Penobscot, ME, USA
N901EM
Agusta A109
The main rotor blade tip cap leading edge fractures initiated due to fatigue cracking with multiple initiation sites. At the time of the incident, the possibility of fatigue cracking of the leading edge of the P/N 709-0103-29-109 tip cap was a known issue, and Leonardo technical bulletin (bollettino tecnico, or BT) No. 109EP-85 and Federal Aviation Administration (FAA) airworthiness directive (AD) 2007-26-52 were previously issued to inspect for incipient cracks before they grew large enough to result in an inflight separation of the tip cap. These inspections primarily relied on visual inspection of the tip cap leading edge using magnification, and only required dye penetrant inspection of the leading edge if there are doubts about the presence of cracks or if damage other than a crack was found. It is possible that the fatigue crack on tip cap serial number L206 was present during its last inspection, but the visual inspection did not detect the presence of this crack. The crack on the tip cap leading edge remained undetected and propagated until the incident flight, when aerodynamic forces led to partial separation of the upper skin from the lower skin and core. While a visual inspection method can detect surface cracks, variances in the inspection environment (such as lighting), inspector experience, and operators’ maintenance practices can affect the success in detecting surface damage, such as a crack. After the initial issuance of BT No. 109EP-85 and FAA AD 2007-26-52, there continued to be a recurrence of partial and complete inflight separations of tip caps, with at least six known tip cap failures due to fatigue cracking. This indicates that reliance primarily on a visual inspection was not sufficient to eliminate the issue of detecting fatigue cracks before they resulted in a failure of the tip cap. A revision to BT No. 109EP-85 required dye penetrant inspection of the tip caps, which should provide a more reliable inspection method for identifying surface cracks.
HISTORY OF FLIGHT On November 16, 2019, about 1845 eastern standard time, a Leonardo A109E helicopter, N901EM, experienced a severe inflight vibration and subsequently performed an emergency landing near Penobscot, Maine. The helicopter was registered to LifeFlight of Maine LLC and operated by SevenBar Aviation. The flight was conducted under the provisions of 14 Code of Federal Regulations Part 135 as a helicopter air ambulance that departed Blue Hill Memorial Hospital, Blue Hill, Maine and was en route to Eastern Maine Medical Center, Bangor, Maine after loading a patient. About 5 minutes into the flight, the pilot reported feeling a “small vibration” on the flight controls, fuselage, and his seat. The vibration was characterized as a lateral shake. The pilot reduced collective and slowed the helicopter airspeed, and discussed the vibration with the crewmembers on board. The vibration subsequently increased in severity, prompting the pilot to immediately execute a precautionary landing. The helicopter landed successfully with no reported injuries to the four occupants on board. DAMAGE TO THE HELICOPTER A postlanding examination of the helicopter revealed one of the four main rotor blade tip caps exhibited a fracture along its leading edge, about 18 inches in length, and the upper skin had separated from the bulk of the core and lower skin (Photo 1). This main rotor blade was identified with part number (P/N) 709-0103-01-109 and serial number (S/N) L265. A subsequent examination found a second main rotor blade tip cap exhibited a crack, about 1.25 inches in length, along its leading edge (Photo 2). The second main rotor blade was identified with P/N 709-0103-01-109 and S/N L251. These two main rotor blades were removed from the helicopter and shipped to Leonardo facilities in Broussard, Louisiana for further examination. Photo 1. The tip cap crack found on main rotor blade S/N L265 (tip cap S/N L206). Photo 2. The red arrows point to a crack found on the tip cap of main rotor blade S/N L251 (tip cap S/N L232), with a US Quarter adjacent to the crack for comparative size reference. A black marker was used to draw chordwise lines to indicate the ends of the crack. TEST AND RESEARCH On January 7, 2020, representatives from the National Transportation Safety Board (NTSB), Leonardo Helicopter Division (LHD), and SevenBar Aviation convened at Leonardo facilities in Broussard, Louisiana to examine the two main rotor blades retained from the incident helicopter. The main rotor blades did not exhibit anomalous damage other than the cracks on their tip caps. On main rotor blade S/N L265, the tip cap was identified with P/N 709-0103-29-109 and S/N