ATSB investigation of j430 vh-ofr

[dmech]

On 6 / 2017 a jab 430 had a forced landing due to piston failure  [ I refer to page 14 aircraft incidents & accidents / light plane down Bundaberg QLD ] . It appears the ATSB has closed this investigation , with no real outcome, as to what has caused the piston failure. In RAAS incident reports , it seems that there are still a number of piston failure's happening under certain conditions . [ some as late as 2019 ] . On looking through latest engine manuals for both gen 3 and 4 it appears as though there isn't any rectification in this area, there is another piston that jab are using for gen4 , which looks like they have reverted back to the style of the old ACL piston skirt ,probably  Asian manufacture ,[ ACL stopped making pistons some time ago] and different metallurgy process than ACL. ACL made good pistons but these would fail eventually in the jab engine under certain conditions.  I'm sure we have found the cause and its not down to metallurgy  or die grinding of skirt but more to do with the style and the application and skirt profile. It is my opinion that these original pistons can be made more reliable with little modification. 

 

My Question is has any one heard of the ATSB investigation out come with regard to why the piston failed.

 

DMECH 

 

 

[KRviator]

It wasn't DFR, it was VH-OFR and the report is HERE.

 

From the report: 

 

During the overhaul, the engine was heavily modified using parts sourced mostly from another engine manufacturer. The pistons were balanced using a belt sander and die grinder

 

Balancing of the pistons using a die grinder or belt sander is not a recommended procedure

 

So if the piston was not a Jabiru-sourced component, then it doesn't matter what piston Jabiru are using for their Gen-4 engines. 

 

As regards the cause, according to the ATSB:

 

The engine experienced a failure within cylinder five, leading to a complete engine power loss. The cause of the failure could not be determined.

 

 

 

[dmech]

have seen this report many times.   No reason for piston failure

 

 

[facthunter]

 I've never liked the use of slotted pistons in any aero (or aircoled ) engine that puts out reasonable power. They are weaker than other types and have a reduced heat path from the crown to the skirt... Nev

 

 

[onetrack]

Excessive heat due to operating under heavy load, is primarily the biggest destroyer of pistons.

 

It's a mistake to try and keep pistons as light as possible in an air-cooled engine, that is required to produce 75% to 100% power, almost constantly

 

 

[facthunter]

No one to my Knowledge has EVER looked  seriously at the piston crown temps achieved in a Jabiru motor which is after all quipped with Max oversize piston (so you can't rebore it) out of a Holden. The head temps we agonise over but cant measure the piston crown temps. so conveniently don't think about it (and it will go away), but we do know the rings often carbon a lot in the grooves and the bores go blue near the exhaust valves.  and the engines often have a lot of blow by.  Nev

 

 

[turboplanner]

No one to my Knowledge has EVER looked  seriously at the piston crown temps achieved in a Jabiru motor which is after all quipped with Max oversize piston (so you can't rebore it) out of a Holden. The head temps we agonise over but cant measure the piston crown temps. so conveniently don't think about it (and it will go away), but we do know the rings often carbon a lot in the grooves and the bores go blue near the exhaust valves.  and the engines often have a lot of blow by.  Nev

 

I’ve mentioned combustion chamber temperature several times but been ignored each time in favour of people waffling on about EGT. Like trying to put a fire out by blowing through the hose.

 

 

[Thruster88]

 I've never liked the use of slotted pistons in any aero (or aircoled ) engine that puts out reasonable power. They are weaker than other types and have a reduced heat path from the crown to the skirt... Nev

 

The Jabiru gen 4 engines now have the same style of piston that lycoming have always used along with dual valve springs and one piece cylinder assembly. All good stuff.

 

https://images.app.goo.gl/k9JToTjhkygijowY9

 

 

[dmech]

Crown temp. can be estimated fairly accurately  by testing the hardness of the crown against the hardness of other cooler part of piston ,or another good piston, is how we do it.   Centre of crown usually the hottest. These pistons contain more than 13%silicon which is more than can be dissolved by the aluminium, which is then frozen in solution  by a heat treating process , [T6] this process increases strength , makes piston lighter ,more heat tolerant , trade off is it also makes piston more brittle , same as carbon does in steel, after various heat treatments .

 

Heating of the pistons past the transition zone which starts at about 600deg. f , will start the softening process .

 

The disintegrated and cracked pistons I have seen [ 16 so far ] don't show indications of over heat on crown, in fact 4 at 120hrs from new, showed no colour under the crown at all, and this engine had one disintegrated piston and 3 pistons showing 2 cracks on major thrust  side and 1 crack on the minor thrust side.  This engine was repaired which included a line bore , has done over 500 hrs. since

 

In my opinion the piston is ok for purpose if the engine is operated properly . My extensive heating test show piston does fit cylinder well once piston becomes fully heat soaked along with cylinder. Cold engine take offs .extensive circuit work ,engine failure sims. Will cause piston failure of some sort in this engine , but can be fixed.

