NTSB: Bell 206L Damage Caused by Aeroelastic Interaction

Buckling of a Bell 206L LongRanger’s tail boom during a “vertical hop” oscillation on Sept. 26, 2024, was due to an “aeroelastic interaction between the helicopter’s main rotor blades and nodal beam system, which resulted in oscillations that exceeded the design limits of the tail boom,” according to the NTSB final report. “Contributing to the accident was the failure of the main rotor blade manufacturer to adequately publish the appropriate recovery technique for a known flight dynamics issue.”

The helicopter—N9984K, operated by JL Aviation of Boring, Oregon—was equipped with composite Van Horn Aviation (VHA) main rotor blades, which had replaced the Bell metal blades about five months before the accident. The VHA blades had logged 348.6 hours when the accident occurred. With no passengers and about 135 pounds of fuel, the helicopter was lightly loaded.

While cruising at about 100 knots and 1,000 feet agl near Fern Prairie, Washington, according to the NTSB, the pilot reported that “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 tail boom and fuselage.”

Memo Procedure

VHA had posted a memo on its website on March 22, 2022, about a vertical hop recovery technique, outlining a procedure calling for banking to load the rotor disk to stop the vertical hops:

“In cases where a 206L fitted with VHA main rotor blades is operating light—for example, with only a single pilot on board, less than a full tank of fuel, and no cargo—the helicopter can experience a severe vertical hop in turbulent conditions. When initiated by a gust at high speed, this very light aircraft configuration can allow an interaction to occur between the rotor blades and the nodal beam mount. The resulting vertical hop can become very uncomfortable. This seems to be an unusual situation, but if the helicopter gets into this condition, the appropriate action is to load the rotor by banking. By loading the disk, the frequency of the system is quickly changed, thereby resolving the issue.” 

Later, this advice was changed to reducing airspeed and raising the collective, with the speed reduction found to stop the oscillations more quickly. The memo was also posted on VHA’s Facebook page and included in its spring 2022 newsletter, which was sent on April 5. According to VHA design engineer Chris Gatley, VHA records showed that the email was sent to two JL Aviation addresses. “A report from the marketing platform indicated one of the two recipients had opened the email. However, it could not be determined if this was indicative of the email being opened or simply ‘marked as read’ by the recipient,” the NTSB noted in its Record of Conversations in the accident docket.

Pilot’s Perspective

JL Aviation pilot Peter Emerson, who was flying the accident helicopter, was interviewed by the NTSB. In the Record of Conversations, the NTSB wrote, “In an email conversation on Feb. 19, 2026, Mr. Emerson indicated that JL Aviation had ordered the main rotor blades that had been installed on N9984K in November 2023. The main rotor blades were received in March 2024. He indicated that Van Horn Aviation did not provide information about the vertical hop or any recovery technique when the blades were ordered or shipped to JL Aviation. He only became aware of Van Horn Aviation’s memo after the accident, when an online search led him to a forum post on pprune.org that contained a link to the memo. Mr. Emerson indicated that if he was aware of the recovery method, he would have used it when the vertical hop began during the accident flight.”

While VHA did publish the memo on Facebook and in the company newsletter, the NTSB pointed out that it wasn’t included in VHA’s 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,” the NTSB noted.

Later, VHA published Service Bulletin (SB) 33000-6 with “more information about the recovery technique [using the collective instead of banking] and maintenance actions that could prevent the hops from initiating,” the NTSB wrote, and once the bulletin was published, VHA removed the memo from its website.

First Occurrence

The reason for publication of the vertical hop recovery technique stemmed from reports from an operator flying LongRangers equipped with the VHA main rotor blades. The first occurrence was in September 2019, about a year after the FAA issued the supplemental type certificate (STC) for the VHA main rotor blades. The NTSB noted, “There was no rotorcraft flight manual supplement for [the STC].”

The operator sent the blades to VHA for examination, but the company found nothing wrong and returned the blades. Then this operator experienced another vertical hop incident in 2021, “during similar operating conditions,” according to the NTSB.

VHA conducted flight tests and, in December 2025, developed a report that said “there were no known cases of severe vertical hops when the VHA-prescribed recovery technique was used by the pilot. Conversely, in all documented cases where the oscillations were reported as severe, the pilots had reacted to the onset of the oscillations by reducing the collective, which prolonged or worsened the oscillations.” It was also noted that setting the main rotor blades with forward sweep “increased the likelihood of a helicopter experiencing hops and increased the severity of the oscillations when they occurred.”

The VHA flight test report “noted that severe oscillations had occurred 26 times in 15 different helicopters within a fleet of about 300.”

The report continued: “In all known cases where severe vibrations were reported, pilots reduced collective in order to descend for a precautionary or emergency landing. Conversely, [in] all instances where the pilots have performed the appropriate recovery, vibrations have been reported as mild or moderate, and it has been noted that the vibration does not reach a severe level. This indicates that the vertical vibrations are not inherently severe but rather become so after collective is significantly reduced.”

STC Updated

The VHA 206L STC was updated in April 2026 and required inclusion of a rotorcraft flight manual supplement that included the service bulletin’s recovery technique. VHA also holds STCs for composite main rotor blades on Bell 206Bs, but those helicopters use pylon support links for mounting the transmission instead of the nodal beam. “VHA has not reported any occurrences of vertical hops in 206Bs equipped with their [main rotor blades],” according to the NTSB.

In the docket material for this accident, there appears to be disagreement about the cause of the damage to the JL Aviation 206L.

According to the NTSB’s record of an Oct. 1, 2024 conversation with JL Aviation owner Jeffrey Linscott, he contacted VHA after the accident, and this is when he learned about the vertical hop memo.

“Mr. Linscott told Van Horn in a phone conversation that the helicopter’s tail boom had buckled as a result of the vertical hops,“ according to the NTSB. “The employee from Van Horn indicated that the damage was not from the vertical hops, but was rather from the helicopter being landed hard.”

The NTSB continued: “Mr. Linscott disagreed with the assertion the helicopter had landed hard. He stated that, in his observation, if a Bell 206 is landed hard enough to incur damage to the fuselage and tail boom, the skids will also spread or collapse, with the aft ends of the skids spreading further than the forward ends.”

“The skids did not appear to be deformed or splayed outward,” the NTSB wrote in its final report.