What would you do in this flight scenario? Potential for shock cooling

[PeteMc]

2 hours ago, 201er said:

What situation? A manufacturer limitation or just reluctant to throttle back because of something you read somewhere a long time ago?

Not sure about your plane, but if I'm already heading down at 500ft/min and at the yellow arc and ATC says to EXPEDITE my descent...  Sorry, pulling the throttle is not going to give the controller what they are expecting you to do.  They want to see an immediate change in the descent rate and there's going to be a delay in my K before the speed really drops enough to make a difference. 

Sure, in the real world if I know it is calm and expect it to be all the way down, I'll push into the yellow to get a better descent and then just let the speed bleed off, but even that may not be enough.  So I suggest "unable" if you're already at the yellow line. 

 

[Vance Harral]

10 hours ago, kortopates said:

Sorry Vance, I thought I had already said that an emergency descents per the ACS where not sufficiently long enough to create an engine management concern or essentially be in that configuration with a cold engine.

Roger, the conversation has multiple tangents at this point.

I remain unconvinced there is any cooling/heating rate concern in a normally aspirated 4-cylinder Lycoming, in any maneuver one might fly in the course of any training or traveling flight.  Prolonged glides, ATC slam dunks, commercial maneuvers, training for engine failures or emergency descents, etc.  Just based on personal experience.

I acknowledge the operating envelope is bigger for higher horsepower and/or turbocharged engines, that might venture into the flight levels where OAT can potentially be 40 below (on either temperature scale).  I still think the concern is oversold in the environment where almost all of us bug smashers fly our piston-powered airplanes.  But again, I'm in no position for anyone to lend weight to my opinions.  I don't own a TSIO-anything, have a limited amount of instruction given in M20Ks, Bonanzas, etc., and have only been above FL180 in a piston airplane a couple of times.

[hazek]

18 hours ago, 1980Mooney said:

Works perfect. 

Seems like no one else thinks so or wants to hear it 

[Hank]

1 hour ago, hazek said:

Seems like no one else thinks so or wants to hear it 

Too many armchair experts debating with experienced CFIIs, all going off course and into the weeds. Special procedures for turbo 6-cylinder Continental engines descending from the flight levels, when the OP had questions about flying his F model, descending from 10K through 6K . . .

But that's life on the interwebs, isnt it?

[jetdriven]

You guys are acting like the yellow arc, the wings are gonna fall off. As long as it’s not seriously moderate or severe turbulence, it’s fine.  There was a salvage Mooney M20k for bid about 10 years ago, and it looked pretty good till you looked up close, and the wings were bent upwards about a foot on each side, and even the fuselage was bent down in the middle so much that you could put your thumb between the baggage door and the frame with it closed. And then I looked up the NTSB report on this thing and the guy flew through Mountain wave that was so strong it was plus and minus 10 Gs rapidly back-and-forth, the factory estimated. The guy even flew to destination and landed before he figured it out how bad it was. 

Edited Saturday at 02:16 PM by jetdriven

[Pinecone]

On 5/22/2025 at 4:19 PM, kortopates said:

And one more point that needs to be emphasized that Rich started with, just do the best you can for your comfort level and experience level. ATC will have no problem vectoring you around if you can’t make the descent rate they were wanting because of traffic. They have more tools and options than we usually do.

One of the most useful words while IMC is "UNABLE."

[DCarlton]

This thread reminds of meetings at work with a room full of engineers before I retired.  If you need to slow down, slow down.  If you don't want to be in the clouds in the yellow arc, slow down around clouds, get ahead of the situation.  Practicing slowing your airplane is a good thing to do.  I've been reminded by an instructor to just throttle back and pitch up.  It works every time.  ATC will see you slowing even if you level for a few seconds.  Keeping a little power is good; no need to overthink engine management.  I personally wouldn't drop gear unless I'm near the airport; it's never been necessary for me but I think my three blade may help me slow when throttled back.  Just do the best you can and tell ATC; they're there to help.  Side note: I'm not a CFI just a F owner that's never had trouble slowing down.  

[201er]

Before slipping through clouds, lowering the landing gear, splitting the engine case, or stating unable, has it occurred to anyone to just ask ATC? “Having some difficulty slowing down enough to expedite descent while coming up on some rough air, any chance of a delay vector for now?”

[Ragsf15e]

On 5/23/2025 at 9:50 AM, kortopates said:

Not to pick on you, but your the 2nd person (IIRC) to say increase RPM when reducing RPM which reduces thrust is correct unless rather than just reduce MAP one pulls the throttle back to idle and then full prop create more drag. 

In summary:

If the engine is producing thrust, reducing RPM reduces thrust/power, while increasing RPM increases thrust/power - see your performance tables.

If the engine isn't producing thrust, increasing RPM creates more drag and is an effective brake, reducing RPM for less drag enhances glide range.

