Apate, As I recall I facilitated his AS355 Line Training. I could be wrong. Apologies if I used the word "Student" which I guess is reserved more for ab-initio.

I am certainly not claiming any Kudos from his outstanding act of airmanship. However, fwhy is this such an issue for you?

Simples, you made a statement that I know not to be true.

You recollection regarding his AS355 Line Training is also incorrect!

I think you are mistaken. As I recall 2 pilots came from CHC/Brintel to Police Ops with Veritiair as somewhat of an experiment to convince good P2s to remain in the Company group by giving them something interesting to do.. I completed the Line Training for both in Cardiff. I am almost 100% sure he was one of the pilots. Unless of course I am having a fantasy moment!!! Always possible at my age. I sent you a PM.

Overdrive

Not at all, it would have made no difference here as any issues were probably not visible. It just seemed a short time to me but I am comparing it with larger types.

Uplinker.

Things that need to be checked regularly are either on the outside (control linkages to the tail rotor, the blades themselves, as examples) or can be inspected either through a specific hatch or other access (for my 120, opening the rear cargo hatch allows me to stand inside the tail and check the battery and look down the inside of the tail boom, for example). Typically the tail rotor drive shaft itself does not require daily inspection and so it (and any bearings along its length) are not made easily accessible. They are, however, subject to specific inspection regimes defined by the manufacturer and performed by mechanics. Where regular inspection is needed (some Rotorway designs have a belt-drive) inspection is facilitated.

There will be many flight-critical parts of a flixed-wing that are similarly treated (looking for corrosion on wing spars is not facilitated by a myriad of little hatches that you can open in a walk-around).

I have not seen a "twin tail rotor" design, and I am not imaginative enough to speculate how that might work; sorry

Most likely dual tail rotors would result in an overall reduction in safety given that some tail rotor failures such as a departed blade can cause other damage. The complexity and added weight would also have a detrimental effect. One comparison are the chances of surviving a (single) engine failure at takeoff in single and dual engine light aircraft, I have heard they are roughly the same.

IF the cause of this accident was a servo loop failure/run to a stop it is not clear that a second tail rotor would have helped, especially given the extremely limited time to take action.

Question for those who know: Would an immediate shutdown of both engines have stopped the tail rotor or is the transmission coupling such that it would continue to spin as long as the main rotor was spinning?

And contra rotating, it's not difficult to do, I designed and built one for a ROV. It worked great when one of the gearboxes failed...

Example of why it is important to inspect the tail rotor drive shaft : http://www.aaiu.ie/sites/default/fil...UGHLIN-0_0.PDF

The tail rotor shaft is geared directly to the main rotor transmission, not to the engines.

Yes it does

From the CAA website Tail rotor inspection

Specifically ATA 64 – Tail Rotor – Tail Rotor Flight Control System – Inspection
Since that AD was issued, EASA decided to require an inspection of the TR duplex bearing, as additional precautionary measure.

When reading AAIB Bulletin S/1 2018 a few things catch my attention:

1). According to the bulletin the general wind direction was north-westerly with a strength of 10 to 12 kt at the surface and around 25 to 30 kt at 1000 ft.
2). The helicopter began a climb on a rearward flight path while maintaining a northerly heading.
3). The climb then paused. (somewhere above 320 ft)
4). Then the helicopter entered an increasing right yaw contrary to the pilot’s left pedal command
6). The helicopter reached a radio height of approximately 430 ft before descending with a high rotation rate.

— The climb out procedure was performed in downwind, (the heading was northerly, but the climb was rearward) so maybe the helicopter was following the same airmass with zero TAS so to speak?.. Somewhere above 320 ft the climb paused. Why?.. Did the helicopter reach a condition that sometimes is referred as ”settling with power” / climb in its own downwash? No descend was reported… If so, did the pilot manage to escape the situation with an unusual procedure by applying max power/max collective?-- The helicopter entered an increasing right yaw and reached 430 ft radio height before going down with high collective and power applied?