 

dmech

 

 

[dmech]

The Jabiru gen 4 engines now have the same style of piston that lycoming have always used along with dual valve springs and one piece cylinder assembly. All good stuff.

 

https://images.app.goo.gl/k9JToTjhkygijowY9

 

Jab piston is a slab sided piston you cant use a round skirt in a jab engine , so can not be same style as lycoming . I have 8 Lycoming 0360 pistons in work shop at the moment .Can you show me a picture of the new piston , the one I saw looks like a reversion back to the old A.C.L style, still same skirt problems but not quite as bad due to less slot under oil ring still has off set as well ,which lycoming don't.

 

dmech 

 

 

[Thruster88]

I don't have a picture but it was described as not having a slot under the oil control ring at the Jabiru seminar at Parkes 2019. Somthing about the cylinder being aluminium now was the reason.

 

 

[dmech]

I don't have a picture but it was described as not having a slot under the oil control ring at the Jabiru seminar at Parkes 2019. Somthing about the cylinder being aluminium now was the reason.

 

Yes , still the same piston style with very little slot would be better owing to a different heat path from crown but not a fix. I believe they had problems with tips being broken off the end of cylinders, they being weaker than the 4140 barrels. The full slotted pistons would still be the better way to go because they flex better. the slotted piston can be modified to work in the jab engine, steel or ally. there is a much better option , but this would require changes to cylinder and con rod.   

 

 

[Jim188]

As to the VH-OFR failed piston,  it was of the type from other company that many Jabiru owners were turning too, because of the frustration they had from the poor time intervals, thus serviceability of the genuine Jabiru engine assembly. 

 

That all.

 

 

[facthunter]

Yes high silicon pistons are very brittle and also quite hard  to machine unless ground which is difficult for the ring lands.  It's a bit like making something out of a tombstone.

 

  Flexing surely (combined with the hardness) will predispose any design to an earlier failure situation. The newer alloy barrels will obviously allow a closer running fit and less tendency to barrel face rings and cause uneven wear on pistons (skirts and ring lands).  Also a more even (cylinder) temperature  might be experienced with less potential cylinder distortion and better  ring sealing. ALL good?   Nev

 

 

[dmech]

Yes high silicon pistons are very brittle and also quite hard  to machine unless ground which is difficult for the ring lands.  It's a bit like making something out of a tombstone.

 

  Flexing surely (combined with the hardness) will predispose any design to an earlier failure situation. The newer alloy barrels will obviously allow a closer running fit and less tendency to barrel face rings and cause uneven wear on pistons (skirts and ring lands).  Also a more even (cylinder) temperature  might be experienced with less potential cylinder distortion and better  ring sealing. ALL good?   Nev

 

I use P.C.D tooling piece of cake and very fast ,expensive but just about lasts forever ,[ impossible to grind ring grooves economically ] , diamond hone for silicon ally cylinders .

 

These pistons do flex and are designed to flex, so long as flex dose not go beyond design limits , in this case only a few thou,[like a spring has limits to] in the jab application I have measured consistently, the flex is over .014" this is what breaks the piston ,marks in barrel show this also. There is absolutely no problem with hyper pistons if operated within limitations , which works in the holden engine, the jab engine has not been designed for use of these pistons , but could easily be. Another thing, the piston top dose not match the cylinder head combustion chamber , but close , probably the saving grace there is the long intake pipes , runners.

 

dmech

 

 

[dmech]

As to the VH-OFR failed piston,  it was of the type from other company that many Jabiru owners were turning too, because of the frustration they had from the poor time intervals, thus serviceability of the genuine Jabiru engine assembly. 

 

That all.

 

JAB PISTONS ARE BREAKING TO ,all the ones i'v been associated with are gen jab pistons , one engine was mine 260 hrs. another one in our area went at 268hrs . these were the valve relief type. 

 

dmech

 

 

[Jim188]

As to the VH-OFR failed piston,  it was of the type from other company that many Jabiru owners were turning too, because of the frustration they had from the poor time intervals, thus serviceability of the genuine Jabiru engine assembly. 

 

That's all.

 

JAB PISTONS ARE BREAKING TO ,all the ones i'v been associated with are gen jab pistons , one engine was mine 260 hrs. another one in our area went at 268hrs . these were the valve relief type. 

 

dmech

 

 

 

Boy dmech, I can hear and see your frustration, but does not caps lock means you are yelling at me and why would this be? 

 

I am sure all of us who actually fly behind either a Jabiru or Crossbred Jabiru engine are all wanting the same thing, to keep flying our aircraft, do our maintenance, achieve the overhaul times and major service interval as specified and if this happens, happy days for all.   

 

I had the chance a while back to buy a very nice Jab 430 with Camit engine, it was nicely maintained, but for me, Camit was no longer an option, what would happen if 200-300 hours down the track the engine needed a top end, unfortunately nothing would have be straightforward with a Camit engine and most likely I would have to head back to Jabiru.     

