"Shock cooling"

[Learjetter]

Fellas, I searched and didn't find an answer: what is "shock cooling", why is it a bad thing, and how does one avoid it? Finally, how is shock cooling different from engine start/runup--isn't that shock heating?

Haven't flown pistons in years...your insight appreciated!

[WebMaster]

Not a problem with Turbines.
If you are cooking along at FL040, it's not a problem. If you are cooking along at FL180, it can be.
Here's what happens.
The engine is at a temp, say 350. Then you descend quickly, and the temp RAPIDLY decreases, to say 180. That sudden decrease in temp to the cylinder head causes thermodynamic stresses, and frequently ends up in a crack.
That's why it's a bad thing.
If you come to Dayton, Dr. Brian Von Herzon will discuss shock cooling at length. It's really a problem for the 520's (which he happens to own) which normally operate at altitude, and then are subjected to slam dunk approaches by ATC.

[Learjetter]

Thanks Larry--I hope to make it to Dayton--even if only for a few hours one of the days.

Not to be argumentative, I get it that you need to reduce power to keep speed under control in the descent. But on engine start, let's say the motor's at 80 deg ambient temp, right after start it jumps to , say, 300+, right? That's tha same thermal stress you're describing, just in reverse, no?

[WebMaster]

Actually No.
If you watch the gauges, when you start an engine, you have max mixture, normally. That helps cool the engine. So, on start up, CHT and EGT are typically low. These temps gradually increase, which is why you want to warm up the engines, before take off. When you are at cruise, you have the mixture leaned out, and that makes the engine more efficient, and raises CHT and EGT.
Hal Stoen has some discussion on this. http://www.stoenworks.com/Aviation%20home%20page.html

http://www.stoenworks.com/Tutorials/...res%20and.html

[Learjetter]

Good stuff, Larry. Thanks!

[Red Air Rambo]

If you are getting a T or P model the speedbrakes really help getting down from alt without pulling the engines back too far...I think the rule of thumb is 2 inches of manifold pressure every 2 minutes.

Brent