"Jaberoo" down (Mildura this time)

[turboplanner]

I'm commenting about engines generally here, and simplifying the discussion to make it easier to understand, do not try to change an aircraft engine just based on these few aspects - there are many others to consider as well.

 

I'm curious at this focus on CHT monitoring, and the discussion seems to keep crossing between

 

(a) The engine design phase where two or three hundred thousand hours might be spent designing, dyno testing, redesigning, dyno testing etc to that point where there are no vices

 

(b) Dyno testing the finished design for the type of applications it will be marketed to, and this is usually based on shrouding etc, and observing the engine rise steadily

 

to the point where the temperature stops increasing and flatlines.

 

The tests you are familiar with Dafydd seem to me to be the equivalent of the (b) tests which are primarily to test an in-vehicle water cooling system.

 

I posted some steel melting and softening points at post#191 regarding valve failure, and to those we could add the melting point of aluminium at 660 degrees C

 

In post #367 I mentioned some oil temperatures related to the Chev 350, one of the world's most reliable engines, frequently used in racing at 100 hp per cylinder.

 

I did some more scouting and found this additional information:

 

Maximum oil temperature for long engine life (highway) - 127 degrees C

 

Typical racing oil temperatures - 138 to 160 degrees C

 

Corvette C6 Manual - For short period: Maximum oil temp 160 degrees C, Maximum coolant temp 124 degrees C

 

The CHT would be higher again than these temperatures.

 

So I'm comfortable with an oil temperature of 127 degrees C for long life in a good engine (with a proportionately higher CHT), these oil temps being at the hottest location as Dafydd earlier referred to.

 

Motz said he had a failure at a CHT of 180 degrees C, which would make the oil temperature proportionately lower, and the oil temps start freaking the instruments out at 110 deg C.

 

If we just talk about exhaust valve failures for the moment, even if you got a torch and heated the valve to twice the CHT reading, it's not going to be shedding metal, so I'm wondering why people are focusing on fins, airflow etc.

 

I found after a couple of years of watercooled heads and even electric fans, which made no difference that when I got the timing and internal cooling right I didn't even need air intakes and could just panel the engine in.

 

I ran one engine for two seasons at 9,000 rpm without any failures and another at 11,000 rpm with just valve spring replacements at the end of the season, and I never ran temp gauges.

 

What I did find on another engine during the development process was when one cylinder went out due to a carby or spark plug issue one of the other cylinders would melt a piston without fail due to a massive rise in combustion chamber temperature.

 

So my thinking right now is that

 

• Maybe there is something like a manifold imbalance in the engine where one or two cylinders make less power than the others, elevating the combustion chamber temperature in the cylinders taking the full power load, or
   
   
  
• Maybe it's that combined with fuel settings which are too lean, or
   
   
  
• Maybe it's a combination with a prop pitch that is too coarse.
   
   
  

 

 

The reason I'm thinking prop is that the Jab cruises around the same speed with 80 hp as reasonably similar aircraft with 912 100 hp engines, so I would expectthe Jab engine to be working much harder, which pushes up the combustion chamber temperatures, making a match of all cylinders very critical.

 

The 3300 on my excel sheet has examples of both exhaust valve failure and thru bolt failure, and although it produces a lot more power and pushes a similar frontal area through the air, it's set for a cruise speed of around 125 kts, and power demand increases exponentially with speed, so it is likely also to have a hard working engine at its cruise.

 

We often hear the comment that Jabs need to be worked hard, and by working hard that means higher rpm.

 

When designing an engine you can obtain the target torque by crank pin offset for leverage and piston area for power, and you finish up with x amount of torque in the test engine.

 

We need Power to push the wind out of the way, and the formula for obtaining Power is:

 

Power (kW) = Torque (Nm) x rpm divided by 9549.3

 

Up to a certain point, which we won't bother with now, the more revs you feed in the more power you get.

