CHTs running cold?

[SepticTank]

I haven't done a lot of cold weather flying, but did a 5 hour trip this weekend in cold (to me) weather with OATs running about 0 degrees Fahrenheit. During my trip, I noticed that my CHTs were averaging about 310 degrees and went all the way down into the white to 250 during descent. I typically run them between 350 and 380, but could not get them up even leaned out a max RPM.

Is there anything wrong with CHTs being this cold or am I just overthinking it and it was just cold outside..? The Lycoming service manual only gives a max sustained value of 400F.

Engine is an O-540.

 

[RyanB]

Low 300’s are fine, mid 300’s are better. What power settings are you running during the descent? Try leaving power in longer if you can, things really cool down quick during descents at low power.

 

[kaiser]

Same thing happened to me this past weekend. Couldn’t get CHTs above 260 in cruise. I heard under 300F is bad for the engine. Something about lead deposits building up.

 

[SethV]

Get a cold weather kit. Not good to run the CHTs that cold and especially don't do touch n gos with them running that cold. Cold weather kit reduces the airflow coming and and is super easy to install and remove on our SR22.

 

[SepticTank]

Low 300’s are fine, mid 300’s are better. What power settings are you running during the descent? Try leaving power in longer if you can, things really cool down quick during descents at low power.

I tried to keep it as hot as possible. I had it leaned out and RPM was 2400 on descent.

 

[Daleandee]

I haven't done a lot of cold weather flying, but did a 5 hour trip this weekend in cold (to me) weather with OATs running about 0 degrees Fahrenheit. During my trip, I noticed that my CHTs were averaging about 310 degrees and went all the way down into the white to 250 during descent. I typically run them between 350 and 380, but could not get them up even leaned out a max RPM.

Is there anything wrong with CHTs being this cold or am I just overthinking it and it was just cold outside..? The Lycoming service manual only gives a max sustained value of 400F.

Engine is an O-540.

In the Lycoming Operator's manual for the 360 engines they state,

For maximum service life of the engine maintain cylinder head temperature
between 150°F and 400°F during continuous operation.

I found this information when researching CHT limits for my experimental aircraft engine. Mine has an auto conversion engine (Corvair) and runs very low temps, even in the summer. I was concerned with it being this cool but ten years and nearly 300 hours later it still runs excellent and uses no oil.

I do know that lead build up could be a problem. In mine I run a lot of Mogas and I use Decalin although I do add some 100LL now and again to keep the octane levels up in summer. On certified this is not an option.

 

[MooneyDriver78]

The lead scavenging chemical reaction requires a certain temperature (~280° IIRC), so running cold would means lead accumulation in your cylinders. Probably not bad if you don’t make a habit of it, otherwise I would fly lower and high power to run the engine hot.

 

[Clip4]

I haven't done a lot of cold weather flying, but did a 5 hour trip this weekend in cold (to me) weather with OATs running about 0 degrees Fahrenheit. During my trip, I noticed that my CHTs were averaging about 310 degrees and went all the way down into the white to 250 during descent. I typically run them between 350 and 380, but could not get them up even leaned out a max RPM.

Is there anything wrong with CHTs being this cold or am I just overthinking it and it was just cold outside..? The Lycoming service manual only gives a max sustained value of 400F.

Engine is an O-540.

If you are flying a plane with cowl flaps, you might do 50% open in the climb and get them closed immediately at level off or you will never get the engine to temperature.

 

[MajorTurbulence]

Get a cold weather kit. Not good to run the CHTs that cold and especially don't do touch n gos with them running that cold. Cold weather kit reduces the airflow coming and and is super easy to install and remove on our SR22.

I don’t know what a winterization kit consists of but if it utilizes a plate to block the airflow over the oil filter, that is what I have for my lyc O360 for winter operations.
Also, my manual calls for descents at 2500 rpm if possible.

