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

[RescueMunchkin]

I fly an F model and had a flight where I was about to start my descent from 10.5k and OAT was around 0C at the altitude, guessing my MP was around 19.5".

Had to get a pop up IFR because of a broken to overcast layer at approx 6k ft, but was also told to expedite my descent at the point where I'm trying to keep MP at 18" for an easy let down and already halfway into the yellow speed arc.

What's the best procedure in this case?  I assume we don't want to go into a cloud while the IAS is in the yellow arc, but pulling power quickly could also cause engine damage?  I know the board is split about shock cooling being a myth, but this is how I learned complex planes and I would rather do an easy letdown than be faced with a $60k engine overhaul and be grounded during it.

[N201MKTurbo]

Shock cooling is a myth.

If the OAT was 0 at 10.5 then it should be 8C at 6k. you should be OK for ice. Or were you worried about the bumps? You could always slow to gear speed and put the wheels down. Or do a slip.

[RescueMunchkin]

18 minutes ago, N201MKTurbo said:

Or were you worried about the bumps?

Yes - going into potentially turbulent conditions (cloud) while in the yellow arc.

Couldn't slow because I was told to expedite my descent. Is a slip safe while in the yellow arc? And my gear down speed is 120mph, yellow arc is 175mph-200VNE so I was in the mid 180mph IAS at this point.

[N201MKTurbo]

Yes, you need to slow down. 

There is expedited descent in a Mooney, and there is expedited descent in other planes.

When ATC tells me that, I give it my best effort. If they tell me to increase my descent, I tell them “unable”.

If you want to get down in a hurry, don’t push the nose over, power back to 14-15”, wait till you get down to gear speed, this is where you would slip to slow down. Put the gear down and keep your speed at Vle. You will get down fast and be good for the bumps. Once you get down, you can put the gear up and speed up if you have a way to go.

[TGreen]

I'd pull power. Even if shock cooling were a real concern, would a single instance of accelerated cooling really cause harm? Also, adjusting mixture ought to keep CHTs in the green even at low power settings.

[RescueMunchkin]

I guess the main thing was that I was already in a 500fpm descent and in the yellow arc at the point I was told to expedite it. To slow down at that point, I would have to level off or gain altitude, which I thought could get me a phone number to call.

What I did was to ask ATC for a 360 while IFR :facepalm, asked why, told him I'm going too fast to go into the clouds right in front of me, then got a heading vector instead.

[N201MKTurbo]

You can also roll you RPMs down to the bottom of the green, but you were carrying too much power.

[RescueMunchkin]

3 minutes ago, N201MKTurbo said:

You can also roll you RPMs down to the bottom of the green, but you were carrying too much power.

Isn't reducing rpm effectively similar to the engine temps as reducing MP? and isn't the smaller pitch on the prop (higher RPM) better for drag purposes?

[N201MKTurbo]

That is true if the engine is dead, but you are still making power. In your situation, you need less thrust.

[N201MKTurbo]

I owned an M20F for 19 years and flew it about 4000 hours. I have slipped it in the clouds, but you should be very comfortable with your plane and doing slips. When you slip in the clouds, be right on heading before the slip, shove the rudder to the floor and note your new heading. Maintain that heading with aileron while the rudder is down. I’ve never done it with an autopilot, I would disengage the autopilot while slipping. I have never slipped more than 30 seconds or so.

[PeteMc]

2 hours ago, RescueMunchkin said:

I assume we don't want to go into a cloud while the IAS is in the yellow arc, but pulling power quickly could also cause engine damage?  I know the board is split about shock cooling being a myth, but this is how I learned complex planes and I would rather do an easy letdown than be faced with a $60k engine overhaul and be grounded during it.

I do believe shock cooling is a myth, otherwise shock heating would destroy the engine every time we take off.  But even so, I also agree with you that if you don't have to suddenly pull the throttle out all at once, it's better for the engine as it is a mechanical thing with lots of moving parts.   But if you're going to fly IFR, you're going to have to accept sudden descents and the need to make a large power reduction. 

[RescueMunchkin]

37 minutes ago, N201MKTurbo said:

I owned an M20F for 19 years and flew it about 4000 hours. I have slipped it in the clouds, but you should be very comfortable with your plane and doing slips. When you slip in the clouds, be right on heading before the slip, shove the rudder to the floor and note your new heading. Maintain that heading with aileron while the rudder is down. I’ve never done it with an autopilot, I would disengage the autopilot while slipping. I have never slipped more than 30 seconds or so.

I'm very comfortable with slipping my plane. Use it all the time for short approach landings, but it's not something that I ever thought of doing to slow down outside of approaches from TPA.

[kortopates]

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.

[N201MKTurbo]

7 minutes 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 agree completely about slipping. It is not a normal maneuver while IMC. It’s one of these skills that your good judgement precludes its use. 

[1980Mooney]

38 minutes ago, N201MKTurbo said:

I owned an M20F for 19 years and flew it about 4000 hours. I have slipped it in the clouds, but you should be very comfortable with your plane and doing slips. When you slip in the clouds, be right on heading before the slip, shove the rudder to the floor and note your new heading. Maintain that heading with aileron while the rudder is down. I’ve never done it with an autopilot, I would disengage the autopilot while slipping. I have never slipped more than 30 seconds or so.

Personally I would be cautious in this case about advising to turn off the autopilot in the clouds and to slip in the clouds.  @RescueMunchkin posted in late 2023 that he was looking for his first plane so it sounds like he is a fairly low time/low experience pilot.  He is asking for advice on how to fly in his plane in clouds in a stressful environment where ATC is telling to expedite faster than he is used to or comfortable.  It is just one more possible form of stress or possible disorientation.  

