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

[RescueMunchkin]

9 minutes ago, Joshua Blackh4t said:

Also, we didn't ask why was the expedited descent required. Could the OP have anticipated the descent and requested top of descent earlier?

 

The reason why isn't really relevant to the hypothetical scenario. Of course the answer would always be "don't get yourself into a bad situation you could easily anticipate"

In my case, I was on flight following at 10.5k ft. There was an overcast layer below me that was predicted to be scattered.  I had to get a pop up IFR clearance in the vicinity of class B airspace and of course at that point you follow their instructions unless it compromises safety.  I was told to descend to 7000.  Then told to descent to 5000 and expedite it.

[redbaron1982]

2 hours ago, kortopates said:

Don't need to slip it on an IFR flight; especially IMC. But you do need to be much more aggressive about getting it slowed down to come down. Reducing MAP to at least 15" is a start, and also reducing RPM as well to get you to gear speed. Lean out the mixture to bring EGTs upto the 1400's if able to reduce the amount of cooling on your CHTs. Then gear down will enable a steeper yet stabilized descent ready for any bumps (if present). 

You'll want to add a "Quick Descent" configuration to your IFR PAC, the one we use for F in the PPP's is from memory 13" at 2250 rpm 120 MPH leaned with gear down - this is intended more for NPA but will get you 1000 FPM or more descent.

I thought that increase the RPMs would help using the prop to slow down. I know that higher RPM >> more power at same MP. But also higher prop RPM means more aerodynamic breaking, right? What I'm missing here?

[kortopates]

5 minutes ago, redbaron1982 said:

I thought that increase the RPMs would help using the prop to slow down. I know that higher RPM >> more power at same MP. But also higher prop RPM means more aerodynamic breaking, right? What I'm missing here?

Actually I thought Rich already answered this above quite well, but I'll try to clarify.

No, you're mixing up drag from the prop at idle and thrust from the prop under power. You have the right concept under power and in fact each 100 rpm is roughly equivalent to each 1 inch of MAP which is where the Key Number concept for power comes from. So using your J as an example we teach the key number of 50 = 75% power which means any combination of MAP in inches + Prop RPM in 100's equal to 50 gets you 75% power e,g. 25" and 2500 rpm or 26" and 2400 rpm are all the same power essentially.

But when the engine power is reduced to idle or less such that the wind is now turning the prop, not the engine, the prop is now creating drag not thrust. As such, the faster, higher RPM, finer pitch, the more drag it produces, which is why power off for best glide we pull the prop back to go further with Less Drag.

But under power to slow down we can reduce thrust with less RPM. 

[Joshua Blackh4t]

45 minutes ago, RescueMunchkin said:

The reason why isn't really relevant to the hypothetical scenario. Of course the answer would always be "don't get yourself into a bad situation you could easily anticipate"

In my case, I was on flight following at 10.5k ft. There was an overcast layer below me that was predicted to be scattered.  I had to get a pop up IFR clearance in the vicinity of class B airspace and of course at that point you follow their instructions unless it compromises safety.  I was told to descend to 7000.  Then told to descent to 5000 and expedite it.

Agreed, I would be in the same place.

In that case, after a descent to 7000, I would be happy that the cylinders are already cool and have no worries of power right back, hold the gear lock switch to silence the alarm, prop full forward for max braking and nose down as fast as you are comfortable.

 

[M20F]

Roll inverted and you can load the positive g’s to get down.

[Justin Schmidt]

25 minutes ago, M20F said:

Roll inverted and you can load the positive g’s to get down.

And keep up international relations haha

[Vance Harral]

6 hours ago, RescueMunchkin said:

I know the board is split about shock cooling being a myth, but this is how I learned complex planes

Let's talk about "this is how I learned" for a minute, because it's really interesting.

One of the topics flight instructors are taught is the Law of Primacy.  Or more generally and formally, the Law of Primacy in Persuasion.  Paraphrased, it says that when debatable subjects are discussed (e.g. shock cooling, prop driving the engine, lean of peak ops), what your hear first is "sticky" in a way that is difficult to overcome in subsequent learning, no matter how much evidence is presented to the contrary.