In addition, while the OP mentioned that he is leaned out at cruise at max rpm, he does not say what percent power he is using in cruise. For my fixed pitch prop, yes for max available power with highest rpm safely below redline and at or below 75% at altitude, but If he had a variable pitch prop, max rpm might not be max power. While I’m usually leaned to peak at 65-70% power on my long cross countries, I guess you could try 75% at peak while watching those CHTs so they aren’t higher than 380.

 

[Domenick]

The lead scavenging chemical reaction requires a certain temperature (~280° IIRC), so running cold would means lead accumulation in your cylinders. Probably not bad if you don’t make a habit of it, otherwise I would fly lower and high power to run the engine hot.

https://www.shell.com/business-cust...entre/technical-talk/techart-18-30071600.html

 

[Hengelo]

Super relevant thread this year. Almost think the mods should repost it every December.

 

[midwestpa24]

One of the many reasons to think hard about flying when its gets near or below 0F. Most of the guys around here stop flying pistons below 0. Its hard not just on the engines, but on all kinds of things on an airplane, from control cables to door latches. Not to mention the survival aspect for the occupants. A survivable off-airport landing can quickly become a life and death scenario if help doesn't get to you soon enough.

 

[Hengelo]

Yeah our club has a "not below zero" rule for instruction. "Nothing good happens below zero.

 

[Martin Pauly]

I have seen very little data on what is "too cold" for CHTs. On the other hand, there is a pretty solid story on how hot the oil should get to get rid of water. 180 deg F is a good target to aim for. So I prioritize oil temperature over CHTs in my winter flying decision making.

I subscribe to the theory that if you can get your oil warm enough, then your cylinder temperatures are likely adequate. I fly in a cold climate, and in the winter I tape off parts of my oil cooler to keep the oil temp high.If it gets really cold, I sometimes cruise rich of peak to get the oil warm. Even then, my CHTs are typically under 300 deg F - and in ten winters of flying my Bonanza like that, I have not yet discovered a downside.

My one other rule is: I don't fly when it's below 0 deg F. Not just to be nice to the engine, but also because the preflight is painful, and survivability in case of an accident becomes a real concern.

- Martin

 

[Kenny Phillips]

At one time, louvers (or some other obstruction for air intakes) were a thing; I don't know why there aren't automatically controlled systems in today's planes. Oh, yeah, the multi-million-dollar cert process.

 

[Captain Sweet T]

If you are flying a plane with cowl flaps, you might do 50% open in the climb and get them closed immediately at level off or you will never get the engine to temperature.

Im not even opening my cowl flaps at all right now. Even during climb. I cant get the temps if do and Im in CA. Something to think about....

 

[Clip4]

Im not even opening my cowl flaps at all right now. Even during climb. I cant get the temps if do and Im in CA. Something to think about....

What are the oil temps doing? There is a Thermostatic Oil Cooler Bypass Valve.

 

[Snowmass]

You may need to verify your CHT gauge accuracy. I have done this several times with a lab quality mercury thermometer in an oil bath heated on a hot plate. Hang the probe in the temperature stabilized oil which I stir. It's the redline temp that is most critical. I found out that the factory calibration was way off and I was running the engine way too hot.

It is better to avoid thermal contraction stress/distortion so I open my cowl flaps in level flight to start the cool down and then close in descent. This makes the cooling more gradual. In any case you will have to cool down to surface temperature.

 

[Clip4]

You may need to verify your CHT gauge accuracy. I have done this several times with a lab quality mercury thermometer in an oil bath heated on a hot plate. Hang the probe in the temperature stabilized oil which I stir. It's the redline temp that is most critical. I found out that the factory calibration was way off and I was running the engine way too hot.

It is better to avoid thermal contraction stress/distortion so I open my cowl flaps in level flight to start the cool down and then close in descent. This makes the cooling more gradual. In any case you will have to cool down to surface temperature.

Have to say that is the first I have ever heard of that technique.

