Sitting there idling at 1200 rpm for minutes, goosing the power to 1800 and shutting down and the turbo is going to be cooler than just after descending with low power and landing?
With the Army Cub (small Lycoming), I usually simply use the magnetos to shut it down, if I know I will start it soon afterward. Like for instance when taxi to re fuel. We also used the same technique on the Pawnee (O540) when shutting down for a short period of time (like taking a leak or a hot dog ) I guess the engine becomes saturated with fuel that way, but that is also the whole idea, even if that is not in general something to recommend. It starts very easily afterward, as opposed to hardly at all. (the Cub has no starter and needs to be hand propped)
I think the recommended Lycoming/Continetal procedures are exclusively meant for shutting down when the engine is going to stay at least as long as it cools down. Funny thing, on the Rotax 912, we (or rather one very clever person) have (after a long time of experimenting), found the opposite procedure works well. Put it on 2000 rpm, shut off the electric fuel pump, let it stay until the fuel pressure is gone, then shut it down. The problem is with fuel pressure when shutting down, the engine gets excessively flooded, and gets almost impossible to start when half warm.
The obvious solution to all engine related starting problems and operating problems is FADEC. IMO it is rather unbelievable that FADEC isn’t a legal requirement on all certified factory new engines today, when thinking about all the other non essential nonsense that certified GA is littered with.
The Army Cubs with Marvel Schebler carburettor do not have idle cut off, even if fitted with mixture control, hence shutting down with mags. As the engine shuts down advance the throttle to lean the mixture and ensure a clean shutdown/avoid dieseling.
I have a big turbo Lycoming and I believe the typical “cool down” considerations are pretty much nonsense. After an approach to land and taxi, the engine has cooled down and the turbo has spun down. Below 29" MP at MSL, the waste gate is wide open anyway.
However, the idle cut off after increasing the RPM does make a lot of sense on Lycomings. The reason is that the oil scavenger pump stops working before the oil pump when RPM is reduced. If I turn off my engine at low RPM, the turbo is full of oil which will drip into the turbo (giving nice smoke at startup) and onto the hangar floor. Therefore I turn off my engine with > 1000 RPM.
The reason for the 1800rpm for shut down is to burn the led in the fuel. In low revs the temps. are not high enough to burn it. As for cooling the turbos. When flying the C-421 I started pulling the throttle while 15nm from the airfield, 1" at a time, than idling for 2min on the ground. It is not just the blades that you want to cool but also the carbon bearing that some turbo systems have.
If a Turbo is shut down when the TIT temperature is too high, one is left with oil that coagulates in the turbo fan.
Remember the Devil: 666!
Let the TIT get below that before shut down.
This is what we do on RAF Tutor (Grob 115E with Lycoming 180hp)
I am not sure what the 1800rpm bit does in a non-turbo scenario. Surely, so long as one is able to start the engine again, one will be at 1800rpm (and a lot more) immediately on the next flight, so anything which needs > 1800rpm to be burnt off will get burnt off.
I used to know a piston twin owner who nearly always got a bunged-up spark plug, but he departed anyway and sure enough the plug always cleared on takeoff.
“I used to know a piston twin owner who nearly always got a bunged-up spark plug”
Why does that worry me ;-)