Originally Posted by gassed budgie
Opening the cowl flaps on final will not 'shock cool' the engine. The engine is already operating at a relatively low power setting, with the aircraft at a relatively low airspeed. The CHT's hardly move if the cowl flaps come open on final. An engine is under its greatest thermodynamic stress at start up. This is where the most rapid change in CHT can be found.
Shock cooling I think, is a bit a of a furphy. I don't no how many times I've seen pilots constantly fiddling with the power settings on descent to avoid shock cooloing the engine. No more the 1" MP change per minute and it has to be at such and such a power setting 5 miles out, so on and so on.
Yet those same pilots in a Chieftan for example at top of climb, will pull the MP back to 31" from 38". A whole 7" all in one go!
In fact the power reduction at top of climb is where you will find the greatest change in CHT's, other than at start up. Not on descent.
Both of those temps are very hot. 232C equals 450F and 265C equals 500F. I like to keep the CHT's in the aircraft I fly at around 350F. Once they go above 385F I start to feel a bit uneasy and if they start to push 400F something needs to be done and quick. The one thing, the prime reason cylinders are junked is because of hot CHT's. I they're kept cool getting them to TBO should not be problem.
If you operate a Cessna 182 with the CHT's constantly at the top of the green arc, the cylinders will be garbage after a few hundred hours. They probably won't even get half way to the engines stated TBO. Many manufacturers over the years have suggested that it's OK to run the engine inside a particular set of parameters. Pilots see the operating procedures in the POH and follow the suggested power settings, only to find the engine lets them down way before it's TBO. To say it's OK to operate and engine as long as it stays in the green arc is not so simple.
Obviously you know the plane, and know whats normal, or similar for that type. If you find the temps/presses are straying from these parametres you would do something about it wouldnt you!