Best Methods for Cooling an Engine?

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CherokeeGirl

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CherokeeGirl
I have a PA-32-300 which means that I need to adjust the prop, RPMs and manifold pressure as I'm climbing, cooling before descending, and descending.

I was taught to start by dropping 2 inches every 2 minutes until I get to 20, at which point I can start my descent without risk to damaging the engine.

There are some other calculations on when you need to start this process based on the difference in elevation between your flight level and TPA, and your ground speed that doesn't really factor into my query, but if anyone has a formula, app, or other general guidelines for this this I'd love to hear it.

Really I'm open to anything that will be a better pilot and save me cost down the road.
 
Sounds over complicated to me and maybe your CFI didn’t do a great job. I’d grab another CFI to get this sorted out and you’ll see it’s nowhere near this complicated.
 
eman1200 said:
Sounds over complicated to me and maybe your CFI didn’t do a great job. I’d grab another CFI to get this sorted out and you’ll see it’s nowhere near this complicated.
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If it's not that complicated to cool an engine properly could you share what procedure you employ?
 
The best way is to bring it down with speed brakes, but many planes don’t have them.
 
Why do you have to cool before you descend? Plan your descents properly so the power reduction is less. I have never heard of the method you were taught.
 
I'm of the firm belief that shock cooling is a completely bogus made up myth.

Do you have concerns over shock heating? When you start the engine and take off do you wait 2 minutes between each inch of manifold pressure?

..
There's certainly is a merit to the idea of different metals expanding at different rates and potential damage to an engine with sudden changes in temperature. However the tolerances on the engines we fly behind are hot-dog-in-hallway status.. to me this is like worrying about accidentally going over 250 knots under 10,000 ft in the planes we
..

Read this: https://www.avweb.com/ownership/shock-cooling-time-to-kill-the-myth/
 
It’s definitely not that complicated. Just try to avoid high RPM and low MP (~15in or less) during the descent. I like to do 500fpm and keep the mixture about where I had it during cruise and enrichen accordingly as I come down.
 
Tantalum said:
I'm of the firm belief that shock cooling is a completely bogus made up myth.

Do you have concerns over shock heating? When you start the engine and take off do you wait 2 minutes between each inch of manifold pressure?

..
There's certainly is a merit to the idea of different metals expanding at different rates and potential damage to an engine with sudden changes in temperature. However the tolerances on the engines we fly behind are hot-dog-in-hallway status.. to me this is like worrying about accidentally going over 250 knots under 10,000 ft in the planes we
..

Read this: https://www.avweb.com/ownership/shock-cooling-time-to-kill-the-myth/
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I asked about the rapid heating when you start the engine but couldn't get a straight answer and my AFM doesn't cover any of these "home grown" solutions. I'm just doing what I was taught, hence me asking here for other options.

Thanks for the link. I enjoy AVWeb's content.
 
I’d have to agree, what you were taught is a bit extreme. When descending just modest reductions not major ones. To time it perfectly is not necessary.
 
CherokeeGirl said:
I asked about the rapid heating when you start the engine but couldn't get a straight answer and my AFM doesn't cover any of these "home grown" solutions. I'm just doing what I was taught, hence me asking here for other options.

Thanks for the link. I enjoy AVWeb's content.
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I just realized you are new here, welcome! And my response was on the exuberant side.. pardon. I just get a little triggered by aviation myths that are past down with no hard evidence. Many of our AFM/POH were written a while ago.. I also find it odd that when people practice engine out procedures, especially in twins, suddenly there's no concern about shock cooling

Anyway.. like the other said, I don't think you can overthink it or break the whole thing down into a pure numbers game. Just plan your approach in advance and as you descend gently ease the manifold pressure back. Once you're under 20 inches you're basically "safe" .. I cruise 2200 to 2400 rpm and leave that alone until I'm in pattern/past FAF. I also don't generally go under 14-16 inches unless I really have to or I'm about to land
 
CherokeeGirl said:
I have a PA-32-300 which means that I need to adjust the prop, RPMs and manifold pressure as I'm climbing, cooling before descending, and descending.