 

So when you work the engine hard, as the rpm increases, the engine is putting out more power and is less stressed in the combustion chamber so runs cooler.

 

So if we fitted a finer pitch prop, we could for example drop cruise speed to 90 kts, but the faster revving engine would be putting out more power, and I would be expecting the combustion chamber pressure to be less, so the temperature to be lower, with less chance of a seizure and so less chance of mechanically jacking the barrels off the case by stretching the thru bolts.

 

The downside would be higher fuel consumption per Nm.

 

Now some will say "But that's no good, I'm going to have to pay more for fuel!", however if that change resolve the issues, then maintenance costs are going to drop by thousands of dollars so overall cost of ownership has every chance of being lower, even at the higher fuel consumption.

 

As I mentioned earlier this is purely a hypothetical; I'm just throwing it out there for discussion.

 

 

[Dafydd Llewellyn]

Hmmm; there's an awful lot there; let me see if I can address some of those points:

 

Firstly, the exhaust valve issue: From what I have seen of this (which isn't a lot, so far), the problem in the Jab occurs when the exhaust valve guide temperature exceeds some limit - and it's phosphor bronze, not steel, so the temperature will be a lot lower than what you see by putting an oxy torch onto a valve. When that occurs, the guide wears very rapidly (This is not confined to Jabiru engines, BTW, Continentals especially seem to operate very close to this temperature limit, and valve guide extreme wear is not as rare as one would like). They are fine at some power rating, but increase that only a few percent and Bingo! Continental made it their policy to get a few more horsepower per cubic inch than Lycoming, to add to their market appeal, and this sort of problem has been the result.

 

The guide wear does two things: Firstly, it reduces the ability of the valve to shed its heat via the valve guide, so the valve temperature increases rapidly. This accelerates the guide wear problem, so the process becomes a "run away" one that accelerates in a short time. Secondly, the increased clearance means that the valve starts to impact its seat off-centre, which causes bending loads on the now overheated valve stem. This produces a fatigue failure in the valve stem. There is also some evidence (Ian Bent found this) that even a minor increase in valve temperature due to the early stages of this process, causes a form of pitting erosion on the neck of the valve stem, which itself acts as a fatigue-crack initiator.

 

Also, an overheated exhaust valve head can initiate detonation. Ian Bent's experiments with his own 3300 have demonstrated these factors, tho because he's doing in-flight experimentation, he's pulling the engine down before he suffers a catastrophic failure. Detonation radically increases the exhaust-gas temperature, so adding to the catastrophic result.

 

All of this means that the ability of the guide to lose heat in its turn, is critical; and the CHT is a major aspect of that, because the rate of heat flow out of the guide depends upon the head metal into which it is pressed, being sufficiently cooler; the "thermal gradient" between the guide and the head must be sufficiently steep for the heat to slide "downhill" fast enough. The head-metal temperature will also affect the temperature of the oil in the rocker box, and hence its ability to do its part in cooling the guide.

 

So the whole process goes "over the edge" at a CHT temperature that is way below the temperature that the valve metal starts to soften. The oxy-torch analogy really isn't relevant.

 

What causes the high head temperature in the first place? Well, obviously mixture mal-distribution is likely to be a prime factor - and because that can be affected by so many things, it's difficult to manage in a multi-cylinder engine with a single carburettor. One of the unexpected causes of it is swirl in the airflow entering the carburettor; this was spotted by Ian McPhee in the work-up of a Jab 2200 installation in a Motorfalke, though I expect it's a problem that has been around as long as the internal combustion engine, but it was a new one to me. That's an engine installation issue that is a bit too subtle for the normal type-certification tests to pick up. That is one of the things that a multi-point fuel injection can fix, but it brings with it its own set of problems; and right now I'm trying to answer Turbs's questions.

 

Mal-distribution of the cooling air supply is, obviously, another potential cause.

 

So it follows that monitoring both the CHT and the EGT is a prudent thing to do.

