Shock Cooling a myth?

[Kejidog]

A great video from Cessna flyers association. The speaker is from Superior Air parts and an engineer

https://youtu.be/5pAsAqQN52c

He speaks about running lean of peak and a lot about max horsepower and cylinder cooling

[CpnCrunch]

Just did some night circuits and kept and eye on CHT after leaning. At 2200rpm (O-360) moving the mixture to full rich only changed CHT by 1 degree (from about 325F). EGT increased about 25F.

The only time I've had warnings about shock cooling is when doing a low power descent at cruise speed. Then I get 60F/min shock cooling warnings, and the only fix is to reduce airspeed or increase power. Given that practically every flight school plane endures much worse than this on practically every flight, it's surprising they cope so well. In the flight school planes I've quite often flown long final at 120kts to keep the speed up, then cut the power to idle. Given what I know now, I'd been subjecting the engines to horrendous shock cooling.

[RedAndWhiteBaron]

The risk of a full power lean go-around is that it reduces your detonation margin to the point where you might actually get detonation. It's hard to do with an aircraft engine but it can be done. Detonation does a massive amount of wear and tear in a very short period of time, and can easily lead to damage. Left long enough (I've seen as little as 30 seconds), it can destroy a cylinder.

As far as I understand, this would be a risk in any very lean condition, not just a go-around, and not just at full power. You'd prbably need high power, but not necessarily full power, right? I don't fully understand why lean mixtures increase CHT and combustion temperature, but I know they do, from both my understanding of cars and airplanes.

So my question is, why are you applying this issue to a go-around if, by my understanding, it could just as well be induced by a climb? Could you not also produce detonation simply by running too lean for too long, at any combination of power, altitude, and attitude?

[photofly]

Unless you have a turbocharger (turbocharged engines are much easier to damage) then the reduced air density at altitude means that the intra cylinder pressure at TDC is still quite low, hence no detonation. The most likely manoeuvre that would include a low altitude full power climb with the mixture left other than full rich would be a go-around, but the engine doesn’t care what name you give it.

The compression ratio on basic engines can be fairly high: 8.5:1 for the O320 in 160hp varieties, because it gives you better fuel efficiency. Turbocharged engines have lower compression, to improved their detonation margin (I’m guessing the heated air intake due to the turbo is a factor the designer has to allow for).

[ahramin]

The risk of a full power lean go-around is that it reduces your detonation margin to the point where you might actually get detonation. It's hard to do with an aircraft engine but it can be done. Detonation does a massive amount of wear and tear in a very short period of time, and can easily lead to damage. Left long enough (I've seen as little as 30 seconds), it can destroy a cylinder.

As far as I understand, this would be a risk in any very lean condition, not just a go-around, and not just at full power. You'd prbably need high power, but not necessarily full power, right? I don't fully understand why lean mixtures increase CHT and combustion temperature, but I know they do, from both my understanding of cars and airplanes.

So my question is, why are you applying this issue to a go-around if, by my understanding, it could just as well be induced by a climb? Could you not also produce detonation simply by running too lean for too long, at any combination of power, altitude, and attitude?

It's applied to a go around because you are taking your engine from a power setting where a lean mixture is appropriate to a power setting where a lean mixture is inappropriate. A takeoff and climb has the same situation, which is why mixture setting is included in the before takeoff checklist. If we follow photofly's philosophy we would remove it since no one should be applying takeoff power without first making sure the mixture is set correctly. This seems reasonable to me since takeoff is much lower stress than a go-around.

Not all power settings can lead to detonation. As you reduce power detonation becomes impossible at some point. Actually detonation in an aircraft engine is quite hard to make happen, so keep in mind that the procedures for keeping the mixture full rich are for enough of a margin that detonation stays impossible at all times. If you have a look at your engine manual, there will be a fuel flow vs power setting chart. You'll notice at some point fuel flow makes a big dive at a specific power setting. This is where the engine manufacturer has determined that sufficient margin exists that you can now start leaning. For all the engines I fly this limit is 75%. Anove 75% it's so risky to lean that they don't want you doing it unless it's for a high density altitude takeoff. Below 75% you can still do extra wear and tear by leaning incorrectly but the margin is sufficient to prevent a catastrophic detonation event. Below 60% it doesn't matter what you do with the mixture, you can't hurt anything.

The above is all predicated on a properly working engine. Throw in a mistimed mag or pre-ignition and all the fancy charts go out the window.

