Light aircraft down Pretoria, ZS-UKU KR2

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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by Arie de Wet » Thu Nov 25, 2021 9:49 am

8B65230A-C725-4156-B0C1-870DD7862953.png
Speculation mode on.
I believe the main spar was compromised in a hard landing by Sakkie in 1985. It was so hard in fact that the landing gear broke on the right hand side ,at the attachment to the main spar. This is incidentally the same place the spar failed in flight. Doing aerobics and stressing that already compromised spar was the last straw to break the camels back. I have attached a report on it by Sakkie (sorry for the poor quality, but it should be readable).
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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by jimdavis » Thu Nov 25, 2021 11:16 am

Frontiersman wrote:
Thu Nov 25, 2021 9:17 am
Airwayfreak wrote:
Thu Nov 25, 2021 1:21 am
How does a PPL hold a Grade II Instructor rating?
It could be that he once held a commercial licence and had it downgraded to a PPL? Maybe an instructor's rating is something that never expires once you have it, but can only be used in conjunction with a comm licence, so will be dormant if you downgrade to PPL until you maybe one day decide to go back to a comm licence?

Anyway, just throwing some suggestions out there, somebody more knowledgeable might be able to confirm it for us.
I suspect it was some sort of LSA or NTC instructor rating.

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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by Trevor Duane » Thu Nov 25, 2021 11:51 am

jimdavis wrote:
Thu Nov 25, 2021 11:16 am
Frontiersman wrote:
Thu Nov 25, 2021 9:17 am
Airwayfreak wrote:
Thu Nov 25, 2021 1:21 am
How does a PPL hold a Grade II Instructor rating?
It could be that he once held a commercial licence and had it downgraded to a PPL? Maybe an instructor's rating is something that never expires once you have it, but can only be used in conjunction with a comm licence, so will be dormant if you downgrade to PPL until you maybe one day decide to go back to a comm licence?

Anyway, just throwing some suggestions out there, somebody more knowledgeable might be able to confirm it for us.
I suspect it was some sort of LSA or NTC instructor rating.

jim
Negative Jim, the LSA instructors rating is categorized on a different scale, for obvious reasons

Frontiersman, that rating would need annual renewal.

As Airwayfreak says it is odd.
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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by Goffel » Thu Nov 25, 2021 12:17 pm

Instructors rating.....more likely a typo error and that he had an LSA instructors rating, "Grade B".......which you do not need a Commercial rating to have.
It has been a bone of contention between the professional pilots and the sport pilots ever since it was initiated,
It was for the microlight pilots in the old days, whereby never in a month of Mondays did anyone ever envisage modern technology getting to where it is now......
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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by Jan » Thu Nov 25, 2021 7:17 pm

The article mentions that the Landing Gear Springbar broke. Is this the actual landing gear and I assume it is steel?
Then the crack he mentioned - was that the noise of the wing skins "popping loose" maybe?
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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by Russell Phillips » Thu Nov 25, 2021 8:16 pm

The rectractable KR's have an aluminium "spring bar". It is not very springy at all and is easily damaged. I doubt very much that the incident mentioned in that article has any bearing on Neville's accident.
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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by Russell Phillips » Thu Nov 25, 2021 8:21 pm

This aspect on the other hand would appear to have relevance.
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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by Russell Phillips » Thu Nov 25, 2021 8:33 pm

fast forward to 9:36.

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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by Arie de Wet » Thu Nov 25, 2021 10:04 pm

Hallo Russell.
With all due respect, I think that if aileron flutter was the cause of this accident, the aileron would have departed the wing before the spar broke, or at least show damage to the aileron to wing hinge points. It doesn’t seem to be the case here ,if you look at the photos of the wing.
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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by jimdavis » Thu Nov 25, 2021 10:55 pm

Arie de Wet wrote:
Thu Nov 25, 2021 10:04 pm
Hallo Russell.
With all due respect, I think that if aileron flutter was the cause of this accident, the aileron would have departed the wing before the spar broke, or at least show damage to the aileron to wing hinge points. It doesn’t seem to be the case here ,if you look at the photos of the wing.
Have I missed something - why is aileron flutter under the microscope? Did he appear to be flying abnormally fast - or had work been done on the ailerons recently?

