leaning/enriching on descent - M20J

[dominikos]

I usually just end up leaving the mixture setting as in the cruise and just reducing MP by 2”. It never caused the problem but today, my #2 temps dropped below 300. The cowl flaps were closed, it was a pretty cold day in TX. I tried to slow down the descent but at some point I had to get to pattern attitude. Any suggestions on whether I should lean more or enrich mixture on the descent? 

Thanks for pointers

[ArtVandelay]

Are you cruising LOP or ROP?

[dominikos]

LOP

[Shadrach]

3 hours ago, dominikos said:

I usually just end up leaving the mixture setting as in the cruise and just reducing MP by 2”. It never caused the problem but today, my #2 temps dropped below 300. The cowl flaps were closed, it was a pretty cold day in TX. I tried to slow down the descent but at some point I had to get to pattern attitude. Any suggestions on whether I should lean more or enrich mixture on the descent? 

Thanks for pointers

I don’t throttle back unless it’s bumpy. CHTs in the high 200s aren’t reall a concern. #1 and #4 are under 300 this time a year in cruise. Been that way since I started flying it 20 years ago.

[Ragsf15e]

You don’t really need to keep chts “up” in the descent.  You should see Washington temps this time of year!  I descend just like @Shadrachdescribed. If im lop, just trim forward and enjoy the extra knots.  You can leave all the knobs alone until pattern altitude or whenever you want to get down quicker.  Don’t plan on slowing in your descent though. Plan to level off early and reduce power to slow down level when possible.

[hammdo]

I drop my MP by 5" from cruise MP to get a 500 FPM decent when the time comes and I still keep cruise speed. Adjust FF with the mixture control as needed. Seems to work for me — I’m carbed though…

-Don

[Hank]

33 minutes ago, hammdo said:

I drop my MP by 5" from cruise MP to get a 500 FPM decent when the time comes and I still keep cruise speed. Adjust FF with the mixture control as needed. Seems to work for me — I’m carbed though…

-Don

I just push the yoke for 500 fpm and adjust the trim, the free extra speed makes up for the low speed in the climb.

But having a carb, every couple thousand feet I'll reduce throttle to my cruise MP or an inch less, and enrichen to regain the cruise EGT.

Then it takes a couple miles to slow down to flap speed. 

The OPs J can descend without touching anything but the yoke and trim before slowing for pattern entry. 

[0TreeLemur]

From cruise LOP, at TOD I reduce M.P. to 19 in. for cruise descent.  As the M.P. increases during the descent, I just pull the throttle out to keep it at 19.   If you keep the M.P. constant you don't really need to change the mixture because the engine still thinks its at the same altitude.  Plus this is a low power setting ~60%, the mixture setting can't harm the engine.  Just don't forget to richen it back up if you open the throttle for any reason.

Regarding CHT's, it's impossible to keep those up when the engine is producing little power.   I've read that the shock cooling is not a major issue for the  IO-360.  Wish I could remember where I read that and provide a source, but I don't.  Maybe someone else can recall.

[ArtVandelay]

From cruise LOP, at TOD I reduce M.P. to 19 in. for cruise descent.  As the M.P. increases during the descent, I just pull the throttle out to keep it at 19.   If you keep the M.P. constant you don't really need to change the mixture because the engine still thinks its at the same altitude.  Plus this is a low power setting ~60%, the mixture setting can't harm the engine.  Just don't forget to richen it back up if you open the throttle for any reason.
Regarding CHT's, it's impossible to keep those up when the engine is producing little power.   I've read that the shock cooling is not a major issue for the  IO-360.  Wish I could remember where I read that and provide a source, but I don't.  Maybe someone else can recall.

As it was explained to me:
Cylinders are made of steel cylinder and aluminum head, they are a screwed together when the cylinder is cold (actually refrigerated) and the head is hot. As the head cools, it shrinks and as cylinder warms it expands, forming a permanent bond. If the head with its cooling fins cools too fast while the cylinder is still hot it could cause the head to crack.
It’s not the absolute temperature, but how fast you drop, something like 50°F per minute is the rule of thumb threshold. I think normally aspirated engines cool slow enough (because they aren’t running at high power at cold high altitudes), turbos can I think.

[Will.iam]

7 hours ago, dominikos said:

I usually just end up leaving the mixture setting as in the cruise and just reducing MP by 2”. It never caused the problem but today, my #2 temps dropped below 300. The cowl flaps were closed, it was a pretty cold day in TX. I tried to slow down the descent but at some point I had to get to pattern attitude. Any suggestions on whether I should lean more or enrich mixture on the descent? 

Thanks for pointers

If smooth air as other have stated just lower the nose and start a 300fpm descent and recover the lost time and speed from your climb earlier. If center will not allow a slow descent then  with power less than 65% i will set the RPM’s to 2200 and peak egt that way the engine is working more at turning the prop and peak egt is keeping my cht’s hotter than when at a lop setting. If it’s bumpy air then i will slowly pull MP back to 20” the speed will bleed off slowly as well. 