So I'm completely sympathetic to your feelings about "this is how I learned".  But it's so important for all of us in this game to remember a few critical things:

1. Flight instructors can be wrong, particularly on debatable subjects (I'm a CFI, consider me exhibit number one - I've been wrong a lot!)
2. Technology and understanding changes over time.
3. Data is more important than opinions.

Respectfully suggest you try to let go of what you were taught when you were introduced to complex airplanes, and form a fresh opinion based on instrumentation and data.  If you draw the same conclusion you were originally taught, you can feel reasonably good it's an informed opinion, rather than just what you happened to hear first.  Others here have already pointed out where you might get data: CHTs from your own airplane on engine shutdown, and maintenance stories from skydiving operators, glider towplanes, and light twins used for multiengine training (the multi-engine rating requires an actual shutdown of an engine in flight, so training airplanes cage one powerplant about every other flight).

 

[RescueMunchkin]

48 minutes ago, Vance Harral said:

Let's talk about "this is how I learned" for a minute, because it's really interesting.

One of the topics flight instructors are taught is the Law of Primacy.  Or more generally and formally, the Law of Primacy in Persuasion.

 

Having just taken my FOI test about 2 weeks ago, I know what you're referring to.  However, there is an even stronger motivator for me in the true cost of engine damage.  I assume that most would agree that an easier engine letdown is "nicer" on the engine and there are no negatives to the engine longevity by performing the easy letdown vs making larger power reductions.

I am curious how quickly a full closing of the throttle would actually have allowed me to reduce the speed into the green arc while maintaining a 700-1000fpm descent at that point of my flight though.

Edited Friday at 12:44 AM by RescueMunchkin

[kortopates]

35 minutes ago, RescueMunchkin said:

I assume that most would agree that an easier engine letdown is "nicer" on the engine and there are no negatives to the engine longevity by performing the easy letdown vs making larger power reductions.

Yes and No, my goal is always to get a pilot discretion descent so that I can descend at 200-300 FPM. Nothing to do with my engine but purely for efficiency sake. But when I have to come down expeditiously I am quite happy to make a large power reduction with zero concern for shock cooling in my turbo - but I avoid pulling back to idle see next comment. 

35 minutes ago, RescueMunchkin said:

I am curious how quickly a full closing of the throttle would actually have allowed me to reduce the speed into the green arc while maintaining a 700-1000fpm descent at that point of my flight though.

No need to pull back to idle unless your making an emergency descent, a short one is okay, but whenever you reduce the engine power so much that the engine is no longer driving the prop but instead the airflow over the prop is, your reversing the stress on the piston rings in the piston groves in opposite direction they are intended for which can lead to damaging ring flutter when done over a long descent. I suggest never reducing the power below 13" till your descending for the runway. But in an emergency you do it regardless. I know a Mooney pilot on the list that did an emergency descent from about 20K that had this happen. Of course nothing to do with shock cooling concerns though.

[Vance Harral]

10 minutes ago, RescueMunchkin said:

I assume that most would agree that an easier engine letdown is "nicer" on the engine

I dissent, primarily because this a nonsensical statement.  "Nicer" doesn't mean anything with respect to a machine, there is no way to measure the cost or risk of it.

If you're asserting that rapid power reductions may cause immediate damage, or reduce time to overhaul, or incur higher maintenance costs, just say that.  Then we can all talk about why that might or might not be true.

 

10 minutes ago, RescueMunchkin said:

and there are no negatives to the engine longevity by performing the easy letdown vs making larger power reductions.

Let's agree on that for the purposes of this discussion.  But what are the potential negatives of an easy letdown independent of the engine?  You've already noted one, which is difficulty complying with ATC instructions, and in the worst case that could lead to a certificate action and suspension that may or may not be more painful to you than dents in your wallet.  There are other risks, ranging from life and death (difficulty deconflicting with a traffic threat), to lifestyle (you don't get your family to the bathroom quick enough and they decide they don't really want to fly around in little airplanes any more).

It feels like you want to hear that super pilots always have some risk-free way to avoid large power changes.  But it just ain't so.  You're occasionally going to have to either make large power changes, or accept other risks in order to avoid them.  Up to you to decide how those other risks stack up against a difficult-to-quantify engine maintenance risk.

