# NTSB CEN16LA401: Agustawestland Philadelphia AW139, N639NA, Galliano, LA, April 7, 2016 - Type: NTSB accident investigation - Classification: Accident · No injuries · Limited investigation · Final report - Date: April 7, 2016 - Place: Galliano, LA - Coordinates: 29.438055, -90.262496 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=29.4381&lon=-90.2625&mi=10 - Phase of flight: [Enroute](https://openairdocs.org/ntsb-phases/400) (OpenAirDocs' grouping) - Defining event: [Enroute: Part(s) separation from AC](https://openairdocs.org/ntsb-events/338) - Highest injury: No injuries - Injuries: 9 uninjured - Status: Completed, report June 25, 2019 - Report: Approved by Office Director - NTSB office: Central Region - Parties: Federal Aviation Administration, Leonardo Helicopters, Italian Safety Investigation Authority, ANSV - NTSB case key (mKey): 97230 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=CEN16LA401 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/97230/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=97230 - Page: https://openairdocs.org/ntsb/CEN16LA401 ## Probable cause The fatigue failure of a tail rotor blade erosion shield extension, which resulted in in-flight damage to a main rotor blade. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Tail rotor](https://openairdocs.org/ntsb-findings/01046400): Aircraft › Aircraft propeller/rotor · Fatigue/wear/corrosion (probable cause); cases with it: [Agustawestland Philadelphia AW139, 1 case](https://openairdocs.org/ntsb-findings/01046400/cases.md?model=0260400), [All aircraft, 23 cases](https://openairdocs.org/ntsb-findings/01046400/cases.md) - [Manufacture/production: equipment manufacture](https://openairdocs.org/ntsb-findings/04013010): Organizational issues › Development · Manufacturer; cases with it: [Agustawestland Philadelphia AW139, 2 cases](https://openairdocs.org/ntsb-findings/04013010/cases.md?model=0260400), [All aircraft, 36 cases](https://openairdocs.org/ntsb-findings/04013010/cases.md) ## Sequence of events The events NTSB coded, in order, by phase of flight. The defining event best describes what happened. - [Enroute: Part(s) separation from AC](https://openairdocs.org/ntsb-events/338) (defining event); cases with it: [Agustawestland Philadelphia AW139, 1 case](https://openairdocs.org/ntsb-events/338/sequence.md?model=0260400), [All aircraft, 563 cases](https://openairdocs.org/ntsb-events/338/sequence.md) ## Analysis 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. ## Factual narrative HISTORY OF FLIGHT On April 7, 2016, at an unknown time, an AgustaWestland AW139 helicopter, N639NA, sustained damage to a main rotor blade during a flight over the Gulf of Mexico. The pilot, co-pilot, and 7 passengers were not injured, and the helicopter sustained substantial damage. The helicopter was registered to and operated by Chevron USA Inc., under the provisions of Title 14 Code of Federal Regulations Part 91 as business flight. Visual meteorological conditions prevailed for the flight and a company visual flight rules (VFR) flight plan had been filed. The flight originated at 0600 from South Lafourche Leonard Miller Jr. Airport, (GOA), Galliano, Louisiana, with an en route stop at an oil rig in the Gulf of Mexico and terminated at GOA about 1135. On May 9, 2018, the Chevron aviation department reported the accident to the NTSB and stated that during the flight the crew felt a slight vibration of the tail rotor. After landing at GAO during a post flight inspection, one of the tail rotor blade (TRB) erosion shield extensions (figure 1) commonly known as a lightning strip (LS) was missing and only its side tabs remained attached to the blade. Additionally, a main rotor blade (MRB) tip was damaged. Figure 1 – TRB erosion shield extension diagram with notations (Courtesy of Leonardo) PERSONNEL INFORMATION AIRCRAFT INFORMATION The separated LS was identified as part number (p/n) 3G6410L00152 that was manufactured in electroplated nickel. Chevron reported that as a normal maintenance practice, an LS would be installed as a brand new part or as an "on-condition" item that had been installed at the discretion of the company maintenance personnel after the