Do you believe in shock cooling?

[E1453]

http://www.avweb.com/news/maint/182883-1.html

I'm not certain if shock cooling is harmfull.

What do you think?

[dmjw01]

The author manages to make the non-existence of shock cooling sound plausible; however I suspect there is some subtlety in his carefully-chosen words.

For example, he says near the end: "Sudden cooling of an air-cooled engine does not contribute in any dramatic way to cylinder-head cracking" (my emphasis added).

I suspect his use of the word "dramatic" means that shock-cooling isn't going to crack your cylinder heads in an instant, catastrophic way. And he's probably right there, otherwise students in 152's would be falling out of the sky every day.

But fatigue cracking is very real over a long-ish period of time, and the more thermal cycles you give your engine the sooner this cracking will happen. Suddenly chopping the power will result in a "miniature" temperature cycle, and if you make a habit of it they'll all add up. I think there's little doubt that if you keep your engine's temperature as constant as possible it'll thank you in the end.

The author also asks: "Does anyone complain that repeated shutdowns are causing head cracking? Of course not."

Well actually, I would claim that repeated shutdowns contribute to cracking eventually. But shutting down the engine is unavoidable - you simply have to bear that pain. If you ran the engine continually without ever shutting it down, I bet it would keep going for at least double its TBO.

[Rupert S]

Correct me if I'm wrong: When you reduce the power rapidly and subsequently, the engine cools rapidly, the engine may be damaged. So it obviously makes sense to reduce the power slowly. However, if you consider the drop in temperature that's incurred when switching off the engine, it is just as great, if not greater and occurs over a shorter period of time, so presumably this would cause the most damage and make shock cooling unavoidable if it really was a serious problem.

[dmjw01]

The issue isn't really one of "damaging" the engine - it's more like shortening its life (and I think this is the point that the author misses in his article). Eventual fatigue cracking is inevitable in practice, but with good handling (and a bit of luck) it shouldn't happen until after your engine has reached TBO.

Sudden cooling of the engine is unavoidable sometimes (like shutting down). It doesn't cause instant damage, but it does cause cumulative fatigue. You can't avoid the "damage" caused by switching the engine off, but you can avoid the damage caused by poor engine handling in flight.

The author uses some fairly twisted logic - he uses the fact that engines don't "fall apart" as proof that shock-cooling doesn't cause any damage. In reality, I think most engineers would agree that it's a case of gradual fatigue.

[Lowtimer]

I am not an engineer and have no claim to great mechanical wisdom, so have no answer to what will / won't damage any specific engine in any specific installation. But l'd like to point out a couple of undisputable facts about the comparison between and airborne throttle-back and a ground shutdown.

These two are not the same case, and inevitably have somewhat different effects on your oily bits, even though certain of the headline figures are similar.

Two significant differences:

1) In the airborne case there is a great deal of cooling airflow being forced over the fins, and that air is colder (often MUCH colder) than the ambient air on the ground. On the ground, there isn't. Furthermore, a nice cozy fug may build up in your cowling after you shut down, the warm engine will heat the air immediately around it. In the airborne case, the cold air constantly sweeps out any such warm air and replaces it with cold.

2) In the ground shutdown case, the engine is at rest, the bearing surfaces are not moving. In the airborne case, the engine may cool rapidly and all the parts are obliged to keep whirring around, going up and down, or whatever they do.

All other things being equal, Sod's Law and engineering realities seem to combine - the larger, more complicated and expensive an engine installation, the more prone it is to damage through improper handling.

[The Nr Fairy]

I can't remember where I read it but a gliding club with a Cub or Pawnee or some other such type reduced its cylinder replacement rate drastically by modifying the technique of throttle and airspeed reduction at the end of a a tow - I think it may have been in Pilot or Flyer this last year.

Shock cooling - definitely alive and well. In piston engined helicopters a period of time after touchdown is required to allow the engine to cool and non-catastrophic but important problems have been observed if this is not done.

[mad_jock]

Shock cooling is very real.

I have seen cracks down cylinders due to it.

It works on the same priciple of sticking a cold glass into hot water.

End in the airflow cools much faster than the other setting up stress through out the body of the engine. Glass is brittle so it goes with a bang. But the steel of the engine is alot more ductile so you get away with it the first few times.

Gliding tugs are famous for it.