I took from his post that he was suggesting reducing mp to idle or close to slow down and then increasing the prop which would also help slow down at that point?  Is that not how you understood what he said?

[1980Mooney]

9 hours ago, Hank said:

Too many armchair experts debating with experienced CFIIs, all going off course and into the weeds. Special procedures for turbo 6-cylinder Continental engines descending from the flight levels, when the OP had questions about flying his F model, descending from 10K through 6K . . .

But that's life on the interwebs, isnt it?

Agreed

On 5/22/2025 at 4:07 PM, hazek said:

Can someone educate me why this isn't a solution?:

• Pitch up
• Slow below Vle
• Drop gear
• Resume descent?

 

10 hours ago, hazek said:

Seems like no one else thinks so or wants to hear it 

Everyone seems to think their engine is made out of crystal.  We are talking about a Naturally Aspirated engine that isn't even making 60% of peak power at that altitude.  I have been doing this for over 25 years in a mid-body with a 300HP IO-550 - and no speed brakes. - flying over the Sandia Mountains and then dropping right into KABQ.

Look at the M20J Emergency Descent procedures from the 1996 POH revision.  Gear Extended will give the same 2,000 fpm rate of descent as gear retracted only at a slower more controlled speed.  Just reduce throttle and retard power - the only thing it says is to try to keep cylinders at 300 F. 

 

 

Edited Saturday at 08:49 PM by 1980Mooney

[kortopates]

I took from his post that he was suggesting reducing mp to idle or close to slow down and then increasing the prop which would also help slow down at that point?  Is that not how you understood what he said?

No, reducing MAP doesn’t equate to reducing to idle to me at all. Nor is it something i ever do till landing or practicing emergency maneuvers.


Sent from my iPhone using Tapatalk

[DCarlton]

On 5/24/2025 at 1:46 PM, 1980Mooney said:

Agreed

 

Everyone seems to think their engine is made out of crystal.  We are talking about a Naturally Aspirated engine that isn't even making 60% of peak power at that altitude.  I have been doing this for over 25 years in a mid-body with a 300HP IO-550 - and no speed brakes. - flying over the Sandia Mountains and then dropping right into KABQ.

Look at the M20J Emergency Descent procedures from the 1996 POH revision.  Gear Extended will give the same 2,000 fpm rate of descent as gear retracted only at a slower more controlled speed.  Just reduce throttle and retard power - the only thing it says is to try to keep cylinders at 300 F. 

 

 

I believe your gear speed is significantly higher than the F.  

[1980Mooney]

41 minutes ago, DCarlton said:

I believe your gear speed is significantly higher than the F.  

Good point. Vle for the F is 120 mph. The J is 140 knots or 161 mph. 

[201Mooniac]

22 hours ago, 1980Mooney said:

Good point. Vle for the F is 120 mph. The J is 140 knots or 161 mph. 

Depends on the year of the J, mine, a 1980 model is 132 knots.

[A64Pilot]

Sigh,

Shock cooling is not a myth, statements like if it was real then shock heating would ruin engines on takeoff etc are non sensical.

As is the belief that sudden engine cooling is going to result in sudden engine destruction.

There are very few if any limitations that exceedence results in nearly instant damage. Like the avoid 1500 - 2000 RPM at manifold pressures below 15” MP on my J. Some people ignore it saying it’s a myth, thinking I did it a few times and nothing bad happened so it’s not real, but the damage is real, it’s cumulative. Like over revving, or exceeding the redline on cyl head temp, just because the engine didn’t seize doesn’t mean you didn’t cause damage. Ever wonder why some seem to crack cylinder heads and others don’t?

Just like dropping gear at max extension speed causes higher wear than if you don’t. In almost all limitations there is no exact point where below it causes no additional wear but above it causes instant damage, even VNE isn’t like that. I know someone that hit VNE with the flaps in T/O position and it didn’t damage anything, does that mean that there is no airspeed limit for flaps in the T/O position? Of course not. Most damage is fatigue induced, and fatigue doesn’t come from one time occurrences.

As much as practicable, avoiding operations at the limits and rapid changes will result in longer component life, and that’s true for everything from slamming on the brakes to overheating starters as well as engines 

 

[1980Mooney]

1 hour ago, A64Pilot said:

Sigh,

Shock cooling is not a myth, statements like if it was real then shock heating would ruin engines on takeoff etc are non sensical.

As is the belief that sudden engine cooling is going to result in sudden engine destruction.

There are very few if any limitations that exceedence results in nearly instant damage. Like the avoid 1500 - 2000 RPM at manifold pressures below 15” MP on my J. Some people ignore it saying it’s a myth, thinking I did it a few times and nothing bad happened so it’s not real, but the damage is real, it’s cumulative. Like over revving, or exceeding the redline on cyl head temp, just because the engine didn’t seize doesn’t mean you didn’t cause damage. Ever wonder why some seem to crack cylinder heads and others don’t?