[Shadrach]

On 2/19/2024 at 1:00 AM, ArtVandelay said:

As it was explained to me:
Cylinders are made of steel cylinder and aluminum head, they are a screwed together when the cylinder is cold (actually refrigerated) and the head is hot. As the head cools, it shrinks and as cylinder warms it expands, forming a permanent bond. If the head with its cooling fins cools too fast while the cylinder is still hot it could cause the head to crack.
It’s not the absolute temperature, but how fast you drop, something like 50°F per minute is the rule of thumb threshold. I think normally aspirated engines cool slow enough (because they aren’t running at high power at cold high altitudes), turbos can I think.

My cylinders are never really hot. It’d be pretty hard to shock cool something that’s already cool (relatively speaking). My cylinders (as with most) experience more rapid temperature changes on start up. Greatful that shock heating isn’t a thing.

[Pinecone]

11 hours ago, ArtVandelay said:

As it was explained to me:
Cylinders are made of steel cylinder and aluminum head, they are a screwed together when the cylinder is cold (actually refrigerated) and the head is hot. As the head cools, it shrinks and as cylinder warms it expands, forming a permanent bond. If the head with its cooling fins cools too fast while the cylinder is still hot it could cause the head to crack.
It’s not the absolute temperature, but how fast you drop, something like 50°F per minute is the rule of thumb threshold. I think normally aspirated engines cool slow enough (because they aren’t running at high power at cold high altitudes), turbos can I think.

The whole shock cooling was debunked when someone actually instrumented an engine.   The highest cooling rate comes when you shut the engine down.  And the heating rate is highest on engine start.

[A64Pilot]

Shock cooling is real, it’s not been debunked, but it’s more of a problem for the High HP big turbo motors usually found on twins running at high HP and therefore high temps trying to get down fast like a turbine can, or glider tugs / parachute aircraft that can go from full throttle to idle in the mission.

For almost all of our little NA motors at altitudes we can’t make the HP and temp for it to be a real issue, if we are only mid 300’s it would be really difficult to cause damage even by reducing ti idle and pointing the nose down, we just aren’t hot enough to begin with.

Only real issue with being LOP and descending and not touching the mixture is the engine may not respond when you add power after descending several thousand feet, of course enriching the mixture will bring it back, you didn’t hurt anything just it can cause some stress if you add power and it starts running rough and doesn’t give you power.

Saying that shock cooling doesn’t exist is as logical as saying you don’t need to cool your turbo because on approach your EGT’s are off scale low.

Lycomings take on it, they call it sudden cooling, not shock cooling.

You can choose to believe the Lycoming are a bunch of idiots that don’t know anything about the engines they manufacture or not, that’s up to you.

They even give you a list of specific components you can damage

https://www.lycoming.com/content/how-avoid-sudden-cooling-your-engine

My personal belief has been forever that for any engine from your leaf blower to large Turbo Diesels that engines live longer if gradually warmed up and gradually cooled down. Even if it’s BS, it doesn’t cause any harm to follow manufacturers recommendations.

[jlunseth]

I pull off an inch of MP, make no other changes, and tip the nose over into a 500 fpm descent. In a turbo, if you are coming down from the flight levels it can be a long descent. If you were at 21k and need to get to 1k, that’s 40 minutes of descending, and making that kind of a descent exactly on the numbers is more art than science. The winds aloft that had you screaming along at 21k may stop quickly as you descend or they may not. Hard to predict. I pull the inch off because as soon as I tip the nose over the plane is going to pick up speed and the ram air effect is going to bring the MP right back up to around where I had it at cruise. Maintaining cruise power will keep the engine nice and toasty. Every time I say this someone jumps in and say, oh horror!, what if you hit a bump on the way down. Obviously, if it starts to get bumpy or you are going to descend into cloud tops where it could be turbulent you change what you are doing. Shock cooling is an OWT unless you are in arctic cold conditions. We do get that on occasion here in MN and when it starts to get difficult to keep the engine warm, remember, if you are at 65% power you can run the engine dead on peak, which helps keeps the temps up. You can also reduce RPMs way down, which allows you to keep percent of power higher in order to keep the engine warm. I am currently breaking in a new engine and TCM recommends pulling the RPMs way back during descents and approaches in order to keep the power up, so I am pulling it back to 1900 or even 1800 and guess what, nothing bad happens except I can keep the engine a little warmer.

[Shadrach]

On 2/19/2024 at 12:38 PM, A64Pilot said:

You can choose to believe the Lycoming are a bunch of idiots that don’t know anything about the engines they manufacture or not, that’s up to you.

