It took zhishengji751 (#341) to point out some of the horizontal stabiliser parts (including end plates) were in plain sight in the NTSB B-roll, and wrench1 spotted them again once the wreckage was in the building. But for everyone else, here are some further views showing both endplates at the pier, with left side endplate in foreground and the right in background. Tear through left endplate seems to be from rotor striking it from outboard side as would be expected. Top of right side endplate seems undamaged. That, and orientation of slice, would tend to support the rotor(s) striking the tailboom after it had already failed:
https://cimg5.ibsrv.net/gimg/pprune....7dd6eb552c.jpg
The parts laid out on the floor (with tape measure spacing them out appropriately) show that a considerable segment of the horizontal stabiliser (and tail boom) are not yet in place. My sense from image shared by wrench1 is that main rotors sliced the horizontal stabilizer at least twice in quick succession from its underside(?):
https://cimg9.ibsrv.net/gimg/pprune....e513b74ec8.jpg

https://cimg1.ibsrv.net/gimg/pprune....4623a341f4.jpg

For ROTOR BLAST (#357) there is a hint that some of the vertical stabilizer may also have been laying near the horizontal stabilizer parts at the pier. If so, it is in at least two parts (with one showing the black-gold-white paint scheme):
https://cimg0.ibsrv.net/gimg/pprune....ed6943c5c0.jpg
https://cimg1.ibsrv.net/gimg/pprune....c0357a8e29.jpg

Here is how it looked when still intact:
https://cimg1.ibsrv.net/gimg/pprune....51c77df1be.jpg

For reference, the 350/355 tailboom structure, while of the same monocoque construction, has a completely different mounting system and would be considered a bit more rigid than Bell’s method. However, even 350/355 system had their share of t/boom issues and failures.

Agree. The vertical fin offset will also drive the tailboom in that same direction.

As to the t/boom failure itself, one needs to understand that the strength with monocoque construction is from the skin and not the internal structure per se. So given the t/boom failed just aft of the intercostal structures, any loss to the structural integrity of the skin in that general area would cause the t/boom structure to collapse under load.

This is why most OEMs have no or minimal damage allowance for t/boom skins and especially near load transition areas. And for reference, the 4 ea. intercostal hat sections immediately forward of the t/boom failure point and inline with the 4 mount bolt fittings, are what transition the flight loads from the 4 mount bolts and rings to the t/boom stressed skin.

Regardless, tailbooms have always been an Achilles heel for Bell as far back as the UH-1 and 206A. Even the 214ST was known to bust t/boom bolts, skins, and pylons on occasion.

Yes, the vertical fin is there in 2 sections with the lower section still attached to a portion of the TR GB mount casting that also includes the lower TR GB cowling still attached to its mount ring on the fin. Thinking out loud, given the clean breaks on the vert fin and hori stab, it appears the tailboom assembly suffered multiple MR strikes under power. So it is quite possible those blade impact forces on the tailboom assy may just have been enough to cause the deck beam attachment to fail resulting in the loss of the MR and drive assy. At least when compared to other similar 206 deck failures I've seen in the past.


As to the Van Horn blades collective bounce issue, even Bell had similar issues with this phenomenon with factory blades and as I recall even an accident or 2 as well. There can be various reasons behind collective bounce, and I believe if this was a more common occurrence with VH blades than Bell blades, then there probably would have been more regulatory action taken.

Wrench1, what were the circumstances of previous upper deck failures on the 206L series? Did the entire nodal beam suspension system and transmission stay with the upper roof structure in those incidents?

I believe that the description of "Collective Bounce" is a misnomor at best.

In the instances I was personally involved with, the pilots literally thought that their time was up and the aircraft was going to throw itself apart.

I'm not going to embellish the facts with how they described it to me "post incident" but suffice to say I was at first sceptical then that gave way to concern.

Both instances occurred before VHA provided any formal feedback.

Subsequently and after I don't know how after how many reports were made to VHA, advice was issued over time in three statements.