I was taught to start by dropping 2 inches every 2 minutes until I get to 20, at which point I can start my descent without risk to damaging the engine.

There are some other calculations on when you need to start this process based on the difference in elevation between your flight level and TPA, and your ground speed that doesn't really factor into my query, but if anyone has a formula, app, or other general guidelines for this this I'd love to hear it.

Really I'm open to anything that will be a better pilot and save me cost down the road.
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I’ve heard the 2” every 2 min or 1” per min. I try to plan my descents, pattern entry and/or approaches to comply roughly with this until 18” or below. It’s not a hard rule with stop watches but I FEEL like it’s not a bad practice. Some don’t like Mike Busch, I do mostly what he recommends. You can read his stuff on AvWeb, AOPA, Savvy, etc and decide for yourself.

My theory is if I operate like this MAYBE I’ll get some more TBO on my engines. If I’m wrong, it’ll be the same. Those who don’t try MAY have to o/h more frequently. You have nothing to lose and slower power reductions will make your passengers notice(potentially feel uneasy) less.
 
Normally aspirated engine you aren't going to do any damage with "shock" heating or cooling. You have a constant speed prop. Adjust to cruise RPM when you get to 500 to a thousand feet in your climb, and leave it there until you get ready to go around on a botched landing. No need to touch it during descent and no need to worry about reducing manifold pressure 2 in per whatever time period. Just start to reduce it when it gets up over twenty-five or so. Use the trim to make your descent and gain the speed back that you lost in the climb. You and your CFI are making this way more difficult than necessary. I'm guessing your CFI is teaching you this because that's what his CFI taught him and he never really dug into anything.

Now if we are talkin a turbo engine that's a whole different kettle of fish.
 
I think I may have similar info to folks above.

The main difference how I think about it is that I decouple top of descent and power reduction/slow down. I’ll typically start my descent with cruise power in. I only reduce power before my descent if I’m low. As an example, I recently flew home (eastbound with a brilliant tailwind) at 15500. My ToD was about ~110nm out, and I didn’t start reducing power until I was about ~60nm out.

My formula for top of descent targets 500fpm and is basically cruise alt - pattern alt / 500 fpm = minutes required for descent. Between GPS and foreflight I can figure out where that is. I’ll add a buffer since I accelerate in the descent.

I’m not entirely sure I have a specific formula for power reduction. When I’m low I’ll just “feel” like I’m going too fast and start pulling power. On a long descent I have gates where I try to achieve specific MP. 15” on downwind means I want 18” ten miles out. A lot more factors here like IAS, ground speed, how good brakes are (I.e. landing gear extension, flaps, etc). Overall I try to be smooth.

I’m not sure I fully subscribe to shock cooling, but I do believe power changes create additional strain/pressure on engines and therefore apply similar power reduction strategies as noted above. I’ve heard 1”/min and 50F/min as well. I don’t have equipment to precisely measure this but it’s a good general ideal. A big pull or chop of power would be the thing to avoid. If I need added cooling, I’ll either introduce mixture earlier, or steepen my descent (more airflow).

For the prop, I’ll leave it set at the cruise setting until I take it off of the governors on final (except for Cirrus which just does whatever).

*disclaimer - I’m still a renter, but I believe in leaving things better off than how you found them.
 
Why don't you just watch the CHT and adjust to whatever your goal? You may very well be able to nose dive your airfield at full power and still be cooling the engine off? If Temps are the goal, watch the temperature gauges.
 
If VFR I know my descent point before taking off. I plan for an easy 400-500’ descent with reduction in power the whole way down.
 
Having a constant speed prop & MP gauge leads to all manner of silliness. I'm betting they also taught you not to run the MP higher than the RPM in hundreds?

I'm not a believer in shock cooling. Even pulling power from WOT to 15" doesn't cool the engine faster than 50 deg/min which is the lycoming recommendation. By the way I'm running the same engine & cowling you have, but with bendy legs...pa-32r-300.

