CEN17LA174

The commercial pilot was conducting a nonscheduled, cross-country passenger flight. He reported that, while en route, he detected an in-flight vibration and made a precautionary landing on an oil platform. As he was shutting down the engine, the vibration increased, and he initiated an emergency shutdown using the rotor brake. Postaccident examination revealed that a tip block and weights had separated from one of the tail rotor blades. Cracks were noted on the tail rotor gear box, which was detached from the tailboom support casting. The casting was entirely fractured and exhibited cracking consistent with overstress separations. The left-side attachment studs were fractured and exhibited reversed bending fatigue fractures. The casting also had cracks at the two forward stud hole locations, and stud hole elongation was noted at the two aft stud hole locations. The separation of the tip block from the tail rotor blade resulted from an incomplete bond area due to a waffle pattern in the adhesive and contamination of the bond line by repair debris. About 50% of the adhesive surface had smooth and glossy surfaces consistent with voids and lack of contact between the adhesive and the tip block, which was likely due to insufficient vacuum pressure being applied while curing the adhesive. The blade tip exhibited worn separation surfaces and the presence of dark material, consistent with engine exhaust, indicating that a crack had formed along the flat side bond line and then progressed until the degraded bond strength was exceeded by operational loads, and the tip block separated. This resulted in a violent rotor imbalance and induced sufficient loads to crack the gearbox attachment studs and produce rapid fatigue cracking in the left-side attachment studs and to crack and nearly fracture the gearbox support assembly. The tip block had been repaired about 65 hours before the flight. According to the helicopter manufacturer, after the repairs were made, the blade passed the postbond pull test of 1,320 lbs, which was equal to the load on the tip block at maximum tail rotor rpm, and it was returned to service. Given the postaccident condition of the adhesive, the postrepair test procedures were not adequate to detect the insufficient adhesive bonding, which resulted in the separation of the tip block from the tail rotor blade. After the accident, the helicopter manufacturer revised its approved repair and inspection procedures. The changes included, in part, a revision to the cure cycle process to use only positive pressure (not vacuum pressure) during the cure cycle that cures the adhesive that bonds the block on the blade tip and an expansion of its postrepair inspection procedures.

***This report was modified on January 8, 2018. Please see the docket for this accident to view the original report.*** On May 2, 2017, about 0635 central daylight time, a Bell 407 helicopter, N457PH, registered to and operated by PHI Helicopters, Inc., Lafayette, Louisiana, made a precautionary landing at Grand Bay Receiving Station near Venice, Louisiana, after the pilot noticed an in-flight vibration. The pilot and five passengers on board the helicopter were not injured and the helicopter sustained substantial damage. The non-scheduled domestic passenger flight was being conducted under the provisions of Title 14 Code of Federal Regulations Part 135, and a company VFR flight plan had been filed. Visual meteorological conditions prevailed at the time of the accident. The cross-country flight originated from Boothville (LS08), Louisiana, at 0629, and was en route to Main Pass 311A in the Gulf of Mexico when the accident occurred. The pilot said he detected an in-flight vibration and made a precautionary landing on the Grand Bay oil platform. As he was shutting down the engine, the vibration increased and he initiated an emergency shutdown using the rotor brake. Post-accident inspection revealed the tip block and weights had separated from one of the tail rotor blades. Cracks were noted on the tail rotor gear box mounting hardware and the tail boom. The tail rotor hub and blade assembly, tail rotor gear box with two fractured studs, and tail boom support casting were sent to NTSB's Materials Laboratory for examination. In addition to NTSB's staff, representatives from PHI, Inc., and Bell Helicopter were present for the laboratory examination. Visual examination of the tail rotor confirmed the tip block and blade tip weights were missing from one of the tail rotor blades. According to PHI, the blade had a total service life of 2,658.65 hours, and had a tip block replacement repair approximately 65 hours prior to the separation. The examination found that the tip block separated along the adhesive interface, leaving the majority of the adhesive attached to the blade skin. The adhesive remaining on both skins appeared as a waffle pattern, indicative of partial bonding and subsequent interfacial separation. Approximately 50% of the adhesive surface had smooth and glossy surfaces consistent with voids and lack of contact between the adhesive and the tip block. The tail rotor gearbox was detached from the tail boom support casting, and the two outboard attach studs were fractured. The entire support casting was fractured and all but 2.7 inches of the skin and the four-inch wide cover were cracked. The fracture in the casting and crack in the skin were consistent with overstress separations. Both fractured left-hand studs displayed reversed bending fatigue fractures. Magnification of the fracture faces revealed features and topographies consistent with multiple origin reversed bending fatigue initiating in the respective root radii on opposed sides ofeach studs. Fretting and rub marks were observed on both the support casting and tail rotor gearbox where the attach studs were fixed. The tail rotor gearbox alignment dowel pins were missing. The tail boom support casting had cracks at the two forward stud hole locations, and stud hole elongation was noted at the two aft stud hole locations. A circumferential crack had formed from the forward left stud hole along the left side. According to Bell Helicopters, after repairs were made to tail rotor blades, a 1,320-lb. pull test was performed on the tip block, a tap test was performed to check for voids, a peel test was performed from skin patch material, and a water leak test was performed. Bell reported that no blade had ever failed the 1,320-lb. pull test, and the facility had made approximately 25 tail rotor tip block crack repairs per year. After the accident, the helicopter manufacturer revised its approved repair procedure, including the block replacement. The changes included, in part, a revision to the cure cycle process to use only positive pressure during the cure cycle that cures the adhesive that bonds the block on the blade tip. The manufacturer also expanded its postrepair inspection procedures. For more information, see the document titled "New Bell Helicopter Approved Tail Rotor Repair Procedures" in the public docket for this accident.