 

In regard to the engine RPM issue:

 

The fundamental fact of life with an aero engine is the propeller tip speed. Propellers become very inefficient if their blades start to generate shock waves, which can start to happen well below the speed of sound, due to the accelerated local flow on the blade aerofoil involved in generating thrust. More lately, this has been exacerbated by more demanding aircraft noise limits. In practice, this means the propeller tip speed cannot go much above about 750 feet per second (230 metres/second). There is also an overall increase in propeller efficiency, especially at low flight speeds, from larger diameters, and the larger the diameter, the lower the RPM must be to stay within the tip speed limit.

 

As a consequence of this, aero engines fall into two distinct categories - geared, and direct-drive. The Rotax series exemplify the first type; the Jabiru, the second.

 

Geared engines in the past, such as the Lycoming GSIO-480 and GTIO-540, and the Continental geared 520 series, have not been particularly reliable, because in an aero engine, the propeller has to act as the flywheel. Therefore, there are large oscillatory torque pulses being exchanged between the engine and the propeller, which give the gear teeth a very hard time. So the more reliable and durable GA engines have mainly been the direct-drive Lycomings, which run at 2700 RPM or less. Continental, in their drive to be seen as having a higher power to weight than Lycoming, went to 2850 RPM with their direct-drive 520 series - which is why a Beech Bonanza make such a loud noise at takeoff. It would not pass current noise certification standards.

 

Direct-drive engines are limited to relatively low powers - above about 300 horsepower, the limited propeller diameters become too inefficient, so gearboxes become necessary - and engine overhaul costs increase accordingly. So a direct-drive aero engine has a lot in common with a tractor engine, except in the use of cast iron.

 

Rotax has gone the other way, and they have managed to make it work - for very limited propeller moment of inertia, by using a spring-element between the engine and the propeller. The gearbox tooth loads on the tiny pinions used in the higher-ratio reduction boxes are rather terrifying; it seems an example of pushing gear technology to its limits. As the engine go up in power, the gearboxes generally have to become vastly more complex, to share the loads between a number of sets of teeth; and the engines get a lot more cylinders, which reduces the magnitude of the torque pulsations.

 

At high RPM, the inertia forces become dominant; at low RPM, the gas pressure loads and the connecting-rod angle become the dominating factor. Horizontally-opposed engines are always at the very limit of connecting rod angle, and extremely "oversquare" because this is the only way to keep the engine width within reasonable bounds. The Rotax gets away with it because of its small capacity; it is using the square-cube law backwards, as it were; but this concept has little potential to be scaled up.

 

A vee engine is inherently compact, without needing to go to extremes of either bore/stroke ratio or connecting-rod angle. A 90 degree V engine also has good inertial balance; but until you get to eight cylinders, it has uneven firing intervals. A V-8 layout like the Chev, is in many ways ideal, but scaling it down to an 80 HP aero-engine would be far too costly. As a result, you can't really compare a Chev racing engine with an 80 ~ 100 HP aero engine; the factors that drive the design are too different.

 

The through-bolt issue has at least six facets, which Ian Bent can explain much better than I can - and he did so at Temora. They are all subtle aspects of detail design, and assembly technique. The peak gas pressure in a Jab engine is not that much different to that in the Holden engine from which its pistons come; but the details that dictate the retention of the bolt pre-load are radically different, because the engine has to be so much lighter. I won't attempt to pass on second-hand information; if you want to know more about this aspect, Ian Bent has the answers; but please remember that he's flat out trying to get his mod. package out.

 

 

[Oscar]

I did say it wasn't directly relevant other than to assert that a mature product should not require constant monitoring over many variables.The last car I had overheating problems with was in about 1980. And then it came about through coolant loss not the normal environment it was operating in.

 

The Jab engine, to my consumer view, is not (yet?) suitable for the application it is sold into.

 

Does anyone believe a Jab engine is cheaper over 2,000 hours than a Rotax?

