Shock cooling

J

JOhnH

Touchdown! Greaser!
Joined
May 20, 2009
Messages
14,189
Location
Florida
Display Name
Display name:
Right Seater
What should you do when you need to lose altitude quickly and you find that your engine monitor is flashing too high an engine cooling rate? I know that advance planning will help. Go to full rich prior to decent to pre-cool the engine. Plan your decent earlier and more gradual etc, but sometimes it can't be helped. When flying into a busy airport under ATC control and they hold you at altitude till the last minute, should you request a go-around? Is this another instance where my engine monitor is giving me too much information? I don't believe what some people tell me; "Don't worry about it because the O360 is bullet proof". I have had an O360 fail in flight.
 
JOhnH said:
What should you do when you need to lose altitude quickly and you find that your engine monitor is flashing too high an engine cooling rate? I know that advance planning will help. Go to full rich prior to decent to pre-cool the engine. Plan your decent earlier and more gradual etc, but sometimes it can't be helped. When flying into a busy airport under ATC control and they hold you at altitude till the last minute, should you request a go-around? Is this another instance where my engine monitor is giving me too much information? I don't believe what some people tell me; "Don't worry about it because the O360 is bullet proof". I have had an O360 fail in flight.
Click to expand...

Just tell them you are unable!
Done it a few times.
 
You can also slow down early when you realize they'll be slam-dunking you to the runway, then use flaps/gear to add drag and steepen the descent without pulling power to idle. Another option is telling them you'll need a 360 to lose altitude.
 
JOhnH said:
What should you do when you need to lose altitude quickly and you find that your engine monitor is flashing too high an engine cooling rate? I know that advance planning will help. Go to full rich prior to decent to pre-cool the engine. Plan your decent earlier and more gradual etc, but sometimes it can't be helped. When flying into a busy airport under ATC control and they hold you at altitude till the last minute, should you request a go-around? Is this another instance where my engine monitor is giving me too much information? I don't believe what some people tell me; "Don't worry about it because the O360 is bullet proof". I have had an O360 fail in flight.
Click to expand...

Don't worry about it, shock cooling is a myth.
 
dmccormack said:
What's the rate the monitor is set to?

If it's a JPI, you can alter the threshold, IIRC.
Click to expand...

I think it is set to - 50. It doesn't flash often, but when it does it worries me.

As to Shock Cooling being a myth, in our veterinary hospital we often hear people (including some older veterinarians) say heartworms in cats are a myth. So I wonder what those white stringy things we pull out of dead cat's hearts and lungs are.
 
JOhnH said:
I think it is set to - 50. It doesn't flash often, but when it does it worries me.

As to Shock Cooling being a myth, in our veterinary hospital we often hear people (including some older veterinarians) say heartworms in cats are a myth. So I wonder what those white stringy things we pull out of dead cat's hearts and lungs are.
Click to expand...

If shock cooling is real, what do we see in cylinders affected by it?
 
Dan Thomas said:
Some consider it a myth, but the operators of glider tugs don't:

http://www.peter2000.co.uk/aviation/misc/shock-cooling.gif
Click to expand...

Interesting info. They talk about applying their cooling technique so that "after a minute" it gets the CHTs below 200 C, which is 392 F. So, they are obviously higher than 392 F when they start cooling, but they didn't say exactly how hot they let them get.

So were they getting cracking because they let the heads cool too quickly, or is it because they let them get too hot in the first place?
 
I worry more about shock heating.
 
Dan Thomas said:
Some consider it a myth, but the operators of glider tugs don't:

http://www.peter2000.co.uk/aviation/misc/shock-cooling.gif
Click to expand...

That's an interesting article that, if true, certainly seems to indicate something is breaking their engines. It's hard to imagine that the rate of cooling seen in their slam dunk approaches to fetch another glider are greater than the rate of cooling when you shut it down at the end of the day. I'm still dubious and have plenty of first hand evidence with my AEIO540 (and the thousands of other AEIO360s and 540s) found in aerobatic aircraft. We're constantly treating them to much worse than tow pilots in climates from Arizona in the summer to Canada in the winter and I've never heard of a single instance of a cracked jug on a standard engine. Acro pilots blow up engines that have been pumped all the time but those are engines designed for 300 hp operation that have been juiced into the high 300's. Standard mills, not so much.
 
