With my 28 yrs of aircraft maintenance involvement I can only agree with nodrama, that report is surprisingly ambiguous - AAIB was always a first-class source of information. Using term "solenoid valve" for a hydraulic system component is softly said, misleading. (except in case there is truly an electromagnetic valve involved - in that case I will have to bury myself )

nodrama, can you comment or confirm that torque applied to castellated nut, that holds duplex bearing in place on control rod, has no effect on the bearing itself i.e. that overtorquing it will not "squeeze" balls between inner and outer race, but "only" overload the rod-threads?
Is duplex bearing actually made of two completelly separate bearings, turned one against the other, or is it one component? From the Fig. 4 in AAIB S2/2018 it seems there are two inner races, but one outer race. Such design implies that nut torque could have an effect on bearing friction? (if there is a gap between inner races-depends on design) Please help clarifying this, if you can!

hoistop

hoistop, I think this is a good question. The AAIB report on page 5 states that this nut was "found to have a torque load significantly higher than the required assembly value".

In addition, on page 7, paragraph 2 "the increased torque load on the castellated nut that remained on the spider end of the shaft is consistent with rotation of the tail rotor actuator control shaft"

Are they saying the jammed bearing would tighten the nut? If the rotor is turning anti-clockwise, the bearing jams and the thread is clockwise it does not seem to make sense. Or does it mean it could have tightened on impact?

on an aircraft the size of a 169 there is no way you are going to move the TR without hydraulic assistance.

Aircraft like the Squirrel or Gazelle are enough of a handful with hyds switched off in yaw.

In the nineties, at BHL Alister Gordon, introduced an amendment to the 212 flight test schedule to include a double hydraulic failure. His reasoning was that if an aircraft had a single failure on a rig, it could still be flown back, on the assumption that a second failure would be controllable. It wasn't, the aircraft each time went into a right hand dive and turn, and the pilot urgently said turn it back on, or words to that effect.
The amendment was withdrawn after Bell said ,"stop being so stupid, your going to kill yourselves".
With regard to the S76 tail control aft quadrant spring centring system, this was fitted to put the system to neutral in the event of a cable brake. It would have no effect on a servo runaway.

malabo - which models? Ones with 2 bladed TRs? I think TR power in these modern aircraft is considerably higher - the 139 for example can deal with a 40 Kts crosswind - I stand by my assertion that manual control isn't possible.

ShyTorque, thanks for your explanation and that's how I initially read the report as well. However if you pick up a bolt and a couple of nuts it would seem that in order to loosen the bolt which came off the torque you need to apply via the inside race of a bearing would tend to loosen the nut holding that bearing on also. I am lost as to how this can happen the other way around. Do the actual directions make sense to you? If so I must be misunderstanding the way it is assembled.

It wasn't you. :rolleyes:

I don't know if 2-bladed TR systems take less control input force than three bladed, I'm just expecting some correlation between weight of helicopter and the tail rotor thrust, or control input force. AW169 gross (increased with a kit) is 10582 lbs. Bell mediums with a single hydraulic to the tail-rotor were the 430 - 9300, 205 - 11,200, 212 - 11,200, 412 - 11,900. All these are flyable and landable with no hydraulics to the tail-rotor servos. I had to do it in training for those types and I trained others to do it later.

I'm hesitating on the AW169 because I'm unsure of the mechanics of the Teleflex system and whether the leverage and control strength is there. Agusta/Leonardo, like Bell, can build some pretty stout pedals though, judging from the abuse they can take on the AW139 just setting the parking brake. It would be a major surprise to me if the 169 tail rotor wasn't controllable without hydraulics Gotta eat crow and agree with crab on this one, given no drama's patient explanation, but it has two systems and the main rotor isn't controllable without at least one, so a somewhat moot point that only serves to pad the thread while we wait for answers.

Not thread drift, but just to correct any misunderstandings, the 212 main rotor is controlled through a stab bar and can be flown with both hydraulics shut off, of course, otherwise Bell could not have certified it with that switch logic. Flight Check Procedures require shutting both off in flight at 70 knots - look it up. In my early days in the industry we expected pilots to be able to land it with both systems off, and we all did, and when we became instructors we taught it. Even spaghetti-armed Brit pilots that found it impossible at BHL, magically found the strength on this side of the Atlantic where failure meant no job.