L206. On main rotor blade S/N L251, the tip cap was identified with P/N 709-0103-29-109 and S/N L232. The tip caps of the two main rotor blades, S/Ns L251 and L265, were removed by Leonardo technicians and subsequently shipped to the NTSB Materials Laboratory in Washington, District of Columbia (DC) for further examination. Main rotor blade tip cap P/N 709-0103-29-109 is constructed of 2 preformed steel shells (the tip cap upper and lower skins) bonded to an aluminum honeycomb core. The skins are welded at the leading edge and bonded at the trailing edge. The assembled tip cap is bonded to its parent main rotor blade and secured to the blade spar via screws. Metallurgical examination of the 18-inch fracture on tip cap S/N L206 (from main rotor blade S/N L265) revealed the fracture was adjacent to the weld along the lip of the leading edge. The fracture exhibited signatures consistent with multiple initiation fatigue cracking. The fatigue cracking emanated from the internal edge of the lower surface at the toe of the weld. Examination of the weld found no evidence of defects. Metallurgical examination of the 1.25-inch fracture from tip cap S/N L232 (from main rotor blade S/N L251) revealed characteristics similar to the fracture on tip cap S/N L206. The material composition of both tip caps were consistent with manufacturer specifications. Additional details of the metallurgical examination of both tip caps can be found in Materials Laboratory Factual Report No. 20-044 in the docket for this investigation. ADDITIONAL INFORMATION N901EM Main Rotor Blade History The incident helicopter, airframe S/N 11614, had both incident main rotor blades, S/Ns L251 and L265, installed new in July 2004 at the time of the helicopter’s manufacture. They remained installed until October 18, 2012, when both main rotor blades were removed at an ATT of 5,527.9 hours to facilitate erosion repairs on the blades; both main rotor blades had a component total time and a time since overhaul of 5,527.9 hours. Both main rotor blades were subsequently installed back onto airframe S/N 11614 on October 18, 2012 at an ATT of 5,527.9 hours. At the time of the incident, airframe S/N 11614 had an ATT of 10,078.4 hours and both main rotor blades, S/Ns L251 and L265, had component total time of 10,078.4 hours. A109E Main Rotor Blade Tip Cap Inspections At the time of the incident, a main rotor blade tip cap inspection was required every 50-hours and could be found in three different documents. First, Federal Aviation Administration (FAA) Airworthiness Directive (AD) 2007-26-52, dated May 9, 2008, required a recurrent inspection, at an interval not to exceed 50 hours, of main rotor blade tip cap P/N 709-0103-29-109. The AD applied to the Leonardo A109C, A109E, and A109K2 helicopters. The AD required a visual inspection of the tip cap leading edge for a crack, using a 10x or higher power magnifying glass, in accordance with LHD technical bulletin (bollettino tecnico, or BT) No. 109EP-85 for the A109E helicopter. If damage other than a crack is observed, the AD required inspection of the tip cap leading edge using a dye penetrant method in accordance with the BT. Lastly, if a crack was present, the AD required the blade’s removal and replacement with an airworthy blade. In addition to the 50-hour recurrent inspection for P/N 709-0103-29-109 tip caps, FAA AD 2007-26-52 also required a 25-hour recurrent inspection, different than the 50-hour recurrent inspection, for certain main rotor blades without tip cap P/N 709-0103-29-109 installed. For the following discussion on A109E main rotor blade tip cap inspections, the focus will only be on main rotor blades equipped with tip cap P/N 709-0103-29-109 that are subject to the 50-hour inspection interval. Second, and similar to FAA AD 2007-26-52, LHD BT No. 109EP-85 prescribed a 50-hour inspection for main rotor blade tip cap P/N 709-0109-29-109 and a visual inspection of the tip cap leading edge for a crack, using a 10x power magnifying glass. The BT stated that “if there are doubts on the presence of cracks” then an inspection of the tip cap leading edge using a dye penetrant method is required. If a crack was present, the BT required the main rotor blade be removed from service. Lastly, A109E Maintenance Manual Section 05-50, titled Special Inspections, required a 50-hour recurrent inspection of the main rotor blade tip cap for the presence of cracks in accordance with Paragraph 62-11-28A. Paragraph 62-11-28A contained inspection instructions similar to BT No. 109EP-85. Main Rotor Blade Tip Cap Inspection History on N901EM At the time of the incident, N901EM had accumulated an ATT of 10,078.4 hours and started the day with an ATT of 10,077.2 