Don - I bet it wasn't a Jag. XJ12, then, which could cruise at 80 -100 mph happily in 32C but boiled due to thermal mass every time you slowed down for a town; I had one trip like that with five stops and waits before I could re-fill the coolant that spewed from the engine at each successive town across central NSW. And the Falcon EA, as I remember, had notorious cooling problems in normal service, while the Datsun 180B (or was it the 200B?) blew head gaskets and warped heads very frequently.

 

However, your general point that Jab engines currently are not as robust as the market generally ought to get, is a fair one, even though the figures that have come out in this thread have shown it to be causing reported failure in something like 4% of the engines in service over five years. That (and just generally improving the realistic TBO) is why CAMit are doing what they are doing now - even though what results won't wear a 'Jabiru' engine plate.

 

 

[turboplanner]

Well that's the academic vs the hands on debate - I fixed mine.

 

And I've heard a lot of the Ian Bent theories but let's remember that we have yet to see production engines in the hands of the same demographic which operates Jabiru and Rotax.

 

 

[Oscar]

Turbs, there's at least one CAMit engine (that probably doesn't have all of the latest stuff Ian Bent has been working on, but does have most of the major developments I think) that should by now have around 300 hours on it of routine 'commuter' use (see: http://www.recreationalflying.com/threads/camit-engines-anyone-got-one.114782/page-8 posts #153 and #155 )

 

 

[turboplanner]

That's my point Oscar = one trial engine. I wish him the best of luck, because as many have said, we need a low cost reliable engine.

 

However normal production involves many engines under dyno test in working conditions for in some cases the equivalent of 700,000 km, and a reliability factor measured in failures per 100 engines, with 3 or 4 being considered acceptable.

 

Incidentally, I looked up the melting point of phosphor bronze and it's 900 to 1050 degrees C. I'm not disagreeing it might be the first symptom of combustion chamber overheat, just that this focus on CHT monitoring is not really going to help solve the problem.

 

 

[Oscar]

Turbs, I think there are about a dozen or more out there, but at what rate they are putting up the hours, I don't know. I believe that Ian 'selected' his first users fairly carefully, on the basis that they would provide reliable use reports amongst other things. Ian tests everything - as far as I know - on his own engine before anybody else gets near it, and he tests it to over-limits conditions; the pile of heavily-tested components sitting in some corner of CAMit by now must be quite considerable.

 

The Catch-22 is, of course, that you have to get the things into service to get the service results, and no manufacturer can really afford to just 'have a bright idea' and throw a fix into an engine and say to a customer: 'there, go try that and see what happens' without having a very, very high level of confidence that it will work, at the very least, safely. The test-cell work will be another step along the way of proving the ability of the engines to meet specific performance criteria as well as provide additional research information, so its all an iterative process. As you cogently pointed out: do not try to change an aircraft engine just based on these few aspects - there are many others to consider as well. The engine is a set of systems that all HAVE to be interlinked, there's no 'silver bullet'

 

 

[Dafydd Llewellyn]

Normal motor vehicle production, you mean I think. They're large-volume products. Aero-engines are not. Bombardier can afford that kind of thing, but a small Australian manufacturer cannot; and even then, as you point out, the demograph of users cannot be reliably tested for in a laboratory.

 

The critical temperature with the guide is NOT necessarily its melting point. If you have ever done any study of metallurgy, you would be aware that metals are, in general, complex solid solutions of the various alloying components, often with some elements precipitating out of solution in the form of minute particles, and that metallurgical changes often occur well below the melting point. The tempering process for steel demonstrates that; and some aluminium alloys start to lose strength at as little as around 120 C (the rate of strength loss is very slow at the start, but it increases with increasing temperature) - the top speed of Concorde was set by this. I don't know what goes on in phosphor bronze, but one of the reasons it it so universally used in valve guides is that it "grows" in use so that it takes up the wear, to a certain degree. Have you never experienced a bronze bushing that has seized on its shaft due to lack of use? That process evidently ceases to work beyond a certain temperature, and the natural lubricity of the stuff ceases to exist - for all I know, it may be a change in the nature of the natural oxide film on the surface of the metal.