 

VH-OFR that started this discussion, no longer has a Crossbred Jabiru Camit engine in it. It does not matters if you dislike Jabiru from your past experience with their engines. Camit has gone and the new Gen 4 Jabiru engine is here and VH-OFR has one in it.  

 

So how does it go temp wise, well all the temps are well within the spec's and room to move even in the middle of these very hot 30 plus days on climb out. I do have the habit when flying a Jabiru to naturally drop the nose for a cruise climb and cooling reason. But with the Gen 4  and it's 6 x EGT's , 6 x CHT's and oil temp, all are well within the temperatures range with much more room to move. I have tried an extended 75 knot climb with temps remaining stable and moving very little. This Jab 430 and it's Gen 4 engine (50 hours) is new to me, having only flown it for 10 hours so far. I don't not know much about metallurgy, but I think the Gen 4 is a good start with Jabiru having got these temperatures thing well under control. 

 

So here's hoping the Gen 4 engine proves to be that engine we are all looking for, just imagine a Jabiru engine that will do a 1000 hours without question, then you do the top end and it's right for another 1000 hours, happy days for all of us.     

 

I guess someone will want to bust this bubble and say that's not going to happen.   

 

 

[dmech]

Sorry Jim 188 , didn't mean to blast any ones ears  , if that's what it means.

 

Sounds like your very close to this a/craft , can you tell me , did the A.T.S.B.  establish the cause of the piston breaking up?. Camit pistons were from same manufacturer as the jab ones [pre valve relief]. 

 

mine reached 260 hrs with no problem at all temps were always good, good power very little oil consumption ,then without any warning a piston broke up.

 

I'm asking the question because I have established the cause of the pistons breaking up . We have repaired one of those engines , which has over 800 hrs so far with out any problems , should easily make 1000 hrs ,is often used on 40 plus days. hi ambient temps are not a factor in the cause of these piston break ups, in fact they are beneficial, cool days are a factor though. The piston problem just dose not happen after just one or to events ,but fatigue over a long period of events over time.     

 

 

[facthunter]

Pistons disintegrating is not uncommon as there's lots of cycling loads and everything has a sensible " safe life". You just stick well under it, if you know what it is and if it's consistent. Ambient should only raise the   temps by the same amount as it is above normal . Heat flow is proportional to temp difference Air cooled engines were very successfully used on war tanks and rail motors.  They just have a lower output /litre max  than you can (safely) achieve with a liquid cooled motor, but the simplicity can be worth it. Nev

 

 

[biggles]

So , DMech what do you believe to be the cause of failure. and is it related to pre-valve relief pistons ? I replaced my pistons after 500 trouble free hours with VR pistons, new valves, dual valve springs etc...... Bob 

 

 

[turboplanner]

Ambient should only raise the   temps by the same amount as it is above normal 

 

Power demand will raise combustion chamber temperature much more and much faster than ambient.

 

Leaning off will do the same.

 

 

[facthunter]

What I wrote is common knowledge , all other things remaining the same. If it pushes the thing into a critical area that's something else. Simple thermodynamics you will see in plenty of references on this matter. In hotter ambients the prop would absorb less power( Less dense air higher density altitude) A general rule is more gas flow more power but that's not directly related to ambient in any way that would increase  a power output. Nev

 

 

[turboplanner]

What I wrote is common knowledge , all other things remaining the same. If it pushes the thing into a critical area that's something else. Simple thermodynamics you will see in plenty of references on this matter. In hotter ambients the prop would absorb less power( Less dense air higher density altitude) A general rule is more gas flow more power but that's not directly related to ambient in any way that would increase  a power output. Nev

 

I'm basically saying the same as you; a 10 degree increase in ambient will produce a 10% increase in nett heat.

 

If you get a 200 degree in nett heat it's not the ambient.

 

 

[Thruster88]

Pistons disintegrating is not uncommon as there's lots of cycling loads and everything has a sensible " safe life". Nev

 

In a lycoming or continental engine piston disintegration is extremely rare given the high number operating world wide. Even a few failures early in the life of these engines would have resulted in an airworthyness directive (AD).

 

 

[facthunter]

 Turbs, I don't get what you are at on this one. but

 

   Any engine has  to be cooled so a certain% of the energy has to be dissipated by the cooling system at all times. Heat flow is proportional to temp difference between the bits to be cooled an the cooling medium, so if the ambient goes up 10 degrees the engines temp will rise the same amount. Aircooled engines actually cope well with high temps and aircooled tanks performed well in deserts. If the 10-20 degrees is too much then it's not ok but no engine should be that critical including any liquid cooled one.

 

 Thruster. I agree but I'm generalising across all Piston motors including diesels earth moving  heavy transport etc and piston design is critical especially with the materials used. Lycoming, Continentals Franklins etc don't really  have any problem  in a normal TBO. I can never recall a piston breaking up in one. It's usually only the valves and I can't see one "top" in say 20 years 2300 hours is too much to bear. Poppet valves are in an extreme environment.  The main problem with those engines is they don't get used enough. Nev