I recognize I'm showing my hand here, and for what it's worth, that's based on a couple of personal experiences.  One is 21 years of flying an engine I own (exact same model as yours), and paying absolutely no attention whatsoever to cooling rates.  Lots of power-off 180s, steep spirals, slam dunk descents from ATC, etc.  Engine is currently running strong at 2500 hours and 31 years since overhaul.  The other is spending much of the last year giving multi-engine instruction, during which I make the ultimate large power reduction about every other flight: shutting down an engine altogether!  No catastrophic failures or early overhauls on that airplane so far.  My experiences are only anecdotes, but others here have provided other anecdotes, and at some point it adds up to data.

[Vance Harral]

17 minutes ago, kortopates said:

whenever you reduce the engine power so much that the engine is no longer driving the prop but instead the airflow over the prop is, your reversing the stress on the piston rings in the piston groves in opposite direction they are intended for which can lead to damaging ring flutter when done over a long descent

Data point: I've been letting the prop drive the engine on a fairly regular basis for decades.  Most commonly during emergency descent training, as required for the commercial certificate.  This is especially interesting in multi-engine training.  The emergency descent in a light twin is brutal: props full forward, throttles full idle, gear down, pitch and bank for just below the gear speed (139 KIAS in a Duchess).  This results in an eye-watering deck angle and a discomforting howl of air and engine noise, but it's required to meet the ACS standard.  I've never flown a glider towplane, but based on watching them at one of our local airports (Pawnees and Super cubs), the profile is about the same.

Counterpoint: essentially all of my experience involves 4-cylinder Lycomings.  The 6-cylinder Continental in turbo Mooneys is a somewhat different design, and is certainly more expensive to overhaul.  I'm in no position to advise or criticize owners of those engines.

[kortopates]

Data point: I've been letting the prop drive the engine on a fairly regular basis for decades.  Most commonly during emergency descent training, as required for the commercial certificate.  This is especially interesting in multi-engine training.  The emergency descent in a light twin is brutal: props full forward, throttles full idle, gear down, pitch and bank for just below the gear speed (139 KIAS in a Duchess).  This results in an eye-watering deck angle and a discomforting howl of air and engine noise, but it's required to meet the ACS standard.  I've never flown a glider towplane, but based on watching them at one of our local airports (Pawnees and Super cubs), the profile is about the same.
Counterpoint: essentially all of my experience involves 4-cylinder Lycomings.  The 6-cylinder Continental in turbo Mooneys is a somewhat different design, and is certainly more expensive to overhaul.  I'm in no position to advise or criticize owners of those engines.

Ring flutter is real, but just like your experiences i’ve never seen it directly damage a Lyc 4 cylinder engine. But i have with bigger engines coming down from much higher altitudes than a 4 cyl typically fly’s at.

But unable to quantity the possible accelerated wear it can have on rings i do emergency simulated engine out training differently in HP Mooney’s and Bonanza’s.

Starting with normal pull to idle, pitch for Vg then observe FPM, next pull the prop back and observe the acceleration and new reduced FPM,
Now with several miles to go to an airport, i’ll simulate the same descent rate by putting gear down and adding just enough power to maintain the observed FPM clean and lean it out (btw, we also compare that to what our calculated FPM should be based on POH glide ratio and Vg)
So now with a little power, we’re reducing the stress in terms of letting get too cold as well as reducing likelihood of ring flutter.
But if we need to go back to full power, we’ll just add about 20” MAP to arrest descent and level off as go over the runway level and not add full power till we CHTs come back. Rather than go full power. Bigger engines have min temp limits before runup and full power. this is just to ensure these aren’t ignored as well
as minimize the prop driving the engine.

But following those minor adjustments i believe the training is well worth it. Emergency descents are never more than a few thousand feet which i don’t think is a concern till they’re prolonged to over more than 10+K and engine has time to get very cold.


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[N201MKTurbo]

36 minutes ago, kortopates said:

Yes and No, my goal is always to get a pilot discretion descent so that I can descend at 200-300 FPM. Nothing to do with my engine but purely for efficiency sake. But when I have to come down expeditiously I am quite happy to make a large power reduction with zero concern for shock cooling in my turbo - but I avoid pulling back to idle see next comment. 

