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Helicopter down outside Leicester City Football Club

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Helicopter down outside Leicester City Football Club

Old 27th Jun 2020, 11:09
  #1201 (permalink)  
 
Join Date: Dec 2005
Location: England
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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.
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Old 27th Jun 2020, 12:40
  #1202 (permalink)  
 
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But the S-76 was designed over 40 years ago. We are supposed to learn from our mistakes, not keep repeating them.
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Old 29th Jun 2020, 20:09
  #1203 (permalink)  
 
Join Date: Jun 2008
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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
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Old 30th Jun 2020, 16:05
  #1204 (permalink)  
 
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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.
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Old 30th Jun 2020, 21:51
  #1205 (permalink)  

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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.
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Old 30th Jun 2020, 22:21
  #1206 (permalink)  
 
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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.
crab@SAAvn.co.uk is online now  
Old 12th Dec 2020, 18:56
  #1207 (permalink)  
 
Join Date: Feb 2009
Location: England
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Have the AAIB published the final accident report on this? I can find no reference at all on the Gov AAIB search facility.
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Old 12th Dec 2020, 19:16
  #1208 (permalink)  
 
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No, not yet.


No, not yet (for the benefit of the min character count).
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Old 12th Dec 2020, 19:18
  #1209 (permalink)  
 
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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."
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Old 22nd Feb 2021, 14:59
  #1210 (permalink)  
 
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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. .
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Old 23rd Feb 2021, 10:07
  #1211 (permalink)  
 
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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"?
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Old 23rd Feb 2021, 11:30
  #1212 (permalink)  
 
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Originally Posted by JulieAndrews View Post
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.
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Old 23rd Feb 2021, 16:26
  #1213 (permalink)  
 
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Originally Posted by skridlov View Post
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.

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!!!)
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