Could anyone help me with a material strenght calculation of this operating horn from crashed plane? 10

[RedSnake]

Picture elevator to the left and stabilizer to the right.

The red cirkel is a tourque tube on the rudder and have a diameter of 38 mm and the thickness is 1 mm.
The operating horn was a welded box design using steel sheet AISI 4130 in condition N thickness is 1 mm welded against the tourque tube.
On the uperside and sides (3/4 of the circumference) has 30% of the normal materiels strenght(severe lack of fusion).
The last 1/4 can be brittle or have been sheared.
The red triangle is the mechanical stop maximum up.
The relationship between hinge, tourquetube and elevetor edge ca..
0 --------1 --------------------7
Can this construction hold for 5,7 G ???


[img [URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1604784211/tips/STABH2_lpxbcz.jpg[/URL]


Properties Metric Imperial
Tensile strength, ultimate 560 MPa 81200 psi
Tensile strength, yield 460 MPa 66700 psi
Modulus of elasticity 190-210 GPa 27557-30458 ksi
Bulk modulus (Typical for steel) 140 GPa 20300 ksi
Shear modulus (Typical for steel) 80 GPa 11600 ksi
Poissons ratio 0.27-0.30 0.27-0.30

 

[Alistair_Heaton]

just to note Va is the speed which we can use the full control movement which theoretically we can't break anything.

There is 30 to 40 of these V speeds and they all have a very exact meaning and limitation and set of conditions when they apply. When you have been doing this game for a few years you speak V speeds and acronyms for distances for performance like it was proper English.

I only add this in as a note to the excellent post above who knows way more than me about the true design criteria, than I do as a stick monkey.

 

[RedSnake]

I do have a lot of diffrent data but never as it seems the ones I need.

Best regards Anna

 

[zeusfaber]

I'm sorry for your loss.

I haven't read the report yet (there are some things I know are best not done immediately before I want to try to sleep - but I will); also, I've been out of the aviation world for more than 20 years and it's probably 30 years since I called myself a skydiver. Having said that, I've done one or two investigations and been interviewed as "the engineer who is responsible for..." for one or two as well. Purpose of saying all that is that this is a general observation, not one specific to this particular investigation.

Thorough, well conducted investigations are really good at discovering problems - including defects in design, material, manufacturing, documentation, operation and corporate culture.

Some of these come to light because the investigator is the first person to have appeared on the scene who is equipped to ask the right questions, many of them because the shock of the accident makes the people involved question their own practice and some because the destruction brought by an accident reveals bits of the structure that were previously concealed, or breaks something that ought to have survived.

Hopefully, they will uncover the defects that caused or contributed to the accident. At the same time, they are likely to uncover a number of other things - sometimes quite shocking - that didn't actually play any part in the accident.

Having identified all those defects, a decent investigation will do two things.

Because the primary aim is to find out why the accident happened, it will evaluate whether the defect contributed to the accident. In this particular case, the investigators probably asked questions like and "did the rest of the accident sequence look like the sort of thing that would have happened if this broke" and "if this connection hadn't failed, would the outcome have been different".

It will still document the discoveries that didn't contribute to the accident and, if necessary, make recommendations around these too. Most investigation reports make the point on their first page that they are there to enhance safety for the future, not to attribute blame - and that aim requires a thorough disclosure of anything that might prevent future accidents - even quite different ones. The best report formats have a dedicated "additional observations" section within their "findings".​

This approach doesn't come naturally. Human nature tends to assume that bad outcomes must have been caused by the most egregious of the potential factors - whether or not there is any causal link. If you don't control this tendency, you can end up with a report whose analysis and conclusions don't bear scrutiny and a culture where the subsequent revelation of any related problem at all leads to a public rush to judgement equally devoid of credible analysis. When Alistair (9/11/20 09:38) talks about British military investigation being broken, I suspect he has in mind an infamous case where both of these things happened in quick succession.

From reading the rest of the thread, the facts that a weld fell far below the standard you'd expect to see in a primary flying control, that the inspection techniques/practices didn't/couldn't pick that up and that the joint actually did fail at some point in the accident sequence sound exactly like the sort of shocking discoveries that automatically attract blame.

