Studs, nuts a couple of Ft/lb and a aircraft crash

[Robert Atkinson 2Participant @robertatkinson2]

This investigation into a light aircraft crash (minor injury only) has a interesting bit of research into the failure mode of studs and the impact of small changes "mprovements" to nuts. It also highlights how critical the applied torque is in some applications.

https://assets.publishing.service.gov.uk/media/6400ae14d3bf7f25f61ff7a6/Reims_Cessna_FRA150M__G-BDNR_04-23.pdf

Robert.

[Robert Atkinson 2Participant @robertatkinson2]

[Michael GilliganParticipant @michaelgilligan61133]

A fascinating, and very thorough report

Useful reading for anyone using fasteners near their design limit !!

MichaelG.

[Clive FosterParticipant @clivefoster55965]

Very interesting reading.

Its impressive how the spiralock system spreads the load over more threads, I'd always thought that system to be a little gimmicky. In comparison the standard thread loads up the first thread much more so clearly things can unzip if the first thread fails and there aren't enough threads to leave at least one carrying effectively zero load to take up the slack.

Checking torque setting tables I wonder how many smaller threaded fasteners are routinely over-tightened. Typically ordinary fasteners of the size discussed in that report, 3/8 – 24, should only go to around 15 to 20 or so ft-lb.

Looking at the sections reduces my faith in the oft repeated statement that using lower thread engagement percentages for easier tapping makes little difference to the strength of the thread.

Mrs Google (Google knows everything so she has to be female!) finds some rather nice graphs from Engineering toolbox illustrating the effect of size and material on recommended torque. I'm minded to do some printouts as graphs give a much clearer view of relationship s than tables although tables are more accurate for individual sizes.

Clive

[Howard LewisParticipant @howardlewis46836]

If a fastener is over torqued, the material will go into yield and the extension vs torque relationship will no longer be linear. You feel the spanner go "soft" just before failure.

Tightening to yield is the most efficient use of the fastener, but tightening HAS to stop, as soon as yield is detected.

Having gone beyond the elastic limit, the fastener will have taken a permanent extension.

I spent six months testing and commisioning a 32 spindle yield tightening machine. We used LOTS of bolts which had been faced at both ends, identified and measured, before and after tightening.

A 1/2 UNF bolt in W range steel would take a permanent extension of a thou or so, indicating that it had JUST gone into yield.

In some instances, this means that a fastener can only be used once.

In industrial yield tightening systems the electronics monitor torque against angular rotation, As soon as the relationship becomes even the slightest non linear, tightening stops.

It also means that the clamp load applied by each fastener is consistent.

For this system to work, the parts being clamped together must not crush noticeably. If it does the control system will detectb the material going binto yield rather than the fastener.

In joint system involving a gasket, this alo has to mbe taken into account, although often the system is intended to mproduce a consistent clamp load on the gasket to improve sealing.

Howard

[HopperParticipant @hopper]

Posted by Clive Foster on 18/03/2023 23:10:42:

Looking at the sections reduces my faith in the oft repeated statement that using lower thread engagement percentages for easier tapping makes little difference to the strength of the thread.

But if you look carefully at the sections, it shows the threads were engaging with each other on the tips of the threads, the thin, weak end of the triangle. Larger tapping holes remove that thin end of the triangle on the female thread but leave the thick mid and base section intact to engage and bear the load. I wonder if anyone has ever done any real-world tests on the thread engagement depth vs shear strength? It seems to be stated as fact by Tubal Cain and others in various books.

Seems like the big takeaway from the conclusion of the report is don't put high tensile nuts on lower tensile studs as it distorts the stud threads before it distorts the nut threads and the result is stripped threads on the stud. Who would have thought?

Bit remiss though that the problem had been encountered on assembly of several engines before but they did nothing, said nothing, and just replaced the studs. As usual, the accident results from an accumulation of several errors or faults.

[Clive SteerParticipant @clivesteer55943]

Interesting reading and worrying where threaded fasteners are being used in tension.

One would have thought that for this application the stud would have had the highest yield strength as replacing a stripped nut is far easier than replacing a stripped stud.

However the thinking here may have been that it is better to strip the thread on the stud than pull the stud out of the Ali crankcase if over tensioned. Unfortunately the designers thinking has probably been lost and the only info is the specification of the stud and nut.

CS

[VicParticipant @vic]

I wasn’t going to check the link as I guessed the report would be quite boring. I’m glad I did though as the report seems well written and more interesting than I expected. Thanks for posting.

[Ches Green UKParticipant @chesgreenuk]

The Report's Conclusion states …

Multiple failures of cylinder base studs on the RR O-240 engine type have been recorded since 2014, but unless they resulted in engine failure in flight, they were not reported to the manufacturer.

