Cracked cylinders

[flyingcheesehead]

One of our Archers is in annual right now, and they discovered that ALL FOUR cylinders are cracked!

What would cause all of them to crack at roughly the same time (at least since last annual, we also did a "modified" 100-hr about 2 months ago but I don't know how closely they check the engine on that)? Are cracked cylinders common, or is this indicative of another mechanical problem or a pilot problem?

This engine has about 1600 hours SMOH, and it was torn down for a prop strike inspection about 3 years and 500 hours ago. (I don't know if it has ECI cylinders or not - What was their problem again? Our cylinders were at least not subject to the AD)

The only recent symptom was somewhat increased oil consumption, but not way out of line for an engine that old. Power seemed good. We also did have all four bottom plugs replaced on the 100-hr a couple of months ago, after some significant fouling issues this last winter.

 

[ScottM]

FYI the ECI Ad for cracking was on the classic cast ones. Those cracks were near the exhaust port if I remember right. The second AD was for the Titan and it was not cracking, it was separation of the barrel from the cylinder heads.

 

[Dan Thomas]

What would cause all of them to crack at roughly the same time (at least since last annual, we also did a "modified" 100-hr about 2 months ago but I don't know how closely they check the engine on that)? Are cracked cylinders common, or is this indicative of another mechanical problem or a pilot problem?

Detonation, maybe? Is someone leaning it too aggressively at really high power settings? Or burning some cheap mogas? Or ramming the throttle in on takeoff? Or is the timing too advanced? Or is there carbon buildup causing preignition? Carbon buildup can be from burning oil, or never leaning the thing.

Or maybe shock cooling, though it's less likely. Climbing hard and getting the CHTs hot, then yanking the throttle off and diving. More common in the flight school environment where they'll climb, then spin right away. Archers don't spin much, I think. Lycomings used to be famous for cracking between the exhaust valve seat and sparkplug hole. Don't see that much anymore. I believe it had to do with the uneven contraction of the head when cooled too quickly. Running the mixture back to full rich too rapidly can do that, too.

Dan

 

[Tom-D]

It is impossible to harm any engine by removing fuel.

Lean mixtures do not cause detonation.

They probably ran the engine with all 4 of the cylinders in the red box.

 

[flyingcheesehead]

Detonation, maybe? Is someone leaning it too aggressively at really high power settings?

How much is too much? This airplane seemed to be very susceptible to plug fouling this past winter, and as I mentioned, we had to replace the 4 bottom plugs at the 100-hour a couple of months ago. Would someone trying to burn the lead off be able to crack all the cylinders?

Or burning some cheap mogas?

Nope, it's mostly fueled from the 100LL truck at the home drome, and I have the receipts for when people refueled it elsewhere. 100LL all the way around.

Or ramming the throttle in on takeoff?

Could be - Could be exacerbated by the carb accelerator pump being finicky, this engine stumbles between 1000 and 1500 RPM when you add power for takeoff regardless of how smoothly you throttle up.

Or is the timing too advanced?

I wouldn't know... But wouldn't that be checked at annual? Maybe they'll find something.

Or is there carbon buildup causing preignition? Carbon buildup can be from burning oil, or never leaning the thing.

There was definitely carbon buildup on the plugs when they got replaced, and issues with it last winter. Now it's a chicken and egg problem. Was the carbon buildup due to us burning oil from the cracked cylinders, or did the carbon buildup cause the preignition leading to the cracked cylinders?

Or maybe shock cooling, though it's less likely. Climbing hard and getting the CHTs hot, then yanking the throttle off and diving. More common in the flight school environment where they'll climb, then spin right away. Archers don't spin much, I think.

These airplanes also aren't used for much training. Our club rules prohibit anyone with less than 25 hours, or pre-solo, from flying them. We've only ever had one, or maybe two, primary students as members (the rule to allow students with 25 + solo just came into being a couple of years ago). They do a fair amount of instrument training, but that should be easy on the engine.