If you plan you descents properly the airspeed should not change and the power reduction is minimal.

 

[Captain Sweet T]

What are the oil temps doing? There is a Thermostatic Oil Cooler Bypass Valve.

I need to check for that bypass valve. Right now the oil stays in the middle of the gauge on cruise climb (245* redline) and the cylinder temps are barely getting to middle on climb and short of middle in cruise fully lean.

 

[Dan Thomas]

I need to check for that bypass valve. Right now the oil stays in the middle of the gauge on cruise climb (245* redline) and the cylinder temps are barely getting to middle on climb and short of middle in cruise fully lean.

The Vernatherm will normally fail in the bypass position (the oil would get too hot). It responds to heat by pushing a plunger out that plugs the bypass port and forces the oil to go through the cooler. Low oil temps are normally fixed by blocking the oil cooler's airflow.

 

[Domenick]

...If it gets really cold, I sometimes cruise rich of peak to get the oil warm. ...

How does running ROP get the oil warm?

 

[EdFred]

The Vernatherm will normally fail in the bypass position (the oil would get too hot). It responds to heat by pushing a plunger out that plugs the bypass port and forces the oil to go through the cooler. Low oil temps are normally fixed by blocking the oil cooler's airflow.

Mine failed the other way. Oil never got warm enough.

 

[Martin Pauly]

How does running ROP get the oil warm?

By producing higher peak internal cylinder pressure (ICP), and higher CHTs compared with running LOP. As the oil circulates, it absorbs some of that extra heat.

- Martin

 

[Snowmass]

Have to say that is the first I have ever heard of that technique.

If you plan you descents properly the airspeed should not change and the power reduction is minimal.

Being an engineer it is simply a logical approach. You will need quite a power reduction if you don't increase AS. You need to get rid of that potential energy somehow. Admittedly long descents are less practical for me as I mainly fly very high mountains with deep valleys.

 

[Domenick]

By producing higher peak internal cylinder pressure (ICP), and higher CHTs compared with running LOP. As the oil circulates, it absorbs some of that extra heat.

Are you sure?

https://aviation.stackexchange.com/...re-ratio-beyond-stoichiometry-give-more-power

Personally, I have a 4-cyl CHT gauge, but no EGT. I just use Lycomings leaning instructions. Lean to stumble; enrichen to smoothness. Year-round, I let the engine warm up at about 1000 rpm until the oil temp needle is off the peg. I don't live in a particularly cold climate, so about the lowest CHTs I see are on steep descents in the depths of winter--high 200s F.

 

[Martin Pauly]

Are you sure?

Yes, I am sure. Maximum CHTs and ICPs (and, as a result, oil temperature) are somewhere around 40-50 deg F rich of peak. From there, whether you go leaner or richer, things get cooler. MUCH cooler on the lean side. Your graph actually shows that.

This isn't a matter of opinion. Anyone can fly in an airplane with a modern digital engine monitor and measure and verify this information.

 

[3393RP]

At one time, louvers (or some other obstruction for air intakes) were a thing; I don't know why there aren't automatically controlled systems in today's planes. Oh, yeah, the multi-million-dollar cert process.

The Polikarpov I-16 and other Soviet designs had that feature in the early days of aviation.

 

[MajorTurbulence]

Yes, I am sure. Maximum CHTs and ICPs (and, as a result, oil temperature) are somewhere around 40-50 deg F rich of peak. From there, whether you go leaner or richer, things get cooler. MUCH cooler on the lean side. Your graph actually shows that.

This isn't a matter of opinion. Anyone can fly in an airplane with a modern digital engine monitor and measure and verify this information.

Nice video.
Question: Your convention for naming how LOP was to use the degrees lean of peak of the last cylinder to peak. If one is trying to set your engine at peak, in this instance do you name that situation with the first cylinder to go to peak with full understanding that the the other cylinders will be rich of peak? It would be nice to set it to the last to go peak, but as you have noted in your video, getting all cylinders in a carbureted engine may be impossible to achieve LOP.