 

Have you never seen a turbocharger bearing heat shield (the bit that looks like a miniature brake drum, tho it does not rotate, and is there to keep the bearings from being cooked) that has warped so much that it has worn a groove into the back of the turbine? Do you know how this warpage occurs? It's NOT due to melting; metals are not that simple.

 

So the CHT is most definitely relevant. If we could measure the valve guide temperature reliably, that would probably be even more relevant, but CHT is what we can measure - if we're careful.

 

 

[Stewy]

The valve that broke did have signs of wear on the shaft where I looks like scratching so to speak, I assume residue from the avgas, along with the pitting that you describe Dafydd. The rest of them in the other heads were all ok apparently but I am no expert only my opinion from viewing the offending head and helping take photos etc of it.

 

Stewy

 

 

[Oscar]

Stewy - do you know the hours that engine had run?, or maybe, if it had recently had a head service with valves replaced, since then?

 

 

[turboplanner]

I agree that with your comments on elasticity Dafydd; I said at the outset I was trying to make it simple so people could understand it rather than engaging in a diagnosis competition.

 

My aim was to show that combustion chamber temperatures are vastly different to cylinder head temperatures and that fiddling around with fins, ducts and so on is not going to change the deeper issue.

 

If I've failed in that, there's always the action RAA has asked CASA to take.

 

 

[Russ]

What action ??

 

 

[Stewy]

I think it had done roughly 300 hrs from a top end overhaul Oscar. He was only changing the oil every 50hrs though I believe

 

I have not sited the log book etc

 

 

[turboplanner]

As a general comment, have a look at that aluminium plenum chamber, stub pipes and extension pipes to the cylinders, and also the squish chamber.

 

 

[Dafydd Llewellyn]

I agree that with your comments on elasticity Dafydd; I said at the outset I was trying to make it simple so people could understand it rather than engaging in a diagnosis competition.My aim was to show that combustion chamber temperatures are vastly different to cylinder head temperatures and that fiddling around with fins, ducts and so on is not going to change the deeper issue.

 

If I've failed in that, there's always the action RAA has asked CASA to take.

Oh, certainly one needs to get to the root of the problem - and the cause of serious thermal runaway in one cylinder is almost certain to be something remote from the airflow through the fins etc.

The relevance of the test-cell cooling system is that it has to satisfy the engine type certification endurance test requirements, which means it has to be able to hold all the cylinders on the engine at or slightly above the red-line temperature limit stated in the TCDS, within fine limits (about 2 degrees C), throughout the specified endurance-run duration. So that's a very specific design issue that has no direct correlation to how engines are used in the field. In the process, it may provide some useful information on such things as the cylinder baffling, and the pressure drop needed to adequately cool the engine at its rated power condition.

 

What the test-cell work can establish is that the engine, if it starts the process in good condition, can withstand operating at the limits for a sufficient length of time to show that no run-away condition occurs in the short term; and that its condition as shown by the strip inspection after the test run, is such that it could be reassembled and go back into service with no repair or replacement. The condition at the end of the endurance run should show that the endurance merely served to "run it in" a bit.

 

I have always considered it a bit simplistic to demonstrate that the engine can survive X degrees for 50 hours, and then project that to suggest that it's OK to operate at that temperature for 2000 hours - but that's the way the Type Certification requirement works; the red line temperatures are what is demonstrated in the test cell for more than half the run duration. The manufacturers generally give a reduced "recommended" temperature limit for best engine life, but that is really something that needs to be found from operational experience; there's no other way to establish it. And of course it needs to take into account the likely errors in the instrumentation, in the field.

 

What we're seeing in these 4% or so of failures, is either that the operating conditions can go outside the certificated limits and produce a failure; or that operating close to the limits for a long time will cause trouble, or that exceeding the limits for even a very short time can start a failure process that has a cumulative effect.