One more time...

You can't cool the cylinders fast enough to cause "quench cracking." That takes cooling rates of 1000's of degrees/sec, and that ain't happening unless you fly at full throttle into the Arctic Sea, in which case you have other, bigger problems. You can, however, cool the heads enough faster than the pistons so the cylinders contract enough faster than the pistons that you can accelerate your piston ring/cylinder wall wear. Lycoming says this can reduce engine reliability and longevity. If you keep your cooling rates under 60 degrees/min, you shouldn't run into this issue. For those without engine analyzers, limiting your throttle reductions during descents from cruise to 100-200 RPM/min with FP props, and 2 inches/min with c/s props, should do the trick just fine. Either way, limiting the number of full power climbs followed by power-off approaches is also helpful (sorry, folks working on their CP-ASEL).
 
Trapper John said:
If shock cooling is real, what do we see in cylinders affected by it?
Click to expand...

We all know that cylinders as a group, tend to crack after about 3000 hours of use.

But

There are so many things that contribute, how can you blame just one thing?
 
Hmmm....you must be really afraid of shutting down the engine on the ground. Think of the shock cooling generated there!
 
sba55 said:
Hmmm....you must be really afraid of shutting down the engine on the ground. Think of the shock cooling generated there!
Click to expand...

Surely someone, somewhere with a nice engine analyzer has logged what happens after shutdown. It would be interesting to see.
 
Ron Levy said:
[snip]

Either way, limiting the number of full power climbs followed by power-off approaches is also helpful (sorry, folks working on their CP-ASEL).
Click to expand...

Ahh, so what does the DPE want to hear?

I just always feel this way about any kind of machine: treat it gently, and it will give you longer life.
 
What is described in the UK article is close to how we handle our glider tow. Full power climbs to 2000 or 4000 AGL, avg 500fpm to 800fpm climb with a glider in tow at 60kias.

At release, reduce power to 2300rpm and push it over to 80-100kias. The descent also unloads the prop so rpms will increase, pull it back to 2200-2300 again.

Some of the pilots once clear of traffic will spiral down pulling 2-3g at 80kias and keeping the power at 2200. This helps with the engine, once you pull power back, don't push it up. Enter the pattern slow it down and rpm down to 1500-1700. Never level off on downwind, keep it coming down hill, 500ft AGL abeam the numbers, 60-65kias and 1400rpm. Smooth 180 turn, base to final and power to idle to land (as required).

We have never cracked a cylinder from shock cooling. And we are on the ground within 2 minutes from a 2-3K climb.
 
Ron I'm not disagreeing with you here, because I'm way too new to know better, but what about my repeated tng's in the training planes? Not just me either, nut other students. I might do 5 touch and go's before a full stop. That can't be good for the engine then? That's a full power climb, a short around in the pattern, and then an "idle" landing. Would this induce what your talking about?

They've replaced the engine in the bird I use for training once, but I think that was corrosion (it wasn't always a flight school bird) that popped up shortly after she was on the line.

Curiosity kills because I'll need to keep night training up JIC I need to fly then.
 
JesseD said:
Ron I'm not disagreeing with you here, because I'm way too new to know better, but what about my repeated tng's in the training planes? Not just me either, nut other students. I might do 5 touch and go's before a full stop. That can't be good for the engine then? That's a full power climb, a short around in the pattern, and then an "idle" landing. Would this induce what your talking about?

They've replaced the engine in the bird I use for training once, but I think that was corrosion (it wasn't always a flight school bird) that popped up shortly after she was on the line.

Curiosity kills because I'll need to keep night training up JIC I need to fly then.
Click to expand...

Maybe a full power climb to 1000ft AGL, and then a reduced power cruise setting on downwind.. and then reducing power again for the descent and landing... sounds like reducing power in stages from a full power (short) climb. staged cooling in progress..
 