2 hours ago, RescueMunchkin said:

I fly an F model and had a flight where I was about to start my descent from 10.5k and OAT was around 0C at the altitude, guessing my MP was around 19.5".

What's the best procedure in this case?  I assume we don't want to go into a cloud while the IAS is in the yellow arc, but pulling power quickly could also cause engine damage?  I know the board is split about shock cooling being a myth, but this is how I learned complex planes and I would rather do an easy letdown than be faced with a $60k engine overhaul and be grounded during it.

 

51 minutes ago, RescueMunchkin said:

Isn't reducing rpm effectively similar to the engine temps as reducing MP? and isn't the smaller pitch on the prop (higher RPM) better for drag purposes?

I agree with @PeteMc.  There is way too much concern about shock cooling, pulling throttle and coasting against the engine and also too much concern in other posts about advancing the throttle too fast.  Your engine and prop are overdesigned to handle stresses of sudden changes at SL making 200 HP.  You said you are at 10.5 and 19.5 MP.  You didn't say your RPM but even if you were wide open at 2,700 rpm you would only be making 120 HP (60%).  

@kortopates has lots of good advice to stay ahead of your plane when in busy ATC conditions.  But if you need to pull the throttle, DO IT and don't obsess about hurting your engine or prop.

[kortopates]

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.


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[dkkim73]

I think the slip is a red herring. Zero interest in anything but coordinated flight at low bank angles in IMC. 

I got stuck high and fast a couple times before I realized the gear are my friend in this plane. They've always been a source of support (sorry). But, more seriously, in my plane Vlo (lowering) is about 140 KIAS and Vle is above 160KIAS, so they're great brakes. If you have speed brakes, use those obviously. 

A trick from John Deakin is to pull MAP (ie. throttle back without fear) and move mixture all the way to the rich-of-peak side to keep CHTs up. If cylinders are the concern. If I moderate the descent with gear and anticipation I may keep it lean until near the end. 
I'm in the shock-cooling-is-overrated camp, further because my plane isn't very susceptible to rapid cooling by cowl design. My personal take is that's the least of the many concerns. 

2 cents only

 

[dkkim73]

4 minutes ago, 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.

Voice of experience there. Don't get guilted into doing things hurriedly. You can always ask for a delay vector. Usually when I am real polite and helpful and say "if it helps, can you box me around? or I can hold at RANDO and lose some altitude" they get the hint. 

D

[Hank]

I've got almost 18 years in my C, 15 since getting serious about IFR. Slipping in the clouds is nit something that I would either try or recommend. Reduce power (throttle and RPM), hold speed steady and you will descend; slow down by raising the nose a bit and you will descend faster. The slower you go, the faster you'll descend, just keep well above stall speed.

Adding drag (flaps and gear) require lower speeds, but will also increase your descent rate.

Practice first! Get good at making these changes, write down what you do and the results, then practice again. When you're comfortable doing this, then you're ready for that next ATC request. 

[hazek]

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

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

 

[RescueMunchkin]

3 minutes ago, hazek said:

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

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

 

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

[201er]

3 hours ago, RescueMunchkin said:

I fly an F model and had a flight where I was about to start my descent from 10.5k and OAT was around 0C at the altitude, guessing my MP was around 19.5".

Had to get a pop up IFR because of a broken to overcast layer at approx 6k ft, but was also told to expedite my descent at the point where I'm trying to keep MP at 18" for an easy let down and already halfway into the yellow speed arc.

What's the best procedure in this case?  I assume we don't want to go into a cloud while the IAS is in the yellow arc, but pulling power quickly could also cause engine damage?  I know the board is split about shock cooling being a myth, but this is how I learned complex planes and I would rather do an easy letdown than be faced with a $60k engine overhaul and be grounded during it.

So, you think that flying through clouds or rain is fine but that reducing power will harm the engine?

Pretty sure the formation flying throttle jockeys have long proven that nothing's gonna happen from rapid throttle changes.

[RescueMunchkin]

1 hour ago, 1980Mooney said:

Personally I would be cautious in this case about advising to turn off the autopilot in the clouds and to slip in the clouds.

I agree with @PeteMc.  There is way too much concern about shock cooling, pulling throttle and coasting against the engine and also too much concern in other posts about advancing the throttle too fast.  Your engine and prop are overdesigned to handle stresses of sudden changes at SL making 200 HP.  You said you are at 10.5 and 19.5 MP.  You didn't say your RPM but even if you were wide open at 2,700 rpm you would only be making 120 HP (60%).  

 

I wasnt in the cloud yet but wanted to get back in the green arc before entering it.

At this point I was around 6500ft and already had been pulling power for my descent and 30 seconds away from entering the cloud.  My cruise and descent is 2400 rpm.

[N201MKTurbo]

Just for clarity, I have only slipped in the clouds twice. Once was on my IFR check ride when the DPE said “what was your crossing altitude at XXX?” I was 1 mile from the waypoint and 1000 feet high. I made it. He said he didn’t think a Mooney could do that. He let the E-ticket ride through and I passed. That time was under the hood actually. Then I did it once in actual a year or so later for the same reason. The last time I did it was 35 years ago.

It is one of those tools in the dusty bottom of the toolbox I haven’t used in a very long time.

[Joshua Blackh4t]

Not an IFR pilot here, but in my VFR experience, I am wary of the yellow arc.

Not that I think the wings will fall off, but its a dead end to descending quickly. Too many times I have hit turbulence while trying to squeeze down a hole in a cloud, and then had the same situation as the OP.

So if there is any chance you need a steep descent, start early to slow down (also less airflow = less shock) and don't be afraid to put wheels out, and remove later if needed.

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

It just feels like a problem I used to get a lot, but now don't anymore. I think I stopped wanting every second of speed and concentrated on more of a smooth procedure.