Avoid shock cooling on decent

[bonal]

I can't remember which one of the gurus on engine management did an article on descents but he indicated there is no reason to richen mixture on decent when power is reduced and that doing so would cause a greater drop in cylinder temp. Most of the flying I do is in hilly or mountain terrain so my descents are often controlled by AGL so my descents are usually at or above 1000fpm and as long as I have smooth air I will be close to the top of the yellow I will adjust power and rpm to keep my prop out of its red line. It's not unusual for me to have to make a 360 or two due to airports that have close proximity to mountains. It's never a problem getting slowed down as long as I stay ahead of the airplane and for me those rapid descents are my favorite stage of the flight. Snoopy loves to go fast.

[Bob - S50]

  On 11/8/2014 at 3:36 PM, daver328 said:

I've been using 20"/23RPM per the MAPA "Fly by the Numbers" for a M20C. We have a JPI EDM-700. Works great!

(I like Bob's 1000' per 6 miles! I'll try that too! I have been using time rathervthan miles. I subtract pattern altitude from cruise altitude, double it, drop the zeros, and that (plus an extra minute or two for deceleration) is how many minutes out I begin our 500 fpm descent. I use the ETE to Dest. Time on the Foreflight HUD to know when to begin our descent.)

Dave,

 

Of course, that is no wind.  I'm assuming my technique will give me about 180 KTAS.  With a 30 K headwind I would use 5 miles/1000'.  With a 30 K tailwind I'd use 7 miles/1000'.  The best judge is to look at my groundspeed.  Since I'm planning on 150 KTAS at cruise, anything different than that gives me a pretty good idea of the headwind/tailwind component.

 

As for slowing, I don't add any extra miles.  A normal glidepath is 300'/mile.  If I'm going straight in, I would want to be at 1000' at about 3.3 miles to be on that glidepath.  Using the 6 miles/1000' I'll get to that altitude 6 miles out.  That gives me 2.7 miles to slow down.  Arrive at 1000' at 6 miles, level off, slow, configure, start down the glidepath.  If I'm going to the normal visual pattern I've got even more time.

 

Bob

[Bob - S50]

  On 11/9/2014 at 12:17 AM, jetdriven said:

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

Nothing magical about 500 FPM.  It is just a nice convenient descent rate for calculation purposes and yet is slow enough that most inexperienced fliers will be able to clear their ears.  If they have problems, it is slow enough that you'll find out about it before they are in serious pain.  If it is just me in the plane or I'm flying with people that I know have no problems with their ears I'm not afraid to dump the nose if I need to.  I had to do that with my wife on the way to SQL in October.

 

I try to stay out of the yellow arc because I cannot predict when I'm going to transition from smooth to bumpy air.  Seems like it is usually about 6000' AGL, or the top of the haze layer if there is one, but I'd prefer to not need to worry about it.

 

Bob

[bonal]

On a good day I can't stay out of the yellow in normal cruise below 6000MSL if I'm WOT. When I am over flat country I try to plan for best rate vs distance and I agree 500fpm makes the math easy.

[Releew]

  On 11/9/2014 at 12:17 AM, jetdriven said:

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

Gravity......

[stevesm20b]

I don't know how you guys stay out of the yellow arc when descending. I'm in the yellow arc when in cruise.

[Bartman]

In my J there are no problems with climbing or cruise cylinder temps, descending, keeping her out of the yellow arc, or shock cooling. Apparently shock cooling is not an issue otherwise we would see problems with cylinders in the older engines, which we don't or at least I am not aware of them. But I can see how it can be a problem with the older fleet trying to descend and keep it out of the yellow arc.

We see cylinder problems with the newer turbocharged fleet staring with the 231, not the older fleet. From what I understand that is due to high temps in climb and/or cruise as well as the fact that these engines are usually run at higher power settings, not shock cooling.

[aaronk25]

Since my mechanic used black rtv silicone and sealed up all the cracks on the front baffles such as around the alternator ram air delete area, between the case and the baffle.....I can't keep the temps above 300 when LOP....so in the decent my temps get down to 220-250....and 280 in cruise.

Its also getting cooler so I think it's time to adjust the cowl flaps tighter.....

[pinerunner]

  On 11/9/2014 at 3:09 PM, bonal said:

I can't remember which one of the gurus on engine management did an article on descents but he indicated there is no reason to richen mixture on decent when power is reduced and that doing so would cause a greater drop in cylinder temp. Most of the flying I do is in hilly or mountain terrain so my descents are often controlled by AGL so my descents are usually at or above 1000fpm and as long as I have smooth air I will be close to the top of the yellow I will adjust power and rpm to keep my prop out of its red line. It's not unusual for me to have to make a 360 or two due to airports that have close proximity to mountains. It's never a problem getting slowed down as long as I stay ahead of the airplane and for me those rapid descents are my favorite stage of the flight. Snoopy loves to go fast.

It was John Deakin in Pelican's Perch #66. The most controversial thing he recommended in that article was a large drop in RPM and not so big a drop in MP to get the power down. He was basicly saying the "never set the MP oversquare" was an OWT (with the exception of during takeoff I suppose).  He didn't go full rich for landing either but kept it leaned all the way to touchdown. A key problem would be if the pilot needed to make a go around and went for full power with just the throttle. Very lean and the engine would sputter as most of us have observed in leaning for idle.  If it were just rich enough to be able to develop power you might overheat (it wouldn't be as bad as at the start of the takeoff roll though since you would have a good flow of cooling air). His answer to that was that pilots should be making adjustments to all three knobs anyway for every major power change. 

 

To do what he suggests safely requires a major retooling in engine management practices from the simple practices that we start with and entails a very personal decision. What do you trust yourself to do consistently?

[aaronk25]

I would because my 77' j has a throttle quadrant....just push everything forward at same time....I'd have to evaluate what I would do with a push/pull set up.

[Bob - S50]

  On 11/10/2014 at 2:35 PM, pinerunner said:

It was Deakins....  His answer to that was that pilots should be making adjustments to all three knobs anyway for every major power change. 

I'm not quite there yet, but may be some day.  Except I might add that any power adjustment should include all ...four... knobs: throttle, pitch, mixture, cowl flaps.

 

I'm trying to always touch all four with every power change, even if I don't need to move them.

 

Bob

[bonal]

My normal GUMPS usually initiated on the down wind and then repeated on base and again on final includes reseting mixture to richen for a full power adjustment I trust myself to do this every time as I also trust myself to drop the gear push the prop forward etc. If i did not I have no business flying an airplane.

[donkaye, MCFI]

  On 11/9/2014 at 12:17 AM, jetdriven said:

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

 

1. Easy on the ears (8'/second).

2. Approximately 3° slope; same as the ILS.

3. Nominal slope below which an engine out aim point will not be met.

4. Simplifies descent rate mental calculation.