 

The data show only the result, not the cause. The cause is often quite indirect and also complex. And whilst close monitoring of the CHT and EGT of all cylinders is all one can do as an operator to get warning of out-of-limit conditions or impending trouble, that does not necessarily mean that "fiddling with fins and ducting'' will prevent such occurrences.

 

What we DO know, is that thermal runaway will occur if the operating temperatures get too high; and there are a number of ways that can happen. The most obvious one is detonation, but valve guide wear and carbon build-up in the ring grooves are also examples of thermal run-away issues. So the basic operating temperatures ARE relevant, and reducing them is the major way to improve overall reliability.

 

Philosophically, the question of whether the engine is good enough for the consumer, could equally well be turned around - is the consumer good enough for the engine? Evidently, from the data, most of them are. (Apologies - couldn't resist that).

 

 

[Oscar]

One possible cause of a single valve being affected while all the others were not, would be increased egt's for that cylinder. If that engine had previously operated happily, that says the maintenance and operating regime was unlikely to be the problem, but a slight change in the angle of the carby attachment to the intake plenum can make a considerable difference to mixture distribution between pots (known effect). So, what may have worked perfectly before, if slightly changed in the TO work, could be the reason here - and that's not intended as any criticism of the owner and maintainer, apparently it's extremely easy to have that happen

 

 

[Dafydd Llewellyn]

As a general comment, have a look at that aluminium plenum chamber, stub pipes and extension pipes to the cylinders, and also the squish chamber.

Yep, and if you have airflow rotation ("twisting") through the carbie, the fuel spray will not be what the plenum was designed for. If you can figure how to design a plenum chamber that can tolerate that, I'd dearly like to know about it. The updraft manifold-in-sump used on most of the carburetted Lycomings looks as though it should be much better, but the Lycoming data on leaning techniques using EGT shows that it is not.

What squish chamber are you referring to?

 

 

[Dafydd Llewellyn]

What action ??

Good question. Next question?

 

 

[geoffreywh]

QUOTE The valve that broke did have signs of wear on the shaft where It looks like scratching so to speak, I assume residue from the avgas, along with the pitting that you describe Dafydd.

 

Hmmmm. Scratching is almost certainly "galling" and not from Avgas lead. It's a lack of lubricant and excess heat causing particles of the guide to "pick up" and damage the valve stem. Result is badly worn guide and poor valve life. As the guide wears the ability of the guide to lose heat lessens and ends in burnt valve ( As Daffyd so nicely explained) . I would seriously reconsider the Jab. figure of .04mm valve/guide clearance.....That's almost 2 thou, many modern aircooled motorcycle engines wear limit on guides is half that........and usually the guide material is cast iron.....Go figure.....

 

 

[Dafydd Llewellyn]

QUOTE The valve that broke did have signs of wear on the shaft where It looks like scratching so to speak, I assume residue from the avgas, along with the pitting that you describe Dafydd.Hmmmm. Scratching is almost certainly "galling" and not from Avgas lead. It's a lack of lubricant and excess heat causing particles of the guide to "pick up" and damage the valve stem. Result is badly worn guide and poor valve life. As the guide wears the ability of the guide to lose heat lessens and ends in burnt valve ( As Daffyd so nicely explained) . I would seriously reconsider the Jab. figure of .04mm valve/guide clearance.....That's almost 2 thou, many modern aircooled motorcycle engines wear limit on guides is half that........and usually the guide material is cast iron.....Go figure.....

Yes, but why on one cylinder only? And if it were that simple, how come there are only 12 occurrences in five years? Continental started with phosphor-bronze guides, went through cast iron and sintered steel, and finally ended-up (at the last time I looked) with phosphor-bronze again. Go figure . . .

 

 

[turboplanner]

What we're seeing in these 4% or so of failures, ...........

I think Oscar came up with that one and it doesn't take long before fiction becomes fact.