Yes it does occur and it really shortens the usefull life of the engine.

MJ

[IO540]

The only plausible evidence I have seen on this subject is the article probably referred to by TNF where they found that shock cooling did not matter so long as the CHT was below a certain value to start with.

[2Donkeys]

The impact of cooling has at least as much to do with even-ness as magnitude.

If it were to be possible to ensure that the entire engine cooled at a rapid, but uniform rate, the impact of shock cooling would be lessened. It is this, combined with the relatively even and low starting CHT that avoids cracking when the engine is shut down at the end of the flight.

By way of contrast, a large engine, throttled back at the top of a steep descent would bring about enormous differences of temperature between front and back cylinders, and a high probability of cracking.

The Avweb article is a nice bit of argumentativewriting that regularly surfaces on bulletin boards, but disbelieving in shock cooling is something I am only happy to do with somebody else's engines.


2D

[Chuck Ellsworth]

" but disbeleiving in shock cooling is something I am only happy to do with somebody elses engines" ? ?

What exactly do you mean?

[2Donkeys]

What exactly do you mean?

Easy tiger!

Disbelief in shock cooling is for fools, and those with unfeasibly large wallets. I would rather that anybody practised their disbelief on somebody else's engines than on my C421's rather delicate pair of geared Geared Continentals. I am not advocating the abuse of engines belonging to a third party...


Alles klar?

[floppyjock]

Mad Jock you say


" It works on the same priciple of sticking a cold glass into hot
water "

So what your saying is applying full power before a sufficient warm up time will cause the damage. I agree with you.

ie Shock Heating

Floppy

[GTOTO]

You better believe in shock cooling damage or pay the price.

Lasham gliding club UK carried out research in this area and can prove it pays.

Basicaly always keep cylinder head temps under 400f and no sudden power reductons untill under 380f.

We have the artical on our web site to prove it.

http://www.horizonflyers.co.uk/Shock%20cooling.pdf

[Chuck Ellsworth]

Two donkeys:

I know you did not mean that you abuse engines...

I just didnt want it to look bad for you. :ok:

By the way just another note on this subject, in the engine manual from Wright for the R1820 they are very clear that power reductions be made slowly to prevent cylinder choking resulting in ring flex damage and piston scuffing. and caution you should exercise care in maintaining cyl temp and avoid rapid cyl. head temp. fluctuations.

Proper care of any engine is manditory, and thermal shock does damage engines....period...

Chuck

[LowNSlow]

Here we go again........

Anybody who has owned an aircraft will attest to the wallet emptying powers of cracked cylinders.

So what cracks cylinders?

Numpties (muppets for those left of the Atlantic) who have been in a fast cruise (or other high cylinder head temperature regimen) and then close the throttle thus lowering the temperature in the cylinders and even worse, sometimes compound the situation by shoving the nose down and increasing the airflow over the, by now, rapidly cooling cylinder barrels. Unfortunately, the engine block is still nice and hot and the cylinder heads are relatively warm as well so you get a pair of steep temperature gradients across the cylinder barrel which meet in the middle. This causes fatigue. Indisputable. Ask anybody who has studied metallurgy. It's a fact. It doesn't go away. It's real. Do it brutally enough (it ain't hard to do) often enough and the cylinder(s) WILL CRACK.

It might not crack the first time you do it, it might not crack the next time you do it. It will crack sooner than it should though. It might even crack when you are asking for max power on the
climb out ......

PS Shutting down the engine on the ground is a whole different ballgame:

a) The lighter components like the cylinders soak up heat from the heavier and thus slower cooling chunks like the engine block so the whole mass of the engine is cooling at a relatively uniform rate. This assumes that there is a cowling over the engine.

b) Unless you have parked nose into a howling wind, there will be very little airflow through the cowling to change a) above.

c) You will have taxiied in (at low revs) and let the engine sit for a few minutes at 1,200 ish rpm to burn the lead off the plugs after the low rpm taxiing and also to equalise the temperatures in the engine.

Thus when the mixture is pulled or the mags switched off, there should be no part of the engine that is so much hotter than it's neighbour that it will create thermal shock between them.

Rant mode OFF,

Tablets taken,

Time for the darkened room :cool:

[A and C]

I dont think that you have to do any more than read the attachment to GOTO, s post to learn that "shock cooling " is a problem.