In order of issue:

1. Offset Cyclic to right,/do not lower collective.
2. Raise Collective and offset Cyclic.
3. Sweep both blades aft two points to reduce likelihood of onset.

Item 3 obviously being a maintenence action.

Yes, the situation when it occurs is controllable providing the pilot has been made aware of the phenomenon and is briefed on how to handle it.

Don't get me wrong VHA deliver a good product, just the information needs to made more readily available for operators and pilots.

Currently the distribution of that information is via Van Horn only and therefore relys on an organisations own internal procedures to pass that information onto the crews at the sharp end.

That may be the "weak point" right there?




​​​​​​

Don't forget that up until last month, the only “official” info you could get on the issue was a blog post on VH’s website. A blog post. I think it had two or three short paragraphs.

I for one think that a lot of the basis of flight testing for the 206L blades may have been based on the 206B blades, on account of the sharing of the type certificate. Given the difference in transmission mounting systems and that VH said (in their blog post) that it was a harmonic interaction between the MRBs and the Nodal Beams, one can assume there was inadequate testing before release and up until now they’ve been doing what they can to minimize the issue to avoid a recall. Blaming Collective Bounce offloads the responsibility to a known boogeyman just as Bell did with LTE for decades.

Even if the VH blades didn’t cause this accident, or if they blame it on the operator due to improper sweep on the blades, or god forbid the pilot for not perusing the VH website enough to read their information letter, there is obviously an issue and it’s been kept pretty quiet up until now.

Agreed.

As stated before I'm not an "engineer" in the true sense as I don't have a degree.

Saying that my technical opinion is that the harmonics and feedback of VHA blades into the 206L Nodamatic system is an issue.

Bell tuned the Nodamatic system to suit the characteristics of their own OEM blades not for VHA blades manufactured under a PMA/STC.

This tuning is not adjustable.

I'm also curious why EASA won't approve VHA Main Rotor Blades.?

Whatever the cause of this horrific incident , I belive some additional attention needs to be paid to the operational and technical aspects of VHA MRB's on 206L series.

Time will bring the facts out I guess.

Edit.

EASA did not certify 206B Main Rotor Blades. Seems VHA did not pursue certification of 206L Main Rotor Blades with EASA.

All were MR strikes. And yes, the entire MR system stayed attached to the roof/roof beam structure. In the more notable one, an L-3 was over the beach when the pilot fell ill and passed out. The LH pax unbuckled and grabbed the controls but the pilot kept falling on the cyclic. A rear pax unbuckled and reached through headrest to hold pilot back in seat. LH pax got it down to the marsh and during the “landing” phase the MR hit the ground and tore out the roof section similar to NYC one. Unfortunately, since they were unbuckled, those 2 pax were ejected out the open roof area and died. The remaining pax (2) and pilot survived. I believe it was food poisoning that caused the pilot to pass out.

I’ve only heard it called collective bounce which is the term Bell used as well. I also seem to recall a couple accident reports using the same term as a factor in the probable cause.

I did hear that supposedly Van Horn recently revised their bulletin on L model collective bounce so I guess that was how they informed their customers of the issue? And being this was a Part 135 ops, I would have expected this bulletin to be part of the GOM or similar document.

Bell 206 Hudson River Crash
 

https://cimg3.ibsrv.net/gimg/pprune....89e47e024.jpeg
Some observations, not assumptions:
1. Rotor, transmission, transmission mounts and roof structure all in one detached assembly. If roof was structurally sound, more force would be required to cause this violent separation than a blade strike on the tail boom would impart. If the roof structure were compromised by corrosion, then the rotor and transmission could have asymmetrically separated causing abnormal rotor plane displacement relative to the fuselage.
2. Mast appears bent and/or twisted (could be damage from water impact)
3. At least one pitch change rod missing
4. Damage in photo is consistent with (not indicative of) MGB seizure
5. Reported MGB chip detection and analysis in preceding months is congruent with this failure tree.

If the MGB seized, would the blades have kept spinning and auto-rotating to the water with the gearbox and everything shown, NOT spinning? Does it look like the whole assembly was spinning at 100+ RPM?