Who are the idiots to which you are referring? This site is full of individuals with differing opinions. I’ve come across a lot of statements with which I staunchly disagree, but I’ve encountered very few idiots here. It’s not a term I use lightly.
The vast majority of the forum members base their opinions on publications they’ve read, the data in the publication used to support the statements made and the ability to reproduce similar outcomes in the field.   Trust in an institution is built or destroyed incrementally over time. Lycoming has not always put their best foot forward.

I’m old enough to remember Lycoming’s  “experts are everywhere memo” that used to be located at the address below on Lycoming’s website (the link below is long since dead):

https://www.lycoming.com/support/troubleshooting/resources/SSP700A.pdf

It did not name names, but was an obvious direct shot at GAMI, APS and other heretics in the industry.   I was actually embarrassed for the author as I read it. Given that it was likely written by an engineer at Lycoming, I rationalized what I was reading by imagining that the legal department held a gun to his head while he was typing. It was indeed idiotic to anyone with an understanding of how mixture works.

I think it’s important to take a manufacture’s recommendations seriously. I also understand that in a small, highly regulated and highly litigious market there is very little incentive to course correct on past statements. Given how slow the manufactures are to move, there has been sort of a small cottage industry of companies that have helped take GA into more modern and data driven operations. Lycoming facetiously called them “experts”. 

Those small companies and their executives have dramatically changed how much information and understanding the general pilot population has about combustion science and applied power plant operations, among other accomplishments. What is mostly mainstream now was controversial at Lycoming in the early aughts which is why the memo was released.

At some point Lycoming realized how bad it had aged as it was removed. The Part number SSP700A is no longer available on the website.

George Brawly having a sense of humor posed for the attached pic in front of a Lycoming trade show booth. I’ve updated it just for you.

Back to shock cooling or sudden cooling or whatever you want to call it. Perhaps it is real, but no one with the ability to do so has been able to generate data to support its detrimental effects. 

The “Idiots” that have stated that shock cooling is or probably is a myth include but are not limited to:

George Braly

John Deakin (RIP)

Walter Atkinson (RIP)

Rick Durden

Mike Busch

and many others…

 

[Mac80]

An airplane mechanic showed me the results of what he said was failure to enrich on descent from cruise. Burnt exhaust valve that fell into piston. According to him bad things happen by overheating cylinders and values  on descent  by not enriching. I enrich my IO360 to keep cylinder head temperatures lower than cruise in descent or climb.

I try to keep cylinder and oil temps up and descend slowly but always try to keep the mixture on rich side while descending.

 

[Shadrach]

1 hour ago, Mac80 said:

An airplane mechanic showed me the results of what he said was failure to enrich on descent from cruise. Burnt exhaust valve that fell into piston. According to him bad things happen by overheating cylinders and values  on descent  by not enriching. I enrich my IO360 to keep cylinder head temperatures lower than cruise in descent or climb.

I try to keep cylinder and oil temps up and descend slowly but always try to keep the mixture on rich side while descending.

 

I would say that the vast majority of mechanics know very little about engine operations. I don’t know the percentage of operations knowledgeable mechanics but I’d bet it barely cracks double digits… and almost all of those are likely operators in addition to being mechanics

[Ragsf15e]

1 hour ago, Mac80 said:

An airplane mechanic showed me the results of what he said was failure to enrich on descent from cruise. Burnt exhaust valve that fell into piston. According to him bad things happen by overheating cylinders and values  on descent  by not enriching. I enrich my IO360 to keep cylinder head temperatures lower than cruise in descent or climb.

I try to keep cylinder and oil temps up and descend slowly but always try to keep the mixture on rich side while descending.

 

The way I’ve understood it and seems to work like this in practice… if you’re descending without touching anything, the fuel servo is somewhat compensating for the increased air pressure of lower altitude by increasing fuel flow.  But not enough to completely keep your same ROP or LOP setting.  You get leaner as you descend (more air, not as much more fuel). So if you started ROP, it will creep towards peak and high power as you descend.  It could potentially get to high power near peak which isn’t great.  However, if you’re cruising LOP, you’ll also get leaner - further lop - which will keep cylinders cool.  Eventually you’ll end up so lean after a long descent that the engine stumbles because it’s very far lean of peak.  It’s not going to hurt anything there, but you’ll need to go rich in the pattern.

Edited February 20, 2024 by Ragsf15e

[Hank]

2 hours ago, Shadrach said:

I’m old enough to remember Lycoming’s  “experts are everywhere memo” that used to be located at the address below on Lycoming’s website (the link below is long since dead):

https://www.lycoming.com/support/troubleshooting/resources/SSP700A.pdf

It did not name names, but was an obvious direct shot at GAMI, APS and other heretics in the industry.   I was actually embarrassed for the author as I read it. Given that it was likely written by an engineer at Lycoming, I rationalized what I was reading by imagining that the legal department held a gun to his head while he was typing. 