Wow.

So were they at least reported to the Light Aircraft Association?

Ches.

 

Edited By Ches Green UK on 19/03/2023 19:19:28

[Ian PParticipant @ianp]

I too found the report technically very interesting, but its got me thinking…..

It makes complete and utter sense to find the cause of a failure or an accident so that we can learn from our mistakes but who pays the bill for the whole investigation process?

Stress cracks, fatigue, fretting, corrosion etc occur in vehicles other than aircraft, would I be wrong in assuming that they only become candidates for serious investigations because aircraft components are always highly stressed due to weight constraints. Not many road vehicle engines suffer catastrophic damage these days.

Ian P

[HopperParticipant @hopper]

The other question is: Whose roof did the missing cylinder and piston land on?

[Nick WheelerParticipant @nickwheeler]

Posted by Clive Steer on 19/03/2023 10:39:52:

Interesting reading and worrying where threaded fasteners are being used in tension.

How can a tightened threaded fastener not be in tension?

[Clive SteerParticipant @clivesteer55943]

Nick

The thread doesn't necessarily take a working load if a fastener such as a bolt is taking its design load in shear ie orthogonal to the bolt axis in which case the nut is just tightened enough to stop the nut coming undone and if there are cyclic or torsional loads acting on the bolt a split pin is added

In the case of the cylinder holding down studs there is a load on the thread due to tightening plus cyclic tension loads during compression and ignition cycles plus complex asymmetric loads due to side thrust of the piston against the cylinder wall.

Most bolts in airframe applications including holding the wings on are working in shear mode. Having said that I had an aerobatic aircraft where the wing was held on by two 1/2 inch bolts in tension when the aircraft was pulling positive G which could be up to +6G !!! The weak point is the transfer of load between the bolt and nut and for this application there were two nuts per bolt.

CS

[John OlsenParticipant @johnolsen79199]

:Not many road vehicle engines suffer catastrophic damage these days.

Ian P

I suggest you take a look at a Youtube channel, "I do cars". The engines he takes apart have generally suffered catastrophic failures. To be fair to the manufacturers, most of them are due to lack of oil. lack of coolant, or just outright abuse. The engines he autopsies are mostly American, although some Japanese and German engines have appeared.

Interesting what Clive says about bolts working in shear, I had the impression that this was not actually regarded as good practice. It is better to use dowels to locate items and take any shear loads, and leave the fasteners to hold things together. Part of the reason for this is that a bolt has to have some clearance in the hole to allow it to go in without damaging the thread.

John

[Robert Atkinson 2Participant @robertatkinson2]

The AAIB is independent but government funded. It comes under the Department for Transport and has a budget of around 20 million pounds. So in short we, the UK public pay for it.

The aviation industry does contribute to specific investigations. For instanace providing expert advice. For a major investigation there will be representatives from the airframe and engine representatives on the team.
Industry will also help fund research that benefits them in the long run. The insurance companies will also contribute to recovery costs as they would have to pay for it all if there was no investigation.

Robert.

[Clive FosterParticipant @clivefoster55965]

To me the biggest worry on this is how poor the OEM components from the design rights holder are.

Component manufacture and materials specification standards for a simple component with no specified life or an inspect before re-use requirement on something like this should be so high that failure without gross installation abuse should not be possible unless the component is defective.

Fairly obviously such standards can be easily met by off the shelf replacements without any side effects so continual supply of known to be (occasionally) inadequate components is inexplicable. There is little doubt that the maintenance community for these motors knew, grapevine communication is pretty effective and fast, that stud failure did occasionally occur and were prepared to change them as required.

Given this knowledge either maintenance instructions should have been modified to life the studs "change after X hours or every Y overhaul" sort of thing or materials changed to give effectively infinite life.

Clive

Edited By Clive Foster on 20/03/2023 10:28:03

[Clive FosterParticipant @clivefoster55965]

Howards contribution concerning tightening to yield convinces me that we really need better terms to describe how stretch bolts behave. When an ordinary bolt or stud is taken past its elastic limit it pretty much looses all ability to recover and stays stretched. A stretch bolt or stud is still capable of elastic recovery after yielding. It can't go back past the yield point but it still recovers elastically within its working range.

Which is why they are so popular and effective for things like head bolts on alloy engines, like the venerable pushrod V8 in my Range Rover. If the bolts didn't recover all head gasket pressure would be lost when cold after about 4 or 5 starts due to stretch from differential expansion when the engine is up to operating temperature.

