Huey "bounce"
Joined: Nov 2010
Posts: 880
Likes: 225
From: Australia
In post #471 of the "Helicopter crash New York City" thread wrench1 included as an example of a vertical oscillation issue on a Bell LongRanger, the case of C-FRCL lost at Legate Creek, BC, Canada while conducting long-line operations. See: Helicopter crash New York City or more specifically see the TSB accident report directly at: https://www.tsb.gc.ca/eng/rapports-r.../a08p0265.html
Since wrench1 indicated this was a case with OEM Bell main blades fitted, I wanted to learn what was reported for that accident. In summary, the 206L was lifting a load heavier than expected and while lowering that load at an altitude beyond the HOGE of the helicopter, the load bounced at least twice when the pilot attempted to lower it onto the drill deck which in turn cause the helicopter to fishtail then spin with a bent tail boom. The load then snagged trees and control of the helicopter was lost and it crashed, killing the pilot.
That accident may well be the same as referred to by cab driver in post #2 of this thread, but the link in that post is to the overall TSB Canada website, not a specific accident.
In Post #17 of this thread Peter3127 speculates whether an interaction with the resonant frequency of the long-line load could be a contributing factor to oscillation incidents during long-line operations and I felt the same about the C-FRCL accident given what was written in the TSB Canada report. There was enough information in that report to attempt a basic time-domain simulation of the vertical motion of the helicopter and its sling load as the pilot attempted to lower the load to the deck. The following graph shows the predictions. Blue line is the height of the load above the deck, green line the height of the helicopter (but shown lower than its actual height relative to the deck to fit it on same graph), red line is the vertical velocity of the helicopter and yellow is the vertical acceleration (in g's) experienced by the helicopter. The lift generated by the rotor was assumes to remain constant (at the stated HOGE limit) even if the TSB report indicates movement of the collective was likely. The simulation shows a pair of bounces and thereafter the load would have come to rest on the drill deck (except in reality the helicopter must have moved away from above the deck by that stage). Had the helicopter remained above the deck with constant maximum HOGE lift applied it would have continued to oscillate against the load via the long-line at a frequency of around 0.67 Hz (or a period of about 1.5 seconds):
The TSB Canada report also indicates that using a 9/16" diameter long-line (rather than the 7/16" 100' Canam Plasma long-line of the accident helicopter) was considered by other pilots to eliminate the load bounce on contact with the ground (or drill deck in this case). But repeating the simulations for a long-line with this proportionally greater cross sectional area and hence stiffness still predicts a similar double bounce under the same circumstances, again assuming constant lift from the rotor throughout the simulation period. The difference now is that the oscillation period after the load has settled on the deck is shorter (more like 1.1 seconds) due to the stiffer long-line:
So why these simulations? I concluded that the rotor dynamics and any "collective bounce" in the C-FRCL accident are likely of a much lower frequency than might be the case for the so-called "VH hop". This accident was due to a combination of an inability to hover OGE (given the load being carried) likely combined with inadvertent collective inputs and long-line resonances that lead to the blades flapping and/or flexing wildly and so striking the tail boom.
Still, it would be worth trying to more fully understand the rotor dynamics of other examples described on this thread such as the UH-1H vertical bounce at 3 Hz as described in the extract provided by The GRINCH in post #20.
Since wrench1 indicated this was a case with OEM Bell main blades fitted, I wanted to learn what was reported for that accident. In summary, the 206L was lifting a load heavier than expected and while lowering that load at an altitude beyond the HOGE of the helicopter, the load bounced at least twice when the pilot attempted to lower it onto the drill deck which in turn cause the helicopter to fishtail then spin with a bent tail boom. The load then snagged trees and control of the helicopter was lost and it crashed, killing the pilot.
That accident may well be the same as referred to by cab driver in post #2 of this thread, but the link in that post is to the overall TSB Canada website, not a specific accident.
In Post #17 of this thread Peter3127 speculates whether an interaction with the resonant frequency of the long-line load could be a contributing factor to oscillation incidents during long-line operations and I felt the same about the C-FRCL accident given what was written in the TSB Canada report. There was enough information in that report to attempt a basic time-domain simulation of the vertical motion of the helicopter and its sling load as the pilot attempted to lower the load to the deck. The following graph shows the predictions. Blue line is the height of the load above the deck, green line the height of the helicopter (but shown lower than its actual height relative to the deck to fit it on same graph), red line is the vertical velocity of the helicopter and yellow is the vertical acceleration (in g's) experienced by the helicopter. The lift generated by the rotor was assumes to remain constant (at the stated HOGE limit) even if the TSB report indicates movement of the collective was likely. The simulation shows a pair of bounces and thereafter the load would have come to rest on the drill deck (except in reality the helicopter must have moved away from above the deck by that stage). Had the helicopter remained above the deck with constant maximum HOGE lift applied it would have continued to oscillate against the load via the long-line at a frequency of around 0.67 Hz (or a period of about 1.5 seconds):
The TSB Canada report also indicates that using a 9/16" diameter long-line (rather than the 7/16" 100' Canam Plasma long-line of the accident helicopter) was considered by other pilots to eliminate the load bounce on contact with the ground (or drill deck in this case). But repeating the simulations for a long-line with this proportionally greater cross sectional area and hence stiffness still predicts a similar double bounce under the same circumstances, again assuming constant lift from the rotor throughout the simulation period. The difference now is that the oscillation period after the load has settled on the deck is shorter (more like 1.1 seconds) due to the stiffer long-line:
So why these simulations? I concluded that the rotor dynamics and any "collective bounce" in the C-FRCL accident are likely of a much lower frequency than might be the case for the so-called "VH hop". This accident was due to a combination of an inability to hover OGE (given the load being carried) likely combined with inadvertent collective inputs and long-line resonances that lead to the blades flapping and/or flexing wildly and so striking the tail boom.
Still, it would be worth trying to more fully understand the rotor dynamics of other examples described on this thread such as the UH-1H vertical bounce at 3 Hz as described in the extract provided by The GRINCH in post #20.
Last edited by helispotter; 22nd August 2025 at 22:34.