Thanks for the ideas... I guess this points toward a JPI being a good upgrade to get next time this engine is out!

 

[Tom-D]

One of our Archers is in annual right now, and they discovered that ALL FOUR cylinders are cracked!

What would cause all of them to crack at roughly the same time ?

AGE! How old were they? can you prove it? at overhaul were they replaced or rebuilt?

 

[Tom-D]

How much is too much? Would someone trying to burn the lead off be able to crack all the cylinders?

Could be - Could be exacerbated by the carb accelerator pump being finicky, this engine stumbles between 1000 and 1500 RPM when you add power for takeoff regardless of how smoothly you throttle up.

I wouldn't know... But wouldn't that be checked at annual? Maybe they'll find something.

Thanks for the ideas... I guess this points toward a JPI being a good upgrade to get next time this engine is out!

Here is an idea, attend a "Advanced pilot seminar" know how to best operate your engine LOP

 

[gismo]

Detonation, maybe? Is someone leaning it too aggressively at really high power settings? Or burning some cheap mogas? Or ramming the throttle in on takeoff? Or is the timing too advanced? Or is there carbon buildup causing preignition? Carbon buildup can be from burning oil, or never leaning the thing.

You might be on the right track with this. From what I've read the only pilot induced cylinder cracking issues involve excessive CHT and/or internal cylinder pressure and these tend to go hand in hand. High ICP can result from overly advanced timing, mixture/power in the "red box" or pre-ignition. It's my understanding that detonation alone doesn't usually directly cause huge ICPs but it can lead quickly to pre-ignition which can destroy cylinders quickly. Other pre-ignition causes are carbon buildup especially on the exhaust valve head, spark plugs with cracked insulators, and plugs with the wrong heat range (too high).

Or maybe shock cooling, though it's less likely. Climbing hard and getting the CHTs hot, then yanking the throttle off and diving. More common in the flight school environment where they'll climb, then spin right away. Archers don't spin much, I think. Lycomings used to be famous for cracking between the exhaust valve seat and sparkplug hole. Don't see that much anymore. I believe it had to do with the uneven contraction of the head when cooled too quickly. Running the mixture back to full rich too rapidly can do that, too.

Shock cooling has been determined (by instrumented testing at GAMI) to be pretty much a non-issue. This is confirmed by the lack of problems with sailplane tug engines which climb at full power and low airspeed creating fairly high CHTs and then dive at high speed with low power yet don't seem to correlate with cylinder cracking occurances. In addition, IME engineers and metalurgists familiar with this issue will tell you that the real concern is reduced clearances between the pistons and cylinders as the cylinders cool and shrink faster than the pistons. Finally, the thermal effect of a rich mixture (fuel has a higher specific heat than air) is rather insignificant and the idea that liquid fuel actually reaches the cylinders when you go to full rich on a fully warmed up engine is pure fantasy.

Kent, I don't suppose that airplane had an engine monitor with data recording? It sure would be nice to review the temps over time and you could even associate any misuse with a particular member. Wouldn't that be helpful?

 

[gismo]

How much is too much? This airplane seemed to be very susceptible to plug fouling this past winter, and as I mentioned, we had to replace the 4 bottom plugs at the 100-hour a couple of months ago. Would someone trying to burn the lead off be able to crack all the cylinders?

It would take at least a couple minutes (probably longer) to initiate pre-ignition while "burning off" a fouled plug with the possible exception of a plug with a chunk of carbon on the tip glowing like a red hot ember. And in that case the ember would likely continue to exist during the subsequent takeoff where pre-ignition would be a lot more likely.

Could be - Could be exacerbated by the carb accelerator pump being finicky, this engine stumbles between 1000 and 1500 RPM when you add power for takeoff regardless of how smoothly you throttle up.

Rapid throttle opening will never cause cylinder damage, the concern over that is all about banging the crankshaft dampers into their stops and "detuning" them plus the potential for overspeeding if there's a constant speed prop (not likely a problem on the ground). The counterweight abuse can lead to broken crankshafts and/or propellers plus maybe problems with the gears in the accessory case.