[E1453]

My Standard Operational Procedure is:

For Bonanza A36:

At Top of Descent:

- I gently reduce the MP to 16 inches;
- reduce the RPM to 1800 RPM;
- maintain altitude while reducing speed;
- at 110 knots I put 500fpm on the VSI and let the speed increase slowly.

That works fine for a 500fpm and 130KIAS descent. For a 140-145 KIAS descent I reduce the RPM to 1900-2000, instead of 1800.

I think that's the least harmfull procedure for my engine.

[mad_jock]

I agree it works both ways round.

Temperature gradient and heat flow causing expansion/contraction is what gets you.

MJ

[dirkdj]

Shock cooling is only possible if the cylinders get too hot to start with.

Good baffling will provide proper cooling at cruise airspeeds, properly set full rich mixture will provide cooling at take-off power and climb airspeeds.

I had a very good demonstration of this in my friends A36. He installed the GAMI Liquidair baffles. I feel these are the most engineered baffles you can find. In-flight testing was done by GAMI with tufted cylinders and under the hood videocamera as well as circumferential thermocouples measuring the temps at eight different points on the cylinder.

Result of the new baffles: dramatic drop in cruise temperatures.

Still remained the unacceptable high climb temps even at full rich, full MP. Switching the fuel pump on and then pulling the mixture knob as required solved the problem.

Previously the CHTs were climbing about 1°F per second as recorded on the JPI. With the increased full rich fuel flow, the climb temps stabilize at a given CHT temp and stay there. Simply the engines as they come from the factory are set too low for maximum full rich fuel flow. The redline on the fuel flow instrument is a minimum not a maximum.

On my own A36, CHT rarely go over 300°F, 320° maximum. Everything over 400°F is to be avoided, the redline CHTs the mfgrs give (460 TCM and 500 LYC) are just criminal.

If you are running the cylinders too hot (CHT) then cracked cylinders are to be expected.

By the way, EGT is NO indication of exhaust valve temperatures, CHT is.

[B2N2]

Take a walk to your nearest engine shop/maintenance facility.
Take a good HARD look at an engine being taken apart.
All of the different components of the engine are of a different metal alloy because of the alloy properties better suited to whatever job the component does. They all contract and expand at different rates.
Pick up a prop and be amazed at how heavy it is.
Now this weight is swinging of the end of a very short crankshaft.
Imagine all the twisting forces when you jockey the throttle either open or closed.
That engine is keeping YOU from making a big smoking hole in the ground. It needs a little bit of respect and common sense.
I'm sorry but I cannot put it any simpler.
"Fly it like you own it" does not even work with some people..

[Chimbu chuckles]

I do fly it like I own it because I do own it, and having paid aud$40k to overhaul it with new millenium cylinders I can assure everyone that that number tends to focus the mind admirably.

As to shock cooling on descent...I believe that the cracking that has been historically associated with cooling is more likely caused by repeated over-heating weakening the cylinders, which in time subsequently crack.

By overheating I mean exceeding 400F, not the 460F mentioned as redline CHT on most piston engines, including my IO550. The problem has been that the traditional CHT gauges are criminally inadequate. Since fitting an EDM700 to my Bonanza I see every flight just how pisspoor the original fit CHT is. In my case the original fit CHT seems to be monitoring one of the coolest cylinders and they are generally 50F+ cooler than my hottest cylinders. That means, with no other indications, you could look at the CHT and see,say, 420F and be completely ignorant of several cylinders exceeding 460F which weakens the cylinder metal permanently.

I have spent thousands of hours doing very short sector bush flying in aircraft ranging from C180/185 through Islander and C404/402...and by short I mean 1 minute airbourne to 10 minutes airbourne and have NEVER seen a cylinder crack doing that. We often took off at DHs ranging from MSL to 8000', climbed over a 12000' ridge and then whistled down the other side for a straight in (or at most a very abbreviated circuit) on 400m strips where we parked and shut down within a minute of crossing the threshold.

15-20 times a day, day in and out, 6 days a week, week in and out, 52 weeks a year, year in and out!!!

Descents were typically flown at cruise power (maintaining whatever MP you started with at TOPD) with, in the singles, one power reduction to about 18 inches crosswind and then reduced to idle at roundout...sometimes then applying LOTS of power to get up a 20% slope after touchdown before idle again and the mixture shutoff. On the 404/402s we might reduce power by 1in/1000' on descent till 20inches then slowdown by adding drag around the (quickly flown) circuit then idle at roundout.