No need to pull back to idle unless your making an emergency descent, a short one is okay, but whenever you reduce the engine power so much that the engine is no longer driving the prop but instead the airflow over the prop is, your reversing the stress on the piston rings in the piston groves in opposite direction they are intended for which can lead to damaging ring flutter when done over a long descent. I suggest never reducing the power below 13" till your descending for the runway. But in an emergency you do it regardless. I know a Mooney pilot on the list that did an emergency descent from about 20K that had this happen. Of course nothing to do with shock cooling concerns though.

Just another point of view. I don’t think the rings are adversely affected by the engine being driven by the prop. During every intake stroke the rings are sealing on the top of the lands. And on every compression, power and exhaust stroke, the rings are sealing on the bottom of the lands. This same paradigm is going on with the prop driving the engine. In both cases the rings are pressed to the top and then the bottom of the lands. So the rings are constantly moving up and down in their lands. What has been documented with shock cooling, is the barrels rapidly cooling unequally with the fin area cooling slower than the non fin area. This can cause uneven wear of the cylinder barrel partially because the barrel is no longer smooth and is undulating. 
 

This is a very rare event that requires going from full power to very cold for a long time like an extended high speed descent in cold temperatures. It will never happen by just reducing your throttle.

[kortopates]

Just another point of view. I don’t think the rings are adversely affected by the engine being driven by the prop. During every intake stroke the rings are sealing on the top of the lands. And on every compression, power and exhaust stroke, the rings are sealing on the bottom of the lands. This same paradigm is going on with the prop driving the engine. In both cases the rings are pressed to the top and then the bottom of the lands. So the rings are constantly moving up and down in their lands. What has been documented with shock cooling, is the barrels rapidly cooling unequally with the fin area cooling slower than the non fin area. This can cause uneven wear of the cylinder barrel partially because the barrel is no longer smooth and is undulating. 
 
This is a very rare event that requires going from full power to very cold for a long time like an extended high speed descent in cold temperatures. It will never happen by just reducing your throttle.

I like your logic on it shouldn’t be an issue with just the prop at idle. Your second point about it only happens when the aircraft gets very cold in a long descent matches with what we see - that it takes a long descent where the engine is allowed to get very slow. But never associated it shock cooling; possible but not necessarily seeing rapid cooling. But the common denominator is getting the engine very cold in a prolonged descent at high rpm. i assume even piston scuffing can occur at that point; although commonly associated with high rpm startup on a cold engine.


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[N201MKTurbo]

9 minutes ago, kortopates said:

I like your logic on it shouldn’t be an issue with just the prop at idle. Your second point about it only happens when the aircraft gets very cold in a long descent matches with what we see - that it takes a long descent where the engine is allowed to get very slow. But never associated it shock cooling; possible but not necessarily seeing rapid cooling. But the common denominator is getting the engine very cold in a prolonged descent at high rpm. i assume even piston scuffing can occur at that point; although commonly associated with high rpm startup on a cold engine.


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These things are very difficult to prove after the fact, how they got that way, and they are rare enough that nobody wants to put the money and effort to get to the root cause.

[Vance Harral]

1 hour ago, kortopates said:

Starting with normal pull to idle, pitch for Vg then observe FPM, next pull the prop back and observe the acceleration and new reduced FPM,
Now with several miles to go to an airport, i’ll simulate the same descent rate by putting gear down and adding just enough power to maintain the observed FPM clean and lean it out (btw, we also compare that to what our calculated FPM should be based on POH glide ratio and Vg)

This is an emergency approach and landing (a.k.a. simulated engine failure), Task IX.B in the commercial pilot ACS.

Emergency descents (Task IX.A) are not flown at Vg, as the purpose is not to travel the farthest distance, but rather to get on the ground as soon as possible; and not necessarily on a runway (think cabin fire).  The ACS requires following AFM/POH procedures, if published.  Those procedures nearly always require closed throttle, full prop, and steep deck angles at relatively high speeds.  I don't have a 252 manual, what does that manual prescribe for an emergency descent?

[Slick Nick]

Don’t be in such a rush to put your airplane in an undesired state just because of some request from ATC. They are there to assist you. Rushing to comply was a mistake. A simple “unable” goes a long way in giving the controller more time to work out a different solution. 