My normal procedure, like many here have said, is to recover some of the energy I spent climbing by leaving cruise power in and trimming to a higher airspeed to get my descent. If it's bumpy I do have to reduce power to stay out of the yellow arc. I use the TLAR (that looks about right) method to reduce power, usually from 25" or so to about 18, which gives me a nice pattern speed. Abeam the numbers i bring it back to 15.5 and drop the gear & start putting in flaps. I've never seen the "shock cooling" warning on my JPI 930 engine monitor pop up...it's set to alarm at the 50deg./ minute lycoming recommendation.

On my io-540 I actually have a harder time getting the engine hot enough. I think those big nostrils might be a bit oversized. Its a good idea to keep the CHT over 300 to prevent lead deposits on the exhaust valves, but it's hard to do, at least in my plane. The fact that idle CHT and cruise CHT are so close is another reason I don't worry about shock cooling.
 
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I learned 1" per minute power reduction, but that was back when shock-cooling was still the thing. Also works for turbocharged engines.
The idea is, to me, (concerns about cooling one way or another, put aside) to have a smooth, relatively consistent power reduction during descent, so as to end up at the desired speed and (lower) mp setting just prior to pattern entry, say a mile or two out from downwind. Top-of-descent (for VFR, without ATC directive), typically I start down and leave power/prop/mixture at cruise setting for awhile. Of course, you want to be aware of your airspeed, but with a fixed-gear bird, and un-pressurized 500 fpm descent, it's unlikely that you'd get into the yellow. Then, at appropriate time/distance from destination, I start mp reduction, using a 'rough' rule of thumb 1"/minute. Not necessary to use a timer at all, so it might be 1" every 40 seconds, or 1" every 2 minutes, whatever. The idea is to go from cruise power to pattern power, smoothly. With normally aspirated engines, this also compensates for increase in mp during descent. Mixture may (or may not) be richened a little at a time too, depending on actual altitude change of each flight. Prop doesn't come forward until short final, or, with some planes, landing is made with prop still at cruise (yes, one more thing to do during a go-around, so be it).
Long story, but, bottom line, no need to over-think with formulas, etc. After flying for awhile, you just get a feel for it, without consciously thinking too much about the timing involved. You're just trying to make a smooth transition, thinking ahead of the plane, the exact method is maybe not so important, so you don't have to pull from 26" to16" all of a sudden, as you level off at pattern altitude, simultaneously trying to lose 30 knots of speed.
Hope this, along with everyone else's good comments, helps.
 
If "shock cooling" was that big of a thing in the real world, no Cessna 206, commonly used as a "jump plane", would ever make it to TBO.

Just sayin'.
 
EdFred said:
Normally aspirated engine you aren't going to do any damage with "shock" heating or cooling. You have a constant speed prop. Adjust to cruise RPM when you get to 500 to a thousand feet in your climb, and leave it there until you get ready to go around on a botched landing. No need to touch it during descent and no need to worry about reducing manifold pressure 2 in per whatever time period. Just start to reduce it when it gets up over twenty-five or so. Use the trim to make your descent and gain the speed back that you lost in the climb.
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Pretty much what I do. Cruise in the Mooney at altitude is full throttle and 2500rpm. As Ed says, I adjust trim for 500fpm down when leaving cruise altitude and leave cruise power just as it is. The Mooney comes downhill at about 180kts, making up time lost in the climb. The MP usually rises to just above 25" as I pass thru 5000, so I pull it to 25" even, then I use the altimeter as my guide. 4500ft, pull to 24.5", 4000 24", 3500 23.5" 3000 23". Works for me and is simple to do. After that I don't worry too much about the pulls as the next one is usually to 20" about five miles out, and then finally 15" as I enter the pattern. Once on downwind and on speed (90-100kts), bring the prop to full.

Rgbeard said:
If "shock cooling" was that big of a thing in the real world, no Cessna 206, commonly used as a "jump plane", would ever make it to TBO.
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Nor would any trainer like a 172, getting it's power jacked all over the place as one does T&Gs in the pattern, yet most trainers make TBO and beyond.
 