The old Rover V8 is an excellent example of why correct installation ensuring the stretch bolt is always within its correct operating range is so important. Combine over torquing and with overheating and the bolt will go out of range so the head gasket fails. The margins are surprisingly slim too. Pushing the operating temperature up from 95°C to 100°C and restricting cold coolant flow to meet emissions requirements shifted the Rover installation from bullet proof to occasionally unreliable. The extra stretch between 95° and 100° is very small but just enough, when combined with stretch bolt production tolerances, to eat up the margins.

Clive

Edited By Clive Foster on 20/03/2023 10:28:40

[Mike PooleParticipant @mikepoole82104]

It seems unnecessary to build a low performance engine with components that are stressed close to their limits. I realise that it is an aircraft engine and probably running for long periods nearer to its maximum performance than a road going vehicle. It is rather different to lose a cylinder on the ground than when high above it though.

Mike

[SillyOldDufferModerator @sillyoldduffer]

Posted by Clive Foster on 20/03/2023 10:19:46:

To me the biggest worry on this is how poor the OEM components from the design rights holder are.

…

I didn't read it that way. I thought:

• The stud was to specification (last updated in 1976)
• The nut met the specification, BUT,
• was made with a shouldered thread that resisted vibration loosening by bearing harder on one side of the stud's ordinary thread.

Although both met the 1976 specification, the combination put extra-stress on an already highly stressed fastener such that tightening the new nut cracked the stud. Lucky pilots got a new stud if maintenance broke the stud on the ground, whilst unlucky pilot had studs fail in flight. Vibration and heat from the running engine propagated the crack.

Interesting that something as standard as nuts and bolts are still being improved, but there's a risk that any change from the original could invalidate the original design. I guess all would have been well if a 1976 style nut had been fitted rather than a modern one. Anti-vibration nuts are good in an aircraft, but only if the studs are rated to take the extra strain. I don't suppose a maintenance engineer would know that the design stresses were exceeded.

Is it wise to be flying 46 year old engines? Materials age and components might change in a way a long dead engineer couldn't anticipate!

Dave

[Clive SteerParticipant @clivesteer55943]

I'm not sure I understand how the nut locking feature caused the failure. Most of the self locking nuts I've come across are a conventional nut with a feature on the top that mimics a lock nut such as a nylon, fibre or crushed threaded tube friction feature. These nut would be wound down to the required torque and checked visually to ensure the thread of the stud or bolt had engaged fully with the locking feature.

I wouldn't have thought that the locking feature would need to extend down into the normal load transfer region of the nut and stud. However if the stud was too short to provide a locking feature on top of the nut or by doing so the nut would become thinner reducing the load transfer area.

CS

[Robert Atkinson 2Participant @robertatkinson2]

Note that the engine is based on 50's and 60's technology and long out of production. The responsible organisation (type certificate holder) has little incentive to make proactive reviews or changes.

Robert.

[duncan webster 1Participant @duncanwebster1]

Posted by SillyOldDuffer on 20/03/2023 11:35:41:

……

Is it wise to be flying 46 year old engines? Materials age and components might change in a way a long dead engineer couldn't anticipate!

Dave

Plenty of Merlin still flying, and they must be 70 ish years old. The trick is to not expose them to full load, unlike the wat they seem to operate preserved steam, thrash it up the S&C or Shap to break some record or other.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Clive Steer on 20/03/2023 12:23:17:

I'm not sure I understand how the nut locking feature caused the failure. Most of the self locking nuts I've come across are a conventional nut with a feature on the top that mimics a lock nut such as a nylon, fibre or crushed threaded tube friction feature. These nut would be wound down to the required torque and checked visually to ensure the thread of the stud or bolt had engaged fully with the locking feature.

I wouldn't have thought that the locking feature would need to extend down into the normal load transfer region of the nut and stud. However if the stud was too short to provide a locking feature on top of the nut or by doing so the nut would become thinner reducing the load transfer area.

CS

That's exactly what I thought at first, but the Spiralock diagram shows their female is cut full length with a wedge ramp:

The problem, I think, is what the wedge did to a stud that was already torqued close to maximum. As can be seen the wedge concentrates the tightening force on the tip of the male thread, giving it an extra hard time. Good idea, but a step too far on this engine! I wonder if the Spiralock instructions suggest backing off the torque when these are substituted for conventional nuts. If so does the maintenance engineer go with the new nut guidance or what's in the aircraft manual?

Dave

[Mick DobsonParticipant @mickdobson]

I found the technical report to very interesting, but since when was torque measured in Ft/lbs?

This is a meaningless unit, since torque is defined as force applied at a standard radius and should of course be written as lbft. For ease of visual interpretation it can sometimes include a dot or dash, such as lb.ft or lb-ft. The lb part can even be expanded to lbf denoting pounds force, so then torque is expressed as lbf.ft.

I think our American cousins write it the other way round, as ft.lbs, but not as the version shown in the report.

[Author]

Posts