[RescueMunchkin]

10 hours ago, Slick Nick said:

Don’t be in such a rush to put your airplane in an undesired state just because of some request from ATC. They are there to assist you. Rushing to comply was a mistake. A simple “unable” goes a long way in giving the controller more time to work out a different solution. 

I agree with you. Part of it was I took it as a pop up IFR and that is something they could have declined but still granted me, so I wanted to minimize their workload caused by me.

This thread has helped open my eyes a little more, especially now being aware I could ask if I could possibly gain some altitude first to slow down and dump the gear for a much faster rate of descent immediately following.

[wood_fly]

Do your best for what you are comfortable with. I agree with the procedure to reduce MP, increase rpm, level off, drop gear and resume. I once had ATC tell me to make a crossing restriction late. I did the math and found the descent rate unacceptable for my pax (had I been empty in the jet, I might have tried). Upon my reply of unable, ATC comes back with "It's only 4,000 feet per min." I let that sink in on the freq for a minute. Response came back from the ATC supervisor, "Do your best for the crossing restriction, contact next controller on X". The controller didn't want to look bad for the next sector in sending in an airplane above his gate and provided a later clearance and his super recognized that was not a NORMAL rate of descent. The word UNABLE is golden. 

[1980Mooney]

18 hours ago, hazek said:

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

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

 

Works perfect. 

[kortopates]

11 hours ago, Vance Harral said:

This is an emergency approach and landing (a.k.a. simulated engine failure), Task IX.B in the commercial pilot ACS.

Emergency descents (Task IX.A) are not flown at Vg, as the purpose is not to travel the farthest distance, but rather to get on the ground as soon as possible; and not necessarily on a runway (think cabin fire).  The ACS requires following AFM/POH procedures, if published.  Those procedures nearly always require closed throttle, full prop, and steep deck angles at relatively high speeds.  I don't have a 252 manual, what does that manual prescribe for an emergency descent?

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. Especially on a 4 cyl engine. So instead I used engine out glide to an airport followed by the power off landing example because that can actually get the engine cold and can lead to go full power with a cold engine which is not great for the engine. 

Maybe a bit of a tangent but the ACS emergency descent in the Mooney isn't that bad as it may seem in some other fixed gear aircraft where you might pitch very steeply since we have the gear. The later Mooney POH's that go into detail on the emergency descent procedure say gear down at max gear extended speed gives about the same rate of descent at gear up with a much steeper descent angle and faster speed. So we always do them gear down (with the required 30-45 deg bank) which limits us to Vle; which is very docile in the Mooney.  (And I've never done this, but the Airplane Flying Handbook says if in Turbulent conditions not to exceed Va.) So because the emergency descent in a Mooney generally doesn't get the engine very cold I instead described how I do maneuver for  IX.B did because IMO the simulated engine out glide should include some engine management adjustments that don't detract from the training value. I do them all the time. Sorry for the confusion. 

[kortopates]

1 hour ago, wood_fly said:

I agree with the procedure to reduce MP, increase rpm, level off, drop gear and resume.

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.

[PeteMc]

20 hours ago, RescueMunchkin said:

Being told by ATC to expedite descent while already in yellow arc and >500fpm and about to enter a cloud

The aviation gods created "unable" for that exact situation. 

[201er]

4 minutes ago, PeteMc said:

The aviation gods created "unable" for that exact situation. 

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

[wood_fly]

2 hours ago, 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.

Fair enough and I agree with your analysis using the charts. I was referring to pulling the power back to a point where the airspeed is moreso driving the prop than the other way around- which I also understand is WAY back and goes back to his original worry about shock cooling the engine - which I am less concerned with in a normally aspirated model. Really good case in point is Captain OGG flying a Boeing 337  10/16/56. Lost prop control so it went flat pitch (4 engine airplane) effectively creating the drag of a disc the same diameter of the propeller - while enroute HNL-SFO. Navigator calculates they aren't going to make it with the available fuel. It was dark, so they found a Coast Guard cutter, circled until the sun came up and put it in the ocean. A few minor injuries, but everyone was saved. The Maui VOR and Airport carry his name in honor of the event (PHOG/OGG).