Flight History
On April 7, 2016, an AgustaWestland AW139 helicopter, registration N639NA, operated by Chevron USA Inc. under Part 91, experienced an in-flight anomaly. The flight originated at 0600 from South Lafourche Leonard Miller Jr. Airport (GOA) in Galliano, Louisiana, with an en route stop at an oil rig in the Gulf of Mexico, and returned to GOA around 1135. Visual meteorological conditions prevailed, and a company VFR flight plan was filed. The crew reported a slight tail rotor vibration during the flight.
Damage and Inspection
Post-flight inspection revealed that one tail rotor blade (TRB) erosion shield extension, commonly known as a lightning strip (LS), was missing except for its side tabs. Additionally, one main rotor blade (MRB) tip sustained damage, primarily on the underside. No other damage was observed. The helicopter sustained substantial damage, but the pilot, co-pilot, and seven passengers were not injured.
The separated LS had part number 3G6410L00152 and was made of electroplated nickel. Chevron reported that the associated TRB was received new from Leonardo on May 22, 2015, without an LS installed; an LS was subsequently installed by Chevron without serial number tracking. On January 30, 2016, the white TRB was replaced due to wear on the main erosion shield from contact with the LS, and the same LS was reinstalled on the replacement TRB. From February 26 to April 6, 2016, the helicopter accumulated 129.1 flight hours with 34 daily serviceability inspections revealing no LS discrepancies.
Metallurgical Analysis
Leonardo performed a failure analysis on the remaining LS portion using a scanning electron microscope (SEM). The analysis revealed microcavity spots (porosity) on the fracture surface, attributed to the electroplating manufacturing process. Fatigue crack initiation occurred at a microcavity, and crack propagation was observed. Finite element modeling (FEM) of an exemplar LS indicated that maximum stress values in the crack initiation area were considerably lower than the fatigue limit for the material, but the model did not account for material flaws. Leonardo concluded that the presence of a microcavity could adversely influence in-flight stresses and reduce LS strength. Flight testing with strain measurement instrumentation did not identify any particular load condition that could lead to similar failures.
Related Events and Corrective Actions
Leonardo identified six additional events involving the same LS part number, including three scenarios: LS crack found during inspections, LS fractured and separated in flight, and LS separation causing MRB tip damage (two events including this accident). All resulted in uneventful landings. In response, Leonardo changed the LS specification from electroplated nickel to higher-strength steel, updating the TRB installation drawing for new helicopters effective July 31, 2017, and issuing an optional service bulletin for steel LS installation upon customer request.