It isn't possible to even move the flying controls on an AW169 without at least one hydraulic system pressurised. That's why I made the point of saying earlier that the MR/ TR pitch control is hydraulically operated and NOT hydraulically assisted. There is a major flying control design difference. The pilot input isn't direct to the rotor pitch control, but to the hydraulic servo input valves. If the pressurised servos aren't commanded to move, nothing is moving. That's why there are two systems, for redundancy. How much clearer does it have to be?

I think we all understand.

M25,

The tail rotor hub and blades on a 169 are on the right hand side of the tail boom and rotate from nose to tail at the top (i.e anti-clockwise as you look at them from the right hand side of the aircraft). The control shaft sits inside the outer drive shaft and hub. When the duplex bearing began to seize, the control shaft would have also tended to rotate in the same direction as the hub, i.e. anti-clockwise. *Any drag on the nut on that end would tend to tighten it because it has a right hand thread; it would be the same as tightening any normal right hand threaded nut and bolt.

Edit: *As the hub rotates in its entirety, this cannot actually have been the case, my error!

If you now move to the left side of the aircraft, the tail rotor and "errant" control shaft shaft appear to be moving in a relatively clockwise direction. The castellated nut on that side also has a right hand thread. With the shaft rotating clockwise, any drag on the nut (i.e. from contact with the stationary pin carrier to which it was bolted) would tend to cause the two to be unscrewed.

If instead the control shaft and its nut on that end had a left handed thread, the relative motion would have tended to tighten them up, as is the case on the right hand side of the assembly. Whether that would have helped prevent the catastrophic failure, I really don't know. If the design of the pin carrier had allowed the control shaft and nut to spin freely together, they would have presumably stayed together, even in rotation.

Regarding the TR control, I think there are several slightly different angles at play here: 1) In the control system on the 169 physically connected so that it's possible, given enough force to change the pitch (apparently no). 2) Would it be possible, considering the forces at play, to design a control system with a manual fallback.

If 2) is true, I guess one could discuss if 1) was a smart design choice. More than the TR control system in isolation should probably be considered if such, if the aircraft won't fly without hydraulics for other reasons, manual fallback for the TR might be completely pointless.

Generally I always prefer manual fallbacks, but they even make cars where the steering wheel and brakes has no such fallback these days. I guess it's part of a trend of over-confidence in system designs that leads to arrogance. They can probably also save some money designing a system without such fallback in many cases. I have no idea what considerations are behind the 169 control system were though, so this isn't meant as a speculation for their reasoning.

I suspect that as has been stated here before, the forces required to move the tail rotor blades on such a big aircraft would be too large for the pilot to manage without assistance from a hydraulic servo. I know that from personal "feet on" experience on rather smaller aircraft where the hydraulics become no longer available (as we know, many smaller types of helicopter such as the Squirrel, A109 etc can revert to manual flight control, but even then it's very hard work to control them).

What would be better would be a design arrangement that "self centres" the pitch angle of the blades to a pre-determined, neutral setting if the pilot's normal yaw control system is lost. This would allow flight to be continued, to some sort of a controlled running landing, as on other helicopters. Pilots can be trained how to handle this lesser, although still serious, type of emergency.

Interestingly, all the 'bearings' in the AW169 tail rotor (flap/ pitch/ lead-lag) are elastomeric. I'm wondering whether, with no hydraulic pressure to the TR control servo, the TR blades would return to their 'neutral' position by themselves under the force of the elastomeric's wanting to return to their normal unloaded position? I could try it with a hydraulic rig sometime, but it wouldn't take into consideration any dynamic loads that would be there in flight.

Where they naturally revert to depends on their CTM / ATM ratio... but the aerodynamic loads are very large and would easily overcome any tendency to "self centre" at rest via the elastomerics.

When the shaft unwound itself from the nut, presumably the shaft was then free to float in and out, so it was the buoyancy of the blades that pulled it out. These blade grips could have had bob weights fitted to keep them neutral if control went limp.

Might have been different if he had done a yaw check after initial lift-off...
Todays helicopters are designed with much shorter and more blades giving a higher `solidity ratio`,and shorter moment arms to the tail rotor,with almost as much `area` in front of the rotor mast as behind,than `classics like the WX,S-K,etc,and giving reduced directional stability...
In that case in the event of a t/r failure,it should be possible to design the CTM/ATM to position to a net thrust against power bias,for control failures,and `fuses` in the hydraulic system in the t/r system in the case of a t/r hyd leak/failure there.