Trapper John said:
Surely someone, somewhere with a nice engine analyzer has logged what happens after shutdown. It would be interesting to see.
Click to expand...
Not many folks leave the electonics on after shutdown -- kinda makes it hard to restart later. However, based on some observations from a friend who instrumented his engine compartment just to look at post-shutdown thermal issues, the temp in the engine compartment goes up, not down, after shutdown, primarily because there's a lot of hot metal and no cooling flow, and it cools comparatively slowly after that.
 
JesseD said:
Ron I'm not disagreeing with you here, because I'm way too new to know better, but what about my repeated tng's in the training planes? Not just me either, nut other students. I might do 5 touch and go's before a full stop. That can't be good for the engine then? That's a full power climb, a short around in the pattern, and then an "idle" landing. Would this induce what your talking about?

They've replaced the engine in the bird I use for training once, but I think that was corrosion (it wasn't always a flight school bird) that popped up shortly after she was on the line.

Curiosity kills because I'll need to keep night training up JIC I need to fly then.
Click to expand...
Jesse, no, that's not what kills these engines. It's operating at very, very high CHTs, and most CFIs are completely unaware of it.

Take a 172 with an engine monitor (not that many around) and see what the CHTs look like when you do a power-on stall a couple of times. I have seen them get close to 460 degrees. _That's_ what's bad for these engines.

Cooling has almost nothing to do with it. Of course, cooling from a very high temperature is bad anyways. So if you are at 460 (where you shouldn't be to begin with!) and then suddenly pull all the power and dive, you're doing some damage. Less than what happened already when you were at 460, though....

Shock cooling is pretty irrelevant. It's quite ironic that most CFIs can blabber on about shock cooling, but they are completely unaware how they're actually damaging their engines all the time. It's obviously better to avoid rapid temperature changes, but there's much bigger fish to fry...
 
JesseD said:
Ron I'm not disagreeing with you here, because I'm way too new to know better, but what about my repeated tng's in the training planes? Not just me either, nut other students. I might do 5 touch and go's before a full stop. That can't be good for the engine then? That's a full power climb, a short around in the pattern, and then an "idle" landing. Would this induce what your talking about?
Click to expand...
What Bill said -- if you make partial-power rather than idle descents, and aren't diving down at high speed, the cooling rates in the pattern aren't that bad. Consider that another reason to use 1.4-1.5 Vs on the downwind and 1.3 Vs0 on final rather than the folks I've seen doing 100 KIAS on downwind and 75 KIAS on final in a 172. Slowing it down reduces the cooling flow through the inlets/outlets (which were designed for limited climb speed/power and continuous cruise speed/power), and that reduces the cooling rates.
 
sba55 said:
Jesse, no, that's not what kills these engines. It's operating at very, very high CHTs, and most CFIs are completely unaware of it.

Take a 172 with an engine monitor (not that many around) and see what the CHTs look like when you do a power-on stall a couple of times. I have seen them get close to 460 degrees. _That's_ what's bad for these engines.

Cooling has almost nothing to do with it. Of course, cooling from a very high temperature is bad anyways. So if you are at 460 (where you shouldn't be to begin with!) and then suddenly pull all the power and dive, you're doing some damage. Less than what happened already when you were at 460, though....

Shock cooling is pretty irrelevant. It's quite ironic that most CFIs can blabber on about shock cooling, but they are completely unaware how they're actually damaging their engines all the time. It's obviously better to avoid rapid temperature changes, but there's much bigger fish to fry...
Click to expand...
Problem is that some parts of flight training just aren't that easy on the engine. But they're things that need to be covered. That said, most of those 172s make TBO no problem, some of them tick on in the training environment for another thousand or so hours after that problem free.
 
Ron Levy said:
the temp in the engine compartment goes up, not down, after shutdown, primarily because there's a lot of hot metal and no cooling flow, and it cools comparatively slowly after that.
Click to expand...

that will be the process of annealing any aluminum. heat to critical temp and allow to cool.
 
Ron Levy said:
Not many folks leave the electonics on after shutdown -- kinda makes it hard to restart later. However, based on some observations from a friend who instrumented his engine compartment just to look at post-shutdown thermal issues, the temp in the engine compartment goes up, not down, after shutdown, primarily because there's a lot of hot metal and no cooling flow, and it cools comparatively slowly after that.
Click to expand...
The engine compartment temperature may go up but that isn't what we're talking about...We're talking about the CHT which would be going down causing that engine compartment temperature to go up.

