N457PHBell 407 · 1997

***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 of each 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.