IMO, determined from reading lots of documentation and threads like this is that if the CHT's are under about 380 you can't really hurt the engine by cooling to fast (shock cooling). If you have the CHT's near redline often 500 degrees and chop the power then perhaps Shock cooling is an issue. Ideally your engine will likely last the longest if you can run your CHT's under 380 most of the time, but some engines you might need to run as hot as 420 degrees. Higher compression engines seem to have more issue with Hot CHT's. If you a running hotter than 380 it could be wise to cool it down slowly. From cruise just the increase speed of the descent might be enough to do this or you can reduce power, richen the mixture, open the cowl flaps or all of the above to get it below 380 or less before you reduce power for your landing descent.

If you want to play it safer see if you can slowly bring it down to 320-350 before reducing power for landing.

BTW, those 206's used for skydiving, usually have a couple minutes of cool down at medium power while the skydivers are jumping before going power off or near power off for the descent. Most trainers get a minute or two of cool down on the downwind and any more few are doing full power off approaches regularly

I have seen some antidotal evidence from one soaring club that cracked cylinders were being caused by shock cooling. Glider tow planes don't get the cool down time that trainers and skydiving plane get unless the pilots intentional do so. Changing the clubs towing procedures seemed to reduce the seemly regular occurrence of cracked cylinders.

Just my opinion.

Brian
CFIIG/ASEL
 
CherokeeGirl said:
I have a PA-32-300 which means that I need to adjust the prop, RPMs and manifold pressure as I'm climbing, cooling before descending, and descending.

I was taught to start by dropping 2 inches every 2 minutes until I get to 20, at which point I can start my descent without risk to damaging the engine.

There are some other calculations on when you need to start this process based on the difference in elevation between your flight level and TPA, and your ground speed that doesn't really factor into my query, but if anyone has a formula, app, or other general guidelines for this this I'd love to hear it.

Really I'm open to anything that will be a better pilot and save me cost down the road.
Click to expand...
Why are you adjusting RPM on descent, or did I read that wrong? I have yet to fly an airplane that needed to adjust the prop for descent.

For a non-turbo high performance airplane, a good rule of thumb that works in most airplanes is to simply maintain cruise manifold pressure during the descent. As you descend, the MP is naturally going to increase. Just keep easing it back as you descend and you'll be just fine. IOW, if you are up at altitude in a normally aspirated piston airplane, you are probably cruising around 22-23". If you simply adjust the throttle to maintain 22" throughout the descent, you'll end up at pattern altitude or on vectors for an approach a...22" You won't require any major power changes and will have a nice steady descent profile. Passengers will be happy. engines will be happy. ATC will be happy. You'll be happy. Works beautifully and hard to get any simpler than that.

IF you are flying a turbo or supercharged engine, then your cruise MP settings will naturally be a little higher, so at some point, you'll need to pull the power back a little more, but it is still a gradual process - unless you are in an emergency or doing something like dropping sky divers, there is rarely a need to make significant power changes in descent.
 
Fearless Tower said:
.....simply maintain cruise manifold pressure during the descent. As you descend, the MP is naturally going to increase. Just keep easing it back as you descend and you'll be just fine. IOW, if you are up at altitude in a normally aspirated piston airplane, you are probably cruising around 22-23". If you simply adjust the throttle to maintain 22" throughout the descent, you'll end up at pattern altitude or on vectors for an approach a...22" You won't require any major power changes and will have a nice steady descent profile...
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this.
 
Jim Carpenter said:
I learned 1" per minute power reduction, but that was back when shock-cooling was still the thing. Also works for turbocharged engines.
The idea is, to me, (concerns about cooling one way or another, put aside) to have a smooth, relatively consistent power reduction during descent, so as to end up at the desired speed and (lower) mp setting just prior to pattern entry, say a mile or two out from downwind.
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Excellent post. I do the same. IFR too, to be at my level vectoring setting.
 
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