While the highspeed/cruise power descents tended to control the rate of cooling on descent we still, by traditional thinking, should have suffered all sorts of cracked cylinders...but didn't.

What I do in my Bonanza is;

Climb balls to the wall on throttle, pitch and mixture (mixture gets leaned a bit as MP falls away) and with IAS high enough to maintain <400F on the hottest cylinder.

Cruise wide open throttle, RPM where ever it suits my purposes, mixture leaned appropriately. Appropriately ranges from lean of peak below 5000', to peak or 60F odd rich of peak at high altitudes where there is so little air you can put the mixture anywhere you like without exceeding 400F on the hottest CHT. The imperative in the mixture is CHTs never above 400F and usually 380F on the hottest cylinder where the coolest will be about 330F. At altitudes above 8000'ish you need to run the fuel flow up a bit to produce sufficient power for reasonable cruise speeds.

Descent (and this is where it gets interesting) is accomplished lean of peak and then just lower the nose for the desired ROD, say 750'/min. As speed builds towards top of the green all I do is wind back the RPM (which reduces fuel flow thereby making me leaner, and reduces IAS). When RPM is at min and IAS is creeping into the yellow I reduce MP. In ONE FELL SWOOP I reduce MP from whatever it is, usually around 25-26inches, to a setting just above where the gear warning horn sounds, say 17-18inches.

When I do this ALL THE EGTs on ALL THE CYLINDERS increase DRAMATICALLY and the rate of cooling of the CHTs DECREASES!!!

The reason the EGTS increase and the CHTs cool slower is that by reducing the airflow into the cylinders I am enrichening the mixture back to a liitle rich of peak EGT... to a fuel air ration that gives a very hot burn...which slows the rate of cooling on the cylinders.

Can everyone agree that peak EGT is close to peak CHT?

In smooth air I might be doing 170 indicated when this power reduction happens and the inbuilt 'shock cooling' warning in the EDM 700 remains mute. I will typically carry this speed and power setting onto downwind where, below 150 KIAS, I lower the gear and reduce to 15 inches MP, where it stays until roundout.

After landing I taxi in and shutdown without delay. Every minute at even idle power and no significant airflow at taxi speed increases CHT. The coolest the engine will be is at touchdown.

Do I believe shock cooling exists at all?

Well if, at TOPD, you reduce power significantly and enrichen mixture heaps (lots of raw, cold fuel into the cylinders) and then poke the nose into a 1000'/min ROD then, yes, that's really bad.

Who does that?

Chuck.

[PPRuNe Pop]

In days gone by I operated 6 x C404 Titans. They were fitted with the Lycoming Continentals - GITSO's. The TBO from new was originally 900hrs. It later went up to 1100hrs. From there I know not.

All 6 had incidents of cracked cylinders - port or starboard - it didn't matter - they were subjected to shock cooling. Cessna were aware of it too.

Each of the affected engines suffered a hairline crack on the last thread of the cylinder just above where it fits on the cranckcase. It didn't show up either, until "one day - white smoke and a straight drop in oil pressure occurred" which caused a shut down.

Shock cooling was a relatively 'new' phenomena. Some dispute it but my operation, and me in particular, got the boss of Cessna over here and we thrashed it out. During our 'thrashing' it became obvious that there was more to it than we suspected and little snippets of serious witholding of information became apparent. I flew with their Chief Pilot to demonstrate our handling techniques, which he didn't fault.

The ops manual already stated that when in a descent the throttles should not be closed to less than 1200-1100rpm. Hopefully this should not then be increased again! After landing idle should be kept at 950 for 5 minutes - there was I think a further idling period which escapes me.

FACT is that shock cooling was/is real and I never would and never have believed these 'engineers' who say it is not possible or shouldn't happen. Perhaps the latter part of that sentence is correct. Largely because of engine mis-handling by pilots.

Each incident was a replacment of a cylinder at £1500 a throw plus time and engineering. By the time they had finished the bill was around £4000.

Fortunately the failures of this engine did not continue at the fast rate I had. Lessons from my operation were learnt - and fast!

