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.