I'm no engine expert but I can't quite see how the CHT's would go anywhere but down. That said, I'd guess they'd go down at a fairly reasonable rate because of the lack of airflow.
 
Last edited:
jesse said:
The engine compartment temperature may go up but that isn't what we're talking about...We're talking about the CHT which would be going down causing that engine compartment temperature to go up.

You right about that. the temps under the cowl may go up when the air flow stops, but the heat source has gone too.

I'm no engine expert but I can't quite see how the CHT's would go anywhere but down. That said, I'd guess they'd go down at a fairly reasonable rate because of the lack of airflow.
Click to expand...

During normal operation of the engine (even in training) you will never see CHTs that will cause any change in cylinder metallurgy, critical temps for 2400 series aluminum is 830F, you must exceed that temp before any change will occur.

most operators do not realize most damage will occur while setting at the hold short line, little or no airflow will over heat the rear cylinders of the engine, because the cowl is not designed to cool when not in flight.

over heating the cylinder will harm the only part in the cylinder that is heat treated, the rings, when they are over heated, the temper will be lost and the rings will collapse into the piston and oil usage goes way up, blow by then over heats the piston and eventually we have a catastrophic failure.
 
sba55 said:
Take a 172 with an engine monitor (not that many around) and see what the CHTs look like when you do a power-on stall a couple of times. I have seen them get close to 460 degrees. _That's_ what's bad for these engines.
Click to expand...

That's why you bleed off all the speed first by reducing power, pitching up, getting close to stall then add full power, so you're only there for about 3-5 seconds or so. Saves the engine, and you don't gain a bunch of altitude.
 
Tom-D said:
During normal operation of the engine (even in training) you will never see CHTs that will cause any change in cylinder metallurgy, critical temps for 2400 series aluminum is 830F, you must exceed that temp before any change will occur.
Click to expand...

I'm not clear on what you're trying to convey. If you're saying it's OK to take an aluminum cylinder or cylinder head casting up to 830 F with no ill effect, that's not the case at all.

AA24xx material isn't used for casting - it's a designation for sheet and plate. Casting alloys are designated Axxx, with the first digit after the "A" being 3, 4 or 5.
 
Trapper John said:
I'm not clear on what you're trying to convey. If you're saying it's OK to take an aluminum cylinder or cylinder head casting up to 830 F with no ill effect, that's not the case at all.

Metallurgy of the cylinders can not change at any temps which can be reach during operation with out burning the paint off the cowl.

AA24xx material isn't used for casting - it's a designation for sheet and plate. Casting alloys are designated Axxx, with the first digit after the "A" being 3, 4 or 5.
Click to expand...

Any of the Aluminum alloys do not change properties at any temps we see in any operations of the engine.

the worse operations we see are acro flight, and they don't see failures occurring at regular intervals.

Lots of OWTs about shock cooling, it simply doesn't exist. Cylinders fail do to too many things to blame just one.

I have never heard of, or seen any cylinders fail due to over heating and quick cooling alone. there is always other factors involved such as poor manufacturing, age, or detonation involved.
 
Tom-D said:
Any of the Aluminum alloys do not change properties at any temps we see in any operations of the engine.
Click to expand...

"Properties" covers a lot of ground. As far as mechanical properties go, generally speaking, compressive and tearing strengths of aluminum alloys vary inversely with temperature. The hotter they get, the weaker they get. And these effects can show up at temperatures much lower than you'd think, on the order of 400 F depending on the composition of the alloy and what kind of heat-treating was used after casting. Fatigue resistance also decreases with temperature, and the propensity for an aluminum alloy to propagate a crack increases with temperature. As far as metalurgical properties go, heat treatment processes can effectively be "un-done" depending on the magnitude of heating in service and the number of heating and cooling cycles.

Lots of OWTs about shock cooling, it simply doesn't exist. Cylinders fail do to too many things to blame just one.

