separated from the tail rotor blade and impacted the bottom of the main rotor blade.

Metallurgical analysis of the fractured LS revealed microcavities in the metal, which resulted in a fatigue fracture. The helicopter manufacturer concluded that the microcavities were a direct result of the electroplating manufacturing process. Finite element modeling (FEM) of an exemplar LS revealed that the maximum stress values expected during flight were considerably lower than the fatigue limit specified for the electroplated nickel material. The microcavity observed on the accident LS could not be replicated by the FEM model, but the presence of a microcavity could adversely influence the in-flight stresses and reduce the overall strength of the LS. Additional flight testing with strain gauges did not reveal any load condition that would lead to similar LS failures that were experienced during the accident flight. It is likely the LS separated in flight due to a fatigue fracture that was the result of increased stress in the presence of a material flaw that had existed since its manufacture.

The helicopter manufacturer, in response to multiple reports of similar LS failures, changed the material specification from electroplated nickel to a higher strength steel. The steel LS will be installed on all newly built helicopters, and an optional service bulletin will be issued to operators to provide the steel LS upon request.

Probable Cause: The fatigue failure of a tail rotor blade erosion shield extension, which resulted in in-flight damage to a main rotor blade.

Sources:

NTSB: https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/97230/pdf
http://aerossurance.com/helicopters/trb-lightning-strip-aw139-mrb/

Accident investigation:
cover
  
Investigating agency: NTSB
Status: Investigation completed
Duration: 3 years and 3 months
Download report: Final report

Media:


Revision history:

Date/timeContributorUpdates
06-Jul-2019 19:37 ASN Update Bot Added
25-Dec-2020 15:20 harro Updated [Operator, Nature, Narrative]
01-Feb-2021 09:24 Aerossurance Updated [Source, Embed code, Accident report]
01-Feb-2021 09:24 Aerossurance Updated [Phase, Embed code, Accident report]

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