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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by Russell Phillips » Fri Nov 26, 2021 10:06 am

A control surface such as this almost full span, narrow chord aileron can with relatively small deflections cause the wing that it is attached to to produce copious amounts of lift without it or its hinges necessarily being subjected to massive forces.
Those gobs of lift are what breaks the spar. The fact that the lift reverses between positive and negative in quick succession makes it even more destructive.
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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by StressMerchant » Fri Nov 26, 2021 11:57 am

A few comments on the flutter aspects:
  • Flutter is not the only demon lurking outside the envelope. Simple structural overload may also cause disaster. Do not underestimate the load generated on a wing by rolling Gs or a “yank and bank” manoeuvre.
  • Ailerons are generally the first control surface at risk. The KR2 design appears to have fully balanced ailerons. As the airspeed increases, the other control surfaces can also flutter. Elevators and rudders. No idea if these are balanced on the KR2.
  • The balance weight needs to be securely attached. We generally design the balance weight attachment to 24G normal to the control surface
  • The freeplay of the control surface is an important factor. The freeplay is influenced by hinge wear and cable tension. For flutter flight testing, the cables are generally slackened to the minimum allowed by the aircraft maintenance manual, and the hinges deliberately bored out to represent maximum allowable wear. Go out of these limits, and you’re on your own.
  • Since the control surface drives loads on the lifting surface, the load on the control surface may be fairly small. The failure point may not be the control surface attachment.
  • Since flutter almost always involves multiple “degrees of freedom”, the stiffness in bending stiffness and torsion stiffness are relevant. A wing that is stiff in bending is not necessarily safe from flutter
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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by Russell Phillips » Fri Nov 26, 2021 7:16 pm

StressMerchant wrote:
Fri Nov 26, 2021 11:57 am
A few comments on the flutter aspects:
  • Flutter is not the only demon lurking outside the envelope. Simple structural overload may also cause disaster. Do not underestimate the load generated on a wing by rolling Gs or a “yank and bank” manoeuvre.
  • Ailerons are generally the first control surface at risk. The KR2 design appears to have fully balanced ailerons. As the airspeed increases, the other control surfaces can also flutter. Elevators and rudders. No idea if these are balanced on the KR2.
  • The balance weight needs to be securely attached. We generally design the balance weight attachment to 24G normal to the control surface
  • The freeplay of the control surface is an important factor. The freeplay is influenced by hinge wear and cable tension. For flutter flight testing, the cables are generally slackened to the minimum allowed by the aircraft maintenance manual, and the hinges deliberately bored out to represent maximum allowable wear. Go out of these limits, and you’re on your own.
  • Since the control surface drives loads on the lifting surface, the load on the control surface may be fairly small. The failure point may not be the control surface attachment.
  • Since flutter almost always involves multiple “degrees of freedom”, the stiffness in bending stiffness and torsion stiffness are relevant. A wing that is stiff in bending is not necessarily safe from flutter
In answer to some of your questions Stress Merchant,
On the KR only the ailerons are balanced. Provided the elevator is not built too heavy and Vne is not exceeded it seems to be ok. For the rudder probably the same applies.
The balance weight on the intact wing was still in good shape and attached I ascertained..
Torsional wing stiffness may have become impaired over 40 years as the polyurethane foam becomes rather powdery and tired. I prefer solid polystyrene cores personally.
The other valid aspects you point out I guess we'll never know the answers to.
We will however soon have answers as to whether or not the aileron was in balance. Once this is available I'm sure it will be posted here and elsewhere.
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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by Volo » Sat Nov 27, 2021 1:25 pm

All this debate and speculation on the reason for the structural failure and the investigative Authority are probably well aware of what failed first and the reason because they have all the bits undamaged by fire or water .

This is what really gets to me about our Accident investigators . What are they going to do - sit and stare at the evidence for a year before pronouncing on what they know today .

If they are really unable to pronounce on the failure mode then say so and invite someone who could probably help them out - its not that difficult - just do it .!!!

I suggest that they also read this thread and they might learn something !!!
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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by Russell Phillips » Sun Nov 28, 2021 7:36 am

Volo wrote:
Sat Nov 27, 2021 1:25 pm
All this debate and speculation on the reason for the structural failure and the investigative Authority are probably well aware of what failed first and the reason because they have all the bits undamaged by fire or water .

This is what really gets to me about our Accident investigators . What are they going to do - sit and stare at the evidence for a year before pronouncing on what they know today .

If they are really unable to pronounce on the failure mode then say so and invite someone who could probably help them out - its not that difficult - just do it .!!!

I suggest that they also read this thread and they might learn something !!!
Hi Volo
They have made contact and done exactly as you suggest. 10/10 for the investigators. Unfortunately the preliminary report is already published however I understand that the final version will contain additional information and findings. The fact that one wing remained intact provides a unique opportunity for analysis and hopefully safety benefits for the rest of us.
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