However, I was asked to join a flight to LFAC one day out of Biggin, in a C404 in 14 seat config. My host thought I might like to sit up front which I accepted. I noticed that when he started the port engine there was a small drop in oil pressure. I asked how long it had been like it and he said a couple of weeks. I decided that I should keep quite but was concerned.

I sat in one of the isle seats towards the rear out of LFAC and kept a look at that oil pressure guage and noticed it drop all the way as we got airborne. The engine had failed. White smoke was pouring from the engine and the pilot did exactly what he should have done. He climbed ahead and did an excellent job of getting the aeroplane back on the ground on one engine - with 14 people aboard. Then on the tarmac the tell-tale oil was on the ground, and the hairline crack that had opened to release the oil was clearly showing by now. SHOCK COOLING caused it but it was NOT spotted. The result was as it had been on all of mine.

One other interesting point was that the cylinder failing on all the incidents I had were on the FIRST cylinder on the outside of each engine - left or right.

Please engineers. Do NOT try to tell me that shock cooling does not exist - it does.

Not all engines are natural candidates for SC. But it is wise to check the SOP's to make ABSOLUTELY CERTAIN that you are operating the engine(s) per the book. I can assure you it is not nice to have one go on you!

[IO540]

I've read all of Deakins articles, plus every other article I could find on the subject of engine management, and I still can't see that anybody has established a correlation, never mind a causal relationship, between rapid cooling and cylinder head cracking.

One problem is why shock heating (happens on every takeoff) isn't supposed to be a problem.

Another problem is why so many flying school engines, badly abused by instructors (PFLs) and by "ignorant" self fly hire pilots, rarely get cracked cylinders.

The real problem is lack of good data. One would have to put a number of engines on test and abuse them and see what gives. This isn't likely to happen, not because Lycoming etc could not afford to do it ( let's face it, they've got plenty of spare crankshafts lying around :O ) but because IF Deakin's hypothesis is true (cylinder head cracks are caused by poor manufacturing) this is the last thing they would want to reveal.

[PPRuNe Pop]

But there was one! Sad thing is I have forgotten his name. The direct cause was as I said above. However, Lycoming still always steered everyone away from them until a smart ar** told them that the TBO would be reduced to 750hrs! Things started to happen then.

Your comment regarding poor manufacture was taken up by Cessna in the case of the GITSO's and was dismissed by the FAA and the CAA!

My view is that there is indeed a link but it does NOT remove the handling problem entirely. There is no doubt that shock cooling occured and I had to ensure that throttles were closed in increments, in the descent, over the North Sea, at night in the winter! Just one example.

I would like to know the current TBO of the GITSO if someone can enlighten me.

PPP

[Volume]

As a pilot I know, that you are lucky if the (especially #3) cylinder heads reach 1300 hours in a tow plane (DR-400 and Christen A-1). The permanent change between full throttle climb and idle descent kills them all much before TBO.

As an engineer I have a problem to relate this shurely existing problem to the shock cooling of the cylinders. It´s quite simple, most heads crack on the inner surface between the spark plug thread and the exhaust port. Cracks are normally initiated by tension in the material. Cooling a metal part rapidly on one side while still hot on the other will cause tension on the cool side, and compression on the hot one. So if shock cooling of the cylinder head cooling fins is the cause, why don´t they crack there, where you would expect the coolest spots and therefor the highest thermal tension stresses ? Why do they crack on the hot inner surface, where they should have thermal compression stresses ?
The problem must be a much more complex one compared to just droping a cold glass in hot water.
With all the modern finite element calculation methods, it must be quite easy to simulate all the thermal stresses in cylinder heads, does anybody know about such analyses ?

[IO540]

PPRuNe Pop

I agree that shock cooling is real; I have an EDM700 which flashes an annunciator whenever the CHT drops by more than 60F/min... but the problem with the simple theory that shock cooling cracks cylinders is the very substantial contra evidence of flying school engines. These almost never have any relevant engine instrumentation, the students know nothing about it, most instructors know nothing about it, yet they rarely crack.

The one theory which is consistent with available data is that shock cooling can crack heads if the CHT is very high to start with, e.g. glider towing or parachuting. That is about the only business where you do the same thing over and over and over some years you will have got through enough engines to see trends.

In reality, shock cooling is easy enough to avoid: just keep the mixture at cruise-lean as you reduce power for a descent and don't drop more than say 2" MP at a time. But I don't think the typical cruise CHT is high enough to cause a problem anyway - if it was, school engines would not last long.

