The type was not grounded, nor was there any immediate service bulletin, as would have been expected had there been an obvious issue.
The cause, a duplex bearing failure, did get a service bulletin as noted by Jahn R81, as a result of the investigation.

The current situation is that the actuator has been modified by replacing the right hand thread at the input end of the control shaft with a left hand thread.
This removes the repetetive inspection on the nut for loss of torque.
The 10 hour inspection on the duplex bearing remains. As does the rotational force check at 100(?) hours.

A number of duplex bearings have been replaced on a precautionary basis, I believe so far they have all been found to be serviceable.

My guess is a one off failure, either a manufacturing fault on the bearing or maybe it just didn't get greased.

Ericferret - Thanks for the input. The input control shaft redesign makes sense. Has there been any redesign of the duplex bearing (I know some were swapped-out under the SB, I don't refer to that)?

I hope the design issue was also a one-off failure.

Not as far as I am aware. We have carried out repetitive inspections probably running into well into three figures with no issues.

Which design issue?

The design issue where a seized bearing resulted in the single nut holding the tail rotor control mechanism in place breaking an ineffective locking pin and unwinding. As stated above, the design has since
been altered so that the nut does not undo; simply by making the thread left handed.

I'm surprised that it wasn't designed like that in the first place.

Precisely! I'd have thought an even safer design would be one where the nut on the control shaft isn't fixed but can rotate on its own bearing.

​​​​​​I believe this would be the safest configuration, because my question is if there was enough force and heat at play to weld the washer to the nut and snap the locking pin when the control shaft started to rotate as not designed is that not significant force to cause just as bad damage on a left-handed thread configuration if the same fault occurred?

WuW

Yes, the binding and subsequent overheating of the bearing must be the main issue.

All helicopters are a compromise, weight,complexity, cost. The 169 has clearly been built down to a weight. Hence the hundreds of titanium panel screws. Around 30 euro per screw.
The basic design philosophy for the duplex bearing is sound. The 139 hit 2 million flying hours, two and a half years ago with no undetected failures. So it is not unreasonable to use this system.

All helicopters have multiple areas where a single failure will cause the loss of the aircraft, if you design them all out the aircraft goes nowhere due to weight constraints bought on by duplication.

At the moment we have the tail servo mod, however, I am sure that a redesign is on the cards. This is not a quick fix and will require re-certification.
The idea of a second bearing at the input end of the control shaft seems a good one.
Helicopters tend to evolve and few have entered service without at least one major defect.

Worth remembering that more than one S76 was lost due to tail rotor control failure, main rotor head failure, not to mention engines chucking out turbine wheels. In comparison so far the 169 has seen a better introduction to service.

But the S-76 was designed over 40 years ago. We are supposed to learn from our mistakes, not keep repeating them.

A fellow pilot referred me to CAA Paper 2003/1 (available here https://publicapps.caa.co.uk/docs/33/CAPAP2003_01.PDF). Makes interesting reading, especially compared to the current climate 17 years later

It's been a few years since I had the temerity to post on PPRUNE but something I just read caused a double-take:
"The current situation is that the actuator has been modified by replacing the right hand thread at the input end of the control shaft with a left hand thread.
This removes the repetetive inspection on the nut for loss of torque."
I used to operate a couple of 1940s ex-US Army 6x6 Studebakers. All the wheel nuts on the L/H side wheels were left-hand thread. That's 1940s: wheels: on a truck...
These days I work on vintage watches, dating back to WW1 in some cases. Almost without exception the crown-wheels, which rotate counter-clockwise, have left-hand threads. There are some esoteric exceptions, but...
It boggles my easily boggled ageing mind to think that this strategy could have been ignored on a HELICOPTER! Blimey.

The part that rotated, the tail rotor pitch control shaft, wasn't supposed to rotate at all. Although it runs through and parallel with the (rotating) main tail rotor shaft it's only supposed to push/pull. Problem was, when one of the bearings that separated the two shafts seized, both locked and rotated together and this spun off the stationary retaining nut, breaking straight through its locking pin.

