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."