[photofly]

Anove 75% it's so risky to lean that they don't want you doing it unless it's for a high density altitude takeoff.

Please don't make it sound llike you're taking your life in your hands by touching the red knob. It's not life on the edge. Lean when it's safe to do so. Don't lean, when you shouldn't.

Lycoming says (O-320 operators manual):

Maintain mixture control in “Full Rich” position for rated take-off, climb and maximum cruise powers (above approximately 75%). However, during take-off from high elevation airport or during climb, roughness or loss of power may result from over-richness. In such a case adjust mixture control only enough to obtain smooth operation – not for economy

The word detonation doesn't appear in this publication.

Cessna says: (C172N POH)

Prior to takeoff from fields above 3000' elevation the mixture should be leaned to give maximum RPM in a full throttle run-up

An article from Mike Busch on the subject:
https://resources.savvyaviation.com/wp- ... nation.pdf

[photofly]

. If we follow photofly's philosophy we would remove it since no one should be applying takeoff power without first making sure the mixture is set correctly. This seems reasonable to me since takeoff is much lower stress than a go-around.

You are mischaracterizing me. If you follow my philosophy it would be perfectly appropriate to have “mixture rich” as a pre- or post- takeoff checklist item because the mixture should be rich (or at least set) before each takeoff, and indeed so it appears on that checklist.

It would also be appropriate to have mixture rich in a post-go-around checklist, because the mixture should be rich at that stage.

I question having mixture rich in a pre-landing checklist because it doesn’t need to be rich at that time and in almost every occasion pre-landing a go-around is not carried out.

[CpnCrunch]

I question having mixture rich in a pre-landing checklist because it doesn’t need to be rich at that time and in almost every occasion pre-landing a go-around is not carried out.

The value is that it's better to do it earlier so it isn't forgotten, and that it has zero negative consequences.

[photofly]

Apparently, it does have negative consequences.
Extra cooling that you don’t want.

Also, as I said, it’s extra wear on the finish on the knob, and I like my plane Controls to look shiny and new.

[CpnCrunch]

Apparently, it does have negative consequences.
Extra cooling that you don’t want.

I wouldn't really call 1F "extra cooling".

Also, as I said, it’s extra wear on the finish on the knob, and I like my plane Controls to look shiny and new.

Each to their own. I'm more concerned about forgetting the mixture than how shiny my knob is. Do you wait until just before touchdown before putting the gear down because you'll never forget it?

[photofly]

Different. The gear is going to be extended anyway. I'm not big on the "just in case".

If you're the kind of pilot for whom a go-around is such startling event that all you have the mental capacity for is to slam the throttles forward before your brain seizes up, then sure, you had better put the mixture rich on final. And don't forget to wipe the drool off your chin, either.

[CpnCrunch]

If you're the kind of pilot for whom a go-around is such startling event that all you have the mental capacity for is to slam the throttles forward before your brain seizes up, then sure, you had better put the mixture rich on final. And don't forget to wipe the drool off your chin, either.

Will do, thanks.

[ahramin]

Anove 75% it's so risky to lean that they don't want you doing it unless it's for a high density altitude takeoff.

Please don't make it sound llike you're taking your life in your hands by touching the red knob. It's not life on the edge. Lean when it's safe to do so. Don't lean, when you shouldn't.

I could have worded that better. The point is the engine manufacturers don't want you leaning below 75% power for normal operations.

On engines with manual mixture control, maintain mixture control in “Full Rich” position for rated takeoff,
climb and maximum cruise powers (above approximately 75%)

Here's the clearest example I can find.

fuel consumption.jpg (150.66 KiB) Viewed 1757 times

[goldeneagle]

Apparently, it does have negative consequences.
Extra cooling that you don’t want.

Also, as I said, it’s extra wear on the finish on the knob, and I like my plane Controls to look shiny and new.

I love AvCanada. I posted a snippet based on conversations with the engineers from Continental regarding the rationale for handling mixture with a high performance piston engine on the descent from flight levels to a sea level airport. Along comes the AvCanada experts and tell us it's all wrong because they took a normally aspirated engine to circuit altitude and did a small experiment by moving mixture, then others add to it based on limited knowledge around small normally aspirated engines that were designed into an installation expected to be abused in flight training environments, and set up as such. But I must admit, keeping the finish on the mixture knob indeed sounds far more important than properly managing fuel to your engine...

This again goes back to my original comment, know your airplane, know your engine, they are all different, and some times the limitations seem unusual, right up until you understand why.

