Event Description
On October 27, 2018, at about 1104 Hawaiian standard time, a Bell 430 helicopter, registration N430ST, sustained substantial damage during a revenue sightseeing flight under Title 14 Code of Federal Regulations Part 135 near Kailua/Kona, Hawaii. The commercial pilot and three passengers were not injured.
According to the pilot, the helicopter was transitioning offshore with the autopilot engaged and commanded to climb to 1,500 ft mean sea level (msl). At an altitude of 1,350 ft msl, a speed of 125 knots, and a heading of about 165°, the pilot heard a loud thump immediately followed by a medium-to-high-frequency vibration of significant amplitude. The vibration was felt through the airframe more than through the flight controls.
The pilot conducted a precautionary landing at Ellison Onizuka Kona International Airport at Keahole, Kailua/Kona, Hawaii. The landing was uneventful, but the vibrations increased in intensity when the throttles were reduced to idle while on the ground. The pilot performed a normal engine shutdown, and all occupants safely egressed.
Postaccident Examination
Postaccident examination of the tail rotor components revealed that one tail rotor blade’s root enclosure, along with the associated weights and attachment bolts, had separated. The other tail rotor blade sustained a 6-inch-long dent about mid-span. Components from the tail rotor blade and hub assembly were shipped to the helicopter manufacturer for examination. Examination confirmed that one blade’s root enclosure had separated from the inboard edge portion of the blade. The damage to the other blade was consistent with impact from contact with the separated root enclosure.
Examination of the root balancing weight plate damage revealed that, at some undetermined point, both end portions of the closed-end nuts used to attach the tail rotor balancing weights to the nut plate had penetrated into the honeycomb structure, leaving a 1-inch void. The investigation could not determine the cause of these voids, but the anomaly exhibited no damage beyond the 1-inch void bore in the honeycomb.
Maintenance History
A review of maintenance records indicated that about 200 flight hours before the accident, the subject tail rotor blade had been removed from service due to a partial delamination of the root enclosure at the root block. A repair was performed in which the blade’s root enclosure was bonded to the root block. Scratches were observed around the edge of the repair area, consistent with surface preparation for the repair. However, while the surface preparation with the scratches created an appropriate bonding surface, the remaining area where the adhesive was in contact with the blade surface was corroded, and the adhesive did not properly bond in this area.
No additional adhesive or evidence of surface preparation was observed in the gap between the root enclosure and root block. On the area opposite the root enclosure, on the leading edge of the blade, paint work consistent with the root enclosure being clamped to the root block during the repair was present.
The surface of the root block where the tang enclosure was bonded was corroded, with paths of oxidation extending outboard. This was consistent with moisture between the adhesive and the blade enclosure.
Spectroscopy examination of the tail rotor blade root block revealed that the material was similar in composition to the specified alloy, and the hardness value conformed to the specified minimum. A sample of the adhesive was examined by Bell Chemical and Processes Laboratory and found to be consistent with the original adhesive used during manufacture, chemically similar to the specified adhesive. The adhesive used to bond the enclosure at the time of manufacture had a cure percentage of about 97%, fully cured. The adhesive used in the repair was not located on the surface areas that were prepared.