Avoid shock cooling on decent

[Tommy]

Hello everyone!

 

What do you guys do to keep CHT up on long decent? I tried keeping MAP up (23") but TAS can get close to 190 knots (!) sometimes (750fpm) and the turbulence as a result is a little uncomfortable for my pax!

 

(I have a 1981 M20J)

[Ned Gravel]

Did I not hear from some esteemed members on this forum that shock cooling is really an old-wives'-tale?  If I don't drop my MP down to 20-21" on descent, my E model will quickly go through the redline @ 500 fpm descent.    For getting out of the yellow arc, I have to go down to 17-18".

[ArtVandelay]

Hello everyone!

What do you guys do to keep CHT up on long decent? I tried keeping MAP up (23") but TAS can get close to 190 knots (!) sometimes (750fpm) and the turbulence as a result is a little uncomfortable for my pax!

(I have a 1981 M20J)

I have a 78 J, occasionally my JPI warns me when I exceed the 50 degrees per min threshold, I think the max I've seen is 60, but I don't worry about it, if I got to a 90 I would.

[Jim Peace]

Hello everyone!

 

What do you guys do to keep CHT up on long decent? I tried keeping MAP up (23") but TAS can get close to 190 knots (!) sometimes (750fpm) and the turbulence as a result is a little uncomfortable for my pax!

 

(I have a 1981 M20J)

How far out are you starting down?

How do you compute your TOD?

I plan no more then 500 fpm. maybe reduce power about an inch or so......

[Marauder]

I typically will keep the manifold pressure at 20" throughout the descent and then drop it to 17" to get to flap speed in my F.

As for the temps, I really don't worry about them as long as I am keeping some power on.

[Hank]

My C isn't as slick as your J. I push for 500 fpm and trim to hold it. Then every couple of thousand feet I'll pull throttle back and push mixture forward to keep MP and EGT at cruise values. That generally puts me right sat the edge of the yellow. If it's too bouncy, I'll just pull a little more throttle or trim out ~100 fpm to slow down.

[Bob - S50]

I have a '78 J and use 20", 2200 RPM, and 500 FPM for the descent.  If WOT does not give me 20" then I leave it wide open until it rises to 20" then I keep pulling it back to keep it at 20".  I don't touch the mixture.  I rarely see cooling rates in excess 15 degrees.

 

I plan on 6 miles to lose 1000' so a descent from 8000' to SL I'll start down 48 miles out.

 

I find this generally keeps me well within the green.

 

If I need a steeper descent but still want to use 500 FPM for my passengers, I'll use 17" and 2000 RPM.  Then I plan on 5 miles per 1000'.

 

The lower RPM moves the peak pressure closer to TDC which tends to heat things up to offset the lower power setting and higher speed.

 

Once I level off I continue to use 20" and 2200 RPM and still don't touch the mixture (assuming I'm approaching the pattern).  This usually gives me about 135 KTAS.  As I approach the 45 to downwind I pull it back to 15-17" and 2200 which slows me to about 105 KTAS in anticipation of lowering the gear.

 

I never get a shock cooling warning from my JPI.

 

We all have our own techniques, you'll find those that work for you.

 

Bob

[Jerry 5TJ]

So, on a related issue -- what happens when you fly into a rain shower?  Has anyone recorded the shock cooling when the rain drops slash onto the cylinder fins?

 

[BorealOne]

So, on a related issue -- what happens when you fly into a rain shower?  Has anyone recorded the shock cooling when the rain drops slash onto the cylinder fins?

 

Or in icing? 

[Awful_Charlie]

So, on a related issue -- what happens when you fly into a rain shower?  Has anyone recorded the shock cooling when the rain drops slash onto the cylinder fins?

 

Yes, I've done it many a time. Biggest number I have recorded from the JPI data is not even as much as 20F/min (reasonably enough on the front cylinders), and that is into some seriously heavy stuff

[N601RX]

My jpi is set to alert at 50 deg minute and the only time it has ever alerted is when doing circuits at a short airport. Basically climbing to pattern altitude at Vy and then immediately nosing over and starting a descent to landing.

[donkaye]

My bird is the '61 M20B. 

 

My instructor advised me to reduce throttle approximately 1" of MP every two minutes.

 

Sorry, but that is absolutely ridiculous and increases pilot workload unnecessarily.  Think about it.  CHTs at cruise even in the worst case shouldn't be above 380°F.  At idle they are about 250°F  That is 130°F differential.  At the recommended maximum cooling rate of 50° per minute, that is about 2.5 minutes.  So you could keep power up significantly longer.  When shooting an ILS into San Jose with lots of airliner traffic, I keep 160 knots until the outer marker and am able to slow to 75 knots at touchdown, a slowdowns distance of 5 miles.  I'm on my second engine now.  The first one went 2295 hours before I preemptively swapped it for a reman.  I have 1100 hour on the reman now.  I recommend rethinking your procedure and if possible taking the APS engine management course, one of the best courses I have ever taken.

[larryb]

I don't believe in shock cooling, but I have read that you do want to avoid using the engine as a big air-brake. You can keep the engine producing thrust by keeping MP >= 16"

 

I budget 5 miles / 1000' for descent. Lets say I'm cruising along at 12,500', LOP, and landing at sea level. I'll start down around 60 miles out, and leave it at full throttle. I'm probably around 7.5 GPH at this point. I'll watch fuel flow and as it increases during the descent I'll back off the throttle to keep the flow around 8.5 GPH, which is out of the red zone. I leave the mixture at the same setting I had during cruise.