I wouldn't know... But wouldn't that be checked at annual? Maybe they'll find something.

It should be easy to check the static timing. The timing can and does sometimes vary enough to matter between annuals especially if the mags aren't inspected internally and the point gap decreases as the rubbing block wears.

There was definitely carbon buildup on the plugs when they got replaced, and issues with it last winter. Now it's a chicken and egg problem. Was the carbon buildup due to us burning oil from the cracked cylinders, or did the carbon buildup cause the preignition leading to the cracked cylinders?

Cracked cylinders (more specifically cracked HEADS) won't lead to increased oil BURNING or carbon deposits but the reverse scenario is plausible (carbon -> hot spots -> pre-ignition -> cracked heads). Also consider that an engine that's burning lots of oil is running with the fuel's octane supressed by the oil. I don't know how significant that effect is but I have read that an engine which uses a quart per hour is only burning a drop or two per power stroke in each cylinder but the fuel per stroke is pretty small as well (something like a few thousanths of an ounce).

These airplanes also aren't used for much training. Our club rules prohibit anyone with less than 25 hours, or pre-solo, from flying them. We've only ever had one, or maybe two, primary students as members (the rule to allow students with 25 + solo just came into being a couple of years ago). They do a fair amount of instrument training, but that should be easy on the engine.

Thanks for the ideas... I guess this points toward a JPI being a good upgrade to get next time this engine is out!

I've installed two engine monitors so far and in both cases the engines were already in the airplanes and unless there's no way to remove the exhaust stack without pulling the engine first I don't see why having the engine out would make much difference. Usually you can do the whole install without even pulling the stacks. IOW I'd consider putting in a monitor ASAP to prevent undetected mishandling in the near future.

 

[flyingcheesehead]

Here is an idea, attend a "Advanced pilot seminar" know how to best operate your engine LOP

Tom - It's only an Archer. Carbureted engine. LOP is most likely not possible.

Even if it was, it's a club plane. I'd have to get a couple dozen people to attend an APS. Ain't gonna happen.

 

[flyingcheesehead]

You might be on the right track with this. From what I've read the only pilot induced cylinder cracking issues involve excessive CHT and/or internal cylinder pressure and these tend to go hand in hand.

FWIW, I don't know if it's pilot-induced or not. Just trying to figure out all possibilities.

Kent, I don't suppose that airplane had an engine monitor with data recording? It sure would be nice to review the temps over time and you could even associate any misuse with a particular member. Wouldn't that be helpful?

Sure would, but I'm afraid we are not yet so equipped. We're getting a JPI put in the 182, this one will be next - If we keep it that long.

Say, does anyone know if the G1000 engine data is downloadable the way JPI's is?

 

[Henning]

Tom - It's only an Archer. Carbureted engine. LOP is most likely not possible.

Even if it was, it's a club plane. I'd have to get a couple dozen people to attend an APS. Ain't gonna happen.

Incorrect. It's quite doable.

 

[John Collins]

Where were the cracks located? Were the cylinders first run or were they overhauls? How many hours on the cylinders since new (this can be difficult to determine in many cases)?

My understanding is that many cylinder cracks are due to fatigue, so that a cylinder with two prior overhauls is more likely to crack than one with a single overhaul and so on. It is also my understanding that cracks are more likely to be a manufacturing defect than pilot induced. Cracks are common near the spark plug and if the engine is injected (not this case), you will frequently find them between the injector and the spark plug.

 

[tonycondon]

Full Rental Power Ahead!!!

 

[gismo]

Full Rental Power Ahead!!!

As long as the mixture isn't leaned into the red box, "full rental power" shouldn't be an issue (other than high fuel consumption), especially WRT cylinder head cracking.