With the GTSIO-520H I was referring to earlier, the 'line up' check includes 'mixtures rich', and that is at any altitude, any conditions, with very good reason, even if you have been running leaned mixtures during taxi to prevent plug fouling. During takeoff and early climb, most of the cooling systems will not be operating up to spec, and particularly the intercooler doesn't provide a lot of cooling at airspeeds under 100 knots. During the takeoff roll, you will be pushing the handles ahead, engine turning at 3400 rpm and props a tad over 2250 (it's a geared engine) with manifold pressures up at 39 inches with little to no airflow over the cooling fins and intercooler. The fuel control unit in that airplane has a set of inputs that detect 'full throttle, mixture rich', and in that condition it introduces a bunch of extra fuel for cooling purposes. The 375hp rating on that engine for takeoff does take into account that there is extra fuel being injected. Under no circumstances should you advance the throttles past 'top of the green' with mixture less than full rich on that airplane, it's all about proper cooling. Top of the green is 32.5 inches but the system is good up to 39 inches with mixture rich at low altitudes. ofc things change when you are up in the flight levels, but that's a whole different discussion, this one started with challenges to 'mixture rich during pre-land', and I"m not aware of any airports where you would be above fl180 doing pre-land checklists.

So yes, you absolutely DO want that extra cooling from rich mixture, and it doesn't matter that AvCanada experts disagree, for some engines it's REQUIRED. It's actually taking advantage of the same effect a water injection system would supply, except using fuel so another complicated system is not introduced to the aircraft.

[photofly]

his again goes back to my original comment, know your airplane, know your engine, they are all different, and some times the limitations seem unusual, right up until you understand why.

Nobody has suggested otherwise.

Most people in this thread don't fly GTSIO--520H engines or other turbocharged high performance piston engines into the flight levels; we're quite happy to take the topic of the thread and apply it to engines that we do fly. If the conversation of those of us who want to talk about flying a normally aspirated piston engine to circuit altitude and back isn't grand enough for you, that's too bad.

[digits_]

So yes, you absolutely DO want that extra cooling from rich mixture, and it doesn't matter that AvCanada experts disagree, for some engines it's REQUIRED. It's actually taking advantage of the same effect a water injection system would supply, except using fuel so another complicated system is not introduced to the aircraft.

But why would you need that cooling for the final 3 minutes of the flight? You wouldn't go full rich when you start descending, right? So what's so special about the last few minutes? I assume you're not generating that much power that you would need to go above 32 map (unless that was your whole point, in which case I missed that).

[goldeneagle]

Most people in this thread don't fly GTSIO--520H engines or other turbocharged high performance piston engines into the flight levels;

The thread original title is 'shock cooling'. In the 45 years I've been flying airplanes, I have never heard of anybody mentioning shock cooling in the context of normally aspirated bug smashers. It's always in the context of higher performance equipment that tends to do longer legs at altitude. If indeed all you are concerned about is normally aspirated bug smashers, then I agree, shock cooling is a non issue, and generally a myth brought about because folks have read about high performance equipment and think the same issues apply to the bug smasher they are flying. O-320 and such with a carburetor are virtually indestructible when it comes to cooling issues.

But why would you need that cooling for the final 3 minutes of the flight? You wouldn't go full rich when you start descending, right? So what's so special about the last few minutes? I assume you're not generating that much power that you would need to go above 32 map (unless that was your whole point, in which case I missed that).

That goes back to my original post on the subject. Short answer, because the engineers at Continental told us it's part of the correct way to manage cooling on a long descent.

Long answer. As originally mentioned, it revolves around physical issues in the engine, combined with bureaucratic issues. The POH says 'mixtures rich' in the pre-land checklist. The folks from continental told us, you can move that to the go-around checklist, or if in a regulatory environment where that's not permitted, then they had an alternate method. During descent slowly push the mixture forward such that when you reach pre-land and it says 'mixture rich' in the checklist, the response is 'already there'. When we were applying for a 703 certificate many years ago, transport inspectors would not approve the change in checklist away from what was in the POH, so we chose the alternate method suggested by the folks at Continental. The bottom line is, to ensure the mixture is in the full rich position if/when handles get pushed to the stops for a go-around, and to get to that point in a manner that doesn't result in a sudden and dramatic increase in the amount of fuel coming out of the injectors shortly before landing.

To understand the actual process that causes the cracks attributed to 'shock cooling' in this case, go back and re-read my original post on the subject, it's fairly detailed.

