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"What is puzzling is that it looks like the rotors take only the top part of the MRGB with them when they separate rather than ripping the whole gearbox out."
I agree - Which leads me to wonder why it was weakened - previous overtorquing? I know it's one of the places I look at when I'm preflighting. It would have to be one hell of a pull, though. |
Originally Posted by paco
(Post 11865365)
"What is puzzling is that it looks like the rotors take only the top part of the MRGB with them when they separate rather than ripping the whole gearbox out."
I agree - Which leads me to wonder why it was weakened - previous overtorquing? I know it's one of the places I look at when I'm preflighting. It would have to be one hell of a pull, though. It’s my understanding that the planetary reduction gear is located towards the top of the 206 main rotor gearbox. A sudden stoppage of this planetary gear assembly ( as in the Super Puma a few years back ) might potentially bust out of the gearbox casing and split it in two. |
Originally Posted by [email protected]
(Post 11865281)
If the MRGB is allowed to move (as a result of one of the nodal beams or other fixings giving way), it would twist the TR drive out put which could, feasibly, create enough imbalance and drag along the TRD shaft to overstress the boom and cause it to fail.
What is puzzling is that it looks like the rotors take only the top part of the MRGB with them when they separate rather than ripping the whole gearbox out. Whatever the initial trigger, it rapidly becomes a catastrophic in flight breakup which even the best pilot in the world wouldn't survive. https://cimg0.ibsrv.net/gimg/pprune....087ff20e53.jpg |
This is interesting about the freewheel on 206's https://www.casa.gov.au/sites/defaul...separation.pdf
1. Effectivity All Bell 206 Jet Ranger and Long Ranger series helicopters. 2. Purpose Alert operators, pilots and maintainers to critical freewheel unit lubrication issues in order to reduce the possibility of torsional main rotor mast yielding and in-flight separation of the main rotor head from the mast. 3. Background Defect report investigations into continuing failures of main transmission freewheel units in Australia and a near fatal incident in Canada demonstrate that a malfunctioning freewheel causes varying degrees of main rotor mast failure, ranging from torsional yield to complete separation of the rotor head from the main rotor mast. In a 206, the transmission is not directly connected to the t/r driveshaft - it runs through the freewheeling unit and then the engine. |
FWIW: word is the MR, transmission, and nodal beam assembly departed together still attached to the roof/deck section.
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The transmission on a Bell 206 is held in by 2 x upside down V links
https://images.proxibid.com/AuctionI...5043/151-2.jpg These have broken before - as in this incident in Australia https://www.atsb.gov.au/media/news-i...e-requirements https://www.atsb.gov.au/sites/defaul...?itok=PUBkzyQG |
Originally Posted by RAFEngO74to09
(Post 11865537)
The transmission on a Bell 206 is held in by 2 x upside down V links
These have broken before - as in this incident in Australia |
The two V links are on the non-nodal beam equipped 206B. The 206L has four independent links between the transmission and the two nodal beam flexures.
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Originally Posted by Torquetalk
(Post 11865384)
Doing that kind of work, from that FATO, a Longranger would spend a lot of time at the top end of its performance on multiple cycles day-in, day-out. Using transient take-off power would be routine. Misjudgments about fuel load and pax weight with a very high turnover would also be easy to make. Without HUMS, there is no certainty that the aircraft had not been previously over-torqued, either occasionally, or routinely.
I doubt you would be anywhere near 100% torque on departure much less pulling any transients. Certainly not cruising around at max continuous Q. |
We have decided to ground our Longranger pending further information coming from the NTSB ,FAA ,CASA and Bell
Worth being conservative in this case I reckon! I do not believe it will be a very long time before the relevant information is available that will allow safe flight |
Originally Posted by Squawk7700
(Post 11865118)
This talk of the fuel call being a disguised mayday is fanciful at best. He called for fuel as he no doubt needed fuel, probably like he would do 10 times or more per day! Presumably to get the fuel guy/gal out at the pad ready to hot-fuel, saving time and money.
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Photo from the NTSB showing rear of the aircraft, and separation of the tail-boom aft of the attachment points.
https://cimg3.ibsrv.net/gimg/pprune....91cea2ae9d.jpg |
That also looks like what is left of the hot section and exhaust of the engine.
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Originally Posted by BigMike
(Post 11865765)
Photo from the NTSB showing rear of the aircraft, and separation of the tail-boom aft of the attachment points.
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Zooming in, what appears to be the remains of the engine and aux gearbox.
https://cimg9.ibsrv.net/gimg/pprune....3e5c27174d.png |
Originally Posted by helispotter
(Post 11865775)
In other words, not a case of the four boom attachment bolts having been the cause of the accident.
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https://cimg3.ibsrv.net/gimg/pprune....3579ef9b11.png
1. Vertical with tail-rotor(?) 2. Tail boom 3. Main rotor 4. Cabin https://abcnews.go.com/US/tourist-he...y?id=120755978 Watch ABC News video at 1:18 and note that main rotor separation occurs a lot later than tail boom and tail rotor. That would seem to indicate that the tail boom failed first after extreme and rapid yaw. |
You can see the T/R Driveshaft section is broken about halfway, ENGINE to OIL COOLER FAN section, just above the white Starter Generator in the above picture.
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Originally Posted by FlexibleResponse
(Post 11865783)
Watch ABC News video at 1:18 and note that main rotor separation occurs a lot later than tail boom and tail rotor. That would seem to indicate that the tail boom failed first after extreme and rapid yaw. What is impossible to tell from the few pixels contained in these videos and related images, are what caused the extreme yaw. Was it something in the tail rotor drive system that went, or, was it something in the main gearbox, transmission or mountings that let go first. That is nothing the currently available videos can answer, beyond confirming a sequence of visible events. |
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