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Helicopter down outside Leicester City Football Club

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Helicopter down outside Leicester City Football Club

Old 14th Dec 2018, 20:27
  #1121 (permalink)  
 
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Originally Posted by [email protected]
So what caused the failure?

Since these duplex bearings are widely used in TR assemblies, their failure rate should be very low unless they are not installed properly.
Well, all bearings have some sort of life / failure profile. Off the cuff, I would venture that this application was quite arduous for a relatively small, sealed lubricant bearing so, I would presume that this bearing could fail quite quickly in use after loss of lubricant, or other causes of failure. Can anyone with type knowledge comment if any method of monitoring is used on this bearing?

OAP
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Old 15th Dec 2018, 03:29
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nodrama,
The step is probably not designed as a stop, it is far too narrow. If you look, there also is a similar step undercut at the inboard end of the bearing seating. This is probably designed so that the inner races very slightly overhang the seating at each end and so, there is no chance of the inner race being wedged up on a radius at the inner end and at the outer end the spacer can also fit against the whole outer end face of the inner race. These bearing races are very hard but quite brittle. Any unequal or point loading can cause fracture.

OAP
This is basically correct. But the "grooves" are stress relieving grooves ground or cut on a radius to prevent a sharp right angle on the shaft material. The outer edge of the inner race of the bearing also has a radius for a similar reason, in addition to ensuring that the pre-load clamping covers the flat face of the bearing without putting any radial force to it which could lead to misalignment.
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Old 15th Dec 2018, 12:44
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Basically, thanks for corrupting my correct post Old Farang. But, the groove and the and the step are there specifically to accommodate the engineering needs of the bearing inner races, as I described. Corner radii are used as standard practice and are incorporated as required. If the bearing did not need the inner races to be precisely seated in this way, the shaft would not be waisted by the groove or the step.

OAP
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Old 17th Dec 2018, 09:58
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Onceapilot you are right. Groove at the right end of bearing surface is engineering need for manufacturing (free space for grinding wheel/tool for close tolerance work on shaft) and a stress relief, as sharp corners are not desired there - stress/cracking promotion. On the left side - same thing - manufacturing process requirement - transition to close tolerance surface-not a step for spacer. Anyway, spacer should contact inner race freely, to transmit clamping force from nut. Am surprised sketch on S2/2018 report is not correct, as Nodrama suggests.
From photo it is obvious there is a standard right hand thread provided for bearing nut.

Would be of interest to hear what maintenance/servicing manufacturer put in Instructions for Continuing Airworthiness? As mentioned in my post#1123 A-109E had duplex bearing design and lubrication requirements changed considerably, Bell 212 and 412 requires lubrication of this bearing every 50 hours with warning - two shots of grease only.
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Old 17th Dec 2018, 10:05
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Thanks for your input hoistop. Between yourself, nodrama, Old Farang and myself, I think we got this covered! The next important thing will probably be some clarity from the investigators on why this failure occured.

OAP
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Old 17th Dec 2018, 10:58
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Originally Posted by hoistop
Am surprised sketch on S2/2018 report is not correct, as Nodrama suggests.
Not a suggestion, fact.

The picture is from the Training School notes, not from an approved Technical Publication.
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Old 17th Dec 2018, 13:52
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The next important thing will probably be some clarity from the investigators on why this failure occured
which will surely come down to installation issues or maintenance issues unless there is a bad batch of bearings out there...then the lawsuits will start flying!
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Old 17th Dec 2018, 19:11
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Originally Posted by Onceapilot
Between yourself, nodrama, Old Farang and myself, I think we got this covered!

OAP
damn, I am so glad you all got this...where would we be without the A team.
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Old 18th Dec 2018, 13:17
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Originally Posted by hoistop
Thanks, crab - I understand well, how follow-up hydraulic servo control mechanism works, believe me.
I am (still) wondering if piston is trully part of control shaft, or control shaft passes thru the hydraulic cylinder and is clamped/attached... to (hollow)piston somehow - in other words, did rotation of (undone) control shaft strain seals of hydraulic cylinder in a way they are not designed to or not? It is an important question.
If answer is yes, it means that in case this failure occured in cruise and pilot managed to somehow maintain (some) control, he might be soon faced with massive hydraulic leak - loss of hyd. system as a consequence. If this servoactuator is powered by both hyd systems, (two pistons one on top of other and dual spool valves as in standard dual servoactuators) both systems would probably soon leak hyd. fluid, rendering helicopter uncontrollable.
That was my original point, which still hasn't been answered. The bulletin clearly says what you say - "the control shaft passes through an outer shaft, which forms part of the Ö piston"
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Old 18th Dec 2018, 13:24
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Originally Posted by [email protected]
It was the pin and nut on the other end - not the spider/duplex bearing end - that failed and the excessive Tq was on the spider end nut which didn't fail and the split pin was intact. How did the excess Tq happen?...
Is it possible a few degrees of movement occurred, enough to slightly deform but not shear the split pin?
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Old 18th Dec 2018, 15:47
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Pozi, I`ll try to clarify a bit, p56 #1117; if you undo the nut at the `spider end’, and the gearbox back-plate, you should be able to remove the hyd. unit complete, leaving the t/r supported on the rotor mast, which has internal splines at its top-end #1102. The spider/slider have splines matching the mast-splines, but it is difficult to determine if there is a lock-nut at the end of the rotor mast as well. The pitch control rod must run through the piston to the feedback link, and the piston limits its travel at each end of the jack... That`s how I see it, unless anyone can produce a full cutaway drawing.
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Old 18th Dec 2018, 18:26
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Originally Posted by sycamore
Pozi,I`ll try to clarify a bit,p56 #1117; if you undo the nut at the `spider end`,and the gearbox back-plate,you`should be able to remove the hyd. unit complete,leaving the t/r supported on the rotor mast,which has internal splines at it`s `top-end #1102.The spider/slider have splines matching the mast-splines,but it is difficult to determine if there is a lock-nut at the end of the rotor mast as well.The pitch control rod must run through the piston to the feedback link,and the piston limits it`s travel at each end of the jack...That`s how I see it,unless anyone can produce a full `cutaway drawing.
Thank you, you seem to be agreeing with me!
A sectional drawing of this particular unit would indeed be helpful, some of the examples posted have been "schematic" and don't show the arrangement of the control shaft and outer (hollow) shaft.

