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One of the items in the helicopter Robinson R22 (and possibly other piston engined aircraft) shutdown procedure is to wait for the Cylinder Head Temperature to drop to a lower temperature. What is the purpose of this?

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According to this document, this is to allow the engine to cool and level the oil temperature:

CHT Drop- Wait two minutes or until you can clearly see the H on Cyl Hd Temp. This is to allow the engine to cool and level the oil temperature. Closing down too early may damage the engine.

This is most probably to prevent damage to the engine due to shock cooling. Any rapid decrease in temperature during shutdown can cause damage to cylinder heads However, note that this theory is quite controversial. This is discussed in detail here and here. Especially, as @Lnafziger points out in his answer,

... in fact the engine cools much more quickly when you shut it down at the end of a flight.

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  • $\begingroup$ I believe an effect of shutting down that way is to allow the cylinder heads to burn off any excess. Leaving the cylinder heads clean makes for an easier startup the next time around. In my opinion it would be equivalent to pulling the mixture to lean. $\endgroup$ – user6035379 Oct 2 '16 at 17:22
  • $\begingroup$ @user6035379 Usually piston aircraft engines are shut down not using the mag switch but by pulling the mixture knob full lean and letting the engine die of fuel starvation. I'm not sure what you refer to when you say "burn off any excess", it probably isn't fuel. $\endgroup$ – Ron Beyer Oct 3 '16 at 2:28
  • $\begingroup$ I was taught that pulling the mixture has a few results. Besides starving the engine to shutdown it also serves as a cleanser to leave the cylinders clean. $\endgroup$ – user6035379 Oct 4 '16 at 6:35
  • $\begingroup$ @user6035379 - I have a few dual hours in the R-22. Straight from my R-22 CFI, the R-22 and R-44 need a cool down period after shutdown. We had to run the engine near idle for at least two minutes after each flight, keeping an eye on the CHT. Since the mixture was all the way in during the cool down period, the max amount of fuel for that rpm speed was being fed to the engine at that time. The excess fuel helps to cool an engine. Remember, the R-22 runs at 80-90% RPMs the entire time from startup. Other aircraft have the time it takes to taxi to their tie-downs or hangars to cool down. $\endgroup$ – Dean F. May 12 at 14:29

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