[digits_]

But why would you need that cooling for the final 3 minutes of the flight? You wouldn't go full rich when you start descending, right? So what's so special about the last few minutes? I assume you're not generating that much power that you would need to go above 32 map (unless that was your whole point, in which case I missed that).

That goes back to my original post on the subject. Short answer, because the engineers at Continental told us it's part of the correct way to manage cooling on a long descent.

Long answer. As originally mentioned, it revolves around physical issues in the engine, combined with bureaucratic issues. The POH says 'mixtures rich' in the pre-land checklist. The folks from continental told us, you can move that to the go-around checklist, or if in a regulatory environment where that's not permitted, then they had an alternate method. During descent slowly push the mixture forward such that when you reach pre-land and it says 'mixture rich' in the checklist, the response is 'already there'. When we were applying for a 703 certificate many years ago, transport inspectors would not approve the change in checklist away from what was in the POH, so we chose the alternate method suggested by the folks at Continental. The bottom line is, to ensure the mixture is in the full rich position if/when handles get pushed to the stops for a go-around, and to get to that point in a manner that doesn't result in a sudden and dramatic increase in the amount of fuel coming out of the injectors shortly before landing.

To understand the actual process that causes the cracks attributed to 'shock cooling' in this case, go back and re-read my original post on the subject, it's fairly detailed.

I did reread your original post, but it seems to me you do not need to end up at full rich on short final. Only that you need to go full rich at the go-around. You wrote yourself that the regulator didn't like the change in checklist. If you fly privately and don't care about the bureaucratic arguments, there doesn't seem to be any physical reason to go full rich on final. Is that correct?

[ahramin]

But why would you need that cooling for the final 3 minutes of the flight? You wouldn't go full rich when you start descending, right? So what's so special about the last few minutes? I assume you're not generating that much power that you would need to go above 32 map (unless that was your whole point, in which case I missed that).

That goes back to my original post on the subject. Short answer, because the engineers at Continental told us it's part of the correct way to manage cooling on a long descent.

Long answer. As originally mentioned, it revolves around physical issues in the engine, combined with bureaucratic issues. The POH says 'mixtures rich' in the pre-land checklist. The folks from continental told us, you can move that to the go-around checklist, or if in a regulatory environment where that's not permitted, then they had an alternate method. During descent slowly push the mixture forward such that when you reach pre-land and it says 'mixture rich' in the checklist, the response is 'already there'. When we were applying for a 703 certificate many years ago, transport inspectors would not approve the change in checklist away from what was in the POH, so we chose the alternate method suggested by the folks at Continental. The bottom line is, to ensure the mixture is in the full rich position if/when handles get pushed to the stops for a go-around, and to get to that point in a manner that doesn't result in a sudden and dramatic increase in the amount of fuel coming out of the injectors shortly before landing.

To understand the actual process that causes the cracks attributed to 'shock cooling' in this case, go back and re-read my original post on the subject, it's fairly detailed.

I did reread your original post, but it seems to me you do not need to end up at full rich on short final. Only that you need to go full rich at the go-around. You wrote yourself that the regulator didn't like the change in checklist. If you fly privately and don't care about the bureaucratic arguments, there doesn't seem to be any physical reason to go full rich on final. Is that correct?

Correct. The Normal Procedures section of the AFM does not have to be followed for a privately flown aircraft. But it's very clear from the original post that this was not a privately flown aircraft.

[ahramin]

Finally had a chance to capture some data on going full rich. Not flight test quality but I did manage to remember to put the mixture full rich as soon as I leveled off so that we would have at least 30 seconds of data before pulling the power. You may recall the charts I posted earlier showed pulling power to idle produced a 30°F / min CHT reduction on this aircraft. A normal descent from cruise while maintaining power gives about 2°F per minute.

normal descent.jpg (441.52 KiB) Viewed 1577 times

I don't typically adjust the mixture in the descent and that was what happened on this flight. After level off, the mixture (blue line) was pushed full rich (8 GPH to 11 GPH) and you can see the EGTs drop almost 200°F in the next 30 seconds. The CHTs do not move. This was at 35-45% power.

Full Rich.jpg (390.43 KiB) Viewed 1577 times

So at low power, the position of the mixture control has no appreciable effect on CHTs on this IO-540.

[zahir 11 rasul]

Dear B200 ser 1911, Recently changed both to new engines and unable to adjust the fcu settings. On one of the engines the acceleration is slower than normal, any suggestions to over come this