component was inspected and determined to be in airworthy condition. Chevron reported that the associated tail rotor blade (TRB) was received new from Leonardo on May 22, 2015, without an LS installed. An LS was subsequently installed by Chevron with no serial number tracking requirement. On January 30, 2016, the white TRB was removed and replaced due to wear on the main erosion shield from contact with the LS. The same LS was reinstalled on the replacement white TRB. From February 26, 2016, to April 6, 2016 the helicopter accumulated 129.1 flight hours with the new TRB installed. During that time, 34 daily serviceability checklist inspections were completed and no discrepancies applicable to the LS on the white TRB (or any other TRB) were noted. METEOROLOGICAL INFORMATION WRECKAGE AND IMPACT INFORMATION A Chevron post flight inspection of the helicopter revealed that the LS on the white TRB was fractured and most of the component was missing. Only the side tabs, where the tail rotor damper bracket bushings pass through, remained attached (figure 2). Figure 2 – Damaged Tail Rotor Erosion Shield Extension (Courtesy of Chevron) The inspection also found damage to one of the main rotor blades; the blue MRB tip sustained damage mostly on the under side of the blade (figure 3). No additional damagewas observed on the remainder of the helicopter. Figure 3 – Damaged Main Rotor Blade (Courtesy of Chevron) TESTS AND RESEARCH Failure Analysis Leonardo, the current AW139 helicopter manufacturer, performed a failure analysis on the remaining portion of the LS, using a scanning electron microscope (SEM), which revealed the presence of microcavity spots or porosity on the surface of the fractured section. Leonardo concluded that the microcavities were a direct result of the electroplating manufacturing process. In figure 4, the area circled in yellow is the microcavity, the red dashes indicate the crack initiation, and the blue arrows indicate the direction of the fatigue crack propagation. Figure 4 – SEM image of the fatigue propagation from a micro-cavity, yellow circle (Courtesy of Leonardo) Figure 5 shows a wide view of the fracture surface with the crack origination circled in red. Figure 5 – SEM image, wide view of fracture surface (Courtesy of Leonardo)Static Testing Finite element modeling (FEM) of an exemplar LS was completed to determine the expected inflight stress values, to include the influence of the centrifugal force and the bending moment due to the deformation of the TRB under beam bending loads. The simulation showed that the maximum stress values in the crack initiation area were considerably lower than the fatigue limit specified for the electroplated nickel material. Of note, the FEM model considered the geometric and fitting stress concentrations without the presence of a material flaw (such as a microcavity). Although the microcavity observed on the accident LS could not be replicated by the FEM model, Leonardo concluded that the presence of a microcavity could adversely influence the in-flight stresses and reduce the overall strength of the LS. Flight Testing A flight test was performed to evaluate the load conditions of the LS during flight. Strain measurement instrumentation was installed on a Leonardo helicopter to monitor and record the load conditions. Test results did not reveal any particular load condition that could lead to similar LS failures experienced during the accident flight. ADDITIONNAL INFORMATION Related Events Leonardo identified 6 additional events involving the same LS part number and the three scenarios are listed below. Of note, all three scenarios resulted in an uneventful landing. • LS crack identified during scheduled inspections (3 events) • LS fractured and separated in flight (2 events) • LS fractured and separated inflight, which resulted in damage to MRB tip (2 events, including the accident flight) Corrective Actions In response to the reports of similar LS failures, Leonardo changed the specification from electroplated nickel to a higher strength steel. As of July 31, 2017, for all newly built helicopters, the TRB installation drawing was updated to prescribe the installation of a steel LS, p/n 