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If you descend from a high altitude with engine leaned, do you use throttle or mixture for setting RPM's on a fixed-pitch propellor

You should enrich the mixture at lower altitudes or you can have the engine stop.......

Ozzy

...and if you have the throttle closed, don't forget to apply carburettor heat, (if you have a normally aspirated engine), or the carb(s) will ice up and the engine will stop....
Cancel carb heat, and make sure that the mixture is rich, just before landing, just in case you need full power to "go around" or "missed approach", or if you are doing "touch-and go's"... ...!

Cheers... ...!
Paul.
(England).

To explain it somewhat better...

At position 1, i'm at 11000 ft altitude, going some 110Kts at around 2000 rpm and leaned for maximum RPM

At position 2, near the PR NDB, I throttle back to 1500 RPM, mixture stays the same

At position 3 I begin the descent  (FIX 13 is around 8000 ft)
When descending in the turn, i can decrease RPM in 2 different ways: by adjusting the throttle or by adjusting mixture.
When i change the throttle, i need to increase throttle again near FIX 17, to keep the RPM constant when entering the glideslope.
When i use the mixture, i don't have to increase throttle to keep the RPM constant from position 2 on. Only in the glideslope i have to adjust the throttle

The aircraft i use is the Il-103, somewhat strange modelled in FS, because it starts to waggle 20Kts from the maximum published cruise speed and is able to fly some 2000ft higher than published.
It has a fixed pitch prop, and 200hp engine power

From a mechanical standpoint I'd say you'd use the throttle since by using the mixture you will either have to lean it out or enrich the mixture so that you decrease the performance of the engine.

If you run it too lean you stand a chance of overheating the engine and if you run it too rich you could foul a plug and not have full power if you need it. By using the throttle with the mixture set where it should be for whatever altitude you are flying you wouldn't have these problems.

Seems like we're using a Bulldozer when we need a shovel!  :-X

 do you use throttle or mixture for setting RPM's on a fixed-pitch propellor

                  Throttle!  



................richen as you descend-if you have a CHT(cylinder head temperature) gage, or an EGT(Exhaust gas temperature) gage use them. If the Aircraft doesn't have these, chances are, it's not a very "high performance" aircraft, in which case, I would just richen at the beginning of the descent.  ;)

A leaned mixture can either increase RPM or drastically decrease RPM (i.e. the engine stops)  If you descend with a leaned mixture you run the risk of losing the engine as there is too much air and not enough fuel going to the cylinders.  Fuel needs a specific amound of air and fuel in order to burn.  
However, if you are flying at high altitudes without a leaned out mixture then you will have too much fuel and not enough air and the fuel will not burn as efficiently which will cause a reduction in RPM.  In this case leaning the mixture will increase engine RPM.