Someone with a wrench and access to the gear box is soon to find out.

I don't know what the internals of this particular gear box can handle, but I have seen some that have been internally pulverized and usually it's just been a bunch of teeth shattered off the gears and the input pinion giving itself a shave against the sharp remainder of the downstream gear that it no longer properly engages with while inertia has kept the big parts rotating and circulating the fragments to do more damage until the last of the momentum is turned to metal chips.

On an internal driven gear one of the internal gear teeth snapped off and got embedded in the pinion; all the other pinion teeth were fine, but in the exact gear ratio it looked like someone took a sledge hammer and bashed out little sectors of the internal gear. People oohed and ahhed over the amount of damage but the system was finished off by a little sliver of metal that snapped off the ring gear. Everything else broken was just decoration. That was for a TF/TA radar system that no one could tell us what it was for, but sometimes the planes came back with chlorophyll stains on the wing tips.

History has shown that sudden accidents like this awful one are seldom caused by one factor.

My guess is a sudden and violent gearbox seizure, however I have also seen mentioned that the cabin roof structure of the type can be affected by corrosion…..

But although the important evidence has probably been secured, everything is just guesswork so I’m sitting back and waiting for their NTSB’s findings.

I imagine three possibilities:

Transmission internally damaged, seized and the whole thing spinning with the main rotor as it autorotated, or,

Transmission not really internally damaged, but input driveshaft damaged/jammed, and preventing normal transmission rotation, so the whole thing spinning with the main rotor, or,

Everything free to turn, but recall the gear ratio inside the transmission now working adversely, so some internal friction, and no torque resistance from the airframe anymore, so the whole thing spinning more or less together.

The apparent near symmetry fore/aft of what separated from the fuselage makes me think that it could find an inertial balance to rotate closely to the mast axis.

There's a video of a 206 in (i think) ground idle having a rotor collision with a landing helicopter:


What is interesting is what happens to the rotor and tranmission of the 206, resulting from the blade strike. I thought it looked not unsimilar; the whole thing is pulled off the roof.

If the pitch-change rod is missing,then that`s it..endex..the rest is just the consequences..

5hz
 

Did I miss the posts where we were discussing the noice in the earlier posted blancolirio ’vid where there was a noice that probably was not very long from 5Hz just before the helicopter desintegrated or was this not discussed yet?
There was a note in the VH Blades info letter about the tailboom having a 5Hz self resonance.

A coincidence?

You've nailed it in the second part of your sentence. Once somethign lets go, it's anyones guess as to what goes next, what breaks and what tears off. You'd need a complex analysis and AI modelling to exactly re-create the chain of events. It could be wildly different every time. That video posted above is a perfect example of this. The blades hit, one loses the tail and the other tries to spin; add in 100 knots of airspeed and what happens next is anyones guess.

There is a thing called “ Whirl mode” seldom heard of recently. It destroyed two L188 Electras not long after they were first introduced. To keep it brief, when two rotating or oscillating components at different frequencies on a common frame are disturbed a problem develops if those frequencies eventually coincide or “ couple.” In the case of the Electras a wobble at the propeller shaft coupled with wing flex frequency in turbulence tore the wing off.

I don't think you have missed any posts directly about that. Juan Browne (Blancolirio) has released three videos related to the accident, the latest being only the overall footage with audio seemingly aligned with video footage. He doesn't actually suggest the audio is 5Hz (or any other frequency for that matter), rather in the preamble to the video he indicates "The RPM matches that of the main rotor of the Bell 206L".

I have briefly tried to work out the 'frequency' of that audio but my mobile phone based app (Physics Toolbox - Tone Detector or Spectrum Analyzer) struggles to determine a primary frequency. No surprise really because the sound would be composed of multiple frequency components and the frequencies certainly slow down over time in the audio simply by listening to it. A better option to try to match the frequency might be to use something like the Physics Toolbox Tone Generator and play with the frequency and waveform. But using that app and plugging in a 5 Hz frequency shows that the 'apparent' frequency heard by (my) ears varies whether I set a square waveform, triangle waveform or sawtooth waveform with the latter seeming like a much lower frequency than the others. Using a 5 Hz sine wave is below my hearing range these days!