Once published, nothing ever disappears from the internet. I searched "lycoming ssp700a" and this was the first hit:

https://www.peter2000.co.uk/aviation/engine-management/lyco-SSP700A.pdf

Interesting reading . . . . but really nothing new, nothing conclusive, only the desultory use of "expert" as a perjorative.

[Shadrach]

57 minutes ago, Hank said:

Once published, nothing ever disappears from the internet. I searched "lycoming ssp700a" and this was the first hit:

https://www.peter2000.co.uk/aviation/engine-management/lyco-SSP700A.pdf

Interesting reading . . . . but really nothing new, nothing conclusive, only the desultory use of "expert" as a perjorative.

I knew it was there and I linked it to the text "experts are everywhere memo" in my post.  I included the Lycoming link as a bit of a jab at Lycoming's efforts to disappear a document that was not only needlessly antagonistic but also embarrassing in terms of the conclusions drawn.  John Deakin's rebuttal on APS's website is a good read and worth a look for a piece by piece breakdown.  

Interesting Deakin column from 26 years ago about fuel management touches on a number of old wives tails including shock cooling and showcases his skepticism on the subject even then.

[Pinecone]

Being on AVSIG in the 90s was an amazing time.  The LOP stuff started there with George and John and Walt.  And others.  And the rest of the OWTs that were tested and debunked.

And a LOT of naysayers.  People predicting destroyed engines in 10s of hours.  Many times after George and John had run engines for hundreds of hours.

The funniest thing, IMO, was that a lot of their information came from much earlier documents on how to run engines, from Lycoming and Continental.  

[jlunseth]

PS there is a normal operating range table in my POH, may be one in yours. Above 250 is in the normal operating range for my engine. Because of the cold here in MN I have looked into the consequences of operating one or more cylinders cold. The only consequence I have been able to find is that some think that lead scavenging degrades, so the spark plugs gunk up. I can’t say I have ever experienced that. Happens for a short time during every single landing in the winter here in MN when the engine is powered down on final or even in the pattern. Can’t say that it has ever caused an issue with my engine.

@ Pinecone. As it happens, I have been reading through Basic Theory of Operation of Turbo Compound Engine, copyrighted in 1957, which the GAMI people distributed at the Ada seminar I attended. It’s by the Field Engineering Department of Curtiss-Wright. Pretty interesting.

[Pinecone]

Cool.

I just looked and it is available here for those interested - theory_turbocompound.pdf (aviatechno.net)

 

 

[Mac80]

On 2/20/2024 at 10:15 AM, Shadrach said:

and almost all of those are likely operators in addition to being mechanics

This fellow and his son were both pilots and his son still work on planes, not mine I trust the other FBO mechanics manager. The other one I trust likes 50 degree rich of peak. I have no idea what caused the deformed piston but it scared the heck out of me. 

I do not want to get into lean of peak vs rich of peak debate. If “I try to keep cylinder and oil temps up and descend slowly but always try to keep the mixture on rich side while descending.” is incorrect help me understand. If I let it go without enriching in the descent then the engine stumbles so I enrich the mixture till it stops.

I am not defending him like I said  I use the other guys. So are you explaining that the mechanic/pilot was incorrect and running to lean and resulting overheating was not probable cause of the loss of the cylinder as he was illustrating to group?

Can you explain how you monitor you exhaust temperature in descent if slightly enriching is incorrect?

Thanks

Steve

 

 

 

 

 

 

[EricJ]

4 hours ago, Pinecone said:

Cool.

I just looked and it is available here for those interested - theory_turbocompound.pdf (aviatechno.net)

Nice.

I think the oft-maligned Lycoming doc is right on many of the facts, a significant one being, as demonstrated by this document, that all of these things about how to run reciprocating aircraft engines in various mixture regimes has been known for a long, long time.   I think it was also right that the general guidance adopted by many operators was developed to optimize the likelihood that the operator wouldn't fubar things too badly in the absence of very much instrumentation, i.e., not that the engine operation was being optimized.    When I was learning to fly in the 70s and 80s I don't recall anybody ever going nuts about potentially damaging an engine due to leaning.   We leaned to roughness and back a little, and that was normal operation.   Perhaps we just weren't sophisticated enough to know the difference.

It's unfortunate that the various documents and personalities at the time engaged in the tit-for-tat stuff, as for the most part they were all saying the same thing if you pushed down far enough.   I don't think there was any brilliant discovery of anything, but a bit of a rediscovery in operation methods was enabled by improved engine instrumentation which was becoming more available to the masses in the form of engine monitors.   The fact that people still argue about how to do it right is kind of to Lycoming's original point that for a long time the published methods were mostly a result of getting operators to do something reasonable rather than try to herd the cats to something more detailed and nuanced.