A.

 

[RedSnake]

I had a picture of how such a survey would go before this happened, which involved
1. Collecting facts, doing interviews and documenting everything, with pictures, producing measured values ​​of what can be measured and that this is done in a methodical and objective way.
2. Analyzes the facts one has and follows up deviations both technically and if such have emerged in interviews, this also in an objective and methodical way.
If you do not find any clear causes for the accident here, you must proceed with step
3. Use experience and cutting-edge expertise in the area to find a cause.

I have asked friends and colleagues if I have too high expectations when I think this is how it should go.
But everyone says they thought this was how they did it.

When I talked to the head of the investigation (I have nothing bad to say about him)
He has been very accommodating and in my opinion he is very unpretentious.
He say that I want it to be a technical error because I am a technician ..
But really, I just want to feel that all the technical aspects have been investigated 100%.

I asked for a picture of the underside of the stabilizer and elevator and got two.
But on both someone has been bending back all the pieces of metal and no picture of it exists before someone touched it ?!
I asked if they measured the hinges for the elevetor .. answer we have felt them ..
I asked that he would check the service manual if these were changed or checked ..
He came back and sade that they were lubricated 70 hours ago the interval is 100 hours .
These hinges are never to be checked in the preflight inspection and the service manual stats that they should be replaced if they become very loose, there is no exact measure of what applies here.

It is possible that much of what I am asking for has been done, but that the report has not explained this in a sufficiently clear way.
That and what is the basis for the technical conclusions drawn.

It is of course good that the shortcomings for the supplier have been pointed out, even if it is not stated in the report.
But I might have expected a service bulletin to be issued that these levers should be checked on other aircrafts because they can be even worse than they were here. Although that may not have been the cause of the accident.
But since I am a woman and need to be 110% sure of my facts before I start a crusade about this ;-) I am stuck a bit because I can not prove my thesis myself.

In Sweden there is a playful expression "boy guessing" I does not think it is possible to translate it in a good way.
But it means that men often claim that what they say is facts even if they only know 75% for sure, unlike women who need to be 110% sure before they speak. ;-)

This is not directed at anyone here.
It's just to show my attitude and thinking.
In my department, it is only used by my male colleagues, to say I'm not really sure. Especially when I'm nearby

Best regards Anna

 

[Alistair_Heaton]

there is a system called Airworthiness directives which EASA holds the systems for Europe.

Personally I find the FAA system easier to navigate and find stuff.

Here is the AD's which are current for this type according to the FAA. CASA which is the Australian CAA also has a similar system. They all tend to follow each other these days.

https://www.federalregister.gov/documents/search?conditions[term]=GA8-TC320

I haven't looked through them to see if they have issued one for checking the welds.

There are several levels of checks for aircraft which come up over certain time periods 50 hours 150 hours and yearly and 3 yearly is normal for light aircraft. What I suspect will happen is that they will add in the weld inspection at the 3 year inspection when the do the corrosion check.

What could also be happening is that as CASA has more of these aircraft type on there boos they have handed it off to them to see if its a rogue bad part or its fleet wide issue. That side of things won't be fed back to the investigators. An AD will just appear with a time frame for completion if its a fleet wide issue.

 

[RedSnake]

Okay, I've just checked their own website.

https://www.mahindraaerospace.com/support/

/Anna

 

[LittleInch]

Redsnake,

anna, It's taken me a good few goes to get my head around the fitting of the parts and the relevant welds.

From your diagram of 8th Nov 16:16 and the subsequent pictures of the failure, my understanding from that is that the key welds in terms of transmitting forces from the control cables to the elevator are the two 23mm welds which run in the same axis as the torque tube. I think this is what the diagram you received from the AIB is trying to show also.

The welds which you show as being poor around the top of the tube would appear to be mainly sealing welds and also those which I am guessing are the closure welds for the box section are also not the ones subjected to high loads.