 

My statistics related to incidents - forced landings, or landings where the aircraft wasn't going anywhere:

 

The numbers only included what the Association Magazine printed - they did not necessarily include all engine failures reported.

 

They did not include partial failures, or early signs, which were caught before a forced landing.

 

And they did not include aircraft which made forced landings, but whose owners chose not to report the incidents.

 

So the total number is likely to be well above 4%

 

You, and Facthunter keep referring to issues on Lyncoming/Continental engines apparently to water down this concern, but the fact is that Continental and Lycoming powered aircraft are not involved in regular forced landings.

 

Owners would naturally be concerned about the cost of unscheduled maintenance, but well above that in importance is the potential for injury.death in a forced landing, and that' why I'm only concentrating on those incidents. Competitive pressure will take care of excessive R&M.

 

 

[geoffreywh]

OOPS! 12 0ccurences in 5 years? what , per engine? I've had 3 in 580 hours.

 

 

[Head in the clouds]

I would seriously reconsider the Jab. figure of .04mm valve/guide clearance.....That's almost 2 thou, many modern aircooled motorcycle engines wear limit on guides is half that.......

This valve-to-guide clearance issue is interesting.

 

One northern Australian operator of Robinson R22 helicopters had a large fleet of them used for mustering and in the earlier days almost without exception they suffered from sticking valves. This usually happened after the main part of the muster and before the yarding, when the helicopter was spending a large percentage of its time at low airspeed albeit not in the hover, so not at particularly high power settings. High CHT seemed to be the issue even though the cooling is provided by a squirrel cage (centrifugal) fan rather than by airspeed. One theory was that the hot air was recirculating through the fan, perhaps while hovering downwind. The R22 is fitted with a Lycoming 0-320 de-rated by MAP limit to 124hp though full power is available to the throttle if required in emergency.

 

That operator had the exhaust valve guides of every helicopter reamed considerably oversize, I don't recall the actual size but I think it was a thou or two above the max wear limit specified in the manual. As far as I know they never had a stuck valve after that and all new engine changes and/or all brand new helicopters, including mine which they sometimes serviced, had the guides reamed before the machines went to work. IIRC this was done without removing the cylinders, by using thin rope to fill the cylinders through the plug holes. That was held tight against the valve head with the piston, to be able to remove the springs and cotters, dropping the valves into the cylinder and then coaxing them back into their guides afterwards with telescopic magnets - or similar. All sounds a bit bush-mechanic looking back on it but it worked well.

 

 

[Dafydd Llewellyn]

I think Oscar came up with that one and it doesn't take long before fiction becomes fact.My statistics related to incidents - forced landings, or landings where the aircraft wasn't going anywhere:

 

The numbers only included what the Association Magazine printed - they did not necessarily include all engine failures reported.

 

They did not include partial failures, or early signs, which were caught before a forced landing.

 

And they did not include aircraft which made forced landings, but whose owners chose not to report the incidents.

 

So the total number is likely to be well above 4%

 

You, and Facthunter keep referring to issues on Lyncoming/Continental engines apparently to water down this concern, but the fact is that Continental and Lycoming powered aircraft are not involved in regular forced landings.

 

Owners would naturally be concerned about the cost of unscheduled maintenance, but well above that in importance is the potential for injury.death in a forced landing, and that' why I'm only concentrating on those incidents. Competitive pressure will take care of excessive R&M.

Turbs, I'm not quoting Continental and Lycoming to water down people's concern; but their prior experience is valid engineering history. And yes, the number may well be considerably higher than 4% - but it doesn't affect my point where it's 4%, 14% or 44%; when you get a problem on one cylinder and not on the rest of them; or on a minority of the engines in service, then you have to look to why the difference.

Now, candidly, I'm fed up to the back teeth with this thread; I have work to do.

 

 

[coljones]

TSnip snip snipNow, candidly, I'm fed up to the back teeth with this thread; I have work to do.

Thanks for your contributions.