I agree completely with what John C. wrote. From what I've read, head cracks have been greatly reduced since new cylinders became the "norm" on any qualilty overhaul vs the "tried and true" use of rebuilt cylinders in the past.

There is one other source of head cracking I've heard of, supposedly resulting from over tightening of the fuel injectors into their boss on the heads. This appears to create a crack running between that hole and the top spark plug hole. Since your engine is carbureted that might not be a likely cause although IIRC the primers go in the same holes and could cause the same problem. At least the primers aren't likely to be removed very often if at all so the chances of an over tightening are slimmer.

 

[tonycondon]

There is one other source of head cracking I've heard of, supposedly resulting from over tightening of the fuel injectors into their boss on the heads. This appears to create a crack running between that hole and the top spark plug hole. Since your engine is carbureted that might not be a likely cause although IIRC the primers go in the same holes and could cause the same problem. At least the primers aren't likely to be removed very often if at all so the chances of an over tightening are slimmer.

I have observed that first hand. had a cylinder crack on the Jacobs radial on our CAF wings Bamboo Bomber. It was obvious the crack had started at the hole where the primer screwed in. We were careful not to tighten them up too much when we put the new cylinder on.

 

[kmead]

You have parallel valve cylinders and they probably have cracks starting from the exhaust flange and extend up between the cooling fins. This problem is very common and often overlooked. The turbo Aztec and TR182 are very bad about this too. Many years ago I started pressurizing the exhaust and spraying soap on the small engines cylinders and have found many cracked.

Some people believe that blown/leaking exhaust flange gaskets accelerate the problem.

Kevin

 

[vontresc]

Alright, pardon my ignorance here, but what do you mean by "Red Box"? CHT/EGT at redline?

 

[jesse]

Alright, pardon my ignorance here, but what do you mean by "Red Box"? CHT/EGT at redline?

At about 65% power 100 degrees ROP to peak. That gap widens as the power increases.

 

[tonycondon]

Alright, pardon my ignorance here, but what do you mean by "Red Box"? CHT/EGT at redline?

its the danger zone!

 

[gismo]

Alright, pardon my ignorance here, but what do you mean by "Red Box"? CHT/EGT at redline?

Read up here:

http://www.warmkessel.com/jr/flying/td/jd/63.jsp

http://www.warmkessel.com/jr/flying/td/jd/64.jsp

http://www.warmkessel.com/jr/flying/td/jd/65.jsp

http://www.avweb.com/news/pelican/183094-1.html

http://www.avweb.com/news/pelican/194452-1.html

 

[ScottM]

its the danger zone!

Hmmm I thought that was something else. At least that is what I learned from Cyndi Lauper songs.

Jesse that 100 degrees is the EGT or the CHT?

 

[ScottM]

Kent where are the cracks located and did the mechanic get any pictures of them?

 

[jesse]

Jesse that 100 degrees is the EGT or the CHT?

EGT.

Generally--much below 60% you won't hurt it no matter where you set it. Which is the beauty of climbing high for cruise, it makes it way less likely you're going to do damage with the red knob..Provided you don't do something stupid with it in the climb.

I can't say I'm an expert at LOP operations because I've never flown an airplane, short of riding with you folks, where it'd be capable of it. But I think I get the idea.

 

[ScottM]

EGT.

Generally--much below 60% you won't hurt it no matter where you set it. Which is the beauty of climbing high for cruise, it makes it way less likely you're going to do damage with the red knob..Provided you don't do something stupid with it in the climb.

I can't say I'm an expert at LOP operations because I've never flown an airplane, short of riding with you folks, where it'd be capable of it. But I think I get the idea.

I have tried leaning with the EGT and never found it to be very easy I do the lean until rough then push it richer. Generally my EGT is 1450F, I watch the CHT to make sure it stays below 400F. I have yet to have any cylinder problems with that but then I am on my third set of cylinders in 5 years thanks to ECI ADs.

So who knows if I am messing it up. But I know my EGT is only on one cylinder as is my CHT. I have over leaned and gotten a rough engine.

