N90326Bell 206B · 1975

On June 21, 2022, about 06015 Pacific daylight time, a Bell 206B helicopter, N90326, was substantially damaged when it was involved in an accident near Stockton, California. The pilot was not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 ferry flight.

According to the pilot, he was enroute to a field, but was carrying no chemicals. He was about 12 minutes into the flight when he felt a vibration that lasted no more than 10 seconds. He then heard a bang followed by a sharp right yaw. He lowered the collective and rolled the throttle down to bring the helicopter back into controlled forward flight. He maneuvered the helicopter for about a half mile to a “young orchard,” and planned to do a run-on landing between the small trees. When the helicopter had slowed to about 25 mph, and while about 20 ft above ground level (agl), it began an uncommanded rotation to the right. The pilot then rolled off the throttle and tried to cushion the landing. The helicopter landed level but continued to rotate, which resulted in it rolling onto its left side, substantially damaging the fuselage, main rotor, and tail rotor assemblies. Before the pilot exited the helicopter through the co-pilot's windshield, he shut down the engine and turned the fuel switch off. The pilot stated that he did not know if he had hit a bird.

The postaccident examination, conducted by the Federal Aviation Administration (FAA), revealed structural damage to the fuselage, the transmission mount, and the main rotor and tail rotor assemblies. The tail rotor gearbox (gearbox) had separated from the end of the tailboom at the four mounting bolts, and the gearbox mounting shims were found scattered on the ground about 10 ft behind the tail rotor mount. The tail rotor control bellcrank rod end was sheared off near the gearbox. One tail rotor blade remained wholly attached on the hub but was damaged. The other tail rotor blade fractured and separated from the hub and about 80% of the blade was missing. The root end of the blade remained installed on the hub. The missing two sections of the blade were found about 2,000 ft north of the main wreckage. The tail rotor assembly, along with the gearbox and its mounting hardware, were shipped to the NTSB materials laboratory for further examination. The metallurgical examination revealed that the pitch change control rod and the four gearbox mounting bolts had overstress fractures. The blade grips, hub, and pitch change links revealed no witness marks or indications of preexisting damage.

The tail rotor blades were examined by the NTSB, and subsequently, by Van Horn Aviation, LLC, the blade manufacturer. The blades were FAA PMA (Parts Manufacturer Approval) components. The two blades, serial number C548, and C549 are shown in Figures 1 and 2.

Figure 1. Tail rotor blade C548.

Figure 2. Tail rotor blade C549.

Blade C548 had partially separated along the centerline, exhibiting a loss of outboard skin, shown near the black stripe. The fracture on blade C548 exhibited a zig-zag pattern, oriented 45° in some locations and parallel with the blade direction in others. These orientations were consistent with fracture along the fiber direction of the composite, across different plies, which were oriented in different directions. The fiber composite also exhibited irregular and erratic fracture patterns, consistent with overstress fracture across multiple plies (translaminar).

Blade C549 had wholly fractured and exhibited three major remnants. The blade fractured along the root of the blade, with the outboard portion cracking along the interior foam core. Only the base of the blade remained installed on the tail rotor hub. Like with the other tail rotor blade, fracture directions were consistent with the fiber direction of the ply when located along the composite layup of the blade. Unlike the other tail rotor blade, this blade was absent the tip, with the outboard 2.5 inches of material missing. This blade also exhibited features that were consistent with a translaminar fracture of the composite, as opposed to an interlaminar delamination fracture. According to the manufacturer, in the fractured tip area the abrasion strip exhibited denting, folding, and debonding/delamination along the aft edge as well as discoloration from a foreign material. The extensive deformation of the abrasion strip was consistent with a high energy impact on the leading edge.

The manufacturer identified multiple scrapes and material transfer onto blade C548, as shown in figure 3, and discoloration from foreign materials on blade C549, as shown in figure 4. A small amount of foreign matter, identified as nitrile rubber, was found lodged in the fractured blade end.

Figure 3. Blade C548, exhibiting scrapes and material transfer on the leading edge.

Figure 4. Mosaic of photos of Blade 549, exhibiting discoloration and a small amount of foreign matter lodged in the blade end.