I've looked again at the other diagram and the word "liv" has been translated as "life", but looking into other sources it can also mean surface or vertical surface apparently

https://www.proz.com/kudoz/swedish-...n-civil-engineering/5632430-liv.html#12504928

Then the post of 8th nov 18:41 starts to make more sense as to what they are saying which is that the forces on the arm are being resisted by the main 23mm long welds and the other welds are really there to simply hold the rest of it together and don't really add anything to the strength of the box section. I can see why that assessment was made now a bit clearer

I may have got this wrong as it is difficult to see everything in a 2 D drawing which doesn't really show the manufacturing sequence or fully explain the factor of safety on the item in question.

I understand your desire to investigate everything which seems odd ( as this does) and be 110% sure which others may not before opening mouth.

I also don't know what was attached to what in the final state, i.e. was the control arm still attached to the plane or had it detached with the elevator / stabiliser?

It would be very interesting to know if the manufacturer is engaged with this and interested to investigate further or whether they've pulled the shutters down.

I would say that perhaps the best legacy you could obtain for your partner is more to educate the skydiving fraternity and new jump pilots on the perils of overloading the aircraft otherwise you won't be the last to mourn. Flying mobile unsecured large lumps of material in a small aircraft when I come to think of it is rather more dangerous than I understood at the time as is the danger of getting more than 3500m. Only once did I get the hypoxia thing when we had bounced around between 10 to 14 000ft for 10 minutes due to air traffic control and when I finally left at 16,000 I didn't really know what I was doing. Pilots thankfully were on O2, so maybe there is something to challenge. Skydivers will always ask for more height if they can. It used to be generally 10,000 ft or 12,000 in the UK, but 3,500 or 4,000m in Europe. I would be going for 3,500m max without oxygen for the pilot, especially if he's operating in controlled airspace where delays could occur. Really - what's one Oxygen bottle going to cost?

I wish you luck on your quest and please keep us informed as to how it goes - your story touches many of here I think who but for the grace of God could be you or your partner.

LI

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[RedSnake]

I agree with that, and I have no problem with the recommendations, they could have been reached in a risk analysis even before this happened.
But another thing that is more personal and not "a fact" is that my partner was Mr. Security Himself.
He was their most experienced jumper and he would never have taken a risk or allowed another jumper to create unnecessary risks and the others would have respected his order to stop if someone did something risky. He has held every position one can have except tandem jumper and chairman.

Everything except the left wing had broken off in the air.
The right stabilizer and elevator had landed before the plane's crash site, all other pices were beyond.
There are videos taken from the ground that show that this was the case after the plane became visible under the clouds.

The fracture has occurred at the location were the horn was welded to the tube mounted in
the elevator. The fracture surface showed no signs of fatigue but the overload fracture had
been initiated in a 10 mm long weld defect. The lack of fusion defect extended more than 10 mm.
Effective weld penetration is here reduced to 10-50 %.

This same lever in three different stages of investigation

 

[LittleInch]

Anna,

It is very rare that there is only one thing to result in an incident like this - the swiss cheese model shows that a lot of "holes" need to line up to create failure.

It could easily be that what had happened many time before in terms of the jumper actions was seen as "normal" but when combined with the other circumstances as shown by the investigations ( slowing air speed, no flaps, high altitude for longer than normal, desire to avoid the clouds, pilot inexperience etc), that they combined to cause the plane to become unstable. I doubt anyone on the jumper side of the plane realised how close to (or even outside of for some times) the operating CoG envelope they were.

The photos above are very interesting. The thing to note here is that this item is intended to operate in one plane only and to translate a back forward motion into a rotational motion with zero sideways force.

The failure / breakages on the photos indicate that the joint was subject to considerable sideways bending forces for which it was never designed as the stabiliser / elevator was ripped from the airplane. This could have been better explained for sure in the report, even as an appendix or "other items investigated" section.

When it comes to the altitude thing I think it would be good to know when people talk about x minutes at y altitude what state the persons were in. What I mean is that on the jump run, everyone's heart rate and breathing increases as the adrenaline kicks in and hence you may need more oxygen than simply sitting still. I've seen heart rate monitors on skydivers that hit 160+ just before launch.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[RedSnake]

In the material report the following can be read, I have asked for a clarification of this because there are many welds around the lever.