3G6410A03053, (figure 6). Additionally, Leonardo will issue an optional service bulletin toprovide the steel LS upon customer request. Figure 6 – Diagram of the updated steel LS (left) and electroplated nickel LS (right) ## Aircraft - Registration: [N639NA](https://openairdocs.org/aircraft/N639NA), registered now, S/N 41326, built 2013 (OpenAirDocs) - Reported make and model: AGUSTAWESTLAND PHILADELPHIA AW139 - Registry model: [Agustawestland Philadelphia AW139](https://openairdocs.org/model/agustawestland-philadelphia-aw139) (registry code 0260400) (OpenAirDocs) - Registry engine: [Pratt & Whitney Canada PT6C-67C](https://openairdocs.org/engine/pratt-whitney-canada-pt6c-67c) (registry code 52174, turbo-shaft) (OpenAirDocs) - Category: Helicopter - Serial number: 41326 - Year built: 2013 - Airframe hours: 2497.8 - Engines: P&W CANADA PT6C-67C 1531 horsepower turbo shaft; P&W CANADA PT6C-67C 1531 horsepower turbo shaft - Damage: Substantial damage - Flight: Other work use, Part 91 - Operator: Chevron USA Inc - Registered owner: Chevron USA Inc - Airworthiness certificate: Standard, Transport - Last inspection: Continuous airworthiness, April 6, 2016 - Departed: GAO, GALLIANO, LA - Destination: GAO, Galliano, LA - Flight plan: Company VFR - Airport: GAO, South Lafourche Leonard Miller - Equipment: Angle of attack indicator, ADS-B, Onboard weather, Handheld GPS, Electronic primary flight display, Electronic multifunction display, Data recorder, Auto-pilot ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 52; injuries: none; airline transport; instrument ratings: helicopter; medical: class 1, without waivers/limitations; last flight review October 31, 2015; seat: right; restraint: 4-point; flight hours: 14,500 total, 1,250 in this make and model, 12,000 as pilot in command, 125 in the last 90 days, 35 in the last 30 days, 6 in the last 24 hours - Co-pilot; age 35; injuries: none; airline transport; airplane ratings: single-engine land; medical: class 1, without waivers/limitations; last flight review October 31, 2015; seat: left; restraint: 4-point; flight hours: 5,792 total, 1,813 in this make and model, 2,866 as pilot in command, 114 in the last 90 days, 32 in the last 30 days, 6 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: 340° at 7 knots - Visibility: 10 statute miles - Temperature: 72 °F, dew point 54 °F - Altimeter: 30.03 inHg - Observed at: KXPY, 19 nm away ## Docket (7 files) The files NTSB released with the investigation on April 11, 2019, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=97230. 5.3 MB in all. - Operator Aircraft Accident Report, NTSB Form 6120.1 (Chevron) (item 1, .PDF, 3.7 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40476742&FileExtension=.PDF&FileName=Operator%20Aircraft%20Accident%20Report,%20NTSB%20Form%206120.1%20(Chevron)-Master.PDF - Chevron Initial Examination Report (item 2, .PDF, 355 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40471847&FileExtension=.PDF&FileName=Chevron%20Initial%20Examination%20Report-Master.PDF - Chevron Root Cause Analysis (item 3, .PDF, 48 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40471848&FileExtension=.PDF&FileName=Chevron%20Root%20Cause%20Analysis-Master.PDF - Leonardo Report Summary (item 4, .PDF, 68 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40472571&FileExtension=.PDF&FileName=Leonardo%20Report%20Summary-Master.PDF - Scanning Electron Microscope Images (Courtesy of Leonardo) (item 5, .PDF, 2 photos, 219 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40472572&FileExtension=.PDF&FileName=Scanning%20Electron%20Microscope%20Images%20(Courtesy%20of%20Leonardo)-Master.PDF - FAA Service Difficulty Reports (SDR) (item 6, .PDF, 123 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40471849&FileExtension=.PDF&FileName=FAA%20Service%20Difficulty%20Reports%20(SDR)-Master.PDF - Helicopter Accident/Incident Supplemental Checklist (item 7, .PDF, 824 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40471850&FileExtension=.PDF&FileName=Helicopter%20Accident%2FIncident%20Supplemental%20Checklist-Master.PDF From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model. --- OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain.