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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, 10:09
  #1201 (permalink)  
Join Date: Dec 2005
Location: England
Posts: 1,278
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.
ericferret is offline  
Old 27th Jun 2020, 11:40
  #1202 (permalink)  
Join Date: Apr 2000
Location: EGDC
Posts: 7,861
But the S-76 was designed over 40 years ago. We are supposed to learn from our mistakes, not keep repeating them.
crab@SAAvn.co.uk is offline  
Old 29th Jun 2020, 19:09
  #1203 (permalink)  
Join Date: Jun 2008
Location: England & Scotland
Age: 60
Posts: 1,370
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
John R81 is offline  
Old 30th Jun 2020, 15:05
  #1204 (permalink)  
Join Date: Feb 2008
Location: sussex
Age: 71
Posts: 114
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.
skridlov is offline  
Old 30th Jun 2020, 20:51
  #1205 (permalink)  

Avoid imitations
Join Date: Nov 2000
Location: Wandering the FIR and cyberspace often at highly unsociable times
Posts: 12,698
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.
ShyTorque is offline  
Old 30th Jun 2020, 21:21
  #1206 (permalink)  
Join Date: Apr 2000
Location: EGDC
Posts: 7,861
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 offline  

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