The fracture had started in a weld defect of the corner weld. The fracture mode was
overload in tension transverse to the welding direction.

Which means that it has most l[highlight #EF2929][/highlight]ikely broken against the maximum stop up.
It turns and pushes upward at the same time since it is fixed in two points. The hinge in the stabilizer and the maximum stop up.

I am well aware of the principle of a chain of events. I myself was about to die in such an "electric accident" three years ago, but I was well isolated from the ground so it went well.

But theoretically, the same chain may have broken the lever!
Is it likely? Maybe not...
But at the same time not completely impossible either ..
As Murphy said if something can happen, sooner or later it will happen ..

An inexperienced pilot, little experience of the aircraft model.
At each landing, the elevator is placed against the maximum stop up before landing. This is a normal procedure.
The landings may be a little hard.
The hinges on the elevetor may be a little loose.
The elevator hinges should not be checked during pre flight inspection.
The mechanical stops may be a little unevenly adjusted.
When taking off, it is often turbulent, especially over the forest beyond the runway.
The struts where badly welded... and so on.

It had taken a few hours to check the technical parts of this, so I do not understand why it was not done.

As those who did the material survey said, it is not possible to see if an overload fault occurred in 3 steps during a day. No rust will have time to form on the surfaces in such a short time.

Best regards Anna

 

[LittleInch]

Is that U bolt looking thing the mechanical stop??

If so it isn't the best for sure as it will be generating bending moments in the horn that I doubt it was designed for and / or placing additional load on the horn to torque tube connection.

There was something a little odd in the maintenance manual which said the right hand stop was set and couldn't be adjusted, the other one could be. no drawings are provided.

I still think though that this is a consequence and not a cause.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[RedSnake]

This is probably due to the fact that on the right elevator there is a trim tab that is to adjust the pitch during long-haul flights so that you do not have to compensate for the oblique blow from the propeller.
So I think it must have a fixed end positions ?!?
I'm pretty sure SHK said both could be adjusted.
And that the ball joint at the end hade a 15 degree flexibilty.

The right hand elevator incorporates a fixed tab that is factory set and must not be varied from that
setting.

 

[MacGyverS2000]

I seem to recall an incident involving wearing of the elevator/rudder control cable caused (and discovered after an "a-ha" moment) by the jetwash of large jets. The airport had removed the blast fence between the jetway and prop plane storage area to make pushing back from the jetway a bit easier in that location. With the the jets powering up to taxi, day after day it buffeted the victim's control surfaces, applying stress to the cables over and over.

Not directly what's being discussed here, just pointing out that stresses-to-(near)failure don't necessarily have to happen in flight.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[RedSnake]

True!
I think I saw that the most common cause of total breakdown on this aircraft model was that they blew away in hurricanes and sandstorms while they were parked outside at the airport! And flew into other parked vehicles etc.
Nothing I have reflected on before ..

A

 

[waross]

As those who did the material survey said, it is not possible to see if an overload fault occurred in 3 steps during a day. No rust will have time to form on the surfaces in such a short time.

I have to ask, why that day?
Was this the first time this aircraft was used in gusty weather?
Was this the first time that this aircraft was used to ferry jumpers?
Had the aircraft never before had a hard landing?

The right stabilizer and elevator had landed before the plane's crash site, all other pices were beyond.

For the stabilizer and elevator to separate from the aircraft, the linkage would have had to have broken at the weakest point.
The pilot may have been able to exert enough force to break the weld, but not enough force to rip the stabilizer and elevator from the aircraft.
Had the pilot exerted enough force to break the weld, then he would not have been able to exert further force on the elevator.
We can say with certainty that the weld was the weakest part of the linkage in regards to the force that broke it.
(This force may not have been in the normal operating direction.)
We can strongly suggest, but not with certainty, that the broken weld was a result, not a cause.
At this point an inspection of similar aircraft for poor welds and evidence of cracking may reduce the uncertainty.
Is it possible that turbulence may have thrust the jumpers to the rear of the cabin and caused an unrecoverable attitude?
Stuff happens and sometimes no-one is to blame?
My sympathies.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[RedSnake]

I know that if things are poorly designed or have manufacturing defects, it is most likely that they will break in the first three months, if they break later, it is often due to fatigue several years later.