 

[vontresc]

Thanks!

 

[brcase]

Alright, pardon my ignorance here, but what do you mean by "Red Box"? CHT/EGT at redline?

Here is a description for the Red box from http://www.avweb.com/news/pelican/182583-1.html

or for more than you ever wanted to know about it...
http://www.avweb.com/news/pelican/182146-1.html



The Dangerous Red Box


Just where is that "red box" I keep talking about? Some rough numbers, good (that is to say, BAD) for most of these engines -- these are "no fly zones," DO NOT set the mixture between them:

Red Box = No Fly Zone

​

• At and below about 60% power, there is no red box. Put the mixture wherever you want it.
  
• At about 65% power or so, 100ºF ROP to Peak.
  
• At about 70%, 125ºF ROP to 25ºF LOP.
  
• At about 75%, 180ºF ROP to 40ºF LOP.
  
• At about 80%, 200ºF ROP to 60ºF LOP.

All those numbers are approximate! Please don't start splitting hairs, here!
You probably don't want to run your engine between those mixture settings. If you do, you are running very high peak pressures inside the combustion chambers, and that peak pressure is occurring too close to top dead center.
There's a chance you read too fast, and missed this very important point, so let me put it another way:

Outside the Box

​

• At 65% power, use richer than 100 ROP, or leaner than peak EGT.
  
• At 70%, use richer than 125ºF ROP, or leaner than 25ºF LOP.
  
• At 75%, use richer than 180ºF ROP, or leaner than 40ºF LOP.
  
• At 80%, use richer than 200ºF ROP, or leaner than 60ºF LOP.

(On most of these engines, with a properly set mixture at full rich, at sea level, full power, the EGT ends up at about 250ºF ROP, with some as high as 300ºF ROP.)

 

[Steve]

http://www.lycoming.com/support/tips-advice/key-reprints/pdfs/Key Operations.pdf

 

[Dan Thomas]

Good book article on detonation:
http://books.google.ca/books?id=mX1...Ktsa4C&sa=X&oi=book_result&ct=result&resnum=8

Briefly, it speaks of detonation (as a result of autoignition) requiring time to take place. Leaner mixtures burn more slowly, so they're a problem. The chemically-correct ratio (stoichiometric, 15:1) is the worst place for it. Gasoline will burn at mixtures between 8:1 (rich) and 18:1 (lean).

Inlet pressures and temperatures and lower RPM also contribute to detonation tendency. Higher inlet temps increase the risk (in engineering terms, the reaction time is reduced) as do higher inlet pressures. We know inlet pressure as manifold pressure, and we can increase manifold pressures at low RPM by opening the throttle too quickly.

Larger combustion chambers contribute, as well, and the larger cylinders found on aircraft engines are therefore more prone to it. So we use higher-octane rated fuel in such engines than we use in our automobiles, with the auto having higher RPM, smaller cylinders, and variable timing to limit detonation. Modern cars usually have knock sensors that control timing and other variables to stop it.

I ran across a really good article about all this some years ago, but it's disappeared off the 'net. It's a subject not covered well at all during pilot training, even in advanced training.

Dan

 

[gismo]

I have tried leaning with the EGT and never found it to be very easy I do the lean until rough then push it richer. Generally my EGT is 1450F, I watch the CHT to make sure it stays below 400F. I have yet to have any cylinder problems with that but then I am on my third set of cylinders in 5 years thanks to ECI ADs.

So who knows if I am messing it up. But I know my EGT is only on one cylinder as is my CHT. I have over leaned and gotten a rough engine.

A good rule of thumb for LOP without multicylinder monitoring is to start out cruising well ROP (like 100-125 F ROP) and note the IAS after things stabilize (takes several minutes). Then lean until the engine starts to get rough and richen just enough to smooth it out. After another several minutes of settling in, check the IAS. If you've dropped at least 3% you're far enough LOP that the CHTs and ICPs should be low enough.