The wind direction for the take off couldn't have been much better, with it fore port side, it mitigates the likelihood of reduced tail rotor effectiveness while maximising forward wind component for a forced landing prior to tdp.

The report doesn't indicate the point at which the climb paused. It references undercarriage retraction at 320', and that maximum rad alt height was 430'. Aside from there being a pause, and that left pedal was applied to no effect, somewhere in the mix, we really know no more.

21 November, EASA issued AD No. 2018-0252-E, which now supersedes AD 2018-0250-E (of 19 November). AW169 & 189 (similar TR flight control systems).

"The incorrect installation of the TR servo-actuator, if not detected and corrected, depending on the flight condition, could possibly result in loss of control of the helicopter." Since the initial ASB from Leonardo, there has been further instruction to look at the duplex bearing.

Still an interim measure, but alters the inspection of the duplex bearing and servo-actuator within 5 flight hrs or 24 hrs.

1. Inspect TR duplex bearing in accordance with the accomplishment instructions - Part I of the ASB
2. Accomplish a breakaway torque check of the TR Duplex bearing in accordance with the accomplishment instructions - Part II of the ASB
3. Accomplish an inspection and reinstallation of the TR servo actuator castellated nut in accordance with the accomplishment instructions - Part III of the ASB

(CAA circulated 22 November, which I read this morning).

Where is the duplex bearing in this assembly?

Ignore the fact that this is actually a 365 but the principal is near enough -

Item 16 is the Duplex bearing and Item 14 is where the nut is roughly.

https://cimg9.ibsrv.net/gimg/pprune....7ca620d1bd.jpg

I will let you work out the rest for what happens if the bearing fails and what could possibly happen to the nut.

14 is where the pitch change arm and the TR servo attachment is on a 365.

Gray - what they call a duplex bearing is what allows you to superimpose horizontal pitch change movement onto the rotating drive shaft - you probably already knew that but I though I'd clarify just in case.

Crab,

My understanding of a "Duplex" bearing is that it's actually two ball (or roller) bearings sitting side by side in one case. They are used in various applications such as vehicle wheel bearings. They provide an alternative to having two separate bearings with a space between them and are used where lateral "rocking" or "run-out" needs to be minimised and also provide load sharing between the two races. They need to be carefully installed to avoid damage, more so than simple ball or roller bearings. If they are knocked in too hard (such as to overcome too tight an interference fit), they can suffer damage.

Are you sure you're not mixing this up with what is known as a "linear" bearing? A car gear change assembly I rebuilt had a small linear bearing in the housing. It allows the gear linkage's horizontal shaft to slide fore and aft (as when going from 1st to 2nd or 3rd to 4th) and rotate from side to side, as when going from 2nd to 3rd. These have rows of small ball bearings sitting in longitudinal slots in a tubular, centre cage which holds them in alignment. I ditched the rusted out one in my gear change and because I had great difficulty in finding a suitable replacement I fitted a plain, sintered bronze bush instead.

The bearing is to allow rotation of the hub around the static control rod. It's a duplex bearing because it has two races precisely matched to maintain alignment when the shaft is moved horizontally.

In my personal opinion it looks like the inspections are checking both ends of the control rod for anything that may cause it to jam or break.

I would hazard a guess that there was a major mechanical failure that caused severe damage to many components and they are looking for the initial cause. Were there not eye witness reports of "gear crunching" noises?

I'm well aware of the function yes. I've overhauled alot of 120 and 130 TGB's.
Its likely a similiar position to it enough that I can relate it to the 169 at least.
They use a double row ball bearing that seems undersized for it's function. Only seen a couple failures though, but nothing catastrophic.
I just wasnt sure where it was located and how it was connected to the nut issue previously.

I am not familiar with the 169 directions of rotation, or left/right hand threads for that nut. but can anyone say for certainty what could happen if the bearing seizes? will it rotate that shaft and un-thread the nut with enough force to overcome the torque, and safety locks?