[A and C]

I think that it is time to have a long hard look at what is likely to be at the root of this thermal stress problem.

1 CHT the hotter the cylinder gets the larger the stresses if it cools quickly.

2 compression ratio , the more compression the greater the heat released from the charge and the higher the gas pressure to be contained.

3 Airspeed , the greater the speed the more chance of the engine cooling quickly.

4 Engine heat cycles.

It would seem to me that a low power engine that is used by an owner pilot for crusing is very unlikily to have cracking from shock cooling by the very nature of the type of flying .

The odds of problems go up if to the same sort of engine you add some glider towing after all in the climb the CHT will be higher and the hot/cold cycles will go up.

The odds of problems increase rapidly if you turbo charge the engine as the heat stresses from high compression along with the faster airspeed give greater oportunity for the rapid cooling of the cylinders add to that high cycles in a commercial enviroment and the chances of damage are quite high.

The flying club trainer falls between the "cruiser" and the glider tug and so for reasons outlined above is not in the high risk group unless grossly mis handelled.

As the reseach by the Lasham gliding club has shown proper treatment of your engine will reward you , this is based on proven facts gatherd over a long time , it should be remembered that some of what is writen on the internet is only the opinion of the writer and for these reasons I conclude that "shock Cooling " is a hard fact and not as some say a figment of the colective aviation imagination.

[E1453]

What would be your rules of Thumb for avoiding shock cooling?

Mines are:

- don't let it to get too hot;
- don't start a descent immediately after a climb. Let the CHT's stabilise in cruise power before starting to descent;
- don't reduce power abruptly;
- when reducing power, maintain altitude, if possible, as the speed is reduced and the cooling airflow is reduced, also;
- when pointing the nose down, let the speed increase gently, if at low power.

[IO540]

A and C

CHT the hotter the cylinder gets the larger the stresses if it cools quickly

I don't think this is objectively true. In metals, thermal expansion is linear, so cooling from say 250C to 200C is going to cause the same dimensional change as cooling from 200C to 150C. What IS different is the metal strength: aluminium loses its tensile strength rapidly above about 250C. This would make sense given the one half decent piece of evidence we have from glider towing.

E1453

My view is that keeping CHT below 400F for power reductions is the best thing. All the other things you mention are worth doing anyway because they make flying easier. Big power changes = big trim changes = more work :O

[A and C]

You are quite right thermal expantion is linear but all the parts of the cylinder are not made of the same metal and all metals cool at diferent rates the aluminium alloy of the head will cool faster than the steel valve seats and sparks.

This is why I feel that slow cooling is important , to keep the temp across all the cylinder parts even and therefore the expantion rate as even as you can.

I find it no coincidence that most cracks on cylinders are around the valve seats and spark plugs.

[Vfrpilotpb]

You also have another major factor in any engine, in fact on any componant using metal, and that is age hardening or age embrittlement, the constant cycles of on/off, full power/ low power all take a toll of even the most modern materials, add this to your abuse of engine or g/box systems and
" Hey you've found a crack":(

[IO540]

A and C

I find it no coincidence that most cracks on cylinders are around the valve seats and spark plugs.

That could simply be that these are the areas where the CHT is at its highest, i.e. where the metal is most likely to be close to its "critical" temperature. Plus the tensile stress could be greatest there too.

I don't wish to argue for the sake of it; I am merely pointing out that we don't actually have anything resembling decent experimental evidence. One must never forget that flying school engines get pretty severe shock cooling regularly yet they rarely crack.

Vfrpilotpb

I don't think the number of cycles correlates closely with cylinder head cracking, because flying school engines get a lot more cycles than most other GA engines, yet they get relatively few cracks.

[Hairyplane]

All those who don't believe that shock cooling will damage an engine and fly club planes - I dare you to cough up your misguided theories to the CFI, or better still, the owner(s) of the aircraft operated by the club.

I think you will receive a word or three in your shell-like.

Isn't this taught any more? It was drummed into me big time, usually when climbing to stall-practice height, especially in the Winter.

I in turn drummed it iinto my students.

I was sufficiently concerned about it to invest in an EDM700 engine monitoring system in my new Regent.