 

When I put the gear down I'll push in the mixture and prop. I know some keep mixture lean until landing, but I prefer to have one less thing to bother with if I have to do a go around.

 

Even if I have to down fast, I've never seen my JPI warn that I was cooling too fast. When coming into Truckee, once I pass the mountains I put the gear down and throtle at 16" and come down at 1000 fpm.

 

Larry

[Hector]

I used to worry about shock cooling. I think during PPL training (25 years ago) it was ingrained in me as something that needed to be managed with great care. Not so. In the Mooney I'll start my descent at 20/2300 but will adjust the MP as needed if I need lower quicker. I prefer long gradual descents, but that is not always possible. I've had ATC ask me lower without delay, yet I never seen the JPI warn me about shock cooling. I keep it in mind and try for gradual power changes whenever possible but I don't have any hard rules to go by.

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[jetdriven]

I keep full throttle, 2500 RPM, and use 4 NM/1000' to lose. Enjoy those 185 knot groundspeeds in the descent, it goes a long way to offset the 130 knots in the climb.  get to pattern altitude ~4 NM from the airport at 180 KIAS, set power to ~22", wait a bit, then set 17".  Gear and flap speed are not a problem, it slows down fine.

[aaronk25]

Shock cooing is B.S. Think about the rate of CHT increase when going from idle to take off power.

Only thing I think a person could do is when at or below 12" MP in decent the prop starts driving the engine and it's been rumored that ring fluter can cause damage....not sure on that one though.....

Any power change as long as gradual should be fine....

I'm not a mechanic, but think about what students do to training engines and they seem to last 3,500 hours......

[1964-M20E]

When I begin my decent I just nose over slightly for 500FPM leave the throttle alone until the MP rises to 23" to 24" then start reducing the throttle to maintain 22" to 24" until I need to slow for the approach.

[donkaye]

Shock cooing is B.S. Think about the rate of CHT increase when going from idle to take off power.

Only thing I think a person could do is when at or below 12" MP in decent the prop starts driving the engine and it's been rumored that ring fluter can cause damage....not sure on that one though.....

Any power change as long as gradual should be fine....

I'm not a mechanic, but think about what students do to training engines and they seem to last 3,500 hours......

 

As was mentioned earlier, it is best not to go below 15"MP in descent to keep the engine driving the prop.  Reducing prop RPM is also a power reduction (100 RPM is about = to 1" MP is about = to 3% power), so reduce prop RPM to anywhere outside of a prohibited area of prop operation as part of a power reduction.  I will typically stay high for as long as possible to get both the benefit of higher true airspeed and often a smoother ride.  While I prefer a 500 ft/min descent rate for passenger comfort, when at 17,000 feet I am not going to start down 34 minutes out.  When I do start a descent I will still go as fast as both turbulence and airspeed limitation will allow.  When it is time to slow down I will gradually bring the MP down to 20", then the RPM down to 2,000, then the MP down to 15".  Gear, speed brakes, flaps, S-turns are still available tools if needed, but should only be needed for descents from very high terrain to low terrain that occurs within a short period of time such as the Sierras into Bakersfield for example.

[Releew]

I too agree that shock cooling is a myth.  Aircraft engines are designed with the necessary tolerances to handle rapidly increasing and or decreasing temperatures.  The rain example proves that..... Having said that, there is no reason I see to ever yank the power to make a quick decent.  Do the math and plan on a 500'/minute decent.  Keep the airspeed below the caution range.  If you're in calm air why not take advantage of the decent speed.  A Mooney just takes additional flight planning but is not a difficult aircraft to control.  I hear fellow pilots complaining how these airplanes are too difficult to slow down. I disagree......plan it, don't force it and the numbers will work out.  

 

Rick

[jetdriven]

I don't get what's so magic about 500 FPM and staying out of the yellow arc.

[Mooneymite]

I don't get what's so magic about 500 FPM and staying out of the yellow arc.

It's all part of the flying "mystique".

Are you trying to make flying "common"?

Sheesh. :-)

[aaronk25]

If I'm by myself, leave cruise power on and adjust decent to stay under Vne...weeeeeee

If the wife is with, 500-700fpm....BORING.....

[Ned Gravel]

I don't get what's so magic about 500 FPM and staying out of the yellow arc.

 

Byron:

 

I don't mind staying in the yellow arc, except when it gets bumpy.  So normal descent for me is around 20-21" MP and 2500 RPM to stay off the red line.  If it gets bumpy and I have to start worrying about Va or Vo, then I have to slow things down.

 

The thing about 500 fpm at those settings is that any greater descent rate will push the ASI past those limits (Vne, Va, or Vo) in my aircraft.  

 

I know, I know.  I fly conservatively.  

[FloridaMan]

I was taught in my initial checkout to not go below 18". Another instructor/DPE suggesting that going to idle below 10,000 ft and diving on a naturally aspirated IO360 shouldn't hurt a thing. I generally follow the most conservative of the two and use the 18" rule and I use it to practice maneuvers to lose altitude/drop through a broken layer. 

 

Keep in mind that your greatest thermal stress on your engine is when you apply full power at takeoff from idle. 

[Tommy]

One plane. Many ways to fly it. 

 

Personally, I have found a good decent is much harder than climb - there is really no art in climb. 

I think next time I might just have to start descent from 100NM away!