WPR24LA319 · Bell 206L

The pilot reported that, while in cruise flight at about 100 kts and about 1,000 ft above ground level (agl), the helicopter suddenly exhibited a severe vertical oscillation, commonly referred to as a “vertical hop.” Believing a catastrophic mechanical failure was imminent, he lowered the collective to initiate an emergency descent. Just before landing alongside a forest road, the pilot raised the collective and slowed the helicopter, and the vertical hop subsided. After landing, the pilot exited the helicopter and observed substantial damage to the tailboom and fuselage. Postaccident examination revealed the helicopter’s tailboom was buckled around its entire circumference. The tailboom buckling was likely the direct result of abnormally high structural loads imposed on the airframe during the vertical hop. This violent oscillation generated dynamic loads exceeding the normal structural design limits of the tailboom, resulting in it buckling about 13 inches aft of the intercostal support, and displacing it downward about 15° and to the right. The full monocoque construction of the tailboom, while robust under normal flight loads, can sustain damage when under the sudden, cyclically-amplified bending forces associated with this type of resonant vibration event. Although the helicopter’s collective lever could be moved with less force than specified by the manufacturer, it is unlikely that the vertical oscillations were due to collective bounce (an interaction between the pilot’s arm and the vibration of the airframe that can be exacerbated by low friction clamp force on the collective control) because the pilot did not have his hand on the collective lever when the oscillations started. About five months before the accident, the helicopter’s metal Bell main rotor blades were replaced with composite main rotor blades produced by Van Horn Aviation (VHA). The underlying source of the vibration itself was likely an aeroelastic interaction between these main rotor blades and the helicopter's nodal beam transmission mount under a specific set of operating conditions. Based on at least 26 similar reports in Bell 206Ls dating back to 2019, VHA identified the triggering conditions as single-pilot operation, low fuel load, and airspeeds above 85 kts, all of which were present in this accident. In addition, out-of-tolerance maintenance conditions, such as main rotor blades configured with forward sweep, increased the likelihood of a helicopter exhibiting hops, and increased the severity of the oscillations when they occurred. Under these conditions, the light helicopter weight and the shift of the center of gravity aft alters rotor system dynamics, allowing a resonant vibration to develop between the rotor blades and the nodal beam mount. When encountered, this vibration can be lessened or stopped by banking, raising the collective, or slowing the helicopter. About 2.5 years before the accident (or about 2 years before the operator installed the main rotor blades on the helicopter) VHA posted a vertical hop recovery technique on social media and in a newsletter, but did not publish it in their technical library until after the accident. The use of blogs, social media, and a newsletter may have initially been an effective means of alerting operators; however, the lack of a persistent, retrievable reference in VHA’s technical library likely precluded operators from learning about the recovery technique if they had not read the newsletter or seen the posts before they were superseded by more recent posts. According to the pilot, he was not aware of the recovery techniques until after the accident. Had the pilot been aware of and implemented the appropriate recovery techniques, he likely would have been able to prevent the oscillations from reaching a structurally damaging amplitude.

Verbatim NTSB analysis and probable cause from the NTSB dataset