Understand how the TR will stay where it is, supported by its own bearings. But wouldn't removing and replacing the hydraulic unit be complicated by the fit of the duplex bearing? However, this isn't really relevant to the incident.
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Old 18th Dec 2018, 20:21
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Originally Posted by Pozidrive
Understand how the TR will stay where it is, supported by its own bearings. But wouldn't removing and replacing the hydraulic unit be complicated by the fit of the duplex bearing? However, this isn't really relevant to the incident.
No. Six nuts hold the actuator (servo, hyd unit) onto the gearbox. Thereís the two nuts either end of the pitch control rod (shaft) and one bolt/ nut attaching the flying controls to the input/ feedback lever. 4 hydraulic pipe unions, and some electrical plugs. Thatís it. Very straight forward.
The pitch control rod (shaft) just passes through the centre of the duplex bearing (which is part of the spider) when the actuator (servo, hyd unit) is fitted to the back of the gearbox.

Last edited by nodrama; 18th Dec 2018 at 22:19.
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Old 18th Dec 2018, 20:42
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What percentage on the #1 Hydro system will auto-close the TRSOV after fluid loss/leakage?
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Old 18th Dec 2018, 20:43
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Originally Posted by sycamore
The spider/slider have splines matching the mast-splines, but it is difficult to determine if there is a lock-nut at the end of the rotor mast as well.
The spider slider isnít splined on this aircraft type. The scissor links transfer the rotation of the tail rotor hub to the spider.
Yes, there is a Ďnutí holding the tail rotor hub on (there is always a Ďnutí), though it doesnít look like a nut in the conventional sense.
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Old 18th Dec 2018, 20:59
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Originally Posted by Sir Korsky
What percentage on the #1 Hydro system will auto-close the TRSOV after fluid loss/leakage?
55% reservoir quantity
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Old 18th Dec 2018, 21:55
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Originally Posted by nodrama


No. Six nuts hold the actuator (servo, hyd unit) onto the gearbox. Thereís the two nuts either end of the pitch control rod (shaft) and one bolt/ nut attaching the flying controls to the input/ feedback lever. 4 hydraulic pipe unions, a some electrical plugs. Thatís it. Very straight forward.
The pitch control rod (shaft) just passes through the centre of the duplex bearing (which is part of the spider) when the actuator (servo, hyd unit) is fitted to the back of the gearbox.
Perfectly explained. So the control shaft is an easy sliding fit through the inner race of the duplex bearing? And it's the castellated nut (and spacer) that clamp the inner race to the step at the outer end of the control shaft?

Only question left is the detail of the control shaft/outer shaft/hydraulic piston, which isn't really relevant.
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Old 18th Dec 2018, 22:12
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Originally Posted by Pozidrive
So the control shaft is an easy sliding fit through the inner race of the duplex bearing? And it's the castellated nut (and spacer) that clamp the inner race to the step at the outer end of the control shaft?
.
Yes.
I canít help you with a detailed diagram of the servo actuator system. Iím not involved with component strip and overhaul.
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Old 18th Dec 2018, 22:23
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Originally Posted by nodrama


No. Six nuts hold the actuator (servo, hyd unit) onto the gearbox. Thereís the two nuts either end of the pitch control rod (shaft) and one bolt/ nut attaching the flying controls to the input/ feedback lever. 4 hydraulic pipe unions, and some electrical plugs. Thatís it. Very straight forward.
The pitch control rod (shaft) just passes through the centre of the duplex bearing (which is part of the spider) when the actuator (servo, hyd unit) is fitted to the back of the gearbox.

I appreciate your contribution, however there is something in your delivery that sets the weemonkey "hang on" alarm going...
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Old 18th Dec 2018, 22:27
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Originally Posted by nodrama


No. Six nuts hold the actuator (servo, hyd unit) onto the gearbox. Thereís the two nuts either end of the pitch control rod (shaft) and one bolt/ nut attaching the flying controls to the input/ feedback lever. 4 hydraulic pipe unions, and some electrical plugs. Thatís it. Very straight forward.
The pitch control rod (shaft) just passes through the centre of the duplex bearing (which is part of the spider) when the actuator (servo, hyd unit) is fitted to the back of the gearbox.
Part of the above is incorrect (I was watching the football, pathetic penalty shoot-out). The nut on the servo end of the pitch control rod doesnít get touched during an actuator replacement. It is part of the hyd actuator component and assembled/ locked at manufacture of the component.
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