Descent profile

[Richard]

I am currently reading, Sky Ranch Engineering Manual, on the operation, failure, and repair of piston aircraft engines. I have also attended the APS seminar put on by Deakin, et al. I'm trying to understand.....

For those of you who operate the big bore engines (-470, 520, 540) what methods do you use to avoid thermal fatigue in the let down for the approach ? Probably for Dave S. but anyone else can chime in: when operating wide open throttle how do you manage your temps in the descent? I mean beyond proper baffling and use of cowl flaps.

I'm probably overthinking this but for quite awhile I've had this nagging question in the back of my mind and I apologize if I've failed to articulate my concern. I trust y'all understand what I'm wanting here.

 

[Dave Siciliano]

Richard:

If you descend with power, CHTs won't take a precipitous drop. Of course, how you do it depends to a degree on the plane.

In the A-36, if I needed to make a rapid descent, I would reduce MP in a couple steps: from 30" to 25" let it stabilize slightly, then put the nose down to get about 250 fpm (leave the prop and mixture alone). While CHTs would drop slightly, the drop was less than 25 degrees per minute. After a minute, MP back to 21" and lower the nose some more. After a couple minutes, CHTs would go from 380 (my max in cruise) to 340. Once at that temp, if you pull back power more (say to 15") and lower the nose, the CHT drop is very slow. Next step is lower MP again, etc. Once below 156 IAS knots, drop the gear if need be. For a normal descent, I leave MP and mixture where they are in cruise and point it down. I can drop 350 to 400 FPM that way without going into the yellow arc. It's all about not cutting the MP back to idle and dropping. Stepped down power settings will work fine.

In the P-Baron, I can drop a little more quickly because I can apply approach flaps and even drop the gear at a very high indicated air speed-around 178 indicated (I'll have to look at the POH). This adds some drag and lets me get a better rate of descent with power.

If one has a JPI, it's very easy to see what power setting will allow what level of descent without CHTs dropping more than 30 degrees per minute (which seems to be where most folks agree is safe). Once CHTs get in the low 300s, we don't worry about 'em because you can't "shock cool" what ain't hot. Even at idle power, unless it's very cold out, CHTs won't drop precipitously from around 300 degrees unless you're doing something extreme.

If Center wants you do expedite the descent, either give 'em 500 fpm for the first 30 seconds, then 1,000 fpm until you get things stabilized and under control; or pull the nose up below gear speed and drop the gear. With the extra drag from the gear, you can get about 2,000 fpm with cruise power and stay out of the yellow arc. Then, reduce power more after a minute or so. I normally step back MP to 25" 21" 17" and have never gotten a warning form the JPI that CHTs are dropping more than 30 degrees per minute (which is where the JPI comes set from the factory).

I normally call approach pretty far out and ask for a gradual descent. Last weekend I called just west of Abilene coming home to Dallas and got a descent from FL210 to 13,000 PD. Pointed it down without changing anything but pitch. TAS was 210 and ground speed picked up about 15 knots while I came down at 250 fpm. Later, I used 500 fpm with no problem in the Baron.

No more guess work with a JPI or other graphic engine monitor. Don't let old wives tales govern: it's now all right there for you to see. This 2" per minute crap drives me crazy. Had a guy telling me this the other day. After chatting a little, he had no idea how to use his graphic engine equalizer in the plane. After we discussed it, the light came on form him. He'd just been doing this the other way (for years) because that's the way he learned (and there was no graphic engine equalizer then). Never occurred to him to monitor temps in the descent on the JPI. (Won't go into what else he wasn't watching.)

Dave

 

[gismo]

I am currently reading, Sky Ranch Engineering Manual, on the operation, failure, and repair of piston aircraft engines. I have also attended the APS seminar put on by Deakin, et al. I'm trying to understand.....

For those of you who operate the big bore engines (-470, 520, 540) what methods do you use to avoid thermal fatigue in the let down for the approach ? Probably for Dave S. but anyone else can chime in: when operating wide open throttle how do you manage your temps in the descent? I mean beyond proper baffling and use of cowl flaps.

I'm probably overthinking this but for quite awhile I've had this nagging question in the back of my mind and I apologize if I've failed to articulate my concern. I trust y'all understand what I'm wanting here.

For the most part, I simply pull back the RPM's a bit to keep the speed from climbing too much on descent and close the throttles a bit every thousand feet once I get to 23-24 inches. I can pretty much leave the mixtures alone whether I'm ROP or LOP when starting the descent although I usually try to maintain about the same EGT's during the descent as I had in cruise.

My engine monitor displays the CHT cooling rate in degrees F per minute and I almost never see anything more than 20-30 and often it's less than 10 unless I really pull back on the power. For reference JPI suggests keeping the cooling rate to something less than 60-80 F per minute.

 

[poadeleted20]

For those of you who operate the big bore engines (-470, 520, 540)

FWIW, the bore on the Lyc O-360 series is the same as the bore on the Lyc O-540 series -- you just don't have as many of them.

what methods do you use to avoid thermal fatigue in the let down for the approach ?

You aren't going to cause "thermal fatigue" (i.e., the intergranular stresses that lead to cracking) with the cooling rates of at most 100 degrees/min you can achieve this way. It takes cooling rates of 100's of degrees per second to cause cracking. The negative effects of the cooling rates typically seen during low-power/high-speed descents are that the cylinder heads cool more quickly than the pistons, and therefore contract more quickly, causing accelerated cylinder wall/ring wear. This can reduce time to overhaul due loss of compression and even blow-by, but does not lead to head cracking.

Probably for Dave S. but anyone else can chime in: when operating wide open throttle how do you manage your temps in the descent? I mean beyond proper baffling and use of cowl flaps.

John Deakin's article on the subject is probably the best summary.

http://www.avweb.com/news/columns/182107-1.html