hours. The incident helicopter was equipped with two main rotor blades with tip cap P/N 709-0103-29-109, those examined in this investigation, which were required to be inspected at an interval not to exceed 50 hours in accordance with AD 2007-26-52. The incident helicopter was also equipped with two additional main rotor blades that did not have tip cap P/N 709-0103-29-109, which were required to be inspected at an interval not to exceed 25 hours in accordance with AD 2007-26-52. For the following discussion on the main rotor blade tip cap inspection history on N901EM, the focus will only be on main rotor blades equipped with tip cap P/N 709-0103-29-109 that are subject to the 50-hour inspection interval. According to helicopter maintenance logbook, AD 2007-26-52 was last complied with on November 8, 2019, at an ATT of 10,065 hours. Additionally, a separate entry on the same day recorded a 50-hour main rotor blade tip cap inspection was performed. Therefore, about 13.4 hours had elapsed between the last 50-hour main rotor blade tip cap inspection and the start of the day of the incident flight. According to the mechanic who performed the last 50-hour main rotor blade tip cap inspection, the inspection was performed in a hangar and no defects were noted. He also recalled not having any distractions or time constraints in performing the inspection. The maintenance logbook recorded compliance with AD 2007-26-52 on November 3, 2019 at an ATT of 10,051.3 hours, but a separate entry for the 50-hour main rotor blade tip cap inspection was not present. Lastly, the maintenance logbook recorded compliance with AD 2007-26-52 on October 23, 2019 at an ATT of 10,026.6 hours; this entry also recorded a 50-hour main rotor blade tip cap inspection was performed. According to the operator’s assistant director of maintenance, for every maintenance logbook entry that recorded compliance with AD 2007-26-52, both the 25-hour and 50-hour main rotor blade tip cap inspections were performed, regardless of time remaining since the last inspection. Certain maintenance logbook entries that recorded compliance with AD 2007-26-52 also recorded a separate entry for compliance with the 50-hour main rotor blade tip cap inspection per the helicopter maintenance manual. Lastly, according to the operator’s assistant director of maintenance, it was uncommon to perform a dye penetrant inspection of the tip cap during the 50-hour main rotor blade tip cap inspection. He could not recall needing to perform a dye penetrant inspection of the main rotor blade tip cap in the past 4 or 5 years across the operator’s fleet of four A109-series helicopters. History of A109 Tip Cap Cracks Before the introduction of main rotor blade tip cap P/N 709-0103-29-109, past tip cap cracks and separations led to the issuance of FAA AD 2001-24-07 R1, effective July 30, 2004, which instituted a recurrent 25-hour inspection of the main rotor blade tip caps. A terminating action for FAA AD 2001-24-07 R1 was to replace an affected main rotor blade with one having an airworthy tip cap P/N 709-0103-29-109. Subsequently, two separate incidents occurred in which a crack or fracture was found on a P/N 709-0103-29-109 tip cap. These discoveries led to the issuance of FAA emergency AD 2007-26-52, which superseded AD 2001-24-07 R1. Since the issuance of BT No. 109EP-85 and AD 2007-26-52, there have been six occurrences worldwide of a partial or complete inflight separation of the P/N 709-0103-29-109 tip cap, including the incident flight involving N901EM. In June 2016, a composite tip cap, P/N 709-0103-46-101 was certified by the European Aviation Safety Agency (EASA). The composite tip cap is not subject to the recurrent 25-hour and 50-hour inspections required by AD 2007-26-52. Post-incident Actions On December 6, 2019, SevenBar Aviation issued an internal maintenance bulletin to perform a daily tip cap inspection on all A109E main rotor blades, regardless of the P/N of the tip cap. On October 19, 2020, Leonardo issued Revision A of BT No. 109EP-085 which removed the visual inspection, prescribed by the original BT, for the 50-hour main rotor blade tip cap inspection; the inspection now prescribes a dye penetrant inspection every 50 hours. EASA issued AD No. 2020-0230 on October 22, 2020 to reflect the changes to the 50-hour inspection per Revision A of BT No. 109EP-085. At the time of this report, the FAA had not yet released an analogous update to AD 2007-26-52.
Fatigue fracture of the main rotor blade tip cap leading edge and subsequent partial separation of the tip cap, resulting in increased vibrations in the main rotor.
Source: NTSB Aviation Accident Database
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