 

[Martin Pauly]

If one is trying to set your engine at peak, in this instance do you name that situation with the first cylinder to go to peak with full understanding that the the other cylinders will be rich of peak?

Hmm, good question... I'm not aware of a convention for something called "at peak" for the whole engine, as an equivalent to "x degrees LOP" (or ROP).

- Martin

 

[Domenick]

Yes, I am sure. Maximum CHTs and ICPs (and, as a result, oil temperature) are somewhere around 40-50 deg F rich of peak. From there, whether you go leaner or richer, things get cooler. MUCH cooler on the lean side. Your graph actually shows that.

This isn't a matter of opinion. Anyone can fly in an airplane with a modern digital engine monitor and measure and verify this information....

First, as a forum is an imperfect medium, sometimes a request for clarification is misinterpreted. So, just for the record, I am NOT trying to start an argument or be difficult. I'm just trying to resolve conflicting data.

Common knowledge says to clear a fouled plug, we lean aggressively and increase power, but for a limited time to prevent burning a valve. If max CHT was somewhere ROP, we wouldn't lean.

How do you reconcile the posted chart which shows max CHTs well LOP?

I'd like to watch the posted video, but not at a 25 minute investment. Got a timestamp to start watching for the relevant bit(s)?

Thanks.

 

[kujo806]

First, as a forum is an imperfect medium, sometimes a request for clarification is misinterpreted. So, just for the record, I am NOT trying to start an argument or be difficult. I'm just trying to resolve conflicting data.

Common knowledge says to clear a fouled plug, we lean aggressively and increase power, but for a limited time to prevent burning a valve. If max CHT was somewhere ROP, we wouldn't lean.

How do you reconcile the posted chart which shows max CHTs well LOP?

I'd like to watch the posted video, but not at a 25 minute investment. Got a timestamp to start watching for the relevant bit(s)?

Thanks.

Leaning to clear a plug isn't so much about max heat as it is about creating an Oxygen rich environment for the gunk to burn off in. Running rich would add more excess fuel, lead, and carbon to further gunk things up. Like Martin said, all you need is an engine monitor to run the experiment yourself. The results are tangible and conclusive. When I picked up my plane on a hot summer day, the cylinders started to go over 400 in cruise. As soon as I leaned beyond peak, the temps dropped dramatically just as shown in the plots.

 

[Troska]

I'm not sure if I'm reading it wrong, or you are reading it wrong -

But your chart shows that max CHT's are just prior to peak as you lean. CHT is already dropping (on your chart) before you get to peak.

Tim

First, as a forum is an imperfect medium, sometimes a request for clarification is misinterpreted. So, just for the record, I am NOT trying to start an argument or be difficult. I'm just trying to resolve conflicting data.

Common knowledge says to clear a fouled plug, we lean aggressively and increase power, but for a limited time to prevent burning a valve. If max CHT was somewhere ROP, we wouldn't lean.

How do you reconcile the posted chart which shows max CHTs well LOP?

I'd like to watch the posted video, but not at a 25 minute investment. Got a timestamp to start watching for the relevant bit(s)?

Thanks.

 

[Martin Pauly]

But your chart shows that max CHT's are just prior to peak as you lean. CHT is already dropping (on your chart) before you get to peak.

You are interpreting it correctly, Tim.