Given the 206L main rotor speed is around 395 RPM at 100% RRPM (strange, the Bell 206L Flight Manual doesn't explicitly seem to state that, rather gives all information only as %), then a full revolution has a frequency of around 6.6 Hz (395/60). But since there are a pair or main rotor blades, the blade passing frequency, which is apparently what we would pick up with our ears, is twice that, or ~13.2 Hz (395x2/60). If I understand the Noda-Matic suspension system on the 206L correctly, it is designed to reduce vibrations from the rotor system being transmitted into the fuselage at this frequency (many Rotorheads refer to this as "2 per rev").

If a pitch link gave way and one blade was at a crazy angle of attack, then I think audio might pick up a 1 per rev (6.6 Hz) underlying frequency component. If the helicopter was flying normally or if the rotor was shaking the Noda-Matic suspension to bits at the frequency it is tuned to, then an underlying frequency more like ~13.2 Hz should be heard in the audio.

If there are Rotorheads that have experience in acoustic analysis, would welcome their feedback.

It’s not really strange - it gives values that the pilot sees in the cockpit. The torque gauge isn’t in Nm or the Ng gauge in rpm, either.

I get it for the gauges, but for the manual not to say what 100% corresponds to anywhere still seems unusual to me. But then again, I couldn't fly a helicopter to save myself!

Well, it's a pilot manual not an engineering one. If there is a complimentary one (second volme) with systems descriptions that might have more details. The TCDS does have all the details, including Tq to Horsepower and Ng RPM, too: https://www.easa.europa.eu/en/downloads/7909/en

Sorry for thread drift.....

Exactly. We write flight manuals to give the pilot the limitations and operating information/procedures they need to fly the aircraft as designed & equipped. We avoid providing additional information which is not necessary, and may actually dilute the pilot's attention to what we need them to know. Sometimes in Section 7 (Systems), or a supplement, this information is provided, but not if it will have no benefit during flight. Pilots are greatly encouraged to read through the type certificate data sheet, and maintenance manuals for more detailed information - in between their flights!

Cited for reference to comments about what might happen if a Pitch Change fails only and not saying that is what happened to the aircraft in NYC.

https://aerossurance.com/safety-mana...430-accidents/

Just to note, while both can use frequency (Hz) as a unit of measurement, the vibration values used with helicopters (1 per, 2 per, etc.) are based on a physical oscillation felt in the structure and not what is heard acoustically as a sound. And in practice the force of those vibration oscillations are measured with specialized balance equipment in units of inches per second (IPS) and not with a sound meter or similar equipment. So the concern in the VH letter applies to the 5Hz vibration oscillation force in the tailboom structure and not any 5Hz sound heard on the video. If that makes sense.

No human ear can hear 5 - 6 Hz. The lower limit is 20 Hz when young, higher in adults. Therefore, all audio equipment heavily filters out lower freqs. Below 20 Hz signal strength is 0 in any audio recording.

The noise being discussed is consistent with a blade tip coming apart. It recures at 6.56 Hz, but is made up of higher frequencies in the audible range.

Harmonic vibration coupling in aviation accidents is rare but has happened. If blade bounce could shake the tail boom at the same frequency, is it not possible the combined resonance could do what whirl mode did to the Electra? I am not an engineer, but very familiar with the Electra story.

I follow your point. The FAA have a good summary of the pair of Electra accidents and suspected cause of both:

https://www.faa.gov/lessons_learned/...nd%20propeller

That page has video showing testing on a dynamically scaled Electra model in a wind tunnel and a still of the model with the wing torn off.

