Story ID: 013
FAA Concerned about Bell 206L Vertical Vibrations
By: Matt Thurber
Caption: Pilots need to know to pull collective to eliminate so-called collective bounce
Word Count: 1312
The FAA has issued an airworthiness concern sheet (ACS) that outlines a vertical vibration issue with Bell 206 LongRanger helicopters. According to the FAA, it “has received reports of severe vertical vibrations on Bell 206L (LongRanger) helicopters. These events are inconsistent in their repeatability. In addition, post-flight inspections have not identified any failure that caused the vibration event. Reports indicate the vibration subsides once additional load is applied on the main rotor by increasing collective. The vibration may worsen with a low friction set on the collective.”
According to the ACS, on Sept. 26, 2024, in Fern Prairie, Washington, a Bell 206L suffered a vertical vibration incident where “continued vibration resulted in substantial damage to the helicopter’s tail boom, but the aircraft was able to land safely.” The pilot was not injured.
In its preliminary report about the accident, the NTSB wrote, “While in cruise flight about 1,000 ft above ground level, the helicopter experienced ‘a severe vertical hop’ and began to shake violently. The pilot initiated an emergency descent, during which time the shaking subsided. The pilot performed a normal landing in an open field. After egressing, the pilot observed substantial damage to the helicopter’s tail boom.”
On March 17, 2025, aftermarket rotor blade manufacturer Van Horn Aviation (VHA) published an information letter (NOTICE No.: 33000-6R1) titled “Collective bounce recovery and mitigation.” The Fern Prairie 206L was equipped with Van Horn’s composite rotor blades. The Bell 206 series factory blades are metal.
“We put out the letter because we had a handful of reports over several years,” said VHA design engineer Chris Gatley. “First and foremost, the sole reason for putting out the information letter is like the ACS mentions—there was the incident in Oregon [the helicopter was on its way to Oregon] where [the helicopter] got a severe hop but the tailboom bent. Both we and the NTSB are concerned that pilots may be doing the wrong thing or not know what to do. We know from customer input and testing ourselves that the reliable recovery method is to slow down and raise collective and pull power to load the rotor system. The sooner you do that, the better.”
Gatley pointed out that collective bounce is not isolated to VHA’s rotor blades and that VHA pilots have experienced this phenomenon on metal-bladed 206s, although it seems to be limited to the 206 LongRanger model. The reports that VHA has received are limited to VHA blades, he explained, because “we only hear from people who have our blades.”
A challenge with this issue is that it is difficult to reproduce. For example, a large fleet operator with VHA-bladed helicopters has not experienced collective bounce or vertical vibration, according to Gatley. VHA is conducting testing on a LongRanger to try to learn more about the causes, although the VHA letter outlines everything the company has learned to this point. The follow-up testing in the LongRanger “will add to that information and narrow down what’s causing it,” he explained.
In any case, he pointed out that it’s important for pilots to report to the FAA any instances of collective bounce or vertical vibration in Bell 206-series helicopters, as outlined in the ACS, and with any types of rotor blades. “We’re very much trying to learn as much as we can,” he told AIN.
Bell has published information on collective bounce, including an Operations Safety Notice dated Oct. 18, 1976, for operators of 206A/B/L, 204B, 205A1, and 212 models that addressed proper setup of collective minimum friction (not the collective control’s friction setting in the cockpit). 
“Maintenance of collective minimum friction,” the notice explained, “is a certification requirement by the FAA and is designed to preclude ‘collective bounce’ by: reducing the effects of pilot over-control; reducing effects of wind gusts and turbulence through the rotor and into the airframe; [and] preventing feedback from external sling loads into the airframe.”
In 2007, a Bell Rotorbreeze article outlined a “vibration or ground bounce caused by the leading edge pitch link main rotor to mast coupling phenomenon.” While it’s not exactly clear whether this phenomenon happens only on the ground, it is interesting to note that the solution is the same as for the collective bounce that pilots have experienced: “Usually a slight increase in collective pitch will cause the rotor to sustain its lift and the vibration will go away,” according to the article.
Asked about the current issues, Bell responded to AIN: "We fully support the FAA’s investigation, and hope that any Bell 206 operators who may have experienced similar events to those described in the Airworthiness Concern letter will submit those to the FAA for review."
In the Van Horn letter, it explains, “Operator reports have indicated the potential for pilots to experience severe vertical vibrations on a small number of Bell 206L LongRanger helicopters equipped with Van Horn Aviation (VHA) p/n 20633000-101 main rotor blades. The phenomenon is characterized as a vertical vibration that develops in level flight or upon the initiation of a descent at 100+ KIAS and quickly increases in intensity. In many cases, the vibration is not readily repeatable even on the same aircraft and in the same configuration.”
The letter describes collective bounce as “a known issue on the 206 family, as well as several other Bell helicopters with two-bladed, teetering rotor systems. Recent investigation has determined that the vibration occurs at approximately 5 Hz, which is consistent with a tail boom natural frequency and available literature on collective bounce.
While investigation is still ongoing, collective bounce is characterized by an interaction between the pilot’s hand on the collective stick and a vertical vibration of the airframe. Several aircraft configuration and maintenance items have been identified as potential contributors.
Should pilots encounter this phenomenon in-flight, the appropriate recovery method is presented. “If collective bounce is experienced in-flight, immediately load the rotor by increasing collective, reducing speed, and turning right.”
In the letter, VHA includes a diagram showing a shaded region where collective bounce is possible and where pilots may want to avoid operating. “This should not be construed as an operational limitation or a certainty that a severe vibration will develop when operating in this region.
“Pilots should simply be aware of the possibility of collective bounce to develop and be prepared to recover, if necessary. All reported instances have occurred with a single pilot operating aircraft in light configurations with approximately 300 pounds of fuel or less. If possible, ballasting the aircraft and/or avoiding fuel burn below 300 pounds will likely avoid any occurrence.”
If pilots have to fly in that shaded region, VHA advised, “reduce speed to below 100 KIAS [and] avoid low-G maneuvers. All reductions in collective and forward cyclic should be applied slowly and smoothly. Increase collective friction via the friction knob, as practical.”
There are also recommended maintenance actions, the most important of which is proper installation of the VHA blades to avoid incorrect blade sweep. “Incorrect blade sweep has initially been identified as the most likely cause of the vertical vibration,” the letter explained. “Correcting blade sweep has so far been demonstrated to eliminate or reduce the vibration to a minor level.” Other maintenance actions include correct trunnion centering and track and balance at speeds greater than 105 KIAS.
Gatley reiterated the key piece of advice for pilots experiencing collective bounce in the VHA letter: “Don’t drop the collective and make it worse and just sit in [the vibration].” Or as the letter put it in bold type: “DO NOT lower collective or enter an autorotation. The vibration will only intensify and continue throughout the descent until the rotor is loaded again.”