- Martin

 

[flyingdirtrider]

My semi-educated take on colder CHT's in extreme cold, are not that big of a deal when we're talking about lower side limits. Almost to the point of being irrelevant. Because when we're talking about 0F with flying speeds worth of wind chill, there's going to be a significantly larger temperature gradient between the outside of the combustion chamber VS inside - VS that same effect when its 100F outside.
AKA when it's 100F outside, the ability for the cylinder head to shed heat is much much less than when it's 0F outside. Thus heat build-up in the cylinder head itself is much more important to keep track of and manage in warmer climates.
But when the outside air is cold enough to shed enough heat to keep head temps well below safe values, regardless of what's going on inside the cylinder - CHT's literally don't matter at that point IMHO.
Now... within reason of course! If things get so cold that we start running into thermal contraction problems of the cylinder itself, well that is another problem! (Probably in the range of 150F as noted by Lycoming)

So with those variables alone, I would expect the ICP's/T's (internal pressures and temps) to be relatively unchanged across seasons (assuming same mixture and % power) meanwhile the CHT's would be significantly cooler.
So to Martin's point, I would switch over to caring more about oil temps and less about CHT's in extreme cold outside temps.

All that rambling to say, when we're talking about 0F, I think CHT's no longer become an accurate gauge of what's going on inside the combustion chamber. As the cooling ability of the air is significantly better at pulling heat out of the cylinder. And CHT's will read quite a bit cooler, for the exact same ICP's/T's - VS when it's 100F outside.

 

[mandm]

At one time, louvers (or some other obstruction for air intakes) were a thing; I don't know why there aren't automatically controlled systems in today's planes. Oh, yeah, the multi-million-dollar cert process.

The club 172’s have this. Said they were $500 each for a piece of plastic with 4 screws. Says (placarded) do not operate above 20 degrees F.

 

[Domenick]

...If it gets really cold, I sometimes cruise rich of peak to get the oil warm. ...

I'm not sure if I'm reading it wrong, or you are reading it wrong -But your chart shows that max CHT's are just prior to peak as you lean. CHT is already dropping (on your chart) before you get to peak.

It is entirely possible I am reading the chart wrong.

The first quote above implies to me that Martin believes going rich of peak raises CHTs. As far as I can see on the chart, CHTs peak between economy and max power and DECLINE continuously as the mixture is richened.

I agree with quote two and it seems to agree with me.Yes, CHTs decline as the mixture is richened, a statement contrary to the first quote.

What "peak" are we referring to? If we are referring to peak CHT or EGT temp, then clearly the chart shows both temps declining as the mixture is richened. If we are referring to peak power, still the chart shows richening the mixture lowers CHTs.

Weren't we all taught to richen to lower CHTs, as the excess fuel serves to carry away heat?

 

[Troska]

Okay, I think I understand the disconnect now.

The first quote above implies to me that Martin believes going rich of peak raises CHTs. As far as I can see on the chart, CHTs peak between economy and max power and DECLINE continuously as the mixture is richened.

What Martin believes is correct, and is demonstrated by the graph. Start in the top left, at peak EGT. LOP and ROP refer to EGT.
If he is operating in the best economy range, to the left of peak EGT, and richens the mixture, moving to the right, you can see CHT RISE significantly as he does so, peak, then drop again.

I agree with quote two and it seems to agree with me.Yes, CHTs decline as the mixture is richened, a statement contrary to the first quote.

It depends on where you start on the chart...

What "peak" are we referring to? If we are referring to peak CHT or EGT temp, then clearly the chart shows both temps declining as the mixture is richened. If we are referring to peak power, still the chart shows richening the mixture lowers CHTs.

This is the crux of the matter. Peak (as in ROP and LOP) refers to EGT. Start there, then reinterpret the discussion.

Weren't we all taught to richen to lower CHTs, as the excess fuel serves to carry away heat?

I wasn't, but consider the chart. As you move further to the right, and dump in more fuel, it shows CHT's dropping. That doesn't negate the portion of the chart relevant to LOP-transition-to-ROP operations.

 

[mandm]

Do you only worry about CHT’s on engines >200hp and/or 6 cylinders?

 

[Domenick]

Okay, I think I understand the disconnect now....

Yep. You got it. I'm on the same page as y'alls now. Thanks for your patience. That may be close to personal best brain-fart.