Helicopter crash New York City

[helispotter]

Originally Posted by wrench1

I think the blades may have hit the tailboom twice after it departed especially when you line up the parts as shown in the pic. Takes a lot of energy to slice the stabilizer at the winglet like that. The inboard break looks more like impact damage.

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:

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(?):




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):



Here is how it looked when still intact:

[wrench1]

Originally Posted by Agile

take the AS350, from the B to the B3, power went from 650hp to 1000hp, MTOW from 4300Lb to 6700Lb (EC130). All they did is add reinforcement plate at the root (fastening to the fuselage) of the tail, but nothing diferent in between.

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.

Originally Posted by helispotter

My current 'guess' is that one point in the breakup may have looked something like this (rotor facing aft is mostly omitted from sketch & gap in tailboom is simply due to graphic limitations of my drawing package):

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.

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):

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.

[SplineDrive]

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?

[Salusa]

Originally Posted by wrench1

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.

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?




​​​​​​

[TwinHueyMan]

Originally Posted by Salusa

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.

[Salusa]

Originally Posted by TwinHueyMan

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.

.

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.

[wrench1]

Originally Posted by SplineDrive

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?

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.

Originally Posted by Salusa

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

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.

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.

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.

[USAFPAVEHAWKDRIVER]

Bell 206 Hudson River Crash

Originally Posted by ettore

According to the NYT

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.

[Squawk7700]

Originally Posted by USAFPAVEHAWKDRIVER

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?

[MechEngr]

Originally Posted by Squawk7700

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.

[ShyTorque]

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.

[Pilot DAR]

If the MGB seized, would the blades have kept spinning and auto-rotating to the water with the gearbox and everything shown, NOT spinning?

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.

[JamesT73J]

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

You Tube

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.

[sycamore]

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

[AAKEE]

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?

[Squawk7700]

Originally Posted by sycamore

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

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.

[ferry pilot]

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.

[helispotter]

Originally Posted by AAKEE

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?

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.

[212man]

strange, the Bell 206L Flight Manual doesn't explicitly seem to state that, rather gives all information only as %

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.

[helispotter]

Originally Posted by 212man

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!