TheDoctorMedic has produced a good objective review of the accident examining possible causes:


Of note are the parallels he identified with the loss of the Air Evac Lifeteam Bell 206L-1 N114AE in Manchester, Kentucky in 2013 which was attributed to mast bumping:

Accident Bell 206L-1 LongRanger II N114AE, Thursday 6 June 2013

The only problem is that mast bumping was not mentioned even once in the final report on 114AE. The cause was the loss of control due to spatial disorientation which lead to an in-flight break up. It’s very misleading to say mast bumping caused 114AE and that because the transmission and rotor head look similar that 216MH was therefore caused by mast bumping. We have video proof that there was no IIMC and thus no spatial disorientation in New York so the accidents are not related.

The similarity to the Electra accidents increases when you consider the tail rotor drive shaft.
Tail boom movement could induce exactly the same condition in the tail rotor as the Electra propeller reduction gearbox oscillation. Combined with rotor bounce mirroring the wing flex
the possibility of this being the cause of the helicopter accident cannot be discounted.

I think the 63 years of service and 37 million fleet hours tends to discount that?

212man: I had never pondered "mast bending," and am not sure if the bent mast in the 2013 accident is attributed to something that happened in the air, or happened about the time of ground impact.
(I have not read the NTSB report on that HEMS mishap, which would likely clear up my confusion).

But with that thought foremost: if the main rotor mast bent while at flight RPM, in powered flight, at some point between the top of the transmission case and the rotor hub, that would probably create some vibrations and loads that the attach points aren't built to withstand. I an not sure what would initiate such a bend.

(So this post is mostly speculation, sorry about that).

Totally valid point. I think the timeline of destruction is key.

Not arguing in favour, but I think I know where the inference to the mast bumping originates. In the investigation documents (I am not allowed to post urls yet, so the long way), from the flightsafety page you jump to the NTSB Docket, there you can access the 'Helicopter Airframe Examination Report'.
On page 12 there is clear evidence that the stops made contact with the mast, though importantly the investigators offered no speculation as to when the contact was made, ie was it the cause or result of the rotor-system through upper deck departing.

And how many of those hours were flown with these blades and in conditions that could induce them to bounce? Look closely at the facts and the history. This accident may have a different cause, but the conditions are so similar to the Electra I will say once again, and with emphasis, this could be the reappearance of whirl mode.

Yes, it wasnt 5Hz mike my first guess but around 6.7 Hz for the initial three thumps using a analyze tool. 6.56-6.57 hz would bee the 1/rev sound from a 206L. Not a too bad match.
https://cimg9.ibsrv.net/gimg/pprune....6b23fc5ab.jpeg


The time increases between the pulses after around 5 of them.

Breaking a blade loosing more than a little of the tip and getting very high imbalance would explain the input drive shaft from engine breaking, or the motor stopping to produce torque = reducing Nr just like the increased time between each sound pulse.

It would also probably explain the detachment of the MGB with the ripping of the roof and tail bom loss.

Sorry, I shouldn't have paraphrased "attributed to mast bumping" rather should have said that TheDoctorMedic was indicating that mast bumping couldn't be ruled out in New York accident simple because the mast hadn't been fractured (as some were indicating). NTSB reported 114AE had about a 5 degree bend of mast just below the stops (which had left an impression on the mast) and a 20 degree bend below the swashplate. When that occurred isn't clear.

Just to add, I've found if a 206 mast is under power it will either twist or shear vs bend.

You'll find that 63 year service history does apply as Bell blades were known to "bounce" as well. However, there are a couple other differences between the Electra whirl mode vibrations and this helicopter. For one is the difference in the cycle rate of the "harmonic vibration coupling," to use your term, between the Electra and a 206 or any other helicopter. While the Electra destructive wing flutter happened over a period of flights or time, any similar "coupling" or bounce would have destroyed this 206 within that single event and probably within the 6 bounces. Its somewhat similar when a helicopter goes into ground resonance where the "harmonic vibration coupling" can and will self-disassemble the helicopter within a dozen or less blade rotations. The other difference, the "bounce" discussed here is induced by the pilot which in turn is usually due to a system discrepancy like an improper collective minimum friction setting vs a design issue like with the Electra. Because of the destructive nature of "harmonic vibration coupling" in helicopters, any design flaws are usually worked out long before the aircraft is even flying.