Just like dropping gear at max extension speed causes higher wear than if you don’t. In almost all limitations there is no exact point where below it causes no additional wear but above it causes instant damage, even VNE isn’t like that. I know someone that hit VNE with the flaps in T/O position and it didn’t damage anything, does that mean that there is no airspeed limit for flaps in the T/O position? Of course not. Most damage is fatigue induced, and fatigue doesn’t come from one time occurrences.

As much as practicable, avoiding operations at the limits and rapid changes will result in longer component life, and that’s true for everything from slamming on the brakes to overheating starters as well as engines 

 

 Bob Hoover shock cooled his engines every flight. He would go from full power to shut off and props feathered. He never cracked cylinders. 
 

“Hoover flew a precise deadstick (no-engine) maneuver with a loop, eight-point roll, a 180-degree turn to a touchdown with first one wheel and then the other wheel, landing, and taxi to air show center.”

[hazek]

2 hours ago, 1980Mooney said:

He never cracked cylinders. 

How do we know this is true? I doubt it's true. But I have nothing to go on. Do we have logbooks from his plane somewhere? Many times people make up stories you know..

[EricJ]

3 hours ago, 1980Mooney said:

 Bob Hoover shock cooled his engines every flight. He would go from full power to shut off and props feathered. He never cracked cylinders. 
 

“Hoover flew a precise deadstick (no-engine) maneuver with a loop, eight-point roll, a 180-degree turn to a touchdown with first one wheel and then the other wheel, landing, and taxi to air show center.”

Likewise twins used for training routinely shut engines down at cruise.   Sometimes there are multiple start/stop cycles per flight.   They don't seem to suffer any extreme ill effect for it.

[PT20J]

I had a conversation with Sacramento Sky Ranch owner John Schwaner back in the mid-1990s when there was a rash of cracked cylinder heads that were being blamed (with no evidence) on shock cooling. John had a lot of experience rebuilding cylinders and didn't think shock cooling was the cause. He told me that, just for grins, he got a wash tub, filled it with acetone and dry ice, heated a cylinder head from (I think) an IO-540 with a torch and dropped it into the tub. It apparently made quite a show, but did not crack.

John had a theory about the cracks. Back then, new Continental and Lycoming cylinders were very expensive. So there were a lot of cylinders getting overhauled. With every combustion event, the head expands slightly and then contracts. Aluminum work hardens. So, a head that is on it's third or fourth TBO run is much more likely to crack. This theory is unproven, but since Lycoming and Continental began offering new cylinders at competitive prices, most overhauls include new cylinders and we haven't heard as much about cracking cylinder heads and shock cooling.

[EricJ]

5 hours ago, PT20J said:

I had a conversation with Sacramento Sky Ranch owner John Schwaner back in the mid-1990s when there was a rash of cracked cylinder heads that were being blamed (with no evidence) on shock cooling. John had a lot of experience rebuilding cylinders and didn't think shock cooling was the cause. He told me that, just for grins, he got a wash tub, filled it with acetone and dry ice, heated a cylinder head from (I think) an IO-540 with a torch and dropped it into the tub. It apparently made quite a show, but did not crack.

John had a theory about the cracks. Back then, new Continental and Lycoming cylinders were very expensive. So there were a lot of cylinders getting overhauled. With every combustion event, the head expands slightly and then contracts. Aluminum work hardens. So, a head that is on it's third or fourth TBO run is much more likely to crack. This theory is unproven, but since Lycoming and Continental began offering new cylinders at competitive prices, most overhauls include new cylinders and we haven't heard as much about cracking cylinder heads and shock cooling.

The overseas aero club that I worked at as a teenager ran 115/145 fuel that we got from the Army.   An AF Flying Club not too far away also ran 115/145, but kept getting cracked cylinders and was blaming the fuel for some reason (scapegoat du jour, easy substitute for shock cooling).   We never got cracked cylinders, we ran the same fuel, and iirc our airplanes flew a lot more than they did.   It was a always a bit of a mystery why their cylinders were cracking, but I think they were the only one of the US flying clubs in Europe that were running 115/145 that were having the problem, so I just figured it must be something they're doing.

Years later I learned that a common reason for cylinders cracking between the plug hole and exhaust port is over-torquing spark plugs on installation.   I think they just had an over-zealous mechanic that maybe didn't own a torque wrench or something.

There were also some local US jump clubs with 182s and Reims Rockets (180hp 172), and they'd climb at full power, throw everybody out, chop the power and descend as quick as they could to pick up the next batch, whereupon they'd climb at full power, descend quickly with the throttle chopped, etc., all day long.    They did not have a big problem with cracked cylinders, either.

It's still pretty common for recip jump airplanes to climb at full throttle, throw everyboy out, and descend fast enough to beat the jumpers to the ground, and repeat the cycle all day long.

There are so many counter examples for why shock cooling isn't really a thing that I'm always a bit puzzled at the people who insist on thinking so.

[MikeOH]