But it can only break once, so it could just as easily have been the day before or any other day.
And the reason could be, as I wrote in the terotic description, that the hinges have simply become bad enough that day as an example .. in combination with the ball joint also being worn etc, etc.

This may have prevented or made it impossible for the pilot to sort out the situation.

On the left side, it was the ball joint at the end of the push rod that went off.

I'm not interested in blaming anyone.
I just want to feel sure that it will not happen again.

/A

 

[Alistair_Heaton]

Anna the trim tab is constantly adjusted during flight when ever there is a change in speed and attitude.

It zero's out the control forces so you can fly the plane with your finger tips.

It is not normal for the plane to hit the mechanical stops during flight at any point. The only time you would do so would be during the just before flight full and free control check before lining up on the runway. They are there so that if you hit a gust of wind while your taxing the controls don't smash round and stress the control pivot points. The only way I could see that arm being on the stops with any real force is if the aircraft was travelling backwards. Which aerobatic pilots do do on purpose but when they do they brace the controls so they can't move. If the the rudder or the elevator come away from neutral then they go hard over against the stops. If this happens and nothing breaks once they are on the ground it requires an engineering inspection before flight again.

 

[RedSnake]

??
This plane has no adjustable trim tabb for the elevator.
It can only adjust the stabilizer.
The text is from the service manual.
The right hand elevator incorporates a fixed tab that is factory set and must not be varied from that
setting.
I am not sure what you're referring to now

Both SHK and the chief pilot comfirms that when they land they "stal" the plane before sett down, as he put it
"the steering wheel in the stomach" I have even seen it om film ?!?

In this picture, it is clear from wear on the lever that it has been against the stop, especially on the left side. This pictures are not from the crashed plane but from a similar one in Sweden.

/A

Perhaps I am the one who is fatigued ;-) time for bed .. Slepp tight every one.

 

[Alistair_Heaton]

Being very nearly at the stall before landing is normal but that is when your nearly at alpha critical angle of attack. The yoke will not be at full movement rearwards when this occurs. The amount of "back" stick is dependant on the CoG and weight of the aircraft. If the CoG is towards the rear you will need less backstick than you would if the CoG was at the forward limit. in fact if you did hit the backstop during the flare it would more than likely result in a rather firm if not heavy landing. Every aircraft has a trimming system for elevator. There is also a second one to preload the elevator to stop flutter against control slackness and rebound in the control runs. Bigger aircraft will also have rudder and alerion trims but this one more than likely won't. in this picture of the cockpit I have circled the trimmer control in red.

The CoG loading envelope is critical for a safe flight. Did they give you the mass and balance/performance sections of the flight manual?

They will go against the stops while the plane is sitting on the ground and the wind is blowing. Most aircraft have things called gust locks which we put in after parking to lock the controls to stop this. Piper aircraft have a hole in the control column that you put a pin in with a big red warning label they are in. Cessna 150 don't have one but its normal to use the seat belt to hook over the yoke to stop it. Other aircraft have pins or other methods of stopping the controls banging about. Maybe this one doesn't ? If you think about it having something swinging in the wind between two hard limits is a bad idea for fatigue more than anything else.

Flying an aircraft even flaring for landing doesn't require a huge amount of strength even the Jetstream which was a manual control setup with no powered controls we landed it with one hand on the yoke and one on the power levers and the J41 was 10 tons and J31/32 was 7 tons. Some of the lady's flying them were all of 50kg the number of times I had to act as ballast with them firmly griping my trouser belt crossing the apron in Shetland in high winds so they didn't get blown away.

 

[Alistair_Heaton]

Actually looking at the airspeed indicator it would appear its max manoeuvring speed is 150 knts when you can apply full control input and its Vne (never exceed) is 190 knts.

And it doesn't have gust locks because they are using a bungie line attached to the rudder to pull the yoke forward to stop it banging about. If they were not doing that at the parachute club it would account for the wear on the lever which will have happened while parked.