GHH,

If you look back through the message thread there are a few pics of the arrangement. The locking in the pic implies a RH threaded nut and the directions of rotation would imply your theory could be correct.

Normally the servo piston is keyed in some way to stop this happening but you can't tell from the IPC drawing. Having a split/cotter pin and locking kind of implies that this is the only locking. If it was keyed you wouldn't go to that length.
Similar things have happened before but not through design issues.

Once the feedback link is disconnected the servo would invariably motor to the end of full travel and stay there.

Pic at #682 and IPC at #630

Surely the issue is that if the bearing seizes then your tail rotor pitch control is now jammed. Whereas if it is just the nut coming off then it means that control is lost.

The bearing is seized in the rotational sense as it isolates the static and rotational part of the control. Then the rotational torque is transferred to the static part of the shaft and could unwind the nut as the torque is now applied
to the static shaft.

The nut connects the static pitch change to the feedback lever. The feedback lever cancels the pilot valve input once the piston reaches the desired position. Standard servo actuator behaviour.

Without feedback the pilot valve input will not cancel.

I may have misunderstood, but doesn't the duplex bearing also help transfer the horizontal load from the static to the rotating part?

If the bearing fails then a number of possibilities then exist. Losing the nut at either end is one, but a misalignment of the static rod leading to it jamming is another.

The servo may run hard over, or may run to neutral depending on how the feedback control is connected. I don't have any details of the servo to be able to say with certainty.

Tired of listening to 'I'm not familiar with the AW169' and the utter rubbish. The duplex bearing is at the end of the grey bit, at the output end of the red bit. Knock yourselves out guys -
https://cimg3.ibsrv.net/gimg/pprune....de4f433990.png
https://cimg4.ibsrv.net/gimg/pprune....40e6d53214.png
https://cimg5.ibsrv.net/gimg/pprune....ad440098c0.png

NODRAMA - For a man with that handle you certainly like to make a "Drama" out of other peoples honest statements.

regardless of experience on type and you're oh so helpful "red bit and grey bit" descriptions, I'm sorry my experience isnt helpful to you in anyway and I'm only triggering you.

Sorry but I'm with NODRAMA. Posting a pic of the 365 system, which is about as different as you can get from the 169 and labelling the duplex bearing as item16 is not helpful at all.

Looking at your 365 diagram, number 21 is about the closest you could get to where the duplex bearing is.The duplex bearing is outboard of the tailrotor and is nowhere near the hydraulic part of the servo.

Thank you for posting accurate diagrams NODRAMA.

For those that can read English.

Where it is physically is irrelevant - it performs that same function.

The issue that is in the debate is the consequences of a failure of the duplex bearing.

Thanks for the pics nodrama.

As you seem to be conversant with the type can you inform us as to how the pitch change shaft is locked against rotation at the servo end apart from the clamp up and a split pin and a bit of lockwire .

Why all this pointless armchair speculation/discussion? The only authoritative body here is the AAIB. Why not wait for their report! I suspect there are many who are seeking to say
"I told you so!"

TF

Hardly believe it's pointless.
if you fly one or fix one, its proactive discussion.
but if you would rather wait a year or two be my guest...no one is forcing you to participate in the discussion.

Because this forum is for armchair speculation/discussion. Whether or not it's all pointless is a matter of opinion...
As for, "I told you so"...here's my theory. Somehow the aircraft was released from the factory in 2016 with an 'incorrect installation of the TR servo-actuator'. In practice, probably meaning that the cotter pin and lock-wire were never fitted, allowing the nut to work its way loose over time until the final (as it turned out) pedal input allowed it to fall off, resulting in complete loss of tail rotor control.
This is entirely pointless (but, I believe, harmless) speculation from my armchair based, almost entirely, on the preceding 42 pages of discussion.

If you look at p11,#216,pic of tail rotor,it appears that the t/r `spider is intact,and appears to have a `device` on the outside of the `spider`...perhaps `nodrama` can comment if this appears normal..? nut /bolt still attached...?