I have never heard of, or seen any cylinders fail due to over heating and quick cooling alone. there is always other factors involved such as poor manufacturing, age, or detonation involved.
Click to expand...
I'm not saying shock cooling is a problem, just so we're clear.
 
jesse said:
The engine compartment temperature may go up but that isn't what we're talking about...We're talking about the CHT which would be going down causing that engine compartment temperature to go up.

I'm no engine expert but I can't quite see how the CHT's would go anywhere but down. That said, I'd guess they'd go down at a fairly reasonable rate because of the lack of airflow.
Click to expand...
OK, let me follow through...

Cooling rate in this situation is primarily a function of three things -- the amount of heat being generated by the engine, the difference between CHT and the cooling air, and the flow rate of the cooling air. Assuming power is relatively low or zero (as it is in an idle descent or after shutdown), if you're getting a high flow rate of relatively cold cooling air (as you do in a high speed/low power descent, with CHT's around 300F and air temp in the engine compartiment of maybe 60F or so), the cooling rate is a lot greater than when sitting on the ground after landing (where there's no cooling flow at all and the air in the engine compartment can reach 200F or more and CHT's are in the high 200's).

The result is that CHT cooling rates after landing may only be a few degrees per minute, while in a high-speed idle descent, they can go well over 100 degrees per minute, which is enough to cause the "clamping" effect of the cylinders on the pistons. Further, after shutdown, even if there were a tiny bit of contraction effect, there's no wear being caused because the pistons aren't rubbing the rings against the cylinder walls.
 
Tom-D said:
During normal operation of the engine (even in training) you will never see CHTs that will cause any change in cylinder metallurgy, critical temps for 2400 series aluminum is 830F, you must exceed that temp before any change will occur.
Click to expand...
You can still do serious damage to your engine with CHT's well below 830F. The 500F redline on most engines is not to be trifled with.
 
IME...leaning to heat on the decent will keep the CHT's near constant. The most I ever saw with an IO-550TN was -30 on the JPI-700. I would rip on back to 18-23" (from 30 +) and push the mix to peak TIT/EGT.

You also have to remember that up high, thin air does a poor job of carrying heat away...

I always felt pulling to idle, doing a quick turn, then pouring on the coals again is where stuff happens....massive fluctuations compared to decent.
 
Last edited:
EdFred said:
That's why you bleed off all the speed first by reducing power, pitching up, getting close to stall then add full power, so you're only there for about 3-5 seconds or so. Saves the engine, and you don't gain a bunch of altitude.
Click to expand...
Yeah, that's a good plan for sure. It's just not how these planes often get flown. Vx climbs, power on stalls, ... and there's generally nobody who thinks about CHTs.
 
I think that we can be a LOT simpler about all this, and even generalize:

Be smooth and gentle with your airplane, on the whole--engine operation, interior, closing the doors, etc.--and you'll get longer life out of everything.
 
Always plan ahead. When you see ATC putting you in a position to slam dunk, ask for a descent earlier, or start stage cooling before they slam dunk you.
 
Inverted said:
Always plan ahead. When you see ATC putting you in a position to slam dunk, ask for a descent earlier, or start stage cooling before they slam dunk you.
Click to expand...
Why? Stage cooling is unnecessary unless you're exceeding CHT limits and abusing your engine to begin with.
 
Ron Levy said:
You can still do serious damage to your engine with CHT's well below 830F. The 500F redline on most engines is not to be trifled with.
Click to expand...

If you run your engine hot enough to harm the aluminum cylinder, you will certainly have valve guide, ring and piston problems too, so how can we blame shock cooling for the problems?

we very very seldom see any cracking or catastrophic failure in low time cylinders that have normal usage. Most occur with manufacturing processes, and detonation problems. you can't blame shock cooling for those problems.

WE must remember that only the head of the cylinder is aluminum, the barrel is steel of one alloy or another. and is never effected by the temps we see in our engines. The head has the exhaust port where we see temps in the 13-1400 degree range or higher, and granted most cracking occur in or near the exhaust port there is not that many cases in low time cylinders.

The only valid argument for shock cooling is that the higher the temps and the wider the cooling range the more rapid the cyclic stresses occur. We do know old cylinders crack, That is why I have always advocated use new cylinders at overhaul and you'll never see this problem.
 
Last edited:
You must log in or register to reply here.
Back
Top