Whilst the instrument settings are fairly conservative when compared to the max cooling rate data in the (Lyco 0360) engine manual, I find that unless I start to back off the power when 5 miles out, you can bet your butt that the shock cool alarm will activate immediately in the circuit - especially at this time of year.

The next owner of my Regent can be certain of a well cuddled donkey up front.

My 2 Gipsy engines are well cuddled too - at least 6 minutes warm up and at least 3 or 4 run down. Never a problem (and with plenty of wood to touch every time I say it!).

Cuddle your donkeys chaps - they'll be less inclined to kick you.

HP

[Dale Harris]

Most low powered a/c, ie c150/152, 172 etc don't have CHT problems. Some of these don't even have CHT gauges That should tell you something about the temperature levels reached by the cylinders on these a/c. However, a Baron, Chieftain or Seneca, closely cowled as they are, and run hard will often get above 420 or 440 deg. It is not the fact that you are cooling the cylinders, it's the temperature that the cooling starts from and the rate of change that is the problem. Cooling a 150 or warrior engine quickly from 350 deg to 275 in 2 minutes is nowhere near as problematic as taking a chieftain engine from 450 deg to 375 in the same time. Aluminium alloys will stand the lower temperatures almost endlessly, they will not stand the higher temperatures the same way. Hence the need for some caution when operating at the higher levels. And the reason simple a/c engines, ie much less temperature stressed models, suffer less from the cracking problem. The temperature range is much less in these a/c types.

[IO540]

Dale Harris

I agree with you entirely - the little evidence there is points in the direction that shock cooling is a problem only if the CHT is above a certain point.

But that isn't what the constantly recycled shock cooling story is saying...

The question of interest to me is whether shock cooking from say 350F (a typical cruise CHT for an IO540 etc) is a problem.

[Say again s l o w l y]

I used to be a shock cooling sceptic until one of our a/c decided to throw one of it pots 200+Hrs before it was due to be changed. This was not a high powered donk, but a nice little O-320.

The crack ran all the way around the flange and at this early stage is believed to be due to the combination of corrosion and shock cooling that caused a quite spectacular failure.

Luckily a very experienced FI was on board and pulled off an excellent down wind landing back on the field after the failure at 700' agl over a large wood. Looking at the engine afterwards scared the c**p out of me, what a mess!

Be nice to your/our engines. Let them warm and cool as gently as possible. That way you may never have to use those rusty PFL skills for real!

[Saab Dastard]

2p

One thing that I remember from my engineering degree is that the rate of change of temperature is dependent on the initial temperature difference.

I.E. the greater the difference between the CHT and the ambient temperature, the faster the CH will cool.

This would appear to support the assertion that high CHT situations tend to increase the possibility of damage from shock cooling.

/2p

SD

[Dale Harris]

IO 540, I will go look for an engineering article I found once. I don't recall the temperatures exactly, but the article in the main was pointing out the operating temperature range of the aluminium alloys involved, and made the point that below a certain temperature there was much less work hardening of the alloy in the heating/cooling process. The other factor that most people are not aware of is that the cylinders on your engine are quite possibly not first life cylinders. Many reconditioners put 2nd or 3rd life cylinders on what is said to be an overhauled engine. The cylinders themselves were originally designed for one life, then scrapping. It is very rare for a first life cylinder to fail due to cracking, whether it be shock cooling related or not. However, on 2nd and 3rd life cylinders, it becomes more common for the cylinder to fail of course. In a relatively highly stressed engine like the L/TIO 540 that we operate, even 2nd life cylinders very rarely make the 2000 hr TBO. First life ones almost without exception do. And that is not just our companies experience........

[IO540]

Dale Harris

Interesting data. Mine is a new engine, fortunately. J Deakin (his articles are at avweb.com but keep moving about so I don't have the current URL) has a graph showing the strength of cylinder head ally falling rapidly around 500F - not good as Lyco give 500F as the max operating CHT! The work hardening point is going to be somewhere below that, I would guess.

Also your TIO540 engines probably work a lot harder than an IO540, in terms of both CHT and the combustion pressure profile.

With most of your 2nd life cylinders not making TBO I take it you are operating twins :O

[Dale Harris]

IO 540, yes we do, Chieftains. Unusual to get a big failure, usually minute cracking picked up at 100 hourly time, usually plug hole oriented, and cylinder gets changed. No major dramas really. Although I've had one or two interesting ones.........