On a critically important system such as the tail rotor pitch control mechanism, the seizure of a relatively small bearing shouldn't have been allowed to cause a completely irretrievable situation without any prior warning. The design just didn't cater for the control shaft being spun up like it did.

Once the tail rotor pitch went to full negative, the crew had no chance of recovery.

yup, no-one practices for that one - it's usually stuck pedals or a pitch control failure that allows the TR to go to min pitch not full negative.

Have the AAIB published the final accident report on this? I can find no reference at all on the Gov AAIB search facility.

No, not yet.


No, not yet (for the benefit of the min character count).

Showing as Consultation Stage on AAIB website.

"Consultation stage - when an investigation is largely complete and a confidential draft report has been sent out for formal consultation in accordance with the regulations. The consultation process includes the time taken to consider representations and amend the draft report prior to publication."

Sort of embarrassing to be still stuck here. Crash in October 2018. Rapid progress to find the initial facts (SB1/2018 Nov 2018) and to find out what actually failed (SB2/18 Dec 2018), then two years later nothing published. The report is in consultation for some time now, which I take to mean that the lawyers for someone don't like the potential consequences of what it says.

Requiring the swap to LH thread on the rod seems like a "and in any event..." fix to me, as the bearing separating the control rod from the tail rotor drive shaft should not have failed in the first place. No doubt the report says why that bearing failed, as this is the information needed to ensure that t does not happen again. .

agree - very embarrassing - not to mention other (more straight-forward?) reports which appear to be taking an inordinate amount of time to be published (S-92). Kind of begs the question "is our renowned AAIB still truly independent"?

Which S-92 are you referring to? If it's the Irish SAR machine (report significantly delayed) then I would remind you that that accident is being investigated by the Irish AAIU, not UK AAIB.

That was a popular solution on truck wheels and also a few vans and high performance cars in the 1960s. This method has died out now that truck manufacturers have found out how to do proper fastener design, tighten them properly, and use reliable tightening methods. Truck wheels are generally no longer user serviceable because of the high torques and specialist tools.

Specialist fastener design is troublesome whenever the format of the threaded components prevent the male threaded element taking the role of a spring. Every standard bolt is designed to act as a very stiff spring and effective reliable tightening requires that spring to be taken to near its limit. If the format of the threaded components, or the low compressive strength of the components being retained by the threaded components, prevents taking the spring to its limit then there is a serious problem. This serious problem is extremely common, even in automotive and aviation sectors where one might think things are all sorted! This explains all the split pins, locking wire, adhesives, and so on that we are familiar with in those industries. It also explains why in modern designs we can find fasteners abandoned and they just glue it if they can!

Aviation also has the Titanium problem. This is because Ti behaviour makes it probably the worst material for threaded fasteners that is in regular use. (S-92 oil housing, Cougar 91!!!)

Find it incredible that the AAIB still haven't published this report. What is going on, or not ?!

The mind of a bureaucrat is different than ours.
The bureaucrat will never have a component failure while sitting at his desk investigating component failures in the world of aviation. Unless his coffee machine breaks down or something.

That is why I always say pprune discussions like this serve a useful function .... within days we will speculate on all the possibilities and it eventually gets narrowed down to just a few ..... pilots and mechanics will pay attention and maybe even check their own machines.

In the case of the Leicester crash we pretty well know what happened ... we just dont know where the finger points as to the cause .... was it people ... metal ... or design .... that is the tedious job of the investigators to sort out.

Same as the Cougar S-92 that went down in Newfoundland ..... within a few hours this forum discussion had pretty much narrowed it down to the titanium stud failure on the MTX oil filter ..... 3 years later investigators confirmed it.

For that reason I do not agree with folks who tell us not to speculate , but tell us to wait years for the official report .

Such folks have the mindset of a bureaucrat.

Any news about the final report of this accident?

https://www.gov.uk/government/news/a...sary-statement

That took long enough.
One recommendation should be that vanity flights in to stadiums should be banned. It was lucky that just the crew and passengers died in this incident.

My deepest sympathy goes to the crew who like the incident at Beccles were servants to those in the back.

When you say Beccles I assume you mean the AW139 accident with Lord Ballyedmond? I fail to see the connection. It is clear the AW169 accident was due to a mechanical fault and the outcome exacerbated by an unfortunate impact with a small wall (that ruptured the fuel tanks). There is no suggestion of pressure on the crew or crew competence, unlike the 139 accident. So what’s your point?

i think that he hasn’t worked out yet the difference between saying no and a mechanical failure, Captaincy has the priority no matter who the talking and paying bagage Is.

There seem to be a number of people on this network who feel that flying is an inherently bad and asocial activity. And that it should be restricted to the pursuit of saving the lives of (worthy) people, or the pursuit of taking the lives of the enemy.

Not surprisingly, many of those who carry this grudge have themselves indulged in the joy of flying their entire career using no expenses spared government-funded hardware, were trained to the highest standards by tax payers money and were exempted from many of the inconvenient regulations that govern the world of civilian flight.

I think it's a bit rich.

'Vanity Flight' my ar5e. Football pitch-sized HLS for a Perf A departure in a highly capable machine, get a grip. Or are you suggesting that helicopters should only be used on the understanding that a new machine will suffer from a mechanical/servicing HF failure at any time? I admit that I always assume something will go wrong just to combat possible 'startle effect' and to be prepared but your mentality means choppers should only land and depart using clear areas - which kind of negates the whole point of a helicopter......#darkages

In two words risk assessment.

I spend a lot of my time in Asia where the owners of King Power have vast commercial monopolies.

The football match prior to the accident was to highlight their chief executive and promote their brand.They arrived near the stadium at the training ground 1.5 miles away but chose to leave from the stadium after the match as this departure was being screened live in Thailand.

The risk although slight of such a mishap is not worth publicity gained by such a stunt.Demands from very wealthy people often lead to the death of pilots put under pressure by their employers.

In this case it went wrong as did this accident at Beccles in Norfolk.https://www.theguardian.com/uk-news/...rd-ballyedmond

Stunt?

Did any authority suggest there was a violation of any Rule or Regulation?

The risk associated with that departure was no higher than a departure from any other site that required that type of departure profile. Given the cause of the accident, it would have happened on the next flight anyway, and who can say what the outcome would have been.

You simply cannot compare this accident to the Norfolk one. This was mechanical failure, the other was pilot error/CFIT of a perfectly serviceable helicopter.
Yes, I agree, if this helicopter took off from a 2km runway and the bearing gave up soon after lifting, then maybe just maybe the outcome may have been different (but not vastly). If the bearing gave up on approach to a 2km runway it could have ended up in someone's front room.
We mitigate against risk as much as possible every time we fly, but if we continually fly a helicopter like a plane, we may as well get a plane.
If you want to mitigate risk to zero, then lock the hangar doors, eat Ryvita and drink tomato juice. 10 years later write a 1 page book on how interesting your life is!

Mike Flynn,

Perhaps you might provide us a copy of the Risk Assessment Protocol that you are using to predicate your evaluation of the Risks for the Takeoff performed that sad day so that we can all have a better understanding of exactly why you have the opinion you do re that event.

It might prove useful to be able to demonstrate the benefit of the Risk Analysis you do for your own operations and perhaps others might consider incorporating yours into their theirs.

Oh c'mon Mike Flynn....I guess you better give up your car as the chances of an accident and someone being killed are 10,000 fold? Was there really a need for you to pop to the shops that night for a bottle of wine, or get that ever so important takeaway, every journey being a risk to someone? If you go out for a walk do you wear a crash helmet incase of falling debris from an airliner? There was nothing risky about that departure.... just bad luck on the crew. I suggest you light up some candles in your house, and take cold showers, so that the boys and girls don't have to risk their lives going to oil/gas/wind platforms/farms in the N.Sea for our energy needs, as this is way more risky than departing